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debequem
09-30-2003, 10:00 PM
Hi,

I have a mill/drill that I might upgrade, but I was wondering about replacing my lead screws with a ball screw?

Right now each shaft has about 0.025" of of backlash and they are very notchy feeling (they have been cleaned and relubed to no avail).

It would be nice to have the tables move with much greater precision and smoothness, but I don't know if ball screws are the right choice or should I go with a good Acme lead screw?

Marv

BFHAMR
09-30-2003, 11:01 PM
Marv,

Ballscrews certainly will reduce backlash, and give a smoother feel but be prepared to pay alot.
The notchy feeling you are talking about could be the bearing support on the ends of the current screw.

Dan

Bruce Griffing
10-01-2003, 02:47 AM
BFHAMR is right on. Many people don't realize that backlash can come from two sources. One is the lead screw/nut combination. But the other is the anchor for the lead screw in the table. This is generally done with a double thrust bearing. Each bearing holds thrust for one direction of travel. These bearings are preloaded with with a nut or ring or sleeve somewhere. Find that and tighten it. Backlash may improve.

debequem
10-01-2003, 07:46 AM
Good idea. I'll check the end bearings (if they exist). This is the Rong-Fu 31 mill/drill. Can't remember what they had for bearings when I tore it down last year.

Seen some reasonable prices on Ebay for the X axis ball screw. It would need to be cut down since it is for a Bridgeport, but I would imagine it could be done on my lathe easy enough.

Thrud
10-01-2003, 08:29 AM
Marv
With ballscrews you may have to put a shaft brake on it to prevent table creep while cutting, If it is CNC'd then the motors act as the brake.

The leadscrew on my maximat 7 has been modified with two annular thrust bearings back to back to prevent movement. I had to do this to do a 000-180 lead screw - it was impossible to cut without it.

[This message has been edited by Thrud (edited 10-01-2003).]

Bruce Griffing
10-01-2003, 12:16 PM
Your Rong Fu for sure has thrust bearings. As to the cutting down of a ballscrew, that may not be so easy. Ballscrews are hardened. If you want to put ballscrews on your mill/drill look for a four article series by Roland Friestadt in HSM about a year ago. He covers everything you will need to know.

debequem
10-01-2003, 01:09 PM
The HSM article sounds good. Is it:

Retrofit a Grizzly G1005 Mill-Drill to CNC - Part 1 - 4?

If so, it is a four part from Jan 2002 through Aug 2002.

How do I get back issues?

The link on the home page only goes back to Nov 2002.

Thanks!

Marv

DR
10-01-2003, 01:59 PM
Marv,

I suggest you treat yourself to a nice full featured DRO and forget the ball screw conversion.

Those conversions are not easy, even if you had a screw/nut setup that was close to the needed size. Besides that, most of the screws you see at "good" prices are rolled. So what's the use unless you install asccurate screws.

Neil
10-01-2003, 02:05 PM
Marv:

You can call our circulation department at (800) 447-7367 8-5 PM EDT.

Neil

Forrest Addy
10-01-2003, 02:20 PM
Try here:

http://www.microkinetics.com/convkit.htm

debequem
10-01-2003, 03:32 PM
Got DRO, but the bloody things (lead screws) are very notchy and grabs as you rotate it. Also gets stiffer as you rotate to the end of thetable.

Marv

Bruce Griffing
10-01-2003, 04:46 PM
Yes the article is on CNC conversion - you found the right one. Most of it is about the ball screw conversion tho. I do think you could make the screws you have work better. They should not grab or have significant backlash. I would realign the nuts and tighten the thrust bearings. If you get them to work well and you still want ballscrews remember that you may need to add a brake as Thrud suggests.

brunneng
10-01-2003, 08:27 PM
I was just about to post a simliar question about my mini-mill. I'm going to CNC it and McMaster-Carr has some ball screws that look good.
Remember: mini-mill 4"x-axis, 9"y-axis and maybe a foot on the z.

They are rolled and rated at .004" per foot. I'm just doing hobbyist stuff and I mostly want it for making aluminum molds for my plastic injector. So tenth accuracy isn't even a factor.

The screws I'm looking at are 5/8" dia with a pitch of 13/64". Cost $1.09 per inch.
The nuts are $21.04 each.

I'd like a better pitch ratio but I also want to keep them sized close to the current leads dia and I'd like to keep the cost low as for the above mentioned reasons. Someday I'll have the cash for better machine but right now it's just for fun.

I calc that with the current setup I have in steppers and controllers I'll get .0002031"/step (if the world was perfect). The software will keep the error accumlation down on the 031" part so I figure it should work pretty nice.

See any holes keeping in mind my intention on accuracy, etc...

Bruce Griffing
10-01-2003, 10:36 PM
brunneng-
One thing to remember about ballscrews is that they do have backlash unless you buy what is essentially a double ballnut. To eliminate the backlash the double ball nut is operated somewhat like a conventional split nut. One half is in either tension or compression with the other half. Look at the size of such a setup for the ballscrew you are looking at and then try to figure out where it will fit in the mill. If you can plan space for that it may make sense.