The path chosen for a simple circular pocket should be a simple circle, stepping down with each pass. This is indeed the case except when the width of the pocket is twice the width of the bit. For example, select the material as 0.75" thick. Create two circle, one of radius 6.5", then a concentric one of 6" radius. Select the pocket toolpath and choose a 1/4" diameter bit and the stock bottom (or close) as the depth. Run the simulation. Instead of a sequence of circles with a shallow depth, it begins with a deep plunge and does 1/8 of the pocket’s arc, digs deep through a short section, then rises some and does an adjoining 1/8 arc followed by a deep dig through a short arc, and only then starts the process at the upper surface with circular shallow passes.
Any time the bit width and stepover exactly divides the width of the circle (or in this case, the annulus) , you are likely to get weird pathing. Round off errors as you go around the circle mean that sometime the bit is just a tiny bit on the inside of the ‘perfect’ circle it’s trying to follow, and sometime on the outside. So, it goes back to clear the areas it think it missed.
Try changing the stepover even slightly, the problem will likely go away. For example, if the bit is 0.250" and the stepover 0.125", change the stepover to 0.124". Or, change one of the circles slightly. If you can reduce the size of the pocket slightly, or increase the stepover slightly, this will usually make the pocketing operation faster as it eliminates a contour.
It shows up a lot for imperial projects because the default stepover is 50%, so the bit widths and stepovers all mesh terribly with the sort of round numbers you’d expect from designs that don’t have to meet existing constraints.
Stepover 0.125 (the default)
Stepover of 0.124: time is 13 minutes
Default stepover, inside circle radius is 6.0010" - time is 6 minutes
Rounding errors caused by the geometry being converted into a polyline — I always adjust stepover until the toolpaths seem even and there is minimal occurrence of short paths which involve lifts.
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