Cutting CDX on a CNC

Exterior grade plywood is hard to work with on a CNC router because it won't lie flat. Here is some hold-down advice and some thoughts on bit choice.October 20, 2013

(WOODWEB Member) :
Anyone have experience getting a decently clean edge with CDX? If so, what bit and speeds?

Forum Responses
(CNC Forum)
From contributor K:
In our experience, getting CDX to hold down on a flat table machine is quite a challenge.

From the original questioner:
I hear you. They lay down like a twisted pretzel.

From contributor K:
Might sound a little radical, but we have actually screwed through the individual pieces (if they're not going to be seen in the finished product) right down to the spoilboard. We make a program showing all our cut paths and then add pre-drilled holes in the parts. Shut off your milling cycle and just run the hole program, then screw down, then turn off the holes and turn on the milling and cut your parts. By the way, Vortex Tool has compression spirals with chipbreakers that give a real nice cut on plywood.

From contributor M:
I do the same thing as contributor K when I need to make fire rated die wall parts or Chinese plywood cores. I use a separate program for the 5mm holes that take a drywall screw nicely. It takes some time but it is better than a lot of wasted and renested parts. Guarantees no losses in cutting due to movement, but is a slow process.

I use an Onsrud 60-037 roughing slow upcut spiral for this type of work. It clears the kerf reasonably well and leaves the top edge well within reason for rough work.

If you cut an awful lot of cheap plywood I have seen a roller system on a Thermwood that holds down the work well. I think they are used by furniture manufacturers that process a lot of cheap board.

From contributor S:
Triple flute compression 18000rpm.
.020-.023 chip load

From the original questioner:
Thanks for all your suggestions. I am going to play with some scrap.