MANUFACTURING TECHNOLOGY

Dr hab. inż. Piotr Skawiński, prof. WUT

Institute of Machine Design Fundamentals

Machine industry realizes many processes that have common theme, controlled material removal, are today collectively known as subtractive manufacturing. Another processes in which material is not removed, as example forging, rolling, casting are today known as chipless forming.

Metalworking is the process of working with metals to create individual parts, assemblies, or large-scale structures. The term covers a wide range of work from large ships and bridges to precise engine parts and delicate jewelery. It therefore includes a correspondingly wide range of skills, processes, and tools. Machining is any of various processes in which a piece of raw material is cut into a desired final shape and size by a controlled material-removal process. The many processes that have this common theme, controlled material removal, are today collectively known as subtractive manufacturing, in distinction from processes of controlled material addition, which are known as additive manufacturing. Exactly what the "controlled" part of the definition implies can vary, but it almost always implies the use of machine tools (in addition to just power tools and hand tools).

Metal cutting Cutting has been at the core of manufacturing throughout history. For metals many methods are used and can be grouped by the physical phenomenon used. Chip forming - sawing, drilling, milling, turning etc. Shearing - punching, stamping, scissoring. Abrading - Grinding, lapping, polishing; water-jet. Heat - flame cutting, plasma cutting, laser cutting. Electro-Chemical - Etching, Electrical discharge machining (EDM). Every method has its limitations in accuracy, cost, and effect on the material. For example: heat may damage the quality of heat treated alloys, and laser cutting is less suitable for highly reflective materials such as aluminum.

Short history of manufacturing technology The precise meaning of the term machining has evolved over the past one and a half centuries as technology has advanced. In the 18th century, the word machinist simply meant a person who built or repaired machines. This person's work was done mostly by hand, using processes such as the carving of wood and the hand-forging and hand-filing of metal. At the time, millwrights and builders of new kinds of engines (meaning, more or less, machines of any kind), such as James Watt or John Wilkinson would fit the definition. The noun machine tool and the verb to machine (machined, machining) did not yet exist.

Around the middle of the 19th century, the latter words were coined as the concepts that they described evolved into widespread existence. Therefore, during the Machine Age, machining referred to (what we today might call) the "traditional" machining processes, such as turning, boring, drilling, milling, broaching, sawing, shaping, planing, reaming, and tapping. In these "traditional" or "conventional" machining processes, machine tools such as lathes, milling machines, drill presses or others, are used with a sharp cutting tool to remove material to achieve a desired geometry.

Since the advent of new technologies such as electrical discharge machining, electrochemical machining, electron beam machining, photochemical machining, and ultrasonic machining, the retronym "conventional machining" can be used to differentiate those classic technologies from the newer ones. In current usage, the term "machining" without qualification usually implies the traditional machining processes.

In the industry we have three principal machining processes are classified as turning, drilling and milling. Other operations falling into miscellaneous categories include: shaping, planing, boring, broaching and sawing. Machining operations: Turning operations are operations that rotate the workpiece as the primary method of moving metal against the cutting tool. Lathes are the principal machine tool used in turning. Milling operations are operations in which the cutting tool rotates to bring cutting edges to bear against the workpiece. Milling machines are the principal machine tool used in milling. Drilling operations are operations in which holes are produced or refined by bringing a rotating cutter with cutting edges at the lower extremity into contact with the workpiece. Drilling operations are done primarily in drill presses but sometimes on lathes or mills.

Drilling Used to create a round hole, usually by means of a rotating tool (drill bit) that has two cutting edges

Typical workpiece materials: Rods and bars (metallurgical materials, profiles), Castings, Forgings, Welded blanks, Punched blanks, Powder metallurgy.

Cutting tools In the context of machining, a cutting tool (or cutter) is any tool that is used to remove material from the workpiece by means of shear deformation. Cutting may be accomplished by single-point or multipoint tools. Single-point tools are used in turning, shaping, plaining and similar operations, and remove material by means of one cutting edge. Milling and drilling tools are often multipoint tools. Grinding tools are also multipoint tools. Each grain of abrasive functions as a microscopic single-point cutting edge (although of high negative rake angle), and shears a tiny chip.

Cutting tools must be made of a material harder than the material which is to be cut, and the tool must be able to withstand the heat generated in the metal-cutting process. Also, the tool must have a specific geometry, with clearance angles designed so that the cutting edge can contact the workpiece without the rest of the tool dragging on the workpiece surface. The angle of the cutting face is also important, as is the flute width, number of flutes or teeth, and margin size. In order to have a long working life, all of the above must be optimized, plus the speeds and feeds at which the tool is run.

Types of cutting tools: Solid Cutting Tools The typical tool for milling and drilling has no changeable insert. The cutting edge and the shank is one unit and built of the same material. Small tools cannot be designed with exchangeable inserts.

Cutting tools with inserts (indexable tools) Cutting tools are often designed with inserts or replaceable tips (tipped tools). In these, the cutting edge consists of a separate piece of material, either brazed, welded or clamped on to the tool body. Common materials for tips include cemented carbide, polycrystalline diamond, and cubic boron nitride. Tools using inserts include milling cutters (endmills, fly cutters), tool bits, and saw blades.

Tool materials Cutting tool must have three characteristics: hardness: hardness and strength at high temperatures. toughness: so that tools do not chip or fracture. wear resistance: having acceptable tool life before needing to be replaced.

We have two main categories of tool materials: Unstable materials (usually steels) are substances that start at a relatively low hardness point and are then heat treated to promote the growth of hard particles (usually carbides) inside the original matrix, which increases the overall hardness of the material at the expense of some its original toughness. Since heat is the mechanism to alter the structure of the substance and at the same time the cutting action produces a lot of heat, such substances are inherently unstable under machining conditions.

Stable materials (usually tungsten carbide) are substances that remain relatively stable under the heat produced by most machining conditions, as they don't attain their hardness through heat. They wear down due to abrasion, but generally don't change their properties much during use. Most stable materials are hard enough to break before flexing, which makes them very fragile. To avoid chipping at the cutting edge, some tools made of such materials are finished with a sightly blunt edge, which results in higher cutting forces due to an increased shear area, however, tungsten carbide has the ability to attain a significantly sharper cutting edge than tooling steel for uses such as ultrasonic machining of composites. Fragility combined with high cutting forces results in most stable materials being unsuitable for use in anything but large, heavy and rigid machinery and fixtures.

Unstable materials, being generally softer and thus tougher, generally can stand a bit of flexing without breaking, which makes them much more suitable for unfavorable machining conditions, such as those encountered in hand tools and light machinery. Carbon tool steels - Unstable. Very inexpensive. Extremely sensitive to heat. Mostly obsolete in today's commercial machining, although it is still commonly found in non-intensive applications such as hobbyist or common machining, where economy-grade drill bits, taps and dies, hacksaw blades, and reamers are still usually made of it (because of its affordability). Hardness up to about HRC 65. Sharp cutting edges possible.

High speed steel (HSS)- Unstable. Inexpensive. Retains hardness at moderate temperatures. The most common cutting tool material used today. Used extensively on drill bits and taps. Hardness up to about HRC 67. Sharp cutting edges possible. HSS cobalt - Unstable. Moderately expensive. The high cobalt versions of high speed steel are very resistant to heat and thus excellent for machining abrasive and/or work hardening materials such as titanium and stainless steel. Used extensively on milling cutters and drill bits. Hardness up to about HRC 70. Sharp cutting edges possible.

Cast cobalt alloys - Stable. Expensive. Somewhat fragile. Despite its stability it doesn't allow for high machining speed due to low hardness. Not used much. Hardness up to about HRC 65. Sharp cutting edges possible. Cemented carbide - Stable. Moderately expensive. The most common material used in the industry today. It is offered in several "grades" containing different proportions of tungsten carbide and binder (usually cobalt). High resistance to abrasion. High solubility in iron requires the additions of tantalum carbide and niobium carbide for steel usage. Its main use is in turning tool bits although it is very common in milling cutters and saw blades. Hardness up to about HRC 90. Sharp edges generally not recommended.

Ceramics - Stable. Moderately inexpensive. Chemically inert and extremely resistant to heat, ceramics are usually desirable in high speed applications, the only drawback being their high fragility. Ceramics are considered unpredictable under unfavorable conditions. The most common ceramic materials are based on alumina (aluminium oxide), silicon nitride and silicon carbide. Used almost exclusively on turning tool bits. Hardness up to about HRC 93. Sharp cutting edges and positive rake angles are to be avoided.

Cermets - Stable. Moderately expensive. Another cemented material based on titanium carbide (TiC). Binder is usually nickel. It provides higher abrasion resistance compared to tungsten carbide at the expense of some toughness. It is far more chemically inert than it too. Extremely high resistance to abrasion. Used primarily on turning tool bits although research is being carried on producing other cutting tools. Hardness up to about HRC 93. Sharp edges generally not recommended.

Cubic boron nitride (CBN) - Stable. Expensive. Being the second hardest substance known, it is also the second most fragile. It offers extremely high resistance to abrasion at the expense of much toughness. It is generally used in a machining process called "hard machining", which involves running the tool or the part fast enough to melt it before it touches the edge, softening it considerably. Used almost exclusively on turning tool bits. Hardness higher than HRC 95. Sharp edges generally not recommended.

Diamond (sintetic or natural) - Stable. Very Expensive. The hardest substance known to date. Superior resistance to abrasion but also high chemical affinity to iron which results in being unsuitable for steel machining. It is used where abrasive materials would wear anything else. Extremely fragile. Used almost exclusively on turning tool bits although it can be used as a coating on many kinds of tools. Sharp edges generally not recommended.

Coatings: Physical vapor deposition (PVD) - describes a variety of vacuum deposition methods used to deposit thin films by the condensation of a vaporized form of the desired film material onto various workpiece surfaces (e.g., onto semiconductor wafers). The coating method involves purely physical processes such as high-temperature vacuum evaporationwith subsequent condensation, or plasma sputter bombardment rather than involving a chemical reaction at the surface to be coated as in chemical vapor deposition. Common coatings applied by PVD are Totanium nitride, Zirconium nitride, Chromium nitride, Titanium aluminum nitride. PVD coatings are generally used to improve hardness, wear resistance and oxidation resistance.

Titanium aluminium nitride (TiAlN) or aluminium titanium nitride (AlTiN; for aluminium contents higher 50 at.%) stands for a group of metastable hard coatings consisting of the metallic elements aluminium and titanium and nitrogen. Selected properties are: Vickers hardness 2600 to 3300 HV. Phase stability ca. 850 °C, start of decomposition to AlN+TiN. Intense oxidation starts at about 800 °C (ca. 300 °C higher than for TiN). Lower electrical and thermal conductivity than TiN. Typical coating thickness ca. (1 to 7) µm.

Cutting conditions Relative motion is required between the tool and work to perform a machining operation. The primary motion is accomplished at a certain cutting speed. In addition, the tool must be moved laterally across the work. This is a much slower motion, called the feed. The remaining dimension of the cut is the penetration of the cutting tool below the original work surface, called the depth of cut. Collectively, speed, feed, and depth of cut are called the cutting conditions.

Speed, feed, and depth form the three dimensions of the machining process, and for certain operations, their product can be used to obtain the material removal rate for the process:

Q= V f a Where: – Q - the material removal rate in mm3/min, (in3/min), – V - the cutting speed in m/min, (in/min), – f - the feed in mm/min, (in/min), – a - the depth of cut in mm, (in). Note: All units must be converted to the corresponding decimal units.

Machining operations usually divide into two/three categories, distinguished by purpose and cutting conditions: Roughing cuts Semifinishing and Finishing cuts. Roughing cuts are used to remove large amount of material from the starting workpart as rapidly as possible, i.e. with a large Material Removal Rate (MRR), in order to produce a shape close to the desired form, but leaving some material on the piece for a subsequent finishing operation. In production machining jobs, one or more roughing cuts are usually performed on the work, followed by semifinishing and finishing cuts. Roughing operations are done at high feeds and depths – feeds of 0.4–1.25 mm/rev (0.015–0.050 in/rev) and depths of 2.5–20 mm (0.100–0.750 in) are typical, but actual values depend on the workpiece materials. Acc.Cl. 14, roughness Ra = 0,63

m.

Semifinishing operations (for example: turning) are carried out at middle feeds and depths – feeds of 0.0125–0.04 mm/rev (0.0005–0.0015 in/rev) and depths of 0.75–2.0 mm (0.030–0.075 in) are typical. Accuracy class 9 – 11, roughness Ra = 2,5 – 5 m. Finishing operations (for example: turning) are carried out at low feeds and depths – feeds of 0.0125–0.04 mm/rev (0.0005–0.0015 in/rev) and depths of 0.75–2.0 mm (0.030–0.075 in) are typical. Cutting speeds are lower in roughing than in finishing. Accuracy class 5 – 8, roughness Ra = 0,63

m.

Essentials of metal cutting operation: machine tool cutting tool method operator.

TYPES OF TOOLS

Lathe Cutting Operations and Lathe Tools

Porównanie tokarki konwencjonalnej i tokarki CNC

Control unit

Comparison of conventional and CNC milling machine

Two forms: peripheral milling and face milling

Drill geometry

Plane in turning operation

Cutting forces - turning