Cutting Tool Materials: Performance and Selection Factors
Cutting Speed and Material Removal
It is apparent that more material can be removed by lowering the cutting speed. However, there are two important considerations:
- The economics of the machining process will be adversely affected if cutting speeds are low.
- Tool life curves can curve downward at low cutting speeds.
Consequently, there would be a specific cutting speed where material removal between tool changes is maximum.
Ceramic and Cermet vs. Carbide Cutting Tools
Ceramic and cermet cutting tools have certain advantages over others. Why, then, are carbide tools not replaced to a greater extent?
Ceramics are preferable to carbides in that they have a lower tendency to adhere to metals being cut and have very high abrasion resistance and hot hardness. However, ceramics are sensitive to defects and are generally brittle, and thus can fail prematurely. Carbides are much tougher than ceramics and are therefore much more likely to perform as expected even when conditions such as chatter occur.
Temperature Effects on Cutting Tool Performance
Why does temperature have such an important effect on cutting tool performance?
Temperature has a large effect on the life of a cutting tool.
- Materials become weaker and softer as they become hotter; hence, their wear resistance is reduced.
- Chemical reactivity generally increases with increasing temperature, thus increasing the wear rate.
- The effectiveness of cutting fluids can be compromised at excessive temperatures.
- Because of thermal expansion, workpiece tolerances will be adversely affected.
Advantages and Limitations of Cutting Tool Inserts
Explain the advantages and any limitations of inserts. Why were they developed?
With inserts, a number of new cutting edges are available on each tool, so that the insert merely needs to be indexed. Also, since inserts are clamped relatively easily, they allow for quick setups and tool changes. There are no significant limitations to inserts other than the fact that they require special tool holders and that they should be clamped properly. Their recycling and proper disposal is also an important consideration.
Multiphase Coatings for Cutting Tools
Describe the reasons for making cutting tools with multiphase coatings of different materials. Describe the properties that the substrate for multiphase coatings should have.
One can combine benefits from different materials. For example, the outermost layer can be the coating which is best from a hardness or low frictional characteristics to minimize tool wear. The next layer can have the benefit of being thermally insulating, and a third layer may be a material which bonds well to the tool.
Using these multiple layers allows a synergistic result in that the limitations of one coating can be compensated by another layer.
Chemical Stability in Cutting Tools
Why are chemical stability and inertness important in cutting tools?
Chemical stability and inertness are important for cutting tools to maintain low friction and wear.
- A major cause of friction is the shear stress required to break the microwelds in the contact area between the two materials.
- If the tool material is inert, the microwelds are less likely to occur with the workpiece material, and friction and wear will thus be reduced.
Precautions for Machining with Brittle Tool Materials
What precautions would you take in machining with brittle tool materials, especially ceramics? Explain.
With brittle tool materials, we first want to prevent chipping, such as by using negative rake angles and reducing vibration and chatter. Also, the brittleness of ceramic tools applies to thermal gradients as well as to strains. To prevent tool failures due to thermal gradients, a steady supply of cutting fluid should be applied, as well as selecting tougher tool materials.
Diamond vs. Cubic Boron Nitride for Machining Steels
Which of the two materials, diamond or cubic boron nitride, is more suitable for machining steels? Why?
Of the two choices, cubic boron nitride (cBN) is more suitable for cutting steel than diamond tools.
This is because cBN, unlike diamond, is chemically inert to iron at high temperatures; thus, tool life is better.
Reconditioning, Recycling, or Discarding Cutting Tools
List and explain the considerations involved in determining whether a cutting tool should be reconditioned, recycled, or discarded after use.
This is largely a matter of economics. Reconditioning requires skilled labor, grinders, and possibly recoating equipment.
Other considerations are the cost of new tools and the possible recycling of tool materials since many contain expensive materials of strategic importance, such as tungsten and cobalt.