on the evolution of drill-bit shapes

Journal of Mechanical Working Technology, 18 ( 1989 ) 231-267 231 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

ON THE E V O L U T I O N OF D R I L L - B I T S H A P E S

C.J. JACKSON, S.K. GHOSH

Department of Mechanical and Computer-Aided Engineering, Staffordshire Polytechnic, Beaconside, Stafford ST18 OAD (Great Britain)

and W. JOHNSON

Schools of Engineering, Purdue University, West Lafayette, Indiana 47907-0499 (U.S.A.

(Received December 30, 1987; accepted January 10, 1988 )

Summa ry

Drilling tools play important roles in woodworking, metalworking, mining and quarrying. The markedly different physical properties of wood, metal and rock have largely influenced the variety of tool shapes and different techiques employed for drilling these materials. From the wide range of shapes four groups can be segregated, viz. semi-cylindrical, flat, helical and hollow, dictated in part by the tool materials and manufacturing facilities available to producers. All users have ex- perienced problems of procuring, servicing and manipulating drilling tools. The present study is devoted to an examination of the evolution of the basic shapes of drill bits and offers a hypothesis which unifies them.

1. Introduction

The shapes of many drill bits in use today appeared before and during the nineteenth century. In the absence of precise data on drill performance, artisans sought to improve the shapes of these tools guided by their intuitive understanding of the cutt ing behaviour of materials, by the manner in which the bits were to be manipulated (manually or by steam power) and by the constructional materials and facilities tha t could be utilised for tool manufacture.

A complete account which explains the evolution of drill bits must consider also creative skills, ethnic att i tudes towards innovation, trade practices, fracture properties of solids and the influences of economic conditions in creating demands for these tools; an analysis of each of these factors lies outside the scope of this paper. The discussion which follows presents the results of some deliberations on features common to drill bits employed in cutting a wide range of materials.

Since drilling and boring operations are performed in many industries, different terminologies relating to these techniques have emerged. In the woodworking and metalworking trades, for example, the terms "drilling" and

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"boring" are traditionally associated with methods of removing materials using cutting tools subjected to combined rotary-axial motions so that the cutting edge, at the end of the tool, follows a helical path. In contrast, during percussive drilling operations, employed in the mining and civil engineering industries, rotation of the bit takes place between successive blows producing indentation and localised fracture of the rock. Indentation, penetration and perforation processes are encountered also in metalworking and have specific meanings [1]. Ultrasonic, electro-discharge (spark) and electro-chemical methods employ indentation, erosion and electrolysis respectively to remove material but are not generally regarded as edged-tool drilling processes, which are the subject of the present study. For the purpose of discussion the following definitions [2 ] will be adopted.

A drill bit is a tool which, when fed into a workpiece, produces a hole where virtually any shape can be obtained with an appropriate design of bit. In doing this, material is ejected as swarf in a direction opposite to the direction of travel of the bit which may, or may not, rotate about an axis parallel to its direction of travel. A consistent terminology has been adopted for the names of drill bits [3], which may differ slightly from that used by some other writers but this will become clear in the relevant sections of the work.

Primary drilling is the manufacture of a hole in solid work material by means of a drill bit, as defined above, where all the material removed from the hole is ejected as swarf or detritus. These are the processes included in the present study.

Secondary drilling is the cutting of an existing hole to a larger size, by means of a drill bit and removing the material entirely as swarf or detritus. This group includes reaming, spot facing and recessing operations, etc., and will not be discussed here.

Associated processes include slot drilling, end milling, die sinking and en- graving, which employ cutters capable of machining holes to shallow depths, but these tools do not comply with the definition of a drill bit given above.

Drill bits possess certain features that make them uniquely different from other cutting tools. The diameter of the hole and the necessity to eject cuttings determine the cross-sectional area of the bit thus imposing a restriction on the amount of metal and its distribution in the tool. The fact that the whole process takes place unseen, inside the hole, makes direct observation of chip formation very difficult.

Clearly the work material, the diameter of the hole to be cut and its depth exert strong influences upon the type of drilling technique and the shape of the bit that is employed in a given situation. For example, orthopaedic and dental surgeons employ different drill bits and avoid thermal necrosis in bone and dentine respectively by controlling the thrust applied to the drill point. The medical profession has, with certain exceptions, tended to adapt commercially available instruments that have been developed for drilling other materials.

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One shape of bit is not necessarily confined to drilling one particular type of material. In general, the markedly different physical properties of wood, metal and rock have exerted the most profound influence on the shapes of drill bits. Therefore, to provide a framework for discussion, each shape will be assigned to one of the three main groups of users of drill bits in woodworking, metalworking and rock drilling.

Many drill bits in current use were developed in Europe, Great Britain and North America after the mid-eighteenth century, a period regarded by many economic historians as the beginning of the industrial revolution in Great Brit- ain. Drilling developments that originated in other areas of the world have not been included and will be the subject of a separate study.

2. Dr i l l b i t s b e f o r e c . 1 7 6 0

Evidence exists of the use of pointed flints during the late Palaeolithic and Neolithic periods for gaining access to the brain through the skull. Prehistoric practices of making openings in human skulls were apparently widespread and in many cases successful because there is clear evidence of healing of the bone around the edges of the holes in recovered skulls. Egyptian stoneworkers employed a similar piece of pointed material attached to the lower end of a wooden shaft with two stone masses attached to the upper end to provide rotary momentum [4].

There are many examples of the early uses of drilling tools [5] but they usually lack sufficient clarity to enable the type of bits to be positively identi- fied. The connection between spears and boring tools appears in Anglo-Saxon literature [6]. The word auger is derived from nafu, meaning "the nave or middle of a wheel", and gar, a word associated with a dart, javelin, spear and arrow. Subsequently, the initial "n" became attached to the indefinite article to form an a[ugar which eventually changed to "augar" and then to "auger", its present form. This description neither indicates the shape of the bit nor how it was used. However, a drill bit found in Hurbuck, County Durham, and dating also from the Saxon period [ 7 ] was of a shell form similar to a gouge.

A collection of Russian carpenter's augers which has been dated at some- where between the tenth and thirteenth centuries [ 5 ] is particularly interesting. In addition to spoon augers, the collection contains spear-point twist augers with an eye at one end to receive a transverse wooden shaft. This fits the description of an auger given some five hundred years later by Abraham Rees [81.

A gimlet is a variation of a twist auger incorporating a screw point which gradually merges into helical flutes. These were probably first made in Nurem- burg about 1525 [9 ]. An unusual design of wooden auger, intended for boring timber, was patented by William Wheeler and John Cropley [10] but no details of the instrument have been given. Some of the earliest evidence which

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enables the identification of the types of augers and gimlets in general use before 1760 is provided by Joseph Moxon [11 ]. His illustrations are not clear but one can discern a flat spade-bit used in smithing, a screw-point helical gimlet and the gouge-shape bit for woodworking.

Metals, unlike wood and stone, can be shaped by casting and forging using well established techniques. Gun drilling had evolved its own specialised techniques. Before the eighteenth century, cannon were cast hollow and the bores finished using multi-blade cutters resembling reamers. Rifle barrels were made in a similar manner from forge-welded twisted strip and the bores finished with the aid of a series of long rods, of increasing size, which scraped away small amounts of metal until the required bore size was obtained. In these methods the cutters had a tendency to follow the axis of the original rough bore and alignment could not be guaranteed. This problem was largely overcome by casting or forging the barrels as solid pieces and then drilling out the central material to form the bores.

Slender drilling tools were used also deep within the ground for seeking water, brine, coal and, later, oil. The need for salt taxed the ingenuity of engineers to devise methods of extracting it from great depths, where it could not be obtained from surface deposits. Needham [ 12 ] has described how Chinese engineers, as early as A.D. 1036, employed a round boring tool, the size of a bowl, and drilled to depths of several hundreds of feet. Blows were delivered to the bit by a team of men jumping on and off a beam while the bit was rotated between blows. A single well could take up to ten years to complete by this means and depths of 3,000 feet have been recorded. Large bamboo stems, with the nodes removed and fitted together by male-female joints, were inserted to form boreholes after which fresh water was poured down the side of a tube forcing brine to ascend and emerge. Pieces of bamboo tube, with a leather flap at the bot tom end, were used as buckets. A similar method was to be used about eight centuries later for oil drilling in California. In Western Europe, water was obtained from shallower depths and the sandy soils in countries such as Holland enabled wells, literally, to be scoped out of the ground. In the early seventeenth century Marinus Mersennus gave an account in his Phaenomena Hydraulica, communicated to him by a "Mr. Hugens", of the sinking of a well, to a depth of 232 feet, in Amsterdam using an instrument resembling a semi- circular iron hoop fitted with a close-mesh bag for holding dislodged sand [ 13 ]. This was essentially a rotary scoop and was unsuitable for use in hard rock at great depths.

For drilling holes through hard strata there is no indication that the bits were much unlike those employed in the late eighteenth century in Europe and North America, see below. Crown drills, which have fixed teeth of corundum (opaque ruby and sapphire) - or gem stones, were used for cutting cores from quartz rocks in Egypt about 6,000 years ago and in Greece about 4,000 years ago [14].

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William Hooson [15] describes three type of drill bits used by miners for cutting holes to receive blasting charges. "Winged bits" (presumably carpen- ters' bits) were sufficient for drilling into soft stone while square-section pointed spikes and wedge-type chisels found applications for cutting holes in harder rock. Hooson claimed that the chisel, made of good steel and well tempered, became the most popular implement since it could be easily made to any size to suit the miner.

Robert Multhauf [ 16 ] has provided a useful summary of drilling techniques employed in the salt industry in various parts of the world and the bits that he describes generally subscribe to the foregoing details.

Details of some early drill bits have not been included in this brief summary since they can be discussed more conveniently in their appropriate group.

3. Developments after c . 1 7 6 0

Around the mid-eighteenth century more reliable evidence began to appear in patent specifications and in books on workshop practice, mainly in Europe and North America, and accounts emerged also from other countries where drilling techniques had not undergone the same dramatic changes. Some techniques were unique to the country or region where they were used, however no at tempt will be made here to draw conclusions from similarities with trans- Atlantic practices, except where names have persisted such as the Chinese method of mineral drilling, already mentioned. The work included in the present study relates to what are believed to be original occidental techniques and this is intentional since the researches of Joseph Needham [12] and his fellow sinologists are still shedding new light on the East-West flow of ideas in technology. There are of course other writers to whom one can turn; for example, Rudolph Hommel [ 17 ] recorded some of the drilling tools used in China during his expedition there in 1921. Reports of ethnologists [18] also provide useful items of information. From all of these sources, and the earlier evidence, a general pattern of drill-bit development begins to emerge and subsequent innovations can be assigned to one of four groups, namely, semi-cylindrical, flat, helical and hollow shapes, each under the headings of woodworking, metalworking and rock drilling.

4. Drill bits for wood

Wood is an important constructional material and is strongly characterised by a pronounced grain structure; for this latter reason it has played a central role in influencing the shapes of drill bits. Unlike metal and rock, it can be charred by the application of hot implements to enable holes to be shaped but the carbonised layer has at some stage to be scraped or cut away using additional tools. The application of heat in this manner dries the wood in the im-

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mediate vicinity of the hole producing a marked change in moisture content which may cause the wood to split.

4.1 Semi-cylindrical bits In the woodworking trades, semi-cylindrical bits, shown in Fig. 1, had the

widest field of application up to the mid-nineteenth century, when they were replaced by twist augers. These bits could be used to approach the grain in different directions, a feature which made them attractive for a variety of applications.

The simplicity of the semi-cylindrical form rendered it amenable to manufacture using basic hand-tools, a factor that undoubtedly made these small implements attractive to labour-intensive rural industries. Pump-makers, coopers, brush-makers, shipwrights and wheelwrights all acknowledged these tools as encumbents of their workshops.

Spoon augers had curved tapered points which were not conducive to ensuring alignment of the bits in the wood; the responsibility for ensuring the straightness of a bored hole rested with the workman who had to monitor continually the progress of the tool through the wood. It is interesting, therefore, to note the use of the plain and screw-point versions during the sixteenth century for boring wooden pipes, used in mines, and illustrated in George Agri- cola's book of that period [19]. The hook-point shape was employed in water- operated boring machines in Nuremburg about 1661 [20]. By the seventeenth century, spoon augers seem to have disappeared from the industrial scene in favour of shell (gouge) and pod (lipped gouge) designs since these tools not only assisted the workman in boring a straight hole but also facilitated extraction of the cuttings.

At the beginning of the nineteenth century, open-ended parallel-sided bits were adopted by many of the woodworking trades. When drilling a cylindrical hole the parallel edges of the bit guided it in the hole once cutting had com- menced, but when using taper augers the straight edges also were required to cut the wood. A shell auger, as these bits were called, was essentially a gouge provided with a handle and the function of the cutting edge was to shear the fibres of the wood transversely across the grain. The names "bung borer" and "tap borer" were given to the square-end and screw-point taper augers respectively, around 1816; this suggests that these tools were used by coopers. The round-nose shape was also known as the gouge bit or quill bit and was in general use around 1846 [21 ]. Neither the parallel nor the taper shell-augers produced shavings by a clearly defined cutting action at their ends when drilling into a solid piece of wood since material was sheared and then displaced into a central core; these bits exhibited a tendency to split the wood. This disadvantage was virtually eliminated by providing a turned-in cutting edge at the end of the bit, thereby partially closing the end to form a pod.

A pod auger, with its cutting edge lying almost in a plane parallel to the

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surface of the wood, enabled a suitable rake angle (see Appendix) to be obtained by trial and error, unlike a twist auger, in which the rake angle was determined by the helix angle. Round-end pod augers were very useful bits, one beneficial characteristic being their facility to drill holes in inclined and curved surfaces; for this reason they were adopted by chairmakers and brushmakers. Cutting occured when the bit was rotated in either direction so that it was not necessary to make a full sweep of the brace; this enabled holes to be drilled in confined spaces. Absence of a screw point meant that an arbitrary feed was not imposed on the bit; there was less risk of splitting in wood and the bit could be used to drill a hole to within a very small distance of the full thickness of a piece of wood without breaking through. Cutting a notch to one side of the point produced a parrot-end pod auger; the function of the notch was to stabilise the bit at the commencement of drilling.

Another version in common use about 1846 was the spear-point pod auger. This bit was used by coopers for drilling dowel holes in tables and indeed it acquired the names coopers' dowel-bit or table bit.

In 1816 John Sorby [22 ] incorporated screw points into a pod auger and into a straight-fluted auger having an "S" section. Unfortunately, regarding the latter innovation, Sorby does not provide details of what would appear to be an interesting and unique cutting tool. It seems that he was at tempting to utilise two curved cutting edges that operated simultaneously, instead of the single shearing action employed in the split-lip pod auger.

Small sizes of split-lip pod augers were employed by cabinet makers, carpen- ters and shipwrights, and the larger sizes from two inches to four inches in diameter were adopted for boring trees for producing pumps and wooden water pipes. Wooden pumps usually comprised two lengths of tree aligned by a conical joint. Each tree could have a length of up to about fifteen feet; it was squared up on site and then bored from each end. It was claimed that sometimes the cutting action of the bit could be heard two miles away! Alignment of both bores, in each tree, was essential for efficient operation of the plunger, or bucket, when in use [23,24].

A flat end on an auger seems to have been the most common design and it was this feature that created difficulty in starting a bit into wood. An obvious solution was to provide a screw point to stabilise the bit initially, although Richard Timmins seems to have been the only manufacturer to illustrate such a bit in his catalogue [25]. There is a similarity between this design and those patented by John Sorby, ment ioned earlier; the similarity is apparent in the straight-flute gimlets which appear also in Timmins ' catalogue. Evidence of the manufacture of twisted gimlets goes back to at least 1526 [9], though the origins of straight-flute gimlets are not known. Gimlets constitute a link between screw-ended pod shapes and twist augers.

An unusual development in pod augers appeared in 1868 when Cornelius Whitehouse, an edge tool maker in Cannock, was granted a patent [26] for his

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open-pod auger. His method of manufacture was ingenious. Whitehouse forged a strip of steel into a deep "U" shape with a stud projecting on the outside of the "U" bend; next the upper ends were forge-welded together to form a closed elongated loop with a tang at the top end. The auger was finished by grinding and a screw thread was cut into the stud. Whitehouse claimed that the space in the middle of the bit facilitated the ejection of wood chips, thus preventing clogging of the bit. Although it could be made with simple hand-tools and did not require the use of special machinery, its obvious lack of torsional rigidity was a disadvantage; this deficiency was not shared by its main rival, the twist auger, which Whitehouse later manufactured. Before proceeding to discuss this important group of drill bits, consideration is made of some which have a very simple shape and which also inspired the introduction of bits where the widths could be adjusted to cut holes to different sizes.

4.2 Flat bits and adjustable bits Shell augers and pod augers competed with centre bits in shipyards for

"wooden sides" and in cooperages, where planking was used extensively. The nicker, which is a distinctive feature of centre bits, shown in Fig. 2 (a), enabled cleaner holes to be cut when drilling across the grain. These bits were unsuitable for drilling holes inclined to a surface. Centre bits had their counterparts in the spade bits employed in engineering metal workshops and in the chisel bits used in rock drilling.

In 1804, M.I. Brunel adopted a centre bit in his Portsmouth block-making machinery for drilling holes in pulley shells [ 27 ]. Other versions are illustrated in Smith's Key [28]. Holtzapffel [21] stipulated that the length of the cutter should be made slightly less than the radius of the nicker, thus ensuring that the nicker produced the required finished hole size and that the cutter removed only the waste material. When drilling holes greater than about 2 inches in diameter it was desirable to break up the chips of wood to facilitate their re- moval from the hole and an additional nicker was provided for this purpose. Some manufacturers made centre bits with a screw point for reasons which are not apparent, since the simpler pyramidal point is sufficient to centralise the bit when drilling soft, fibrous materials such as wood. When tapping casks containing liquids the conical shoulder on the centre bit plugged the hole when

Fig. 2 (a). Centre bits.

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the cutting edge broke through the wall of the cask, thus preventing discharge of the contents. The provision of a cylindrical plug in front of the drill point enabled a recess to be cut into the surface of a component concentric with an existing hole.

To obviate the necessity for using one size of bit for drilling one size of hole, the notion of an adjustable bit attracted the attentions of inventors during the latter half of the nineteenth century and centre bits provided the inspiration for these cutters. Between 1855 and 1894 all the British patents for adjustable

(i) Sliding Blade (Newton,1855)

I,. , . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

(ii) Hinged Blade (Brooman,1856)

(iii) Eccentric Blade (Tucker,1858) Fig. 2 (b). Adjustable bits [29-31].

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and renewable bits were, with two exceptions, granted to, or on behalf of, for- eigners, mostly Americans. Some of the patentees displayed an understanding of the cutting properties of wood in the design of their bits. Those who did not devote the same attention to detail soon saw their invention become obsolete. In addition to the sliding-blade design [29], shown in Fig. 2(b) , there were hinged-blade arrangements [30] and one incorporated an eccentric cutter [31 ]. A useful feature of these tools was that the adjustable blade could be renewed if it became damaged.

Renewable blade bits were patented [32,33] but the use of a separate blade was generally deprecated and the trend was to adopt a single-piece solid construction to obtain maximum rigidity.

It was only a small step to adopt a separate profiled blade and this consti- tuted the essential feature of a group of cutters employed in surface sculptural work [34 ].

4.3 Twist augers Neither semi-cylindrical bits nor flat bits effectively eject cuttings out of a

hole but a helical flute had long been recognised as a means of performing this function. This innovation had appeared as early as the tenth and thir teenth centuries and it was a distinctive feature also of certain types of gimlet. There is a similarity between gimlets that have a screw point which merges gradually into the flutes, and the Viking auger which had a curved, tapered or spear point that merged in a similar manner. The evolution of the modern twist auger was centred around the search for solutions to problems associated with the manufacture of augers and it is the a t tempt to overcome these difficulties which sheds some light upon certain metalworking practices, notably improved forging techniques that were being developed in North America during the nineteenth century.

Some basic shapes of twist augers are shown in Fig. 3. The origin of the form that is known today may be at tr ibuted to Phineas Cooke who, in 1770, was awarded a bounty of 30 guineas by the Society of Arts, for his newly constructed spiral auger [35]. It was claimed that this auger pierced wood much easier and truer than common augers (tests were carried out on fir, oak, beech and mahogany) and that it obviated the necessity to start the hole by picking with a gouge. An improved version appeared in Smith's Key in 1816 [28]. To produce this shape by forging demanded a certain degree of skill and in North America, where craft skills were at a premium, a t tempts were made to produce alternative versions using machine methods.

Dr. William Church was granted a British patent [36] for his twisted auger and for a machine for producing it. He was born in North America and lived in Birmingham, England, for a number of years. This invention was not one of his more notable achievements but it provides a glimpse of a t rend that was taking place in his native country at tha t time. Another version [37], fitted

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(Cooke,1772)

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Fig. 3. Twist augers [26,28,35-40].

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(Gedge, 1854)

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(Whitehouse, 1868)

Fig. 3. {Continued.)

with a cutting blade, originated from the same country sixteen years later. John Cleaveland Palmer dispensed with the central rod in his single-flute twisted version, but described how it could be manufactured [38]. Although twisting methods were not abandoned entirely, die-forged double-flute shapes began to appear in North America around 1854, when Ransom Cook introduced a variety of point shapes. In England these became known as the Gedge pattern, named after the British representative in whose name the British patent was filed [39]. Twelve years later the famous Jennings pattern made its appearance [40]. Between 1824 and 1896 twelve British patents were granted to, or on behalf of, Americans for innovations to twist augers, out of a total of eight- een granted during that period [41 ].

The solitary unique British contribution came from Cornelius Whitehouse of Cannock [26 ], for his eyed and winged augers, for which he obtained patent protection in North America. These types of auger can still be found in woodworkers' toolchests today. The Whitehouse family had learned the craft of auger-making at the factory of William Gilpin nearby.

In 1891 Gilpin experimented with the manufacture of triple-fluted bits. The helical versions included a winged point, for drilling wood, and a vee point for drilling metals. His straight-flute designs were reserved for parallel and taper reamers. Though he was skilled in the production of hand tools, for use by farmers and woodworkers, his venture into metal cutting tools was abandoned.

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Edge-tool manufacturers began to realise that they had to specialise in making either woodworking tools or metalworking tools if they were to stay in the hand-tool market. Later it became necessary for them to acquire some specialist knowledge in order to meet the demand for rock-drilling tools.

4.4 Cylindrical saws, trepans and trephines It is not always desirable, or necessary, to break up the material being re-

moved by the cutter and in fact to do so when drilling large-diameter holes would require a large torque. This can be minimised by cutting an annular groove in the material to remove a solid cylindrical piece; the cutters that fall into this category are shown in Fig. 4.

A cylindrical saw consists of a piece of saw blade bent into the form of a cylinder. This type of cutter was used for a wide range of work, including the shaping of boards for tubs and wheel segments and for making corks and bungs using the lathe. Brunel used this method for shaping pulley sheaves in his Por tsmouth block-making machinery.

This technique was employed also in bone surgery. Charles Holtzapffel [21 ] claims that the trephine saw "appears to have been by far the earliest circular (cylindrical) saw of this kind". This instrument was rotated either by means of a cross handle, similar to a corkscrew, or by means of a brace, similar to those used in carpentry. A small guide point was located in a central hole in the spindle and held there with a side screw. At the commencement of drilling, the point was set to project ahead of the cutting teeth to make a small central hole in the bone. When the point had penetrated a sufficient distance the side screw was released and the point allowed to fall back as the teeth began to cut.

A single cutting tool mounted on the end of a bar, rotating at a fixed distance about a fixed axis, consti tutes another type of trepanning cutter used for making holes and recesses or for producing discs from thin sheets of material. Vari- ations of this tool, which was essentially a single-tooth version of a cylindrical saw, are used to cut radii over a wide range of sizes.

4.5 Polygon bits The shaping of non-circular holes, such as those which have square and hex-

agonal profiles, requires additional operations after the initial cylindrical hole

(a) (b) (c)

Fig. 4. Hollow bits. (a) Cylindrical saw, 1861; (b) trephine, ca. 1846; (c) trepan, ca. 1846.

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has been cut. There is the obvious attraction that a saving of time can be achieved if a cutter can be devised that eliminates the additional operations. This problem was approached in three different ways, in America.

In 1864 Benjamin Merrit patented a piece of apparatus comprising a vertical shaft with pinions attached to the upper and lower ends. Rotation of the upper pinion was through a rack under the control of a follower in a square track. This motion was transmitted to the lower pinion meshing with opposed racks attached to cutters [42]. However, the motion of the follower in the track is not clear, since it would appear to lock in the square corners.

A simpler piece of apparatus was patented by Alexander Allan in 1869. He adopted a guide plate, into which a square hole had been cut, to receive a drill bit having an equilateral triangular section; the length of the side of the tri- angle corresponded to that of the square. When the triangular-section bit rolled around the inside of the square hole in the guide plate, a square hole was generated in the workpiece [43 ]. A similar arrangement has been retained in modern equipment.

A yet different approach was adopted by Azariah Yeldon Pearl, in 1896, by actuating two opposed cutters from a starwheel. The popularity of the broach- ing process effectively stifled further interest in this type of drilling technique.

5. D r i l l bits for metal

The gradual replacement of wood by metal, from the mid-eighteenth century, introduced new standards of precision into hole-making processes, which had become such an important feature of engineering manufacture. Many of the processes which have since become established in engineering workshops appeared during the period covered by this study. Hole sizes ranged from the large bores of steam-engine cylinders and cannon, demanding standards of accuracy which had not previously been achieved, down to the smallest rivet holes required in large numbers in boilers and bridges. In addition to the point geometry, dimensional and geometrical accuracy is influenced also by the alignment of the drill bit in the drilling spindle and the shank of the bit plays an important role in satisfying this condition. In consequence there is a closer relationship between the drilling tool and the machine in the metal engineering workshop than between the corresponding elements in either wood-drilling or rock-drilling equipment. Each of the four groups of drill bits, namely flat, semi- cylindrical, helical and hollow, assumed roles of varying importance in the metalworking industries.

5.1 "D" bits and gun drills It is common practice in some workshops to make special-size drill bits by

taking a piece of steel rod, of a diameter the same as that of the hole required, and filling or grinding away almost one half of the diameter to form a "D"

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247

C

Fig. 5. "D" bits and gun drills. (a) "D" bit, ca. 1846; (b) gun drill, ca. 1868; (c) gun drill, ca. 1890.

section, see Fig. 5. A point is ground on the end and some care is needed when "backing off" the point to obtain a satisfactory cutting edge. Finally the point may be hardened by heating it in a flame followed by quenching in either oil or tallow, depending upon the size of the bit. This technique has been practised by instrument makers since at least the early nineteenth century. The same type of bit, with minor modifications, was adopted for drilling gun barrels of up to 16.5 inches diameter bore at Woolwich Arsenal in the 1860s and it has formed the basis of the subsequent design of modern gun drills.

5.2 Flat bits When it was decided to bore cannon and mortar barrels from solid castings

at Woolwich around 1770, the Verbruggens [44 ] adopted a flat steel strip that was pointed at one end and constrained in guideways to slide into the rotating casting. Small-arms manufacturers employed a similar technique for drilling rifle barrels in the lathe. The principle was to rotate the barrel and provide a

248

C

k

Fig. 6. Spade bits. (a) Watchmaker's drill bit; (b) bow; (c) double-cutting spade bits; (d) single- cutting spade bits.

rigid sliding constraint for the flat drill-bit. A similar type of bit was adopted in lathe work and is still used today.

It was customary for watchmakers and jewellers to make their own spade bits, see Fig. 6. The type of cutting edge provided at the point was dictated by the manner in which the bit was rotated. A bow, or fiddle drill, a pump drill and an Archimedean drill all provided an oscillatory motion and the bit scraped away metal during each stroke. A brace enabled a continuous rotary motion to be imparted to the bit, which could therefore be provided with a positive rake angle. Mechanics succeeded in drilling larger sizes of holes using a bow in con- junction with a thrust pad strapped to their waist.

A spade bit wears quickly across the corners, causing a reduction in diameter, and does not readily maintain its al ignment in the hole. Neither "D" bits nor spade bits eject cuttings from the hole. There is the additional, and important, limitation of obtaining suitably shaped cutting lips. Woodworkers had overcome all of these difficulties with twist augers and it was natural that mechanics should adopt a similar approach.

249

5.3 Twist drill-bits A twisted steel bar, originally of square section and forged to a point at one

end, met with only moderate success in practice since this tool had a tendency to crush, or grind up, the cuttings in the hole. Twisted flat strip, on the other hand, offered more space for the cuttings and bits made from this material were manufactured commercially. Details of these bits are not available but Cameron Knight [45] provides a clue to their possible likeness, see Fig. 7. He describes both these bits as "screw drills" and goes on to explain that the bit in Fig. 7 (a) has been made from a piece of strip which has been flattened and then twisted, the diameter of the screw part being very nearly equal to the short straight portion at the end. The screw portion guided the bit, when drilling deep holes, without much friction. No description is given of the methods used to produce the helical grooves shown in the drill of Fig. 7 (b), which more closely resembles a machine-cut twist drill-bit. Flat, twisted strip has its limitations and in fact any gain in torsional stiffness which accrued from improved grain flow in the steel was offset by the lack of material in the bit itself. In addition, the absence of a front rake angle, although militating against the tendency for a bit to pull itself into the hole, nevertheless imposed a restriction on the range of materials for which this type of bit could be used. One solution therefore was, instead of using flat strip, to twist the bits from specially rolled section (Fig. 7(c)). This technique was practised in Britain and North America between 1834 and 1899, possibly for manufacturing larger sizes of drill bits. Meanwhile, attempts were being made to employ forging methods.

In 1877 three Americans, C.F. Jacobson, G.E. Maltby and J.C. Jones, obtained a British patent for a machine to forge twist drill-bits. The bits were made by forcing heated blanks, which had been grooved on opposite sides by rolling, longitudinally through a die containing a helical groove of the appropriate lead; in the latter, small segments were located and retained by an outer cone that enabled adjustments to be made to accommodate different sizes of drill bits. There is no evidence to suggest that this method was implemented on a large scale. Manufacturers relinquished forging and twisting methods; most of the effort that had been devoted to improving these techniques, after

a ~___~~------ ~ 3 r ~ >

b

Fig. 7. Twisted bits. (a,b) "Screw" drills; (c) rolled section.

250

1860, was in response to developments that had been taking place in the machine shop.

American mechanics made twist drill-bits by filing them out of solid pieces of rod and, in 1861, Joseph R. Brown, of the Brown and Sharp Company, Rhode Island, was approached by Frederick W. Howe to resolve this situation ur- gently by devising a method for machining helical grooves in twist drills to replace the slow and expensive process of hand filing [46]. Brown's Universal Milling Machine proved to be not only an ideal solution to this particular problem but also found wider application in the machine shop. The principle of synchronising the rotation of the drill blank with the table motion as it trav- ersed past an inclined rotating cutter embodied in this machine was quickly adopted by other machine-tool builders. Problems associated with point geometry and the manufacture of these bits in large numbers were resolved by Stephen Ambrose Morse, who set up a workshop in East Bridgewater, Mas- sachusetts, for producing these bits. Smith and Coventry of Salford, Man- chester, was one of the first British companies to manufacture twist drill-bits commercially around 1878 and tests carried out by them over the next five years demonstrated the superiority of these bits over spade bits.

5.4 The taper shank Time spent in producing a correctly ground symmetrical point is wasted if

the axis of the bit it not set and maintained coincident with the axis of the machine spindle. Furthermore, provision has to be made to achieve this condition quickly and consistently each time a bit is changed. Woodworkers had solved this problem, with some measure of success, by adopting a square taper (pyramidal) shape for the shank which ensured a reasonably secure fit in a corresponding hole in the brace. A square taper offered convenience for manufacture in either wood or in metal; in addition, a positive drive was ensured from the brace to the bit. Where a brace was provided with a single hole to receive a shank then all the bits intended for use in that brace would each require identical shanks. If the fit between the hole and the shank was not a particularly good one, a stud was sometimes provided to prevent the bit from falling out of the brace.

The next development was the adjustable chuck which enabled a range of shank sizes to be used in one brace. Though improvements in drill chucks enabled cylindrical shanks to be gripped securely whilst maintaining concen- tricity of the bit, still the friction force between the shank and the chuck jaws imposed a limit on the size of bit which could be accommodated. An efficient drill chuck did not provide a universal solution because alignment and concen- tricity between the chuck and the spindle, unless the chuck was made an in- tegral part of the spindle, still had to be ensured. The solution was to provide a conical shank. Writing in 1856 a British engineer, Thomas Forsyth, de-

251

Fig. 8. Samuel Colt's taper shank and taper sleeves, 1854 [48].

scribed how eccentricity associated with a cylindrical shank was avoided by adopting a slightly tapered form drawn up tight by an end screw [47].

Two years earlier Samuel Colt had adopted a taper shank for locating and holding cutters in his small-arms machinery plant, see Fig. 8 [48]. It would appear that Colt and Forsyth arrived independently at the use of a taper shank within about two years of each other. Colt's use of a tang and taper sleeves puts him ahead of his time in the use of interchangeable tools in metalworking machinery.

5.5 Trepan-boring bits Drilling relatively large-diameter holes deep in solid pieces of metal required

large machine tools and this places such work outside the capacity of small workshops. In 1869 two Manchester engineers, William Sumner and Eric Hugo WaldenstrSm, were engaged in the manufacture of copper cylinders for calico

t Fig. 9.Trepan boring of copper billets, 1871.

252

printers and one of their problems was to machine holes down the centres of the billets. For this purpose they made a thin steel tube with teeth at one end set to provide clearance between the tube and the surfaces of the hole and the central core. Coolant was supplied down the centre of the tube to flush away the cuttings [49], see Fig. 9. This principle, but with a modified cutting head, has been retained in modern trepan-boring machines. In this type of work the words "drilling" and "boring" are used synonymously - the technique is associated with gun-boring practice.

6. Dri l l bits for e x c a v a t i o n

In the mining and civil-engineering industries, drilling tools are used for activities which may be classified according to size, under two broad headings. For the first are examined tools employed for drilling holes to relatively shallow depths, for example, to receive explosive charges, fencing posts, telegraph poles and land drains and for the second are considered larger tools employed in geological exploration and in the sinking of wells and mine shafts, which will be treated in Section 7 below. These operations require additional equipment for removing or by-passing broken bits and for lining bore holes. These activities are not encountered in other drilling situations and require specialist skills.

6.1 Impact bits In quarrying and when laying railway track, men worked in groups of two or

three, one man holding and rotating the drill rod and the others striking the rod with sledgehammers. A modification of this method was known as "churn drilling" in which a large drill rod was swung upwards by two men and was either allowed to fall or was struck by sledgehammers. When cutting very deep holes the weight of the rod itself was usually sufficient to deliver the blow, the long rod being lifted up and thrown down by two men. Before the introduction of powered machinery at the rock face, attempts were made to alleviate the workman's task by improving the method of delivering the blow more effectively and by devising point shapes which maximised the amount of rock shat- tered during each blow.

The former approach was adopted by William Pidding [50], who modified the simple jumper drill-rod by surrounding it with a tube and inserting a compression spring between the end of the rod and the inner end of the tube, see Fig. 10. Radial clearance was provided between the rod and the bore of the tube while the end of the drill rod projected beyond the open end of the tube so that when the surface of the rock was struck by the point of the drill rod the tube continued forward, under its own inertia, vibrating laterally and thus opening out the hole in the rock face. The intention was to remove a small central region of solid rock and at the same time chip away the surrounding area.

253

@ b

i ~ j?'

e a

Fig. 10. Percussion bits. (a) Pidding's impact tool, 1852; (b) cruciform bit, 1854; (e) Low's bits, 1863; (d) percussion bits, ca. 1865; (e) percussion bits, ca. 1892.

254

Evidence of the now familiar cross bit appeared in a patent granted to Jon- athan Worthington, of Llancaiach & Gilvach Main Collieries, near Cardiff, and Fennell Allman, a consulting engineer of London, in 1854. The bit consisted of an iron body with four cutting edges arranged at right angles, to which thin steel plates were welded, brazed, screwed or rivetted. As the soft iron was eroded, hardened steel was left standing proud providing a cutting edge that was constantly maintained by the action of drilling - in the same way that the hard enamel of the teeth of some animals always maintains a sharp edge; the inner part of the tooth is more rapidly removed by mastication than the enamel, which it leaves proud and sharp [51 ]. Inventions arising from biological observation are rare in the history of drilling tools, but one other instance is that of Brunel who referred to the activity of teredo navalis boring into ships' timbers [27 ].

Improved point shapes are of particular interest in this study. A Notting- hamshire engineer, George Low [52], used both a curved edge-bit and one of zed shape. He found that the latter gave better results, presumably because it provided a longer length of cutting edge in contact with the rock. Other shapes were tried out but ten years later he had returned to the chisel edge. It must be remembered that tools were continually re-forged and re-sharpened on site and this practice exerted a strong influence in favour of simple shapes, although at the end of the nineteenth century crown bits were being used for drilling soft rock.

4

I - J l - . . -

i j ~ . . . . . . . . , L '

Fig. 11. Rotary drag bit, 1852.

255

6.2 Rotary-drag bits Drain laying is essentially a problem of cutting channels or small tunnels at

relatively small depth in the ground. Consequently, when hard obstructions are encountered there is insufficient space to use chisels and hammers. In 1852 Emil Schott, of the Duchy of Brunswick, devised the bit shown in Fig. 11; it comprises a series of chisel points offset from the centre of rotation [53 ]. This was the forerunner of the modern designs sometimes referred to as wing bits - an appropriate term - or rotary-drag bits.

6.3 Helical drill-rods When using percussive methods in mines there was the ever-present danger

of explosions caused by sparks generated at the tool point. Manually controlled rotary cutters, sometimes water cooled, largely eliminated this problem. Twisted bits, operated by the miner through a crank at tached to a screw mounted in a frame adjacent to the coal face, appeared around 1860 and have remained in use up to the present day. A typical arrangement is shown in Fig. 12; in these illustrations the emergence of the helical drill-rod and the separate end bit is witnessed. The stepped-diameter drill rod comprised two helical sections and was designed specifically for penetrat ing into underground streams of high pressure water. By reducing the sectional area of the rod at the leading end that was in contact with the water, the initial force on the rod was reduced.

6.4 Terriers In complete contrast, terriers, or ground augers, were designed to excavate

soft ground to receive fence posts and telegraph poles. The broad deep helical blade, shown in Fig. 13 [ 54 ], illustrates the manner in which this was achieved; the same arrangement is still used in its modern counterpart.

6.5 Hollow bits The broad helical blade employed in the terrier is difficult to manufacture

using simple hand tools and before this was introduced an iron cylindrical tube, with a cutting lip at the bottom, was used for excavating peat. The one shown in Fig. 14 was in use around 1700 when land drainage was being actively pur- sued in agricultural areas.

Land drainage and conveyance of drinking water was achieved by means of wooden pipes and machines were patented for boring these items. The deteri- oration of these pipes, with consequent risk to health, led to their abandonment.

Before the introduction of earthenware pipes for conveying water, a t tempts were made to bore them out of stone blocks. A notable example of this technique is that adopted by William Murdock in 1810. He set the stone block vertically with a metal tube situated on the top. An annular ring was fitted to the bot tom of the tube and sand was used as an abrasive medium. Two men pulled alternately on a rope that was wrapped around the top of the tube and

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,

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257

Fig. 13. Terrier, or ground auger, 1869. Fig. 14. Peat borer, ca. 1700.

passed over pulleys, causing the tube to oscillate and gradually remove a core from the block [55]. This slow cumbersome process became obsolete with the introduction of ceramic pipes in large numbers.

7. Cut t er s for b o r e h o l e s a n d m i n e s h a f t s

The economic and social changes that gradually manifested themselves in Western Europe, from the mid-eighteenth century onwards, exerted pressures upon engineers to explore and evaluate mineral resources. Borers were employed by geologists and land owners for this purpose. Scientific interest in minerals had prompted M'Neven to translate Geissau's work in 1788 [13]. Before 1700 mines at least twenty fathoms deep were common in most British coalfields and to meet the demand for this fuel during the next two hundred years shafts would have to be sunk to increasingly greater depths as surface measures became depleted. Mine shafts today are much larger in diameter than bore holes and another group of specialists, shaft sinkers, took over when the mineral surveyors had completed their work.

258

As the population increased, so the need for greater quantities of clean water increased. Early records of breweries provide useful evidence of borehole sink- ings and of the mineral content of water. Sanitation, swimming baths, laun- dries and fire brigades are some of the other services that depend heavily upon water made available by boring.

The search for petroleum introduced into Britain drilling practices from other countries, e.g. Japan, China, Burma, North America and Russia; Sir Boverton Redwood's treatise [56] provides a useful summary of the methods used in these countries. A French view of the same subject has been provided by Henry Neuberger and Henri Noalhat [57 ], who refer also to contributions from other European sources. British well-boring practice, in the water industry, has been described by C. Isler [58 ], a hydraulic engineer and manufacturer of well-boring equipment. He has provided a reasonably clear account of tools, apparatus and methods used in Britain around that time.

7.1 Bits for bore-hole sinking In France and the low countries, where strata is generally soft and sandy,

the usual method employed for sinking artesian wells, at the turn of the nineteenth century, was by rope boring. Shear legs, with a pulley at tached to the top, were erected over the site to be bored and a pointed bit, a t tached to the end of a rope passing over the pulley, was lifted and allowed to fall repeatedly in the same place. Loose material was removed with a sand pump, comprising a cylindrical tube with a flap valve at the bottom, substi tuted for the bit. This method was suitable only for penetrat ing to shallow depths were directional accuracy was not important.

Iron rods, screwed together, were used for sinking holes through hard strata and this method produced a straighter hole. This technique was the basis for developing methods to achieve greater depths. As the total rod lengths increased, impact stresses reached unacceptable limits when chisels began to fracture. A broken chisel wedged across the bot tom of a bore hole was a situation that all boremasters feared. The solution presented a choice of two ap- proaches. In one it was possible to smash the bit and retrieve the pieces; this was no mean feat when it is considered that a bit and associated items could weigh up to one ton. In the other an a t tempt was made to deflect the hole and by-pass the broken bit. Both methods involved considerable delay in the sinking operation.

This hazard was greatly reduced by the introduction of the Fabian free-fall system [57 ], which raised and released a bit from a fixed height, thus ensuring that the impact momentum was the same during each blow. An essential feature of this system was the introduction of modified jarring. Jars were introduced to jolt chisels out of the rock where quite often they had become wedged. The device used consisted of a rod sliding in a tube, the rod having been fitted with a transverse peg which located in a vertical slot in the tube such that on

259

8

I

]I

i L

J

Fig. 15. Fabian free-fall system. (a) Rod assembly, I = free-fall instrument, II = auger stem, I I I = drill; (b) detail of free-fall clutch; (c) successive modifications to the shapes of the bits.

the upstroke the lower end of the slot contacted the peg and snatched the bit upwards. Fabian provided a ledge at the upper end of the slot, to hold the peg, and a curved edge to guide the peg on to the ledge, see Fig. 15. In the walking- beam apparatus the drill string was jolted at the top of the upstroke and a quick turn of the sleeve by the master borer allowed the rod, with bit attached, to fall. When the sleeve descended again the curved edge of the slot re-engaged the peg on the ledge. An additional piece of equipment provided a twist to the drill string to ensure that the bit did not strike the at the same spot twice in succession.

It is interesting to observe above, successive modifications to the hollow cone bit through to the flat chisel. It illustrates an alternative approach to obtaining an edge profile which maximises fracture of the rock during each blow.

7.2 Annular bits As more knowledge of geological stratification was acquired, mineral sur-

veyors could proceed with greater confidence. In the absence of more precise methods, borers could only obtain data on the quality and thickness of strata by removing sample cores intact. This necessitated the use of a tube in which to capture suitable lengths of core. The difficulty was to develop a hollow bit that did not wear away too quickly; steel crowns suffered from this defect before the introduction of hardenable alloy steels. In 1862 an engineer engaged on the Mont Cenis Tunnel project, introduced a diamond crown, shown in Fig.

260

Fig. 16. Leschot's diamond crown bit, 1862.

16, which consisted of an iron ring into which recesses were cut to receive diamonds, or "carbons" [59]. This idea was immediately taken up by Captain F.E.B. Beaumont, R.E., who, on ret i rement from the British Army, established a business to develop the technique. He competed successfully with French engineers and carried out underwater drilling contracts in the Middlesborough area. Improved drilling machinery speeded up the abrasive process that Mur- dock had been compelled to perform manually.

There was a resurgence of interest in steel crowns from an unexpected quarter which led subsequently to the introduction of a new drilling process. The price of diamonds for drill crowns, when they were originally developed, was relatively low but as time passed the stones became more expensive. In addition, diamonds were not readily available in all areas of the world and some skill was required to set them into the crown, which activity precluded their manufacture in jobbing workshops. This difficulty was surmounted in one remote area by constructing a drill for core extraction from readily available materials.

The Davis calyx drill which appeared towards the end of the nineteenth century was of Australian origin and Francis Harley Davis explains how the tool developed [60]. Surface indications in one region of the Australian bush suggested the presence of coal below the surface but no drilling tools were available. A makeshift drill was constructed from some lengths of water pipe 2 inches in diameter, a boiler tube 4 inches in diameter, an axle box, old dray wheels and a few cart springs. The water tubes served for drill rods, and the boiler tube for a core barrel, connected by means of the axle box, which was first firmly rivetted into one end of the large tube and made water-tight by running in tea-chest lead. Teeth were cut into the cart springs, which were then bent around the lower end of the core barrel and rivetted into place. When in use, the appliance tended to become distorted, which made the work of removing worn cutting teeth and their replacement by a sharpened cutter somewhat difficult. In spite of this, a hole 4½ inches in diameter was put down to a depth of 350 feet and a core 3 inches in diameter was extracted for the entire distance entirely by manual power. A commercial version of this drill consisted of a metal cylinder with the lower end shaped into a series of long sharp teeth (Fig. 15 (a) ), which, Davis claimed, was "perfectly scientific and novel". This drill-

261

ing tool received favourable comments from the United States Patent Exam- iner and during trials the bit penetrated hard bluestone rock from the Hudson Palisades at the rate of ½ inch per revolution.

Later, steel balls were introduced into the bit to break up the rock by a rolling-crushing action.

7.3 Trepans for mine shafts By the end of the eighteenth century, techniques for sinking water boring

holes had become established practice and engineers applied this knowledge to

O

Fig. 17. Kind-Chaudron trepan.

262

the sinking of mine shafts. The most impressive application of the borers art was in the sinking of a shaft through water-bearing strata. Wet strata presented serious hazards to men working inside the shaft, with flooding and col- lapse of the sides the chief dangers. The Kind-Chaudron process resolved these difficulties in an ingeneous way. The cutters (" t repans") employed were a small one about six feet wide weighing about four tons, for cutting the pilot hole, and a large one for cutting the main shaft that was about ten feet wide and up to twelve tons in weight. A large trepan, constructed from wooden spars bolted together with metal plates, is shown in Fig. 17. Sometimes a centre- piece was provided with the larger trepan, to guide it into the pilot hole. These trepans were reciprocated up and down and rotated between blows to chop out the ground. At a suitable depth, drilling was stopped and a complete annular section of lining (" tubbing") was floated down the shaft, aided by the water within it. A dome was fitted inside the tubbing to effectively seal off the shaft. A central hole in the dome allowed the drill rods to pass through and a second hole was provided with a valve to control the escape of water as the tubbing sank into position. Large pumps drew off the surplus water while workmen cleared debris from the hole.

The large trepans employed in the Kind-Chaudron process were not a spon- taneous innovation as smaller versions had been used by Leon Dru [61 ] and a cluster of chisels at tached to a cylindrical body unit was a feature of a piece of apparatus patented by William and Colin Mather [62] - two brothers of the well-known engineering company whose business later became part of Mather and Plat t PLC.

Towards the close of the nineteenth century, the introduction of freezing methods enabled shafts to be sunk through wet strata using more conventional tools.

8. Conc lus ions

Drill bits occupy an important place in the evolution of cutting tools as a whole. Along with files, saws and woodworking planes they belong to a group of tools having cutting elements inclined at fixed angles relative to the work surface; unlike knives and chisels, for example, which can be presented in various directions when paring wood. It was the introduction of the self-acting lathe that imposed constraints on the positions and motions of single-point cutting tools; the turner was no longer able to alter the position of the tool point with respect to the workpiece at will. Drill bits, however, had to be provided with point geometries suitable for use in a manually operated brace or in a power-driven machine. Charles Holtzapffel was among the first to appreciate the importance of adopting suitable tool angles and to compare common geometrical features of various cutting tools. The emergence of opt imum rake angles can be observed in the groups of bits that have been discussed.

263

A split-point square-ended pod auger required a positive rake angle and a clearance angle in order to shear wood effectively. Users of spade bits realised that a positive rake angle (obtained by providing a groove behind the cutting edge ) was preferable to a negative rake angle that produced a scraping action. When cutting holes in rock it was natural to adopt a cutting edge supported by an adequate amount of metal, e.g. a chisel shape, since drilling operations in this material are usually accompanied by percussive blows. Gimlets, used in woodworking, had long provided a progressive shearing action by means of the curved edge of the flute immediately behind the screw point and it is surprising therefore that it was not until 1770 that Phineas Cooke's twist auger appeared. A helical flute provides a rake angle if the flute is continued to the point but English mechanics, during the nineteenth century, do not appear to have ap- preciated this fact; they employed twisted bits having fiat points (zero rake angle) for drilling metal while their American contemporaries recognised the advantages of using helical bits filed from rod. Eventually twist drill-bits sup- planted twisted bits and spade bits in engineering workshops. Drill bits provide evidence of the many problems that have to be resolved to facilitate technological and industrial progress.

Rolt [63 ] recognised that the speed of technological advance during the first half of the nineteenth century was due to the part played by "brilliant flexible engineers". The achievements of eminent engineers such as the Stephensons, the Brunels, Watt and many others were made possible by a strong supporting cast of artisans or technicians whose skills in resolving minor, but collectively important, practical difficulties when they arose have gone largely unrecog- nised. This area, which Mathias [64] calls artisan technology has not yet been fully explored and the present record of the evolution of drill bits is uniquely placed to make a contribution to this subject.

Each of the drill bits that have been discussed can be assigned to one of two broad categories, namely, special-purpose or general-purpose applications. The important contributions made by special-purpose bits, e.g. diamond crowns in tunnelling and mineral exploration, are readily apparent, but what deductions can be made concerning general-purpose bits? From the evidence that has been assembled a pattern begins to emerge.

Semi-cylindrical and flat bits performed their functions reasonably well and, in an age when pressures to achieve production targets were not as great as they are today, there was no real stimulus to seek alternative designs. However, many of the semi-cylindrical bits exhibit an unbalanced torque which could lead to unfortunate results when used in a drilling machine [3], and they did not eject cuttings effectively from holes. While holes were being drilled by hand any innovation which reduced physical effort was welcome. Twist augers ame- liorated the latter disadvantages and entrepreneurs, notably in North America, realised the potential benefits these tools had to offer. After many attempts at composite constructions, solid designs were finally adopted but completely au-

264

tomatic production by machinery was never achieved. In contrast, complete twist drill-bits could be produced on machines and the points could be re-ground until virtually the entire flute length was exhausted. In addition a similar pic- ture emerges in the rock-drilling industries in which helical augers and drill rods were machine-made and provided with separate cutting bits at the ends. Helical bits, factory made in large quantities, eventually ousted many of the semi-sylindrical and flat shapes. The introduction of cemented carbides for wing bits and but ton bits placed drill-bit manufacture in the hands of specialists and this trend was reinforced by the development of roller cone bits and down-the-hole drilling equipment. Drill bits became a specialised group of tools.

From this trend it can be hypothesized that the acceptability of a given drill bit depends upon the degree of convenience it affords the user. In this hypothesis the important words are acceptability and convenience. The acceptance of a drill bit over a long period of time depended upon the extent to which it had been found convenient to procure, manipulate and re-sharpen it. Clearly, if a drill bit proved a constant source of irritation and inconvenience to the user it would have been replaced eventually by a more efficient design and the unsat- isfactory tool would sooner or later disappear from use. Of the two qualities, viz. acceptability and convenience, the former is the most difficult to measure and has to be assessed subjectively. In order to employ these two parameters as a means of comparing the efficacy of drill-bit designs it would be useful if numerical values could be assigned to them. For example, acceptability could perhaps be assessed on the basis of the length of time a given drill bit remained in use and on its popularity, which could be judged from manufacturers ' sales ledgers or the frequency of its appearance in their catalogues. It must be remembered however that standards of acceptance change with time and what was acceptable to craftsmen at one period is unlikely to gain approval at a later period when att i tudes have changed due to insistence on better performance. Manufacturers have to decide between producing a drilling tool that cuts holes with little or no interruption and has a long life, or making a cheaper version having a shorter life but which can be supplied to customers quickly. From this it is clear that the difficulties of devising a reliable measure of acceptability are considerable. However a useful purpose may be served by a precise measure when applied to an ongoing technique to compare several drill-bit designs that have become available. In these circumstances some quantitative comparison of the relative acceptabilities of the designs may be possible. Convenience is a more readily identifiable parameter since it can be represented by energy con- sumption, which can be measured and is directly related to physical effort. This in turn depends largely upon the geometry and the condition of the drill point and the ease with which it can be maintained in its correct shape and to the appropriate degree of sharpness. It has been shown above that availability gradually took precedence over serviceability. Availability and convenience are achieved through capital investment in manufacturing equipment, which re-

265

quires an adequate supply of fuel for its operation and consumes large quantities of non-renewable metalliferous ores. Countries which are deficient in these resources could benefit from simple tools that are capable of being produced and maintained by manual labour using readily available materials. This is one potential application of the information contained in this paper.

A study of the development of any group of tools raises questions of wide scientific interest such as, for example, "Was the pattern of development of a group of tool shapes inevitable? Were other solutions overlooked?" Answers to these questions could provide insight into creative skills in a community - one of the wider issues that emerge from studies of tool technology. The two conditions, namely, the limited range of available manufacturing facilities and the inadequacies of tool materials, imposed severe constraints on the designs of drilling tools that could be produced up to the end of the nineteenth century. Demands for these tools did however stimulate innovations in forging machines and the invention of the helical milling machine for producing twist augers and twist drill-bits respectively [4]. Now that restrictions on manufacture and tool materials have largely been removed and more data on chip- forming and other cutting processes have become available, the opportunities to introduce new tool shapes and alternative drilling techniques [65,66] have been considerably enhanced.

References

1 S.K. Ghosh, Principally on sheet metal forming defects as described in the 11th Biennial Congress of the International Deep Drawing Research Group (IDDRG), Int. J. Mech. Sci., 23 (1981) 195-211.

2 C.J. Jackson, Some aspects of the design and classification of drill bits, Prod. Eng., 54 (1975) 537-541; 589-593.

3 C.J. Jackson, A history of the development of drill bits and drilling techniques in nineteenth century Britain, Ph.D. Thesis, The Open University, Milton Keynes, U.K., 1983.

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A p p e n d i x

l

~ angle

T ~ ~ Motion of cutting tool

_._--------3-- - 1 Clearance angle

Metal /

b

~ Rake angle, maximum (positive) value at outer edge reducing to a negative value at the web.

Clearance reduced angle, ~'~- - ~ - 7 I ~ by helical path of cutting - :~.P'- - - j lip. I

~ . . . . . . I Helical path of cutting lip. I "1

Fig. A.1. Tool angles. (a) Single point tool, orthogonal cutting; (b) twist drill bit.

on the evolution of drill-bit shapes

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