[chuckknigh]

Well, I'm slowly but surely gathering up parts, and finalizing my design for my router. I'm working on the Z axis, and various subassemblies right now.

One:

All of the designs I've seen on this forum show a Z axis supporting a router, and they all seem to be cantilevered. The router is on one side, and the supports are off to the other side...analyzing the forces, it's a short cantilevered arm!

Since a router's weight is non-negligible, that means that lateral stress is going to be put on the bearings and guides. This should reduce their useful life, and wear them out faster than they should be. (Of course, cutting drag also introduces lateral forces, but those can't be eliminated very easily)

Has anyone addresed this "problem?" In a real machine, is it even a real problem?

I've had the inspiration to place my Y axis rails next to each other on a horizontal plane, and place the entire weight of the Z axis and router in their center. This will be a relatively simple design to implement, and will eliminate any lateral stresses involved with that axis.

I could conceivably do the same basic thing with the Z axis, mounting it such that the router's center of mass falls between the rails, but it would complicate the design a little bit...just trying to figure out if it's worth the extra trouble.

Of course, by doing this, my drive screw is going to be moved off center -- if the fit on the linear bearings is tight enough, it WILL move straight, but again there will be lateral forces involved. Has anyone come up with a simple design which overcomes this?

I could do it by making my Z axis longer and putting the drive screw in line with the cutting tool, probably above it -- this would keep everything aligned. Another obvious way is to mount the router *inside* the screw drive itself, but that'd require an enormously large diameter screw, and would likely introduce other problems, too. But, something like that could be implemented with relative ease in large diameter PVC... Hmmm...I might have to play with that idea.

Two:

Anti-backlash nut design. I think I've come up with a simple design...but it seems too simple. Does anyone see a problem with this basic design?

Premise: The drag on a nut is directly proportional to the area of contact -- in other words, as little of the nut should touch the screw, as is possible, to minimize drag.

Solution: PVC pipe fittings. Internally (female) threaded cap and an externally (male) threaded plug that screw together. The fittings for SCH40 are fairly thick, so they have some structural integrity, but this arrangement results in a hollow box, and only the ends will be in contact with the lead screw.

So, you take this assembly, drill a hole in both ends, and tap them to accept the lead screw -- screw the fittings together, or apart to dial out any backlash. Since the thread pitch of the fittings is different than the pitch of the lead screw, the "nut" will put pressure on the lead screw's threads. Voila...we have adjustability and minimal surface for contact. Should produce a cheap, homebuildable (by anyone) anti-backlash nut. If it slips, it's easy to add a set screw to the outer half, to hold it in place.

I look forward to your comments.

-- Chuck Knight