The use of screws for linear rails as well

[CNCDevil]

I already made my first wooden cnc-router, but got obsesed and want to build one more

I´ve found two acme lead screws measurering 30mm/~1 1/4" in diameter and 600mm/~23" long. They got a pitch of 6mm/~4 TPI. Both of them are very straight. My idea is to make a "table-moving" machine with a fixed x-axis, primeraly to cut small wood parts. The table should then be around 300mm/~11" long.

What are the chances to get a good result, if the table is only supported by the screws, using no linear guides?

Estimated table weight is around 3kg/6.6lbs

Brian

[WilliamD]

Interesting idea. That has sparked my own imagination a little bit. I'll post some drawings later. I have some 6' 1-5 tpi acme screws. I'm thinking one on each side, so two stepper motors. The nut that drives each side would have a slit across the bottom, so you could support the screw. We'll have to see.

[toastydeath]

It depends on how much sag you care about. The reason dedicated surfaces are used for support is so they can be made flat under the effects of gravity and machine stresses. They can me made very rigid, and corrected to support the machine under a wide variety of operating conditions.

The screw will bend, and since the bend will be dependent on the changing process, there's no way to correct for it.

[steve323]

Also, the screw will bend relative to the inside diameter of the threads, not the outer diameter. They do sound like some really beefy screws for a 23" span.

Steve

[Glacern]

A 4tpi ACME screw has a thread height of 0.125", so the minor diameter is 1".

It will easily support 6.6lbs over 23" with very little deflection. Hopefully friction isn't too big of a problem. Big ACMEs tend to have a decent amount. What material is the nut made of?

Edit to add: The tricky part will be machining the ends of the screw. Any radial inaccuracy will directly translate to machine inaccuracy. Machines with linear motion rails can usually tolerate some radial error without affecting motion.

[spoiledbrat]

Will your cutting forces (lateral, vertical for helical cutters) exceed 6.6 lbs? I get lines in the wood when cutting with my 1/2" solid carbide up-cut bits, and I have found that this is entirely caused by the lifting effect of the bit. By making multiple and smaller passes this goes away...

Just wondering if you think this will effect your application.

Rob

[CNCDevil]

The nuts are made of steel. They are 60mm/~2 3/8" in diam. and 30mm/~1 3/16" deep. If I surface the rim on the diameter, I should get a decent flat area on the nut to fastening the table on.

6.6 lbs is the weight of the table, which presses on the screws. I don´t know about the other forces. I intent to use Daves 305oz motors.

What about you WilliamD, have you made some drawings?

Brian