Factors that typically improve a material's performance often degrade its machinability, presenting a significant engineering challenge.
[3] Other important factors are operating conditions, cutting tool material and geometry, and the parameters of the specific machining process being performed.
This can be useful when comparing materials that have similar properties and power consumptions, but one is more abrasive and thus decreases the tool life.
This leads to a rating method where higher specific energies equal lower machinability.
Stainless steel and other materials with a high strain hardening ability also want to form a built up edge.
The American Iron and Steel Institute (AISI) determined machinability ratings for a wide variety of materials by running turning tests at 180 surface feet per minute (sfpm).
[9] The machinability rating is determined by measuring the weighted averages of the normal cutting speed, surface finish, and tool life for each material.
These additives may work by lubricating the tool-chip interface, decreasing the shear strength of the material, or increasing the brittleness of the chip.
Historically, sulfur and lead have been the most common additives, but bismuth and tin are increasingly popular for environmental reasons.
[10] Since lead has poor shear strength, it allows the chip to slide more freely past the cutting edge.
Sulfur improves the machinability of steel by forming low shear strength inclusions in the cutting zone.
[11] Aluminium is a much softer metal than steel, and the techniques to improve its machinability usually rely on making it more brittle.
[10] This creates heat that builds up in the cutting zone, which degrades the tool life and locally melts the plastic.
Machinability can be improved by using high lubricity coolant and keeping the cutting area free of chip build up.
Composites often have the worst machinability because they combine the poor thermal conductivity of a plastic resin with the tough or abrasive qualities of the fiber (glass, carbon etc.)
The machinability of rubber and other soft materials improves by using a very low temperature coolant, such as liquid carbon dioxide.