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Bryce.R
08-01-2011, 07:23 PM
can someone help me tighten up my backlash in my 1980 macmillan metal lathe. The backlash is in the cross slide and i have no idea how to tighten it up :( Please help

Dr Stan
08-01-2011, 07:27 PM
can someone help me tighten up my backlash in my 1980 macmillan metal lathe. The backlash is in the cross slide and i have no idea how to tighten it up :( Please help

Without pics and the manual there's virtually no way any of us can help.

DFMiller
08-01-2011, 07:27 PM
Got any pictures you can post? I don't recognize that brand.
Dave

Black_Moons
08-01-2011, 07:43 PM
First you have to figure out whats moving.

A dial indicator can be a big help in this case. Backlash is caused by a few things.

First off, the entire acme shaft usally has some play in and outward due to imprecise thrust supports. This can be fixed with shims, or by securing parts that may of come loose, Or by replacing the bushings with nice bearings that are locked in place.

Second, and the most annoying, basicly impossable to remove except for buying ballscrews, Is backlash beween the nut and rod. As the rod wears, you basicly can't tune out backlash without it jaming near the end of the rods travel (where its worn less)

Third, You can have the nut not be secured to the cross slide and move, This just requires tightening the bolts that secure the nut to the cross slide, Maybe adding some shims if the nut isent quite aligned to the rod correctly (Causes binding at ends of travel).

Note that basicly ALL manual machines have backlash, I think its around 0.02" of backlash on my lathes crosslide outta the box. You just learn to live with it by allways going back at least 0.02" (Usally much more) and then moving forward again every time you 'pass' a mark and need to go back to it. (Ie, you allways ensure the cross slide is loaded against one side of the backlash)

The only way to truely get rid of backlash for any length of time (Nut wears out fast) (Well, Reduce backlash below 0.001" really) is to use ballscrews.. Not needed OR recommended for a manual machine as the tool cutting force can end up pushing the handles around with ballscrews.

SGW
08-01-2011, 08:15 PM
What Black_Moons said: it's inherent in all machinery. Learn to live with it by always taking up the backlash in the same direction. After a while it becomes automatic and is no big deal.

At some point the amount of backlash can get to be so large it's annoying. 0.050" would probably be annoying to deal with, for example, because you would have to back up so far to take the backlash out.

You can do as B_M suggests to get rid of any endplay in the screw or movement of the nut, which may help. Your problem though, is more likely due to simple wear on the screw or the nut or both, and the only fix for that is probably a new screw or nut or both.

You could try partially splitting the nut so the two halves can be spread apart slightly to compensate for the worn screw, but if the screw is worn you'll have trouble with it being too tight at the ends (less worn) and too loose in the middle (most wear).

Black_Moons
08-01-2011, 09:19 PM
You could try partially splitting the nut so the two halves can be spread apart slightly to compensate for the worn screw.

Two halfs compressed to compensate. Not spread.

Often the nuts are allready designed like this. But again, It may just make it tight at the ends where the rod is worn less.

SGW
08-02-2011, 07:26 AM
I think the nuts on my milling machine are set up to be spread...but I've never had to do it and haven't examined the adjusting screws in the nuts closely, so perhaps they are set up to compress the nuts. Either way ought to work, but you do run into the problem of the nut being tight at the unworn ends of the leadscrew while it's still loose in the middle.

Joe Rogers
08-02-2011, 07:55 AM
How could spreading something make it tighter and eliminate backlash?
Joe

batt-man
08-02-2011, 08:17 AM
How could spreading something make it tighter and eliminate backlash?
Joe

Imagine two nuts on a bolt - you can have them with a gap between them and the bolt will turn easily within them. Now tighten the two nuts with each other; by default the bolt is now "tight".

SGW
08-02-2011, 09:48 PM
Either compression or expansion will shift the relative thread orientation in the two halves of the nut. It's just a question of which side of the leadscrew thread the nuts will bear on. One way the nuts are squeezing the leadscrew threads together, the other way the nuts are pulling the leadscrew apart. Either case eliminates backlash. Remember that the two halves of the nut cannot turn, they are locked together.

Don Young
08-02-2011, 10:03 PM
How could spreading something make it tighter and eliminate backlash?
Joe

The nuts (or parts of one long nut) are moved along the long axis of the screw, further apart or closer together. I think you envisioned a nut split so that it could 'squeeze' the screw which is possible but not the method under discussion.