very long linear rails and screws

Not exactly high precision, but I once saw a rig for cutting big sheets of architectural lucite that used a skillsaw mounted on a slowed-down garage door opener.

What you are building has been done before other than the width and length.

have two plasma tables where I work, neither of them have screws to run the carriage, just motors with drive wheels and sensors so it knows where it is. Think DRO. Greg

elevator rails are pretty long

How accurate? For moderate accuracy I haven't found anything that can beat the price of the timing belt systems from Item. I'm building a machine right now that has a gantry positioner with a 2.5m stroke. About $2000 for all the components for one axis, not including the drive and controls.

If you need very high accuracy, IKO, THK, etc., will supply their linear rails jointed to be spliced in any length you specify - at a hefty price.

A 6m lead screw is not a trivial undertaking, particularly if you require high speeds and/or accuracy. A timing belt drive would be much easier and cheaper. It would be easier to make recommendations if you specify speed, accuracy and loads.

Ned Simmons

"John Smith" wrote in message news:42191080$0$28983$ snipped-for-privacy@news.xsall.nl... | I am planning to build a very large CNC plasma cutting table for cutting | plates of 6000x2000 mm metric, so about 20' by 7' | Anyone know which company supplies rails and screws that long?

Have you considered 80/20 tubing by Bosch? This stuff is usually used for framing and such but it also has excellent applications for moving parts of good precision. Precise, strong as hell, and easy to work with. You can shim to get even tighter precision. You can have Bosch work out the design for you or you can do it yourself. I'm in a shop at work now that uses lots of this stuff for all kinds of projects, from simple to phenomenally comples, with computer controls for all the moving parts. The current project I'm working now on uses lots of it. We've come up with a few small jigs to make drilling it easier, which is nice. Here's a couple sites that came up with a quick web search, but there are competitors that have some great stuff and is even cheaper. Each kind has advantages.

and The Bosch stuff comes in like 20 foot pieces, and can easily be spliced together with no loss in accuracy. I like it a lot.

I would use a timing belt drive, Brecoflex sells the most precise belts. As for the rails, thk style rails can be bolted on end to end. I have designed long feeding mechanisms in this exact fashion and the resulting machine was able to hold tolerances under 0.010" atlthough a lookup table might have been used for error compensation.

stan

Some cnc software has this option. You can use a laser or similar to map the errors and tell the control where it really is. ERS

why not consider using gear racks and spur gears? These and linear rails can be joined.