I’ve seen the same problem many times. The spindle and gantry assembly for the Shapeoko is not particularly rigid. When you’re cutting a workpiece around its perimeter, the spindle has a tendency to deflect away from the workpiece by a small amount as its moving in X-Y motions. When the spindle is plunging, its X-Y position dwells at a fixed point and thus it straightens out and cuts a little deeper into the workpiece resulting in a scalloped cut on the outer edge of the part.
Here’s something that I tried to address the problem: Its probably not the easiest way to cure the issue, but it was very effective. The example below shows the geometry that I created to cut a simple obround part out of MDF.
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When programming in Carbide Create, my first step is typically to make a rough-cut by offsetting the outer perimeter of the part by .030" (STEP 1)
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Carbide Create always plunges in the lower-left-most part of the selected toolpath. If I were to create the finish-cut toolpath as one single step, then the mill would plunge tangent to the lower-left are of the part and create the scalloped groove that Julien mentioned in the first post. Instead, I revised the outer profile of the workpiece so that it extends outward on the lower-left side of the part (making it look like a cartoon speech bubble with a pointer to the lower-left). This results in the cutter plunging outside of the finished profile of the workpiece.
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STEP 2 cuts a portion of the finished profile. The cutter plunges at the pointer of the speech bubble, and then makes a finish-cut completely around the perimeter of the part (except where the pointer is).
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STEP 3 finishes off the final outline of the workpiece. The cutter plunges to the lower-left and follows the inside path of the triangle shape, thus milling away the pointer that remained from STEP 2.
