CNC End Mill Question: Up-cut, Down-cut or Compression?

Hi All,

I’ve been reading a few articles on the topic, but I wanted to get a consensus for what would work best for our space. I’m wondering if it would be advantageous to invest in a down-cut or compression end mill if I’m planning on making something that requires a finished surface on both sides of the workpiece.

A downcut would obviously give a better finish on the top of the workpiece, but I’m wondering if this would create issues with chip extraction, not to mention some potential tear-out on the bottom of the piece.

A compression end mill would have a portion of the bit up-cutting on the bottom of the workpiece and downcutting on the top, but if my tool paths are plunging in 1/8" increments, it would potentially tear out on the top, as it would be cutting with the up-cut portion of the mill. I think the only scenario where this would make sense is if the machine were able to do a full depth cut in a single pass. Is this a possibility the PA 1000? Is it recommended with the right cutter?

In my mind the best approach would be to do a tool change starting with a downcut bit on the first pass on the top of the workpiece and switching to an up-cut bit for the subsequent passes for the cutter path.

Oh - I should probably mention that this would be cutting plywood, most likely baltic birch or a finished veneer on both sides.

What advice do you all have?


My $0.02, you probably won’t get a lot better results with a compression bit, largely because of what you just describe with doing multiple step downs. If you’re seeing problems with the bottom side, a new sharp endmill will probably help a lot (there should be more in the drawer). Or, you could “onion skin” the parts. Essentially, set your depth so that you leave a very small amount at the bottom (5-10 thou). Small enough that you can free the parts with a knife, and then use a deburring tool, flush trim bit, or just a razor blade to clean up the bottom side.

That said, the bottom side should be well supported with the spoil board, so you shouldn’t have too much or a chip-out problem with a sharp bit. Adjusting the feeds and speeds to find the right MRR will really help as well.

I was having issues with the top side of the material, but after further examining the cutter, I realized I used a cutter that probably had some “extra mileage” on it. For the project I ran last night, I used a newer cutter and the top side finish was great. No issues with the bottom other than the cut not being deep enough is places, but like you mentioned, I was able to cut through that with a knife.

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