Introduction This chapter focuses on machinery

which is used to cut and shape steel. We will discuss:

› drilling machines › lathes, milling machines› power saws and › Grinding machines.

Drilling machines Drilling machines are used mainly for

drilling holes. In addition, their functions include

reaming, countersinking, boring, spot facing, honing, lapping and tapping.

Safety is the starting point for any machinist.

Drilling machines cut by rotating a multi-edged cutting tool.

Basic Drilling Operations

Drill multiple diameters

Multiple drill, countersinkand counterbore

Drill and countersink

Drill and chamfer

Drill, countersink and counterbore

Drill and counterbore

Sensitive Drilling Machine› used for light drilling on small parts

Upright Drill Press › used for heavy-duty drilling

Radial Drill Press› used for drilling large, heavy work pieces

This machine allows you to ‘feel’ the cutting action as you hand-feed it into the work.

This drill press is usually belt driven. It is either bench- or floor-mounted They are designed for light-duty applications

only, drilling holes up to 12,5 mm in diameter. It consists of a base, column (which holds the

motor), vertical spindle and horizontal table.

The drive is very powerful and isused for heavier work than a sensitive drill press.

They can drill holes of 50 mm diameter or more, and have thecapacity to drill holes of 38 mm diameter in thick steel.

It also consists of a base, column,spindle and horizontal table

These are the most versatile drilling machines.

Machine size is determined by the diameter of the column and the length of the arm, measured from the centre of the spindle to the outer edge of the column.

Used for operations on large castings too heavy to be repositioned for drilling each hole.

The most commonly used tool in a drilling machine in various sizes is the twist drill

Twist drills have either straight or tapered shanks

Straight-shank drills are held in the chuck Taper shank drills fit in a tapered hole in the

spindle, with the tang of the drill engaging in a slot

pointbody

shank

tang

The following diagram illustrates the main parts of a twist drill bit and displays differently sized drill bits

To ensure good work without damaging or breaking a drill bit, it is important that the correct cutting speed and feed are used.

The correct speed and feed are influenced by:› the diameter of the drill bit, › the type of metal being drilled and› the type of drill bit being used (carbon or high speed

steel). The cutting speed for the drilling operation is the

peripheral speed of a point on the surface of the drill in contact with the work piece.

Feed is the distance the drill advances into the work piece for each revolution.

Generally the larger the hole to be drilled, the lower the cutting and feed speed.

The following factors influence the choice of the best cutting speed:› kind of drill being used› composition and hardness of material› amount of feed› condition of machine› use of coolant (will increase cutting speed).

Usually drill presses are supplied with tables indicating correct cutting speeds.

Drifts are wedge-shaped pieces of steel used to remove taper shank drills from

their sleeves or spindles Sleeves are used to increase the

tang diameter of taper-shank drill bits so that smaller drill bits can be used

in larger spindles.

1. Name the three principal parts of a drill bit.2. Make a neat labelled diagram of the taper-shank

twist drill bit.3. Name three drill presses and briefly explain their

differences. Describe how the primary uses differ in each of these drill press types.

4. Make a neat labelled diagram of the sensitive drill press.

5. What is the function of a drift?6. Name five factors that influence the drilling speed.7. Name six operations that can be performed on a

drilling machine.

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The centre lathe produces a cutting action by rotating the work piece against the cutting edge of the tool.

As the cutting tool is moved lengthwise and crosswise to the axis of the work piece, the shape of the work piece is generated.

The shape produced is basically cylindrical.

The swing and the length of the lathe bed designate the sizes of a screw-cutting lathe.

A 330 mm x 1 524 mm lathe bed has a swing of 330 mm (sufficient to take a work piece up to 330 mm in diameter) and has a bed length of 1 524 mm.

The maximum distance between centres determines the length of a work piece that can be turned.

Headstock location

Gap piece is removable Bedways

It is a rigid casting made of cast iron A lathe bed is constructed to withstand the

stresses and strain created by heavy machining

It is usually in a robust box-like form, ribbed (strengthened by metal webs) on the inside and ported (certain sections bored out) so that coolant and swarf can pass through it easily.

It is the backbone of the lathe

The headstock assembly is fastened permanently to the left side of the lathe.

It contains the headstock spindle, gears and speed change mechanism.

Gears (or a combination of gears and pulleys) rotate this spindle.

A hole extends through the spindle. The front end of the spindle hole is tapered to hold a tapered

sleeve because lathe centre and tools having a tapered shank. Since it turns with the work piece, the headstock centre is

called a live centre. The headstock spindles are supported by spindle bearings at

each end. The headstock spindle is driven either by a cone pulley and a

belt (belt-driven lathe) or by gears in the headstock (geared-head lathe).

It supports the free end of the work and is also used in drilling, reaming and taper turning operations of work held in the chuck or on the faceplate.

The tailstock can be moved longitudinally or can be locked in any position to the ways of the lathe by tightening the clamping bolt.

The tailstock movement makes it possible to hold work pieces of different lengths between centres.

The tailstock contains a spindle which does not rotate.

The one end is bored to a standard taper and may be used to hold tapered-shank drills, reamers, taps and the dead centre.

The spindle can be moved back and forth by turning the hand wheel.

You can clamp the spindle after setting so that it cannot slacken off during working.

The dead centre may be removed by reversing the hand wheel.

A screw running through the hollow spindle strikes the end of the dead centre, loosening it.

The tailstock is constructed in two parts – an upper and a lower unit (base unit).

The upper unit can be moved either towards or away from the operator by turning the adjusting screws in the base unit, to offset the work for taper-turning.

This moves the tailstock out of alignment.

Along with the lead screw the feed shaft is employed in operations of the carriage or the cross-slide in automatic turning.

The feed shaft, with a key way, runs alongside with the lead screw in front of the centre lathe below the rack.

The feed shaft passes behind the apron where a keyed worm wheel, mounted on the shaft, turns with it and is free to slide along it.

The worm wheel drives a gear wheel and from this, the feed can be directed either to the cross slide or to the carriage which is operated by a control on the apron.

Automatic turning is useful in long traverses, the steady movement of the carriage giving a superior finish to that usually obtained by hand feeding.

With automatic feed it is possible to maintain a constant cutting speed since the speed at the work periphery will be at its maximum and this will be reduced towards the centre.

Most modern centre lathes are fitted with safety devices which will either prevent accidental engagement of more than one feed at the same time, or, in the event of two feeds being engaged together, prevent damage being done, for example the clutch mechanism slipping or a shear pin breaking under an abnormal load.

The lead screw ( master screw), which transmits feed motion for screw-cutting and extends the length of the bed, passing behind the apron, is provided with an acme screw thread.

The lead screw speeds are obtained through the gear trains in the quick-change gearbox.

The lead screw can be engaged or freed from the carriage by a clutch mechanism, which can be operated whilst the lead screw is turning.

This clutch mechanism is quite simple, consisting of a large split nut (or half nut) that can be opened and closed over the lead screw by the movement of the lever on the apron.

This mechanism is used only when cutting screw threads.

This forms the base of the unit which supports the cutting tool and can be traversed along the whole length of the bed by hand control (manually) or by power feed (automatic).

The lathe carriage is attached to the lathe bed.

It consist of five different units:› saddle, › apron, › cross slide, › compound slide and › tool post.

The Saddle is a H-shaped casting that fits over the bed and slides along the ways from the headstock to tailstock.

The saddle contains the cross slide.

The cross slide can be moved in and out, perpendicularly to the centre of the lathe, with the cross feed lever.

The cross slide supports the compound slide.

The compound slide is situated on top of the cross slide, and can be rotated in a full circle and locked in any position.

The base of the compound slide is calibrated in degrees to facilitate the set up of the compound slide at the required angle.

The cross slide and the compound slide screws are equipped with a micrometer collar, which is graduated in millimetres.

The micrometer collars are used for turning work pieces to close measurements and for cutting screw threads.

This unit is fastened to the saddle and is mounted over the front of the bed.

It contains levers, clutches, friction clutches for automatic operation and gears that control the movement of the carriage either for manual or automatic feeds.

The hand wheel is attached to a pinion gear that meshes with the rack under the front of the bed.

A split nut which is present in the apron can be closed over the lead screw threads and is used for screw thread cutting.

For lathe work, cutting tools must be supported and fastened securely in the proper position.

A cutting tool is supported and held in a lathe tool holder that is secured in the toolpost of the lathe.

All modern lathes are fitted with a quick-change toolpost.

These toolposts allow tools to be changed speedily and are more adaptable than standard toolposts.

Tool holders used on this toolpost are accurately held because of the dovetail construction of the toolpost.

The toolpost is securely clamped to the compound slide.

Work pieces to be machined that cannot be clamped between centres on a lathe can be held in a chuck. Various chucks may be used.

Machining operations like centre drilling, reaming, facing, tapping, screw cutting, taper turning and boring are some of the operations that can be executed in the chuck.

A number of chucks including face-plates can be used on the centre lathe.

One end of the work piece is mounted in the chuck while the free end is supported by the centre in the tailstock.

This chuck is also known as the self-centering chuck. The three-jaw chuck is simplest to use. It grips the work piece from three sides. The three jaws advancing to the centre at an equal

rate achieve the self-centering effect . The self-centering effect is achieved when the work

piece is automatically brought to an on-centre position when the jaws grip it.

The jaws are simultaneously put into action by a scroll mechanism.

The scroll mechanism is a circular apparatus situated within the chuck.

This chuck is mainly used for cylindrical work pieces.

A chuck key moves all three jaws simultaneously. This chuck comes with two sets of jaws. The inside jaw is set with the high end toward

the centre while the outside jaw is set with the high end toward the outside of the chuck.

The two disadvantages of this chuck (compared with the four-jaw chuck) are:› The work piece will run off-centre when the scroll

becomes worn.› No separate adjustment is achievable like in the case of

the four-jaw chuck.› This chuck does not grip the work piece as securely as

the four-jaw chuck.

This chuck is also called the independent chuck and has four reversible jaws.

These jaws can be independently adjusted to fit an irregular shape of the work piece, no matter how irregular.

By gripping the work piece at four points, the jaws exert a great amount of pressure.

Most work pieces are gripped on the outside; however, in some cases, because of the shape and size of the work piece, it is necessary to grip the work piece on the inside.

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A milling machine provides cutting action to a rotating cutting tool.

In milling, a multi-toothed cutter rotating at a fixed position on the machine shapes the work as it is traversed across the cutter.

The work piece is firmly and safely secured in a machine vice or on the machine table, which can be adjusted to set the depth of cut and can be traversed in at least two directions in the horizontal plane.

With a swivelling table and a dividing head, the universal milling machine can be used for work mounted between centres and for gear cutting, in addition to normal work.

It can also be used for vertical milling by fitting a vertical head attachment.

The major assemblies of the milling machine are the base and column, knee, saddle, machine table, spindle and overarm.

The base along with the column forms the major structural component of the milling machine.

A dovetail slide is machined on the vertical face of the column, providing an accurate guide for the vertical travel of the knee.

Another dovetail slide is machined on the top of the column, providing an accurate guide for the overarm.

The column also contains the machine spindle, main drive motor, spindle speed selector mechanism and the spindle rotation direction selector.

The knee engages the slide on the face of the column and is moved vertically by turning the vertical hand-feed crank.

When adjusted it can be locked, and is given extra support by an adjustable leg which bears the machine base.

A slide on the top of the knee provides a guide for the saddle, which can be traversed in line with the spindle by hand-feed.

The saddle engages the slide on the top of the knee and is moved horizontally toward or away from the face of the column by turning the cross-feed handwheel.

The machine table is mounted on the saddle.

The machine table engages the slide on the top of the saddle and can be moved horizontally right and left by turning the table handwheel.

The machine table is equipped with T-slots for direct mounting of the work piece, machine vice or other fixtures.

The machine spindle is located in the upper part of the column and is used to hold, align and drive the various cutters, chucks and arbors.

The front end or spindle nose has a tapered socket in a standard milling machine.

This taper aligns the milling machine adapter or cutter arbor. Two keys located on the spindle nose provide a driving force.

A draw bolt keeps arbors and adapters in place. This bolt extends through the hollow centre of the

spindle to the rear of the machine. The draw bolt is threaded at one end and is designed to

fit into the thread in the end of the arbor or adapter shank.

Tightening the draw bolt lock nut draws the taper shank of the arbor or adapter into the spindle taper.

The overarm engages in a slide on the top of the column and may be moved in and out by loosing the overarm clamps and sliding this part to the desired position.

The arbor support engages the dovetail on the overarm.

The arbor support contains a bearing that is exactly in line with the spindle of the milling machine.

The arbor support provides a rigid bearing support for the outer end of the milling machine arbor.

Cutters are spaced by spacing collars on the arbor, all being held firmly together by tightening down a nut on the end of the arbor.

Key ways in cutters and arbor are provided. The nut should be tightened until the arbor is

supported at the outer end of the bracket. For most work this friction grip from the collars is

sufficient to drive the cutter and provides some protection from damage, should the cutter or work jam.

A great variety of cutters is available and of these, a number are made for direct mounting in the spindle nose, for mounting on stub arbors or for holding in special chucks.

Milling cutters can be grouped as follows:› Plain or cylindrical cutters : used in machining flat surfaces and only

cut with their sides. E.g. helical cutters.› Face cutters : do their main work with teeth formed on the end.

e.g. end mills.› Side and face cutters : cut on both the periphery and face.

E.g. cutters with straight and staggered teeth.› Saws and slotting cutters : produce plain, T- and dovetail slots.

e.g. slitting saws, slotting cutters, slot drills, T-slot cutters, woodruff key seat cutters and dovetail slot cutters.

› Form cutters : produce rounded corners, hollows, gear teeth, etc.E.g. concave, convex, corner rounding and involute gear cutters.

› Inserted-tooth cutters : usually made in larger sizes of face and cylindrical milling cutters.

On a plain or horizontal milling machine in your workshop, identify the following parts and describe their functions.

Do not switch on the milling machine without permission from your teacher.

a. overarm

b. power feed levers for longitudinal feed, cross feed, and vertical feedc. on/off switch for spindled. arbore. arbor supportf. machine tableg. kneeh. saddlei. Base

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Control systems, not limited to vertical milling machines and lathes, that activate machine control movements from information stored have been developed.

Information is retrieved either from a punched or magnetic tape called numerical control (NC), or from a computer numerical controlled machine (CNC)

These machines can generally be divided into two groups:› cut-off machines (which include reciprocating

saws, horizontal endless band saws, abrasive saws and cold saws) and

› vertical band machines that can be used as band saws or with other band tools.

Their primary function is to reduce material to lengths suitable to be held in other machine tools.

Reciprocating machines› These machines vary in design from light-duty crank

driven to heavy-duty hydraulic driven machines.› Actual cutting takes place in one direction and the

saw blade is slightly lifted on the return stroke.› This action saves wear on the teeth.› The saw is fed into the work piece by gravity,

through the weight of the saw frame and the blade.› The saw frame is usually lowered a fixed amount on

each stroke.› Some machines are designed for a faster return

stroke of the saw blade.

Horizontal Band Saws› This saw has a continuous blade that travels in a

horizontal plane or a plane slightly inclined from the horizontal.

› The blades are made of high-carbon steel with a flexible back and hardened teeth.

› They may have from six to 24 teeth per 25 mm with raker set.

› Some of these machines have a hydraulically operated feed, an adjustable vice, an adjustable stock stop and a means of varying the cutting speed and downward pressure.

› They can cut stock square or at an angle.

Vertical band sawing machines are available with either fixed or variable speeds.

If the machine is operated too fast, the teeth are not allowed sufficient time to dig into the material and they merely rub over the work.

This creates friction and dulls the cutting edge of the teeth. Blades for profile sawing are always raker set. This type of tooth provides the necessary side clearance. Profile or contour sawing is a fast, accurate and efficient

method of producing intricately curved or irregular cuts in almost any machinable metal.

Where internal contours are to be cut, it is necessary to first drill a hole within the contour to enter the saw blade.

The blade is cut at a convenient point, threaded through the pilot hole, and rewelded on the butt welder.

Machines have a built-in butt welder for this purpose.

On a horizontal band saw in your workshop, identify the following parts and describe their functions. (Do not switch on the band saw without permission from your teacher.)› a. coolant reservoir› b. feed control› c. machine-vice open/close handwheel› d. blade tensioner› e. on/off switch› f. blade guard

Grinding is one of the most accurate and basic machining methods.

A grinding machine differs from other machines in that it uses a tool made from emery, carborundum, Cornish silica, diamond dust or similar material.

The grinding wheel, made up of tiny cutting points, cuts with the entire surface area that comes in contact with the material being ground.

Grinding machines cut with an abrasive grinding action, removing material in the form of tiny particles.

A grinding machine generates the cutting action by rotating the grinding wheel.

It may also rotate the work piece, reciprocate the work piece, or reciprocate the tool head.

All these movements produce a sequence of cuts.

Grinding machines are designed to machine metal parts to very fine tolerances with very accurate finishes on work pieces.

With the aid of grinding machines, parts with the same size, shape and finish can be produced in large quantities.

The grinding operation depends on the abrasive or cutting qualities of the grinding wheel.

Each abrasive in the wheel is a very small sharp cutting tool.

As the grinding wheel rotates, it removes small chips from the work piece.

The wheels on bench or pedestal grinders can be dressed with a mechanical wheel dresser also known as an emery wheel dresser.

The wheel of a surface grinder can be dressed with a diamond wheel dresser, which is placed on the magnetic chuck of the surface grinder.

Answer the following questions.1. What are the two principal components of an

abrasive grinding wheel?2. What are the safety rules that must be observed in mounting a grinding wheel? (see Chapter 1)3. What are grinding machines mainly used for?