backlash comp confusion

I used backlash comp yesterday for the first time. The instructions are pretty simple but I experienced a behavior that has me a bit confused. My Y backlash was 2 thou, so I added the line BACKLASH=.002 into the ini file under the Y axis section. To my surprise, the backlash INCREASED to 4 thou ! I went back to no comp and verified the 2 thou again, then only comp'd out one thou, t he backlash increased went to 3 thou.

To get backlash comp to work properly for my Y axis I had to put the number in as a NEGATIVE number! (BACKLASH= -.002) It then properly comp'd out the backlash. Strangely, the X axis, which had .0005 backlash did NOT require the negative number to work properly. Z did not require any comp at all. The backlash comp doc's make no mention of needing to use negative numbers in some cases and thus my confusion.

It works nicely but leaves me wondering.... Any clarification would be appreciated.

Running Debian from the live CD, LCNC current release with all updates applied to 2.7.8

Thanks,
George

Thats a bit surprising, do the X and Y axis have different scale signs?

PCW wrote:

Thats a bit surprising, do the X and Y axis have different scale signs?

Not sure which scale sign you are referring to? Encoder counts/inch?

Yes (for a servo system with encoders), the sign of this number

( typically INPUT_SCALE in the ini file )

PCW wrote:

Yes (for a servo system with encoders), the sign of this number

( typically INPUT_SCALE in the ini file )

Machine is a typical Bridgeport. There is no INPUT_SCALE in my ini file but there is ENCODER_SCALE
Encoder scales are X +76000
Y +76000
Z -45000

(numbers rounded off for discussion purposes) Initial config was done using PncConf then modified to include various control panel switches etc. Axis setups are as setup by PncConf, no changes there except adding the BACKLASH= statement. I should also note that I have NativeCam embedded but doubt that would have anything to do with this issue.


George

Well, since X and Y scale signs are the same, its a mystery,
though with only .0005" a of X backlash it could conceivably
be a measurement error

PCW wrote:

Well, since X and Y scale signs are the same, its a mystery,
though with only .0005" a of X backlash it could conceivably
be a measurement error

I agree. The Y had 2 thou backlash, the X .0005 Both were quite repeatable and comp'd out nicely. It was just a head scratcher why I had to use a negative .002 for the Y entry in the INI file and not the X. Again, if I put a positive .002 in the INI file it doubled the Y backlash to .004 and very repeatable.

It does not really matter, it did the trick very nicely. I just thought it was worth questioning here on the forum.

George

(this bridgeport is my 3rd retrofit using the 5i25/7i77 combo, I just LOVE those boards ! Your effort is very appreciated)