Opinions sought on designing skate-bearing slides for 1m x 2m (3.5'x7' roughly)

[posix]

I have drawn and am pondering an idea of a 1000mm x 2000mm (cutting area) router for woodworking. Joes 2006 seems to be out of the question because of the size. And also because I got some HUGE steppers really cheap (nema42 I think they were). And they would go to waste if they weren't mounted on something big. Thus 1000mm x 2000mm cutting area. Think doors.

Now, since I'm on a really tight budget and the availability of linear components is non-existent in my neck of the woods I'm going for the skate bearings approach.

Having come up with a design where bearings ride on the standard structural 80x40mm steel tube (gantry section detail attached) I was wondering how accurate can that square tube actually be over a 2000mm length? How bent will it be? What are my options?
Will actually mounting a 2000m long pipe atop the 80x40 be more precise?

What I need is accuracy down to 0.1mm as I think woodworking can't benefit from better accuracy but anything bigger than that and you're looking at major headaches when it comes to aligning cut parts. Maybe I could get away with 0.5mm but 0.1mm would be ideal.

Is it even sane to ask for such accuracy from a DIY gantry router that has a 1000mm x 2000mm cutting area?

I am going with belt drives (Here a big nod an a smile to Mike Everman, if he doesn't mind me using his idea at home?) as even getting hold of threaded rod is impossible in my neighbourhood, let alone ball screws, and then at that length!

So ideas please, how straight, accurate, flat etc are these structural steel tubes?

Thanks for your ideas guys, I'm skinnt right now and getting this machine up and running (after so many failed attempts) would bring me back to life, definitely.

[posix]

maybe I should rename the thread? I'm thinking people don't understand what I'm asking. I know sometimes I don't really put across what I mean to say...

[posix]

if I were to lay long strips of glass on the 80x40 tube? would that help? is glass "straight"?

[Trucks]

I have a table thats 3' x 5' right now and i used the skate bearing idea, and steel tube and steel angle, and it seems like it is really straight, i used a large welding table to assemble everything and when i clamp a piece of flat stock onto my gantry and move it around the table, its so close all over that i can't tell if its off or not. I don't know if glass will work, i just decided that 1/8" steel should be strong enough for a DIY machine.

[posix]

thanks trucks! do you have a picture of your setup that you could post?

[Trucks]

sure.

its in this forum...

http://www.cnczone.com/forums/showthread.php?t=87606

[Matt Adams]

I have a few thoughts on this. Hopefully what you're looking for is honest feedback.

Overall accuracy is different than repeatability, and both of these have as much to do with your transmission components (belts, screws, rack & pinion) as your frame.

It should be noted, that there are people that have spent big bucks on profile or linear rails with linear bearings and have precision machined components on a precision aligned frame and aren't getting anywhere near the 0.1mm accuracy you're looking for, especially at the size you're looking at.

As a rule of thumb, a generally descent ball screw might have an accuracy of 0.003" per foot. You're looking for this for your whole system. Belts stretch, rack and pinions have backlash...

My point is that I think you have expectations pretty high. The beauty of wood is that there's sandpaper for the fine tuning of mating parts. You'll probably need to compromise your accuracy requirements or your budget. That choice is up to you.

Good luck!

Matt

[posix]

ah ok so that's a good start. my expectations are too high. so what am I looking at then? 1mm? 0.5mm? and how difficult is it to get those numbers from plain stock structural steel tubing? and is 80x40 a little "oversize"? can I go lower, maybe 40x40?

[posix]

oh and thanks for honest feedback, much appreciated, indeed!