Question about new 4 axis router (lead screws)

[Tom Kay]

Hello All;

My first post on the forum. First, congrats to Art, who really seems to be making a lot of CNC users happy with is software.

I want to make a 4 axis router to cut wood, plastics and perhaps alum. Mainly I want to make wood propeller blades, 45" long. Hence the 4th axis.

To save money, I might use Acme lead screws and plastic nuts, preloaded for smallish backlash (not quite zero backlash as this causes plastic wear more quickly).

I want to use cheaper lead screws, say 1/2" diameter, but at 6 feet in length, stiffness becomes a real concern, especially if I load the screw in compression. By the way, this will be a typical gantry system, with 2 lead screws moving the gantry, as we've seen in many designs.

So here's my question: Is it OK to find a way to permanently load the leadscrews in tension to keep them from buckling under compression? Obviously the amount of tension preload that I "stretch" the leadscrews with would have to be greater than the max cutting force that I apply to them in compression. But has anyone ever seen this approach to using smaller leadscrews?

I might have to use a steel frame to take advantage of its stiffness if I permanently stretch the leadscrews.

Any thoughts?

Thanks, Tom.

[zenbot]

Yes, tensioning the lead screws is often used to increase the stiffness, and to decrease the screw's whip at higher rpm's.

[wcarrothers1]

How ever due to length might want to consider the 5/8" screws or could do the spinning nut trick..

b.

[ger21]

Roton sells a 3/4-8 4 start screw for $11/ft. Plastic nuts for about $30.

[Cartierusm]

I agree try Roton, but don't be mislead about Lead Screws Roton makes some very inexpenisve Ballscrews 5/8. I started with McMaster Carr Acme Lead Screws and then came across Roton and switched out my lead screws for about the same price.

[pminmo]

The cheap acme screws will wind up costing you more when you go to drive them. Maybe this will help: http://pminmo.com/PMinMOwiki/index.p...chanical_Power

[Cartierusm]

I agree the Ballscrews are way more efficient meaning you will get more power from your motors.

[Tom Kay]

Hello;

Thanks to everyone who replied. I appreciate the feedback.

I have heard of the usual suspects when it comes to buying ballscrews, Nook, Daniher, THK, etc., but I had never heard of Roton, so that's worth the price of admission right there.

Some related questions:

1. What sort of backlash do the Roton ballscrews/nuts have?

2. What sort of accuracy in thous per foot do the ballscrews have? I could not find this info anywhere on the site.

3. Does Roton make a preloaded (zero backlash) ball nut? I suppose I could make my own with 2 nuts and a wavy spring, but curious if Roton does the job for us.

Thanks Boys, Tom.

[pminmo]

I can't speak to Roton's accuracy, but remember a couple of things. Cheap ACME isn't all that precise, when you get to precision ACME it gets more expensive. Second, how accurate do you want to be for woodworking? I've been a woodworker for 30+ years, yes there are times when I take a second or third pass to take a very fine cut on a crosscut which maybe 6 or 8 mils, but when I put the tape measure on the piece, the tape measure isn't that accurate.

If your going to use stepper motors I'd take a hard look at the 5/8" x .2" pitch ball screw. At $11/ft and a nut for $24 might sound expensive, but not when you see the resulting rapids.

http://www.roton.com/Mating_Componen...family=7059321