So I've been thinking about clamping a CNC machine. The nicest way to clamp is with vacuum tables and these are very expensive with elaborate ways to fence off just part of the table. But maybe there is a better way. In a CNC machine, you are only ever cutting in one spot at once. And so you only ever need to clamp one spot at once with vacuum power. So imaging a vacuum hose underneath that moved around on a CNC rig to follow the cutter. It could be on a belt-driven two axis or three axis setup. It doesn't have to be as rigid as the top so it could be built more cheaply. No more need for a super-powerful vacuum that can clamp the whole table at once. No more need for complicated manual setups to clamp only a small area at once. There are definitely difficulties. You'd need to pause cutting while you turned off the vacuum, moved the hose, and turned it on again. And the system underneath would have to work around the supports for the router bed. But these seem solvable. What do you think? Crazy enough to work? -D
It might be more efficient to break the underside of the table into a series of zones with shut off valves for each of the zones. You wouldn't need to need to pause cutting to move the hose, you would just merely adjust shut off valves as you go.
the big problem with this is that the smaller your zone the less pressure and friction you have. you want as much area as possible being sucked on so that all of that area is creating friction with the table and preventing the part moving. as you cut the vacuum reduces due to leakage, so you really do need all you can get.
I hadn't realized that vacuum clamping required a large surface area to be effective. So I suppose that pretty much scrubs this idea. I might play around with a 'pressure foot' style clamp. My basic thought is that there is only one spot on the table where clamping force is needed at any one time. So there might be ways to make clamping simpler by only ever clamping at the spot where clamping force is needed: around the bit. -D
Essentially it's the atmosphere pressing down on your work, so with a perfect vacuum on the back side it would have a force of 14.7 lb/sq in. If you had a work piece of one square foot, that would be over a ton of pressure holding it down. That should be sufficient for any machining operation. I saw a really neat system for woodworking where the guy made a flat puck out of plastic with a gasket on each side. In the center of the puck was one of these recessed into a hole: It's called a bullet catch, and it's used on cabinet doors to hold them closed. There's a little spring inside that holds the ball against the end of the barrel. These guys were using them as valves. The back side of the catch opened up on a chamber that was under vacuum. The vacuum wasn't enough to pull the ball back against the spring, but if you put something flat down on top of the catch, and pressed down, the ball would get pressed down and allow vacuum into the chamber formed by the gasket. The vacuum was then enough to hold the piece in place. The vacuum was also applied on the back side of the puck to stick it to your workbench, and several pucks could be coupled together with a hose. The larger the work piece, the more of these little pucks you could use to hold it down, and the greater the holding force. You could also make a larger version of this with small gasketed "cells" that would each have their own valve. This way you could stick down almost any size work piece just by covering up the cells. Problem comes when you have to machine through holes...
Hi Metal Guru and Mark, Not sure if you have seen our product which is a vacuum clamp which is powered by compressed air. These were originally designed for CNC machines some fourteen years ago because I had the same problem that you are facing. The CNC has long since died, but the original clamps are still working just fine. I'm sure everyone hates blatant advertising on forums, however I though that this might be of interest to the people on this particular forum. Naturally enough I'm happy to go through the pro's and con's of our clamps if more information is required Website is http://vac-clamp.com/wp/ Best Regards, Errol
