I am a newbie to the board. I am interested i the concept of a simple (inexpensive) plywood panel cutter. Only X and Y axis would be required. The cutter would be a plunge router that the operator could engage as necessary.

Both axis would be rack and pinion due to the desire for a 4' by 8' working area.

I believe this idea would save complexity and cost and still get the job done.

Also, since tolerences for woodworking stuff is usually around 1/32", could the rack and pinion be cut from UHMW plastic? It would be a lot less expensive than steel. I would think that backlash issues could be taken care of in the programming to drive the gantry in one direction for all cuts. It might work.

Any thoughts?

There is a lot of force required for cutting plywood. I'd use steel rack and pinion. Backlash can be taken care of like Shopbot does, by having the pinion on a pivoting mount that can be tighted into the rack.

IMO, if you're going through all the trouble to build this thing, add the Z-axis. A small travel Z-axis can be built pretty cheaply. Just but a short set of rails like these. http://cgi.ebay.com/4-IKO-LWH-15-Linear-Ball-Bearing-Slide-Rail-CNC-NSK-THK_W0QQitemZ7534423225QQcategoryZ55826QQrdZ1QQcmdZViewItem

This will make the thing faster by a factor of 10, but open up many more possibilities.
Also, trying to eliminate backlash with software will always have problems.

Have you considered chain drive instead of rack and pinion? A chain looped around sprockets at each end of the travel with both ends attached to the moving part and pulled tight can have very little backlash and the bits are much cheaper than rack and pinion.

While what you suggest is possible, heck you could replace the operator with an air cylinder, do realize that there are huge limitations here from the aspect of capabilities.

Interestingly another thread here is covering design trades offs as we speak. One idea is to avoid desinging out future upgrades. So do what you want but just keep in mind that you are likely to do an upgrade in the future.

In any event think seriously about that vertical axis. You won't regret increased performance there.


Thanks
Dave

I am not throwing out the possibility of a z axis. Having one in the future would indeed make the device more useful. My main concern was cost. I have seen several plans for smaller machines that were quite well done. My interest is in getting something that can do a 4'x8' sheet of plywood and not break the bank in the process. At this point I'm just going through the mental exercise.

I like the idea of chain drive. I would be concerned with supporting the drive rod over a long distance. THe chain idea takes that out of the equation.