RE: [CAD_CAM_EDM_DRO] lead error versus backlash compensation

Backlash is the amount the axis does not move when changing directions.
Ie. If you're going +X to 1.000, then move -X to 0.500, if the axis
winds up at 0.503, you have 0.003 backlash.

Lead error, on the other hand, would be where the axis is in relation to
where it should be, due to the threads on the lead screw not being where
they should be. So, for example, if you start at 0.000, and move to
5.000, the axis may actually be at 5.0002, moving to 10.000, the axis
might be at 9.9997, move to 15.000, and the axis could be at 15.000001.
There is also cumulative error, where the error gets greater the farther
you go. This would indicate that your 10 TPI lead screw could actually
be 10.01 TPI. Using this example and starting at 0.000, if you turn the
lead screw 100 turns, the axis would be at 9.990. All these errors are
also affected by atmospheric conditions, so you might do better work in
the winter than you do in the summer. If you're doing heavy machining on
one end of your table, then move down to the cool end, it could be more
or less accurate on that end, since the screw is longer on the warm end.
And of course all the errors add to each other making it all very
interesting.

High precision motion devices use software mapping to account for these
errors. Usually the stage (axis) is mapped using laser interferometry.

If you can find the handbook on one of the Danaher sites, it contains
all this info and a lot more.

Regards,
JJ

Be Kind, Be Careful, Be Yourself

> -----Original Message-----
> From: flyspeck1@... [mailto:flyspeck1@...]
> Sent: Monday, August 26, 2002 2:49 PM
> To: CAD_CAM_EDM_DRO@yahoogroups.com
> Subject: [CAD_CAM_EDM_DRO] lead error versus backlash compensation
>
>
>
> First off...
> Thanks to all who responded to my question about way
> covers. I'll pipe in with a comment or two as well.
> Neoprene is strange stuff; it comes in all sorts of
> forms... Everything from a permeable "sponge" type that
> is used on wet-suits, or a totally water tight material
> for dry-suits, and finally to.... you guessed it.... way
> covers. In fact, MSC sells a set of for BPort type
> mills that are specifically listed as being neoprene.
> Anyhow, neoprene does work well and is more than
> suitable for light milling applications; Jon Elson
> touched on this. It might not be suitable for 15HP Haas
> power mills, but for the mills that 99% of us are using,
> neoprene works great. McMaster-Carr, by the way, seems
> to have a fairly nice selection. Again, thanks to all.
>
> Now for my next question, and one I'm almost embarrassed
> to ask (because I should know the answer). What is the
> exact difference between lead error and backlash?
> Backlash is pretty easy. It is the amount of rotation
> your screw turns (translated to distance, of course)
> without moving the table when reversing direction. My
> understanding of lead error is that it is the amount of
> error between the commanded position and the actual
> position that a control will tolerate before tripping a
> fault.
>
> Is that accurate? My Hurco control that I'm slowly
> bringing alive (great project, by the way... I've had a
> ball with it and learned a ton about archaic controls)
> has the ability to set DIP switches for both. I can
> measure the backlash easily enough, but the lead error
> is where I'm questioning how to proceed. If my
> summation is correct, I assume that you just keep
> increasing the lead error until the machine no longer
> trips out with an axis error (assuming such a situation
> really does not exist). Is my thought process correct?
>
> Thanks in advance,
> Alan
>
> Addresses:
> FAQ: http://www.ktmarketing.com/faq.html
> FILES: http://groups.yahoo.com/group/CAD_CAM_EDM_DRO/files/
>
> OFF Topic POSTS: General Machining
> If you wish to post on unlimited OT subjects goto:
> aol://5863:126/rec.crafts.metalworking or go thru Google.com
> to reach it if you have trouble.
> http://www.metalworking.com/news_servers.html
>

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