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Milling Tools: Rake Angles

Random information I thought it a good idea to collate.

From Wikipedia: Rake Angle

Rake angle is a parameter used in various cutting and machining processes, describing the angle of the cutting face relative to the work. There are two rake angles, namely the back rake angle and side rake angle, both of which help to guide chip flow. There are three types of rake angles: positive, negative, and zero.

Generally, positive rake angles:

  • Make the tool more sharp and pointed. This reduces the strength of the tool, as the small included angle in the tip may cause it to chip away.
  • Reduce cutting forces and power requirements.
  • Helps in the formation of continuous chips in ductile materials.
  • Can help avoid the formation of a built-up edge.

Negative rake angles, by contrast:

  • Make the tool more blunt, increasing the strength of the cutting edge.
  • Increase the cutting forces.
  • Can increase friction, resulting in higher temperatures.
  • Can improve surface finish.

A zero rake angle is the easiest to manufacture, but has a larger crater wear when compared to positive rake angle as the chip slides over the rake face.[1]

From The Home Shop Machinist: Carbide Rake

It’s my impression that the positives are recommended for home-shop and relatively small machine use. They take less HP and work better with less-than-rigid machines. The negatives are better for high-production-rate, high-metal-removal applications with heavy machines and rigid automated machines. That’s not a hard-and-fast rule, of course, but that’s generally it, and why people here tend to recommend the positives.

A carbide inserts cutting edge is pretty fragile. By having it at 90° this strengthens the edge as much as possible. The inserts are negative to give the edge more strength – carbide is brittle and the acute angle formed by positive rake doesn’t last well. Negative does require more cutting force meaning more hp and rigidity (hence the common recommendation its not the best choice for light home shop machines), but creates longer lasting tooling when using carbide. Negative or neutral rake inserts are also best on grabby materials.