Cutting tool selection begins with materials.
There are three basic problems to be overcome:
* Wear which takes place at the cutting edge,
* Heat generated by the energy required to remove material from the workpiece, and
* Shock involved in the cutting action.
The main properties which any cutting material must possess in order to carry out its function are therefore:
* Hardness to combat the wearing action,
* Hot strength to overcome the heat involved, and
* Sufficient toughness to withstand any interruptions or vibration occurring during the machining process.
The following materials are those generally used for cutting. Except for hardmetals (which cover a wide range of hardness and overlap cermets and sialons at their harder end), they are listed in order of hardness:
High speed steel Stellite Hardmetals Cermets Sialons Ceramics Silicon nitride Cubic Boron Nitride (CBN) Diamond (PCD)
In general, increasing hardness brings with it a reduction in toughness and so those materials in the higher hardness region of the list will fall by breakage if used for heavy cuts, particularly with workpieces which have holes or slots in them which give rise to interruptions in cut.
Sialons and silicon nitride are also regarded as ceramics. There are two generally recognized groups of ceramics, silicon-based ceramics (sialons and silicon nitride fall into it) and aluminum oxide-based ceramics.
High speed steels
Main areas of application: Drilling, end mills, solid milling cutters, slot drills, circular and dove tall form tools, taps, reamers, broaches, hobs, buttweld turning tools, regrindable tool bits for smaller and lower power lathes. High speed steel is restricted to comparatively low cutting speeds. Higher speeds will cause the temperature of the cutting edge to rise above the softening point.
High speed steels have the lowest hardness and the highest toughness of the cutting materials in general use. Their major disadvantage is that their hardness is brought about by a heat treatment process so they are not naturally hard. If the temperature at the cutting edge rises to around 600 [degrees] C, then the high speed steel will soften and the edge will fail. For this reason they are limited to comparatively low cutting speeds up to a maximum of the order of 50 m/min.
In turning they are mainly used as circular, or dove-tailed form tools, on so-called automatic screw machines. These machines are found in establishments producing high volume parts directly from bar or tube. Many of them are multi-spindle machines where cutting is going on at most of the stations at the same time. The form tools take a broad cut and because the machines lack rigidity by the nature of their design this broad cut must be comparatively light.
This in turn calls for high rake angles on the tools (usually a minimum of 10 deg) which results in a weaker geometry at the cutting edge. The use of harder cutting materials would require reduced rake angles and higher cutting speeds but the multi-spindle automatics are not rigid enough to work continuously under such conditions and high machine maintenance costs together with frequent tool breakage would result.
Probably less than 10% of all turning applications are carried out using high speed steel as the cutting mate, rial. Its major area of application is drilling. At least 80% of all drilling is done with high speed steel. It is ideally suited for most of the machines in use today which have insufficient power and lack the rigidity so necessary for drilling with hardmetals.
The second most important application area for high speed steel is milling. Solid high speed steel end mills, slot drills, and router cutters form a large market and, together with high speed steel face- and comer-milling cutters up to 75 mm and 100 mm in diameter, they make up about 40% of the total milling cutter market.
Although the development of CNC machines and machining centers equipped with robust rotating spindles and the introduction of stiffer drilling machines and milling machines with more power available has aided the increased use of hardmetal, high speed steel is still likely to be the predominant material for drilling and a widely used mate rial for milling.
Another important development which has enabled high speed steel to cut at higher speeds has been the utilization of a very thin titanium nitride (TiN) coating on the surface of the tools. This is particularly so in the case of drills where both increased feedrates and cutting speeds have resulted. This TiN coating, which is gold in color, is about 3 micrometers thick and is extremely hard and stable. It is applied by a process known as Physical Vapor Deposition (PVD) whereby the high speed steel base material does not reach a temperature greater than 500[degrees]C and thus its hardness is unaffected. Coating is ideally suited to tooling which is not reground when the cutting edge is worn.
Three types of high speed steel are available. The first uses tungsten as its major alloying element and in the UK is known as the T series. The second type contains molybdenum and considerably less tungsten is present. This is known as the M series of alloys. The third type contains cobalt and can be either a T or an M series of material.
The T series without cobalt is not quite so tough as the M series but their heat treatment is easier to carry out. The M series are more widely used, especially with drills and end mills. The introduction of cobalt increases hot hardness and wear resistance but reduces the toughness. High speed steels containing cobalt appear to be more advantageous when machining steel with a hardness over 275 Brinell. The British Standard is B.S. 4659.
The hardness of high speed steels after they have been heat treated is usually quoted in Rockwell C units and generally falls within the range 62 to 68 Rc. Hardness of all the other cutting materials is quoted in Vickers Diamond Hardness (VDH) which is used in this discussion. For comparison purposes high speed steel lies in an approximate range of 800 to 900 VDH.
The most popular alloy for producing drills is M2 and this is also a favorite for the production of taps. It is extremely unlikely that any of the other cutting materials will succeed as a basis for standard taps. The harder T42, about 1000 VDH, is used when abrasion resistance of the cutting edge is the vital factor. If hot strength is the main requirement such as in the machining of heat resisting alloys then M42 is used. M42 is also the ideal substrate for coated inserts made from conventional high speed steel whereas M35 (favored on the continent for similar applications) is currently the preferred material for powder metallurgy high speed steel with a TiN coating.
Stellite will perform on heavy cutting operations at medium to low speeds. It is not one of the important cutting materials and has a narrow specialized field of application.
Stellite is the trade name for a cobalt-based alloy which is naturally hard and does not require heat treatment to attain its cutting properties. Stellite Alloy No 100 is a cobalt alloy containing chromium, tungsten, and carbon. It is produced by melting and casting and is as hard as the hardest high speed steels but its hot hardness at dull red heat is 535 VDH compared with 175 VDH for high speed steel. It is mainly used for turning operations and is supplied as solid tool bits and as turning tools which are tipped with the Stellite alloy.
The cutting geometry is ground into the tool and once the cutting edge is worn it is reground to bring it back to new condition. Stellite tools are used to cut surfaces which are extremely difficult to machine with hardmetals and where the cutting edges of hardmetal would be liable to fracture. A typical example is the machining of welds. Welds tend to be hard and have inclusions in their surfaces. They are uneven and give rise to interrupted cutting. Stellite is tough enough to cope with these conditions even with positive rake geometry. The range of cutting speeds in which it will perform satisfactorily is lower than that for hardmetals but a little higher than that for high speed steels.
Hardmetals cover a very wide band of machining applications. It is estimated that some 70% of all turning tasks are done using hardmetal tooling. A range of compositions is available and each alloy is tailor made to provide the properties needed to perform the special requirements of an application, e.g. high hardness for finishing or good toughness for roughing. Coated hardmetals, in the form of indexable inserts, enable very high productivity levels to be achieved.
This family of alloys is the hard core of all the cutting materials in use today. There is no international standard based on composition and mechanical properties for hardmetals. There is, however, an ISO standard for machining applications. Hardmetal manufacturers then nominate alloys from their range which they recommend to carry out the ISO applications. The alloys are usually called grades.
This application standard is ISO R513 and it classifies workpiece materials into three major groups. Each group is given a letter and a color to identify it. The cast irons and non-ferrous metals applications are given the letter K and their color is red. The steels group has the letter P and is colored blue. The third group covers more difficult materials like heat resisting alloys and is given the letter M and is colored yellow.
The groups are then sub-divided into the types of application involved. Lighter, finishing cuts are at the top of the group and heavy roughing cuts at the bottom. Each type of application is given a number. The smaller numbers relate to lighter cuts and the larger numbers identify the roughing applications. Thus fine finish turning of a mild steel cylinder with no interruptions would be termed a P05 application while planing a cast iron lathe bed with interruptions and sand inclusions would be termed a K40 application.
The grades which any two hardmetal manufacturers nominate to carry out a P05 application will almost certainly not be identical in composition but they are likely to be near to one another and their properties will be similar. This comment applies all through the range.
Although this system does not classify competitors' cutting materials as direct equivalents nevertheless it has to be said that, by and large, it works.
Prior to the introduction of coatings in 1969, two groups of hardmetal existed for machining purposes. Both these groups are still used but they have been joined by a third group of coated hardmetals.
The simplest hardmetals are the first group and are composed of tungsten carbide (WC) bonded by cobalt (Co). Tungsten carbide has a hardness in excess of 2000 VDH while cobalt has a hardness only 10% that of tungsten carbide. Pure WC is comparatively brittle and Co is tough. A combination of these two materials results in a compromise between wear resistance and shock resistance according to the amount usually reported in weight percent. Because the density of WC is almost twice that of Co the volume of binder material is considerably greater than would appear from the quoted Co percentage.
Two factors affect the cutting properties of a simple WC-Co hardmetal. They are:
1) The cobalt content, 2) The grain size of the tungsten carbide.
Increasing the Co content increases the toughness of a hardmetal but reduces its hardness and therefore its wear resistance. Courser grain WC is better for shock resistance and for a given Co content reduces the hardness of an alloy compared with finer grains.
Conversely, reducing the Co content reduces the toughness and increases the wear resistance by increasing the hardness of a hardmetal. Fine grain WC also increases the hardness and therefore the wear resistance for a given Co content.
The useful range of Co content for cutting purposes in weight percent is from around 5% to 12%. Grain sizes of WC go from around 0.5 micrometers to 5 micrometers. The hardness span of these alloys ranges from 1250 VDH to 1800 VDH.
A cutting speed of 50 meters per minute is at the low end of the range of speeds typically used for machining with hardmetals. Even at this speed, when cutting steel, the temperature at the interface between the chip and the tool tip is well over 1000 [degrees] C. At these temperatures iron is able to absorb tungsten carbide by a mechanism known as solid solution. The way in which this operates when cutting ferritic steels with a Co-WC alloy is that a crater is formed immediately behind the cutting edge. Metallographic examination of the chips reveals grains of WC which have been removed from the tool tip. The higher the cutting speed, the higher the temperate, the more rapid is the cratering effect and breakdown of the cutting edge occurs in a very short time.
In order to be able to machine ferritic steels it is necessary to make the hardmetal resistant to cratering. This is done by adding titanium carbide (TiC) to the basic Co-WC alloys. These materials form the second group of hardmetals used for machining.
TiC has an extremely low solubility in iron and therefore as the chip flows over a cutting tool tip containing grains of TiC they act as a barrier and deter the cratering action. The hardness of TiC is even harder than WC and therefore wear resistance is maintained. The amount of TiC varies from 5% to 25% by weight.
The proportion of TiC added depends on the cutting speed the hardmetal is required to perform at. Finishing operations need to be carried out at higher speeds for economic metal removal. High speeds will cause cutting temperatures to increase and cratering will be more pronounced. To counteract this a high TiC addition is made. Adding TiC tends to reduce the toughness of the alloy but with finishing operations the cutting is very light and the hardmetals containing up to 25% TiC by weight are tough enough to perform satisfactorily.
At the other end of the scale heavy roughing operations are usually carried out at lower speeds and so the cutting temperature is lower and cratering is reduced therefore less TiC is needed. The smaller amount of TiC does not adversely affect the toughness of the hardmetal.
All the family of hardmetals is produced by a powder metallurgy process. The basic Co-WC alloys are made by mixing cobalt and tungsten carbide powders, pressing the mixture into shapes and then sintering these shapes.
Reverting back to the ISO application standard, the grades which are used for the K applications i.e. to machine cast irons, austenitic stainless steels and non-ferrous metals are the plain Co-WC hardmetals. The K30 and K40 applications require toughness and therefore need a hardmetal with a high Co content to withstand the shock. The grain size must be at least medium and tending to coarse for the really tough applications. The very light finishing operations, K01, present no problems of toughness and so the hardest, most wear resistant, plain Co-WC grades are used i.e. 5% Co and fine grain WC.
The P applications need hardmetals containing TiC to combat the problem of cratering. The heavy, interrupted, roughing operations need a high Co content and a medium to coarse grain size of WC to withstand the shock during cutting. This will result in a hardness in the region of 1400 VDH.
A typical hardmetal for finishing operations will have a low Co content e.g. 6% to 7%, a high TiC content of around 20% and TaNbC of the order of 10%, The hardness of such an alloy will be 1700 VDH.
The range of hardmetal alloys used for the M applications is much narrower. Co contents are from 6% to 9%, TiC from 4% to 8% and TaNbC from 5% to 10%. Over the last 20 years remarkable improvements in cutting performance of hardmetals have been achieved by applying very thin coatings of TiC, TiN, TiCN, [Al.sub.2] [O.sub.3].
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|Publication:||Tooling & Production|
|Date:||Sep 1, 1994|
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