I was wondering if there is a formula or something else to eliminate any racking in building a machine with 2 rails and a 14' long bridge on top of them. I was planning on having the y axis manual but want to make sure it doesn't rack at all. A friend has one that I looked at today and it racked very easily.
Thanks,
Doug

Spread the bearings out as far as possible and make the rails and gantry as stiff as possible. Got any pic's of your friends machine?

Chris

Hi Chris,
I don't have any but can get some. His side rails were like 3' long which I thought would be plenty to eliminate any racking. I was planning on having my rails around 3' also. The only guaranteed way of not getting any racking I thought was to have gear rack on both sides with a shaft across the bridge connecting each side. But I also thought that it would really increase the resistance of manually pushing the bridge back and forth. So now i'm not sure. :(
Regards,
Doug

are you talking about a timing issue between the horizontal or side rails?

Hi Chris,
I don't have any but can get some. His side rails were like 3' long which I thought would be plenty to eliminate any racking. I was planning on having my rails around 3' also. The only guaranteed way of not getting any racking I thought was to have gear rack on both sides with a shaft across the bridge connecting each side. But I also thought that it would really increase the resistance of manually pushing the bridge back and forth. So now i'm not sure. :(
Regards,
Doug

Doug-

Get some pictures. It's difficult to talk about this without seeing how it was built. If you drive a gantry with a double gear rack and gears connected with a shaft the only way you could get racking would be because of torsional deflection of the shaft which could easily happen if the shaft is 14 ft. long. You could instead use two steppers (one for each rack) that are simultaneously receiving the same step/dir signal from the computer.

Chris