Compression bit - better cutting tip


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This may be common knowledge amongst you CNC guru's but I just figured it out today. I don't research cutting profiles and tips often - ok, rarely - and it would probably be in my best interest to do so from time to time.

However, assuming this little issue is one you've encountered or not thought of then this little tip will save you a bunch of time and a fair amount of sanding.

My normal cutting profile on 1/2" BB (12mm) for Longworth chucks is to cut the slots in a rough pass climb cut leaving 0.007" on the side walls (radial) and 0.020" on the bottom (axial). Then I come back and clean up each slot with a conventional cut at full depth and that has left a very clean slot. Except for the bottom face veneer sitting on the spoilboard. That has splinters/fibers/fuzz and I have to hand sand those. All this is at 175 ipm and 18k rpm.

Now this may not be a big deal if you're cutting one Longworth chuck. But on days like today, where we had a 16" going to the UK and three 12" sets for the States, then that adds up to 64 slots! That's way more hand sanding than I want to be doing on these given the low cost we charge and I don't want to increase the cost just because I feel the need to do some sanding to make a better product.

However, time is money and as I was cutting the plates today I began to finally think about how I can eliminate the fuzz. It dawned on me before I cut the final of eight plates that I need to cut the full depth in one pass rather than leave 0.020" because what's happening is there's nothing to support the upcut of the compression bit in that final 0.020". So I quickly modified the cutting profile in Fusion 360 and cut the final plate. 

Turns out my thinking was correct, albeit 7-8 months late (we've cut over 270 Longworth chucks and I've had to sand slots on most of these). So now I can't wait to cut more chucks and NOT sand slots! LOL!  In thinking about why this worked I realize it's something that just makes sense and I should have thought about this a long time ago.

Leaving 0.020" for final pass -


Cutting to full depth in one pass -



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Hi David. While there's nothing wrong with cutting to full depth in one pass on your router, you only NEED to cut past the depth of the upcut portion of the compression bit to avoid the fuzzing. Even with a ramped lead-in, the toolpath should take you past the start point far enough that any fuzzing on the ramp portion is removed or cleaned up by the full depth pass. There should never be any need to sand after using one of these bits.

Also, there's a variant of compression cutters called mortise compression bits which have a shallow, or shorter upcut portion so you don't have to cut as deep on the first pass. 

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Thanks, Mick - good info as usual. 

My bit is configured with 0.200" upcut and on some cuts other than the BB, like in Walnut, I just make sure I cut deeper than that (although I often cut to about 0.380" in Walnut).  On the BB I would rather cut the full depth in one pass because we can.  But I thought I was doing the right thing by leaving 0.020" for a clean-up pass and that proved to be the source of the issue. 

And I agree that there shouldn't be anything to sand when using these bits so that's what was puzzling.  When I realized that I had created the situation that now required sanding I was eager to share what I had learned, even if it was already common knowledge.  I frequently 'discover' things that once I get my head out of my own shop I come to the realization I'm the only one who didn't know what I 'just discovered'! LOL! :o

I'll look at those mortise compression bits; they are pricey but I can see why.


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