Carbide Create - 3D Roughing and Finishing

I thought I understood most of the terminology and calculations, but I need some help as the software is not calculating the stepover numbers I was expecting to see in 3D roughing and 3D finishing.

3D Roughing with the 201 1/4 (flat) endmill. In my tool library the 201 is setup to use 40% stepover.

I thought the calculation would be (0.25 * 0.40 = .10) 1/4 tool diameter * 40% stepover percentage. But Carbide Create (621) returns with a stepover of .125? Is the “finish allowance” somehow involved in this calculation? I thought the finish allowance turned into the “stock to leave” value, but it appears I am wrong there as well. Just trying to understand how the calculation works.

The 3D finishing with the 202 1/4 ball. In the tool library the 202 is setup to use 10% stepover.

So I was expecting (0.25 * .10 = .025), but the software calculated 0.125

Thanks, I just want to understand the process.

this might be a place where the code doesn’t use the setting in the tool library. Your understanding is correct, 0.1 would be that 40% stepover.

finish allowance is something else, it is basically how much material to leave for the finishing pass to clean up. this is important because if your roughing tool isn’t super precise or undergoes deflection due to larger forces (after all, you’re roughing)… it might go a little further in the cut than you’d want, and if you go EXACTLY to the final surface, that shows up as scratches and otherwise looks ugly…
so it’s always a good idea to leave a little bit of margin (e.g. keep some distance from the final surface) and have the finishing pass clean it up, which is assumed to be a more accurate bit (in terms of dimensions) and also not have the same amount of force on it during the cut (so less deflection)


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