Clean Plywood Edge Cuts

Bit choice, feed speed, RPMs, and other factors that affect how cleanly CNC equipment will cut routers. June 5, 2006

I cut mostly 3/4" pre-finished plywood, using a 1/2" diameter up/down spiral bit (C.R. Onsrud) at 400 i.p.m., 18,000 R.P.M. cutting in a conventional direction.

I could be happier with the cuts I'm getting. The cross-grain plys in the board come out slightly raised. It's not nearly as bad as a it would be if I had climb cut, but still not as good as a saw cut. This has sometimes led to edgeband failure. I've been running the parts through the table saw, taking 1/64 off the front edge to clean it up but would like to avoid this for obvious reasons.

Forum Responses
(CNC Forum)
From contributor T:
We cut a lot of plywood as well. I got tool from Leuco a couple of months ago that cuts great. It’s a 3/4" diamond, 3+3 compression bit. I am running it at 16 meters/minute (630 ipm) at 18 000 rpm. It leaves an incredibly clean cut, definitely ready to edgeband straight off the machine.

I just finished a job of all vg fir veneer on MDF core. I cut all the panels to size with this bit, and even cross grain routing come out perfect. I was very impressed. The sales man recommended 11 to 16 m/m and I run it at 16 m/m with great results.

From the original questioner:
3/4" diameter seems a bit wide. Is the same type of tool available in 1/2"? I had always heard that the main down side to using diamond is it requires a slow feed rate.

From contributor Y:

I cut mostly 3/4" VC pre finished plywood as well. I'm using Onsrud 3/8" max life compression 2 flute bits, 650 IPM at 16500 rpm. I've been getting acceptable edges over the span of the bit life which for me has been about 125-140 sheets. I've never had an edgeband failure due to the plywood edge. I've found that the quality of the edge is as much related to the plywood core as it is the bit and speed. I can cut a job and all the 3/4" pre finished has great smooth edges but when I change to the 3/4 cherry plywood for slab applied ends and toe kicks the edge is not as nice. Find a product and bit/speed combination that works for you and stick with it for predictable results.

From contributor A:
I agree with Contributor Y. No matter how good of cutter you have if you have crappy plywood you will never find a good cut. The one thing I recommend is to try running you ipm faster or maybe turning your rpm down a little because your chip load is quite small at the rate you are running.

From contributor E:
I also run a lot of plywood with a 3/8 Onsrud compression cutter. I've had some problems like you are having with the imported pre-finished ply these days. The cores are really getting worse, they even look different than I used to get and reek of formaldehyde. Try a little less rpm or a higher feed to get your chip load right. Also, have you tried doing a finish pass of say .04? I would suggest trying that and seeing if it helps.

From the original questioner:
Contributor E - your idea of making a finish pass might be the ticket. The quality of board – that shouldn’t be an issue since I'm using Canadian Norboard. Are you guys getting the identical quality cut off the router as you would off the saw? Like I said, the quality isn't that bad, just not as good as what the slider would produce. I'm using a Virutex hot air machine, not a glue pot type, so that might account for the occasional failure.

From contributor Y:
Slider cut and router cut are definitely different. The slider produces a smoother cut to the touch in plywood for sure but I've not noticed any problems or failures in edge banding due to the router. You probably have something with regards to the hot air machine.

From contributor R:
I was having the same problem and switched to a 3/8 compression bit from Courmat and the problem was solved.

From contributor G:
You are running the tool incorrectly. What is happening is your chips are being re-cut twice, prior to being evacuated from the flutes. If you could see the inside of the flutes (don't poke your head near there) the flutes would be completely full of the chips. These chips are being imbedded into the material causing you this problem. Slow your rpm's to 14K and you should see a better cut and longer tool life.

From contributor J:
I agree 100% with Contributor G. Your rpm's are way too high and this also reduces your tool life as well. You may want to increase your feed rate as well as slowing the rpm’s somewhere between 12,500 and 14,000.

From the original questioner:
I’ll take that advice to heart, but let me play devils advocate for a moment. I onion skin all my sheet goods. After the first pass, the path is completely full of chips. Aren’t these getting cut twice no matter what the feed rate/RPM? The machine has an articulated air nozzle which would allow me to direct a stream of air at the cut to blow these chips out. Unfortunately, my compressor can’t keep pace if I use it. If I got a larger compressor, would it make a significant difference in terms of cut quality/bit life?

From contributor G:
If you can run the air while machining, the chips (not all) will be evacuating into the dust collector. But re-cutting on the second pass, with all the chips in the groove, they will be re-cut again. We have a number of different cut lengths on the up/shear portion. .200 is for dado cuts, .400 is for material 1/2" and greater, and we also have a .700 up shear to remove most of the material for 3/4". Doing the onion skin cuts is for really small parts that may move on the table. If you have enough vacuum, or a lot of small parts, continue to do the onion cut, but use the .400 or .700 up shear.