Newbie- Blacklash with glass scale Heidenhain

[tapinlu]

I have a plan to buy a new VMC with glass scale Heidenhain. But I wonder "Does it has blacklash when using glass scale Heidenhain". Someone says yes, others say no.

anyone can anwers this question.

Thanks a lot.

[neilw20]

Backlash is a mechanical issue. Yes.
A glass scale can give an accurate position. No, it can't stop backlash.

You can have accurate position, with backlash, but as you change direction, the other axis must wait (pause) while the bad axis 'catches up'

So the answer is yes and no.

[appusivadas]

Because of direct mesurement, most of the backlash related problems can be avoided when using Glass scales.
Also the positioning accuracy of the machine will be improved when using linear scales.

So always good for using linear scales

Sivadas

[KCT]

Well, if I might add, a glass scale has no backlash but it has a very small linearity error. In most cases the error is so small that it does not matter. The glass scale positioning feedback can help to compensate for most existing mechanical backlash to some degree. It is always best to have none or extremely little mechanical backlash. Even with glass scale positioning feedback and some mechanical backlash on the axis, the controller can not do a perfect interpolation without a 'hickup' during an axis direction reversal. Best seen at the 90 degree points when interpolating a circle. Of course, this also depends on your accuracy expectation. If 0.01mm is good enough for you than you might not ever run into such a problem. But when you get down to 0.001mm things are quite touchy.

You may want to investigate where the backlash occurs or what contributes to it.

Lead screw, definitely
Ball screw, perhaps
Motor coupling, likely
twisting shafts, perhaps

Then improve things to get rid or at least reduce some backlash were possible.

Then you can enjoy the full advantages of precise glass scale feedback systems on a CNC controlled machine.

just my 2 cent worth

[Valen]

We are using linear scales on our mill
http://wiki.linuxcnc.org/cgi-bin/emc...akeAndRussells
and soon to have a detailed write up at http://www.cnczone.com/forums/showthread.php?t=109407

Anyway, we have done our best to eliminate the mechanical backlash in the system but theres still enough there that it poses a bit of a problem. The issue is when you go through a reversal and you wind through the backlash in the system the motor builds up speed such that when it comes out of the backlash its moving fast and overshoots.

We are hoping to add encoders to the system as well for velocity feedback into the system so we wind up with the best of both worlds.

[christinandavid]

Linear glass scales are generally more precise. However...

Uni-directional positioning, distortion elimination through correct method, and single point boring tools are recommended to ensure the most exact positional relationship between features. A CMM check will confirm this...

DP