A reader recently used the “Ask an Expert” feature of our High Speed Machining Zone to ask a series of questions aimed at better understanding high speed machining.

Question

What is the best surface finish I can acheive while contouring an outside radius with a ball endmill during high speed machining? We use a large-radius cutter now (5/16 inch).

Response from Tom Delio, co-founder of BlueSwarf

Surface finish can be affected by many things, including chip load, stepover, tool geometry and inclination of the tool.  The benefit of high speed machining is that higher machining rates can be achieved with lower cutting forces and lower chip loads—both of which will improve surface finish.

However, one risk is the vibration resulting from imbalance and also the frequency sensitivity of the cutter at higher speeds.  To best combat this you must make sure your tools are well balanced. For high speeds, a level of G2.5 is the generally accepted standard.  Also, it would be best to identify the frequency sensitivities with a tap test and then compute the sensitive speeds that need to be avoided.  Alternately, this can be done with testing by choosing your desired speed and simply trying speeds about 5 to 10% higher and lower and observing which ones result in a better surface finish.

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