Need to figure out lead screws before I begin

[Darren]

Ok, I've been trying to catch up since I put my project on hold for the last year. I have collected most everything I need to begin my build except for the lead screws. I'm using 360oz/in servo's. From what I've read it appears I'll need to do a reduction so I'm assuming I'll need to drive the screws with a belt driven gear reduction.

I'm trying to build this cheaply so I'm considering acme screws. My only concern is machining. Well, that and how to mount them properly. I have no way to do it and really would like to buy the screws machined to my needs. Is there a place that can do that? I really need to figure this part out before I begin building so I can have them in my possession as I design. I'm a bit overwhelmed as I've been reading the forum for the last few days and am a little frustrated.

You learn more about my planned build in this thread here:
Overwhelmed, have many parts... need guidance

Any suggestions or links to helpful threads would be much appreciated! Thanks.

Darren

[Evodyne]

Darren,

Hi again. You might take a look at the HomeShopCNC web site. He sells rolled ball screws which are a step up from acme. And he cuts to length and machines the ends for you. Don't know if he can/does provide support bearings and mounts, but you can ask him. I also see he now has complete "dual v-slide assemblies". They look a little more expensive, but they are self contained.

I've bought similar self contained "motion stages" off of eBay-track, linear slides, ball screw, etc. in one unit. Again, the advantage is they are ready to go. Sometimes they come cheap, sometimes they don't

Lance

[Darren]

Ouch, can't afford the V-slide assemblies. I'm interested in the ball screws though but they appear to be fairly spendy as well. I'm really limited on budget so I gotta stay on the cheap. Every corner I turn is another $500 and I can't continue like that unfortunately. I didn't see different prices for different sized diameter ball screws though. I was thinking 1/2" would be good for 48" length but that is just a guess. I'll send HomeShopCNC an email.

[rustyolddo]

Look at Roton, you can get 5/8 ball screw 4 feet for $30 and the ball nuts with fitted balls will run you $20 a piece. Outstanding customer service too.

[ger21]

http://www.nookind.com

They'll machine the ends, but it may not be as cheap as you'd like. You'll also have to fabricate yout own mounts, as their's will cost far more than you could imagine.

[Darren]

Thanks guys, I'll look at those options as well!

[chuckknigh]

Maybe this is a silly question, but why do you need the ends machined? Just buy a ball bearing with the ID the same as your screws. Then, use a washer and a nut on the end, to secure it!

There are couplers that can be used to mate the ends of your screws and the ends of your motors, as well, with no machining required.

ACME thread has all the same types of nuts as regular triangular thread...get a lock nut and attach it that way.

It's not rocket science...and it certainly doesn't require precision machining just to mount a screw. Simple solutions...

-- Chuck Knight

[ViperTX]

Chuck, Well on an ACME that might work...but pre-loading the bearing may be a challenge...have you done it that way?

[rustyolddo]

One thing to note on ball screws is that the outer surface is hardened, the inner is reasonably machinable. You could drill & tap the end of the ball screw for a small bolt, 8mm or 1/4-20 use a large washer to secure it in the bearings. If the screw was shorther than the span between the bearings it would allow you adjsut the pre-load. Or You could simply shim the bearings.

[Darren]

Chuck,

I like that solution but wonder about pre-loading. Can it be done that way?

[rustyolddo]

Originally Posted by Darren Chuck, I like that solution but wonder about pre-loading. Can it be done that way?

Sure. It takes very little to preload a bearing. Too much can cause it to fail prematurely.

What I'd probably do is order some bearing springs, they're wavy and designed for the purpose of establishing preload. You can get them from Reid Tool & McMaster Carr. They are sized to the industry bearing numbers.

Machine the pockets for your bearings to the spring compression spec, put the spring in, the bearing then the bolt & washer for the ball screw and tighten it all up. You're good to go then.

The only issue that you have to worry about is extreme force on the ball screw. If you were to push really hard as in a bind, it would be possible to compress the spring a little and introduce some error. You can solve that simply by adding a set screw to the bearing pocket. Assemble everything, allow the springs to tension the bearings, then gently lock the bearing in place with the setscrew, you could get fancy with the mount if you wanted. Chances are that your stepper motors would stall before you'd compress the bearing springs but the setscrew would eliminate the possibility. I wouldn't use the bearing springs in a Bridgeport mill conversion but in a router or plasma table, they are the way to go.

If you are tool poor, you could even skip drilling the ball screw and just buy some 5/8" locking collars with the double setscrews, they're less than $1 each. Assemble everything and compress the bearing springs and tighten the locking collars onto the ballscrew.

[Darren]

Thanks rustyolddo,

This is sounding more reasonable cost wise. It sounds like I could do this with ACME or ball screws just as easily so I might start with ACME first to keep cost low, then upgrade later down the road if I feel it is necessary.


What do you think about this idea for pre-loading using the idea of bearings over the threads...

Fix one end of an ACME thread rod with a bearing sandwitched with appropriate nuts/washers and trap the bearing in it's machined mount on one end of the axis. On the other end of the axis machine a seat for the bearing so that the rod can pass through the mount, then on the "outside" you slide the bearing on and into the seat, follow that with a nut and torque the nut appropriately... this would create pre-tension by pulling on the ACME thread rod but would it be a satisfactory method?

I've got plenty of tools and woodworking machinery, I'm an avid woodworker and most of those tools will work aluminum "ok". I have plenty of carbide

I'll have to draw up some plans and post them to be sure I'm getting things right.