Backlash in mill-drill

Hi,
I have maintenance responsibilities for The Rong
Fu mill which is about 15-20 years old. I am
trained as a physicists not as a machinist. So
please excuse my lack of understanding and misuse
of terminology. This milling machine along with a
small lathe are used in a junior/semior physics
lab to give the students a taste of what
metalworking machine can do.
There was a lot of backlash (0.020'- 0.030' on
the hand wheels) in both the front to back feed
and the right to left feed. I cut a small hole in
the table below the mill and tightened up the
partially split leadscrew nut. This reduced the
front to back backlash to a few thou. At this
point the tightening screw was getting hard to
turn and the handwheel for the motion was pretty
stiff, so I stopped.
On to the right to left backlash. This
leadscrew seems held in tension from both ends, so
I tried shimming one end out just a bit to
eliminate any slop in the connection between the
leadscrew and the table.
Next, I tightened the adjusting screw on the
leadscrew nut as much as I could and now am down
to about 0.015" of backlash on the handwheels.
Better but not great.
Questions:
1. Any good ideas as to how to further reduce the
backlash in the left right feed?
2. How does one tighten up the jibs on this
beast? There is a single large screw running the
in the same direction and the motion the moves the
jib in that same direction.
Thanks
Roger Haar
Reply to
Roger Haar
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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
Reply to
Harold & Susan Vordos
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:
Reply to
shabtai
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.
Reply to
DoN. Nichols
Great point, Don. And one that is not so obvious unless you think really hard about it. Bob Swinney
Reply to
Robert 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.
Reply to
William Sugg
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:
Reply to
Roger Haar
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
Reply to
jim rozen
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
Reply to
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.
Reply to
DoN. Nichols

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