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I am cutting 3/4" Birch Plywood with a downcut 1/4" bit with a feed rate of roughly 39 IPM with a 2mm depth of cut(all information I pulled from the Shapeoko Wiki) and fastest speed on the router
The cut starts great, super repeatable and consistent over multiple passes/parts, but during the cutting operation there is always one or two passes that step over to far in the x direction. I have tried to counteract this with slowing the feed rate from 50 IPM to 39 IPM and it still manages to do it.
Its important these parts are the same or within reason of sanding distance for the finish of the project to be successful, but when the machine likes to take a random step over for seemingly no reason after cutting perfectly for 20 minutes its kind of frustrating, and I hate wasting all this material.
Just a thought. You mentioned that you were using a down cut bit. Is it possible that the kerf from the bit is getting packed with sawdust? An Upcut bit will pull all of the chips up and out of the kerf where as a downcut bit will pack the chips at the bottom.
I have found that the best time to use a downcut bit is when you are concerned about chipout or the fuzzies on the top edge of the cut. Basically for Inlays. The rest of the time an Upcut bit is more beneficial.
They do make a compression bit that is both an Upcut and downcut bit. But I do not think in our situation these will do anything other than what a downcut bit will do. Compression bits work best when trimming off slivers of wood from the full edge of a piece of wood. Say a 3/4" piece of plywood, a compression bit will be cutting across the full distance of the 3/4" plywood. Where as with a CNC we are only making cuts about 1mm thick at a time.
You might have to include some relief cuts first. I believe this is basically the same thing as “step over” but I may be incorrect. If you are cutting a 1/4" wide channel, the bit might be binding in the channel. This is where instead of a 1/4" wide channel you cut a 3/8" wide channel. The path that touches the perimeter of the actual product should be the final pass, this will mean that the actual product gets cut with the bit only cutting into the wood by 1/2 of the diameter of the bit, in this case 1/8". A lot less stress on the bit this way.
A compression bit will leave him a clean top finish and also evacuate chips as it moves lower after a couple of passes. It would be quite beneficial for 3/4 birch plywood. But, I don’t think this is his problem. I cut 3/4" birch all the time with a downcut bit.
He should NOT be using the fastest speed on the router. a setting of 1 would be more appropriate. At the highest rpms, with a downcut bit you could be looking at a fire hazard, or at the very least scorch marks and a blackened bit. And on the Dewalt, he’s burning through his brushes - the don’t last long at the highest setting.
