Coping with backlash

[Tony Wells]

Generally, the gibs are adjusted tight enough to provide a little drag on the moving member. This would prevent the creep of the axis feed. Backlash should be taken up leaving the slack in the direction of travel, so the cutting pressure is put against the nut. In other words, if you are turning, dial towards the cut, and stop at the intended DOC. Do not back off the dial position. It should stay put.

 

[randyjaco]

If nothing else works satisfactorily, a DRO is the best way to deal with backlash ;D

Randy

 

[fluxcored]

If nothing else works satisfactorily, a DRO is the best way to deal with backlash ;D

Randy

Hi Randy,

Excuse my ignorance, but how would a DRO allow you to cope with backlash. I've heard this before but never really had the courage to ask the question.

Regards
Cas

 

[randyjaco]

The DRO measures the travel directly from the carriage, cross slide or to what ever it is attached; and not from a dial. It does not have to deal with slack in the gear train. The slack will still be there, but the measurement will be accurate regardless of the position of the dial. They might not look good with a vintage machine but they will damn sure increase its accuracy,

Randy

 

[fluxcored]

The DRO measures the travel directly from the carriage, cross slide or to what ever it is attached; and not from a dial. It does not have to deal with slack in the gear train. The slack will still be there, but the measurement will be accurate regardless of the position of the dial. They might not look good with a vintage machine but they will damn sure increase its accuracy,

Randy

Randy, thanks for taking the time to post your explanation. Regards.

 

[rleete]

Sounds to me like your screws on the cross slide are too loose. There is a way to adjust them and take out some of the play. Go over to minilathe.com and look at the setup pages. Very good info over there. With some of the play taken out, the screws will be tighter, and it won't have quite as much tendancy to back off when you're cutting. Caution: do not tighten up too much, or you will just wear the sccrew/nut faster.

 

[Tony Wells]

DRO's enhance a machine's accuracy, true. They are independent of any possible lead screw/nut error or wear. However, they have no direct effect on backlash. The issue at hand is the back-driving of the screw from cutting pressure causing pushoff to actually move the entire cross slide. It is likely that wear on the screw and nut are the root causes. That said, adjustment of the gibs, whether they are straight or tapered, should help. In an extreme case, it may not give total relief, but should allow new parts to be made on the machine itself. Trouble with that is you need to disassemble the machine to get all your dimensions, reassemble to machine the parts, then tear it all back down to repair it. Unless you choose to source the parts out of some vendor. I don't know what is available. I imagine most hobbyists would rather pride themselves making them.

In no case should the dials on any machine be used in a bidirectional fashion. As Bill said, backlash is present, and has to be. On NC/CNC machines, they do use bidirectional travel, BUT, they either use an encoder on the stepper motor driving the axis, or a scale that measures actual travel. The computer used has all the backlash comp data stored, so it can offset, or overtravel as needed to maintain accurate positioning. You will see the effect of bad comp data if you program a ball on a lathe, for instance. That requires a cut that reverses direction while forming the profile. A step is discernible almost all the time, because you just about can't keep up with the normal wear on a machine. An old, worn out machine that hasn't been compensated will leave large steps. The same will apply to a mill or machining center if you interpolate a circle.

I remember years ago working with a factory rep on a small mill we had just bought. It would not place holes in a pattern accurately. We took a few test bars and holed them every inch, then every half inch, and plotted them on our CMM. Taking that data, we created an offset profile for X and Y. The machine was locating within 0.0002 throughout the work envelope when we were done.

 

[randyjaco]

You are welcome Fluxcored.

Granted, a DRO will not correct backlash, wear, etc. But as I read, the topic is "Coping with backlash" not eliminating, repairing .... it.

Randy

 

[Tony Wells]

I don't believe the problem is so much the existing backlash, but the supposed effect it has, like axis creep. I'm not sure creep in this case can be blamed in totality on backlash. I think we got a little sidetracked is all.

 

[Hawkeye]

A big part of solving problems is to teach yourself how to think about the problem. This thread has a lot of different points of view. I changed directions several times in reading down the page. I love it.

Mayhem, to get a picture of what the guys were saying about backlash, wind your cross-slide a turn or two toward the work. Then, push on the cross, in the same direction. It will move. Wind it in again and pull back toward you. It shouldn't move. (By now, you've adjusted all your gib screws, so slop shouldn't be showing up.) Next, simulate what you were suggesting by winding in a couple of turns (you don't want to actually be touching any work for this), then wind back a half turn. If you pull back on the cross, to simulate the tool pushing out of the work, it will move back toward you. This is why they were saying to only advance into the work.

Now, the bonus idea. Whenever I've been in the middle of a job and I've noticed the cross or compound feed dial has moved back a few thou from vibration or whatever, I'll clip a small metal spring clamp onto both the moving dial and the fixed dial. This provides enough friction to prevent it from backing out.

You can just make out the SHCS above the clamp. The original gib screw was missing when I got the lathe, so I added this one as a lock. As previously mentioned, the end position isn't the best, so I'll either move it to the middle or add a new position.