Originally Posted by
dlcw
Personally I cut everything with a 1/4" mortise compression. Like mentioned, a mortise compression has much less up cut spiral thus you don't need to plunge as deep into the material.
For 5/8" Applyeply (like baltic birch) with my particular setup (PRSAlpha 2.2hp spindle, pretty strong vacuum holddown system), I will make 3/8" deep passes at 5IPS @13K RPM.
In softer 3/4" plywood, I will make my first cut all the way to the point of just leaving the .025" skin. Then conventional cut all the way thru. I go 6IPS at 14K RPM. The bits I use have no problem cutting that much plywood at those speeds and feeds on my machine.
One thing Gary taught me, many years ago, you can push these machines pretty hard. I was always hesitant about cutting fast. But as the saying goes, a bit that is screaming as it cuts is a hungry bit. Either increase your feed rate or decrease your RPM's. When you've reached the sweet spot for your machine, the vacuum hold down system (if it resides in your shop with you) is MUCH louder than the cutting bit. When done cutting, you should be able to grab the bit with your fingers (TURN THE SPINDLE OFF FIRST) and it should be room temp. If hot, the bit is spinning to fast (friction) for the feed rate rate you've selected.
The problem I've seen with the Shopbot machines is the inherent flex in the machine. That is why I've switched to the two-way cutting process. Like Gary says, you can cut a piece of plywood with a 3/8" compression bit in one pass. I've found the accuracy of part dimensions using the method to be off a little (but that's probably because my machine isn't as tight as it could be). A climb cut will push the bit away the your cut line. A conventional cut pulls the bit towards the cut line. For parts I cut on my machine, I can go straight from the CNC to the edgebander with no sanding or edge cleanup needed.
Your mileage may vary!! So you need to experiment and figure out what works best for your machine.