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But remember, even VMC's with ground ball screws and all the rest also use backlash comp.  It matters for best results.

I suspect this has more to do with marketing than performance.

Jeff, you're wrong about that.  They don't spend much of any time marketing their backlash compensation. 

But, it matters for precise positioning before you begin many operations.  For example, positioning a boring head.  The boring head itself is not subject to problems with backlash once positioned.  But even a little backlash that would not be noticed for other operations matters for positioning the boring head at the center of the bore.  If you're using a boring head in the first place, you want it positioned as precisely as possible.

Many of the commercial controls even have a positioning mode (M-Code) that ensures this is taken into account.

BTW, the good controls allow you to set different backlash in each direction.

Cheers,

BW

Never saw a good VMC using a boring head They can orbit a circle as good as within 0.01mm (and very often much better than that) by interpolation and get a better surface finish than they would with a boring head and plus it's much faster than using a boring head.

Dan

Well, you have to keep looking then, because its happening out there.  Opinions vary on where the exact cutover is from helical interpolation, but 5 tenths (0.0005) is not a bad place to start depending on your VMC's accuracy.

Here is one of many threads on the subject:

http://www.practicalmachinist.com/vb/cnc-machining/advice-using-boring-head-vmc-177986/

Note the remarks in that thread such as, "Always approach the hole from the same direction and at the same speed so the table is in the same place."

Cheers,

BW

Hi Bob,

Don't know... I wouldn't use a boring head if I had a good VMC with ground ball screws. My mill has rolled screws and a bit backlash but with clever programming I can orbit a circle within 0.015mm!

A bore accurate to 0.01mm is good enough to nicely fit a bearing inside, so don't think I get your point about depending on the VMC's accuracy...

Are you saying you can get better than 0.01mm using a boring head...? I (and probably most CNC workshops out there) don't have the tools and can't even measure that accurate

Dan

Dan, read the thread.  There's a whole bunch of CNC machinists discussing the topic.  They will tell you what you want to know, and as I say, this is far from the only thread on the subject.

As for measuring the bores and fitting bearings, that's another topic.  Have you looked at fitting and tolerances for precision bearings?  It's a lot tighter than you think.  See for example this Barden document (one worth downloading and having on hand if you work with bearings much):

http://tinyurl.com/23fs5et 

It gets worse for higher precision bearing work.

The measuring instruments required are not that crazy, dial bore gages or Intramiks.  You don't want to do this work interpolating, whether with a rolled or a ground ballscrew.

Cheers,

BW

Hi Bob,

Read the thread (a nice forum BTW, thanks for the reference), and that's exactly what I was saying. See replies number 6, 17, 21, 22 and 25. You can get 0.01mm tolerance and even better by interpolation on a good VMC.

I am aware about fitting tolerances recommended for precision bearings - have fitted a few and designed a few fittings And I am still saying that getting better than 0.01mm is much of trial and error and by clever programming can be achieved by interpolation milling as well.

And just to give an idea about the "theoretical" tolerances we are discussing, a 10 degrees Celsius temperature difference would change the size of of a 30mm bore in aluminium by 0.006mm and twice that in steel - 0.012mm!!

Dan

Dan, if you want to install your bearings that way, go for it.  There is a right way, a wrong way, and a way that you know works for you.  Maybe this is it.

You will notice the posts you called out didn't really prevail in the thread.  Since you've only just discovered PM, you wouldn't know much about the posters.  AlfaGTA, for example, is one of the foremost engine builders in the world for rare vintage cars.  You may want to spend some time on PM, there is a wealth of knowledge there beyond the hobby world.

At this point, it should be pretty clear why a VMC might care about backlash compensation.

Cheers,

BW

Hi Bob,

The posts I listed are by people who say they actually are making parts by milling whole day long to tight tolerances. These are NOT speculations. If you know how to do it right with a boring head and it works for you then no reason to not use it, especially if your machine is not a very expensive VMC.

I think this discussion is not far from becoming a scientific discussion  so I think we have to agree to disagree on it.

Thanks,
Dan

I was looking for the latest on SS and this thread somehow devolved into boring vs interpolation. I'm gonna add my 2 cents here because I tried to get away from it and I can't. I have 14 yrs experience working mainly on VMC's, current machine hurco vmx64 .0002 in. repeatablilty, I don't know how anybody could want to interpolate when they could bore? Your finish, size holding ability and time spent cannot be beat with boring equipment. the only reason I can come up with that someone would want to mill a bore instead of bore is,A they don't have boring equipment B. don't know how to use it C. want the job to take more cycle time.  The fact that you have stated that there probably isn't a device that can measure .0015mm(.00059in) is wrong. It's called an airgauge the ones i used we were checking bores to .0002 in. Also you stated that steel grows more than aluminium tells me who ever told you this stuff thinks they know, but they really don't know.   

Qouting Greg from the Warp 9 site:

Re:no backlash 1 Week, 3 Days ago     
I wanted to give an update. I am working on it and I am realizing I am needing to make a significant change in the way the data
 inside the SS is handled. It will be for the better though. Once I get it changed I will be releasing an update without backlash comp,
 followed by another one with backlash comp. I hope it won't be too much longer.

I would suggest you follow the Warp 9 Site to see what's happening relative to the SS.

RICH