The first step-over for a pocket path doesn’t respect the step-over setting for the bit.
This is a 3/4" bit, 8mm step-over, grid is 10mm. The first stepover is clearly much greater than 8mm.
overlap test.c2d (44 KB)
If you set the grid size to 8mm:
then the toolpaths line up with the grid:
If you then increase the stepover to 1mm:
one gets the much smaller spacing expected.
But the First step-over is way bigger than 8mm. All the rest are fine, but right down the middle the bit moved over way more than the step-over.
None of the lines should be more than 8mm apart in this scenario.
In NCViewer, it’s trivial to see that the first stepover is a move from -19.905mm to -4.955mm, a travel of 14.95mm.
In every other case it make sense to move over by 2x the step over, because the machine is cutting ‘around’ an area that is already cleared. This is not the case for the first pass, the step over there should be limited to at most the step-over value and not 2x the stepover value.
No, that’s not how it happened at all. Original width is 98mm. Subtract the width of the bit to get the center line of the outside passes → 98-19.05 = 78.95 apart.
Take that, and divide by double the stepover (i.e., 16), and round down 79.95/16 = 4.934 ==> 4. This is the number of fullwidth passes made on each side.
Now you are left with 79.95-64 = 14.95, and that is the final stepover distance. The toolpath should see if the final stepover > requested stepover, and if so run another pass right down the middle.
The way the math works, the final step-over will always be >0 and < 2x stepover. I just found a case here that nearly maximizes the final stepover, but really any (rectangle minus bitwidth) that is just under a multiple of 2x the stepover will do it.
The problem is that the toolpath is planned from the outside in, but cut from the inside outwards. If it was both planned and cut from the outside in, there would be no problem, since when moving towards the middle it only uses the stepover, and there is no ‘final stepover’ that is an odd size - the final area is cut already.
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I’m removing my comments because I’m an idiot. I re-read what i wrote and realized how wrong i am. The path is doing exactly what you wrote, and create doesn’t do the middle pass because of the 3mm overlap. If you absolutely had to achieve a middle pass you’d have to add a single vector line with a zero offset contour pass. I have brought great shame to my famiry.
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And now that i have wasted enough time, i feel the need to say that if you absolutely need to have the extra center path, you could reduce the stepover from 8 to 7.8125 (in probably wrong again)
As a new discovery, this high initial step-over can cause parts of the pocket to not get machined.
If the stepover is more than half the width of the bit (perhaps not the best practice, but still a valid thing to do), you can have material left behind. Since the initial stepover can be double the requested stepover, and the stepover is more than 1/2 the bit diameter, as a worst case scenario the initial stepover is more than the width of the bit.
An exaggerated example - 3/4" bit, 14mm stepover:
But even with a not-unreasonable sounding 3/4" bit, 10mm stepover:
