To be effective in cutting a gear, a cutting tool material must have a combination of the following qualities:
• Durability: The ability to withstand—to a high degree—wear that occurs at the interface of the tool, and the work due to pressure of the cutting process.
• Strength: The ability to withstand the load applied on the tool by the pressure required to cut the work material.
• Red hardness: The ability to retain a sufficient degree of hardness when the material is at a high temperature due to friction at the cut.
It is unfortunate that, in any material, an increase in one of these qualities will always lead to a deterioration of the others. Generally, we can say that durability and red hardness are increased only at the expense of strength. Cutting tool materials fall into one of four general categories:
• High speed steel (HSS)
• Powder metal high speed steel (PM)
• Bridge materials
HSS and PM HSS Materials
The most commonly used material, high speed steel, is so named for the ability of this range of alloy steel to cut iron, steel, and other fairly hard materials at a higher speed than plain carbon steel tools. This is due to HSS’s ability to maintain hardness at an elevated temperature. This quality, known as “Red Hardness,” varies according to the grade of high speed steel. This hardness is the primary factor in the tool’s resistance to wear.
HSS is also tougher at a given hardness than is carbon steel, and it is able to stand higher cutting stresses. Again, this desirable quality varies with the grade of material. Toughness and hardness are usually inversely proportional—the higher the hardness, the more brittle the material becomes. The attributes of HSS then can be summarized:
• High hardness at room temperature
• High “Red Hardness”
• Reasonable toughness
• High wear resistance
HSS is an alloy steel—it includes metals other than iron in the basic iron/carbon chemistry. The number and amounts of these alloying metals determine the characteristics of the final material and result in a wide range of HSS grades, not all of which are used in gear cutting tools.
In addition to iron, the chemistry of high speed steel will contain one or more of the following (carbon, which is not a metal, is a component of all steels):
Carbon C Molybdenum Mo
Chromium Cr Vanadium V
Tungsten W Cobalt Co
The factors of durability, strength, and “red hardness” distinguish one grade of HSS from another. Durability is the resistance to wear while in contact with the material being cut. Strength is the resistance to tooth breakage under high cutting loads. Red hardness is the ability to retain a high hardness at elevated temperatures.
As in all materials, it is difficult to obtain maximum strength and maximum hardness in one combination. Harder materials last longer, cut harder steels, and break more easily. Many of these are classed as powder metal (PM), which ensures finer grain structures, better strength, and durability after heat treatment.
Non-PM materials are referred to as “wrought” high speed steels. Both of these types are available in bar form, but wrought material bars are rolled from poured ingots. PM materials are rolled from ingots created from pressed, sintered powder. The result is a very fine grain structure and, consequently, superior strength and durability.
PM materials have substantially higher cost than wrought materials. They have the same alloying agents and the same grades as “wrought” high speed steel. The difference is in the manufacturing process and the resulting grain structure. Wrought HSS is mixed in a crucible and poured into ingot moulds. These ingots are then rolled into appropriately sized bars and delivered to the tool manufacturer in a soft, machineable condition. PM materials are converted to a fine powder by spraying the molten alloy into an inert atmosphere. The powder is then compacted in a cylindrical form under very high pressure. Oxygen is removed from the form, which is then heated to weld temperature. The powder forms a solid billet, then is rolled in the usual manner.
The advantage of the process is that during the tool hardening process, the resulting grain structure is extremely fine and the hard grains (carbides) are small, densely packed, and even. This results in a much higher structural strength and durability than that of a wrought steel tool.