It’s a small world, I guess. No sooner did I get done adding a section to my Turner’s Cube page showcasing Widgitmaster’s lathe-turned versions when I find him asking a question on CNCZone about how to calculate stepover to achieve a desired finish when profiling with a ballnosed cutter. As it happens, this function is built into G-Wizard like so:

Ballnose information appears below the RPM and Feedrate when the “Ballnose Cutter” choice is checked. It provides the effective diameter based on depth of cut and for a given scallop height will tell you the RA/RMS Surface Finish and required stepover. In this case, to achieve a maximum scallop height of a tenth (0.0001″) for an RA finish of 100, we need a 0.011″ stepover. That’s about 2.2% of the tool’s diameter.

Feeds and speeds for ballnosed cutters can be a bit tricky because the effective diameter of the tool is based on the cutting depth whenever you’re less than have the diameter of the tool deep. Think about it, the end of the cutter is a ball rather than the usual cylinder shape. You have to calculate the effective diameter based on how far the ball is into the workpiece.


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