Backlash in mill-drill

Hi Roger,
Backlash is nothing about which you should worry. Machinists deal with it regularly, although it does take a little experience to do so comfortably. If you have the nut tightened such that it has a few thou, I'd suggest you simply use it as is. The best way to deal with backlash is to always work a given direction, which, in my case, is as the dial reads. You count turns and stop on the appropriate line, or mark. If you happen to go past your mark, all you have to do is wind back past where you desire to be, then approach it again. Make all your dial settings in this manner and backlash is a non-issue. A respectable machine is surprisingly accurate when used that way. You can make table (left to right) or saddle (front to back) adjustments in either direction by simply always turning the handle the proper direction for the final setting.
The word is GIB, not jib, and is pronounced as it is spelled, with a hard G. How they normally work is for the head to be captive in a slot in the gib, so when you turn it right handed, it tightens the tapered gib, and when you turn it left handed, it loosens the gib. There are other designs, so yours may not work that way. Better quality machines use gibs like I described. You don't want yours so tight that you fight the feed handles when the machine is in any extreme position. It's quite normal for machine tools to be tight on the ends, and loose in the middle as they age.
Hope this helps,
Harold

Hi Roger, Funny my degree is in Physics also. Are you at UofA or ASU? As another email stated you can work with the backlash, it is done all the time and is part of learning to work with machines.
Another option is to convert the Rong Fu lead screw to a ball screw. I got my backlash down to .5 thousands. Good luck, Simon Shabtai Evan
Roger Haar wrote:

Agreed.
Hmm ... I would have my doubts about going that route, simply because it requires you to either tighten the clamps on whichever axis you are not feeding, or to keep a hand on each handwheel (a bit of a problem if you need a third hand to change spindle speed, or feed either the quill, or the knee. The reason is that while standard Acme screws are very resistant to being driven backwards (pressure on the nut turns the screw), with ball screws, it is very easy, and you can have exciting times if one of the axes self-feeds.
If the machine is at least part-time CNC, that is a different matter, as you have the servo or stepper motors adding drag to the axes.
But I don't remember you mentioning CNC. For CNC, ball screws are a major benefit, but not so for manual machines to anyone accustomed to compensating for the backlash.
Good Luck, DoN.

Great point, Don. And one that is not so obvious unless you think really hard about it. Bob Swinney

It's very obvious when you have ball screws. Just the same, having a table that goes exactly (-+) where you tell it to go is pretty nice. Well worth having to install a lock / drag mechanism.

Simon, et al.
This is at U of Az.
I know enough and am confident enough to handle some backlash. But I am teaching physics students most of whom have never used a hacksaw and possibly not even a screwdriver. I wish to eliminate as many problems as possible so that these student gain a little confidence. On a good day, the students come away amazed that they actually can make something.
Thanks Roger Haar
******************************************************************* shabtai wrote:

Is this an undergraduate lab? If so then that's a nice feature for the department. When I was there things were pretty minimal. I used to work in the physics electronics shop for a man named Buck Crouch, that was 77/78. I think there are probably a few tektronics scopes in that building still around that I repaired.
Jim

While I understand that you want a confidence builder for students with no mechanical experience, I don't think that learning about backlash is too much to ask. My experience w/ physics students (bold generalization coming) is that they require that the world conform to their simple models. After all, the goal of physics is to explain the universe in the most elegantly simple way. It's better to break that thinking early and keep their head in the real world, especially if they would like to be experimentalists. Backlash is a fact of life and it's not hard to explain. It's also not hard to remember to always approach from the same direction. They'll get more out of if it if you don't shield them from the realities of it and they won't be frustrated later in their careers, when late at night, after all the techs have gone home, they need to make that important gizmo on the old Bridgeport or Rong Fu, with backlash.
My $.02, Greg

But when recommending it, be sure to recommend the lock/drag mechanism at the same time. Some people have no experience with ballscrews, and if a milling cutter starts to drag an axis, it will inherently be climb milling, so things could get *very* exciting *very* quickly.
So -- warn them about it -- don't leave them to discover it on their own. They may be new enough to metalworking so they will not think of it on their own.
Enjoy, DoN.