Cutting speeds 1

[SomptingGuy]

Way back in my sponsored student days I was sent to a technical college for a few months to do loads of hands-on stuff (machining, drawing, engine stripping, etc). One question our training manager asked us afterwards was why different materials have different optimum cutting speeds. I never managed to work this out at the time and another thread (thread404-240191) has reawakened my interest.

Can anyone explain simply why there is an optimum and why it's different for different metals?

- Steve

 

[CorBlimeyLimey]

I don't have a Machinery's Handbook handy, but I believe it has a good explanation.

(Paraphrasing)
Cutting speeds and feeds are affected by variables other than just the material being cut. Cutting tool material, geometry and cutting fluid or coolant all have significant effects.

Hardness obviously plays a big role and is usually used when considering cutting speeds and feeds. However, the structural composition of a material being cut is probably the most significant attribute affecting it's cutting speed. Materials with the same hardness but different structures (induced by heat treating) may have very different machine-ability.

 

[SomptingGuy]

I thought (from dim memory) that too slow was just as bad as too fast. That's the bit that I don't understand.

- Steve

 

[btrueblood]

Too slow just means you are wasting time, I thought.

 

[TVP]

Remember to keep in mind the difference between feed rate (usually called feed) and the tool's rotational velocity (usually called speed). Materials that have very high toughness such as austenitic stainless steels or nickel-based alloys often have problems with slow speeds because a distinct chip is not produced-- instead of the chip fracturing in the desired comma-shaped manner, the chip just keeps extending into a stringy mess. You want a very sharp, high rotational speed tool constantly cutting new material so that work hardening is avoided, and distinct chips can be produced. Separately, for Al & Mg alloys, slow feed rates can lead to excessive heat generation at the tool/workpiece interface, which can lead to softening of the workpiece, ignition problems in the presence of H gas, etc.