Carbide
Carbide is the basis for most cutting tools used today in non-gear cutting applications. It has been in use approximately 50 years. It cannot be machined by normal methods, preventing it from becoming readily available for tools of complex shapes such as hobs and shaper cutters. Any machining of these tools performed in the hard state, including service re-sharpening, must be done with diamond wheels. Initial machining is performed with standard cutting tools but when the carbide is in its pre-sintered state. At this stage the material has the strength of chalk and must be handled with extreme care. This is a non-ferrous material, more accurately called by its full name of cemented tungsten carbide. This is a tungsten alloy made into powder form and pressed into the rough shape of the finished product along with a cementing binder material, such as cobalt, with a lower melting point than the tungsten carbide.
After pressing into shape, the material is then sintered (baked) at a temperature to melt the binding material and cement the tungsten carbide particles together but will not affect the properties of the tungsten. At that point the material is fully hardened and may only be machined with diamond tools. Most production machining is today performed using a form of this material. It can be used dry and has the highest cutting speed capability of any of these materials.
The latest development in these materials comprises much smaller particles than previously used and these are called “microfine” carbides.
In the manufacture of complex shapes such as gear tools, a great deal of machining is required on the pressed blank. This can be done with standard machines and tools, and since the blank material is very weak, it can easily be broken (by hand, in fact). Heat treatment makes the blank extremely hard and can only be machined with diamond tools.
The great advantages of this material, which is entirely non-ferrous, is its initial hardness, allowing for cutting speed increases in the order of 300% when compared to common high speed steels, and its hardness at elevated temperatures allowing for the elimination of coolant from the cutting process. The disadvantages are in the high cost and vulnerability to breakage.
Bridge Materials
These are very highly alloyed high speed steels and are also known as “Super HSS”. They are made by the PM method. The composition includes very high percentages of cobalt and vanadium for very high red hardness. (Hardness at high temperature at the point of the tool). These tools can be used to cut steel in “normal” hardness ranges without coolant and as such they “bridge” the gap between HSS and carbide. To be effective they must be used in conjunction with specially developed coatings. They are more brittle than other high speed steels but not nearly so as carbides. Examples of these steels are ASP 2080 and CPM Rex 121. The coatings are generally proprietary to the coating manufacturer.
The coatings fall into two categories — layered and lattice. These are combinations of different materials applied as a combination or in layers.
Application of HSS and PM HSS materials
The most common High Speed Steels used in gear tools today are:
• M4
• M4+5
• Rex 45 (Rex is a trademark of Crucible Steel) ASP2030
• Rex 76 ASP 2060
• T15
These materials are most often of the PM type, with the wrought HSS materials used only in non-critical applications, such as very low production. All of these materials need to be used with a coolant when cutting steel and without coolant when cutting cast iron.
The most common all-round high performance HSS for this application is M45. This has good strength, wear resistance, and red hardness. It readily out-performs the earlier common materials of M2 and M3 and can be used with confidence in most situations.
In the search for higher performance it is necessary to consider the conditions under which the tool is expected to perform. There is a difference in requirement for a tool to perform at higher speed in materials of high machinability and those of a tool to cut materials that are harder and tougher than the norm.
For readily machinable steels, the next higher grade to consider would be M45. This could be expected to allow an increase in cutting speed approximately 15 to 20 percent above that of M4. For the same materials, the application of Rex 76 would give even higher cutting speed.
Figure 1 gives a rough comparison of the materials and is based on Crucible Steel’s designations. At the end of this module is a chart that compares the steels of various suppliers.
For every increase in performance there is generally an additional cost and a lowering of strength. The tools may be too brittle to support the cutting load, especially toward the end of cutter life when the tooth becomes thin.
For tougher part materials the need is generally to increase the wear resistance of the tool and that calls for a different approach. In this case the next higher grade material would be M4+5, and then T15. The same considerations of cost and strength apply.
