I've RTFM and Googled but I can't find an idiot's guide to creating a
lead screw compensation table for Linuxcnc. Is my googlefu weak or is
there no such thing?
...or do I actually need one at all?
As background (Hello! and Warning Newbie Alert!) I have a metric
WMD30LV (big brother to the G0704) mill using the standard lead screws
being driven by 7Nm NEMA 34 steppers (oldham couplers), DQ860MA drivers
and a cheapy BOB connected to a parallel port with Linuxcnc 2.5.3 in
control. The mill has glass scale DRO's on all axis (fitted when I
thought I could resist going CNC... Pah! :-)
However, I think I might have a problem with the accuracy of my lead
screws or perception of accuracy and/or something else... which is why
I'm thinking that a lead screw compensation file might be better than a
crude backlash setting (yes I have backlash and no, I'm not surprised
but I'd like to make chips before embarking on a ballscrew conversion).
If I do:
G90
G0 X0
X50
<reset DRO to zero in the X axis>
X100
and take a reading from my DRO I get slightly under 50mm shown.
I've tried various microsteppings with the following results on X & Y:
Micro
Steps X-Axis Y-Axis
400 49.960 49.950
800 49.960 49.930
1000 49.970 49.970
2000 49.965 49.970
4000 49.955 49.970
Max variation: 0.015mm (0.59 thou) in X and 0.04mm (1.57 thou) in Y.
Does that look like lost steps? or as much accuracy as I'm likely to
see? (one can get hung up on accuracy reading the various cnc
lists'n'forums and watching the DRO for that +-5 flicker in the third
decimal place :-)
Is this the sort of thing that a screw compensation table is meant to
deal with? If so, how do I actually measure the numbers for the table?
Are the measurements absolute or relative to the previous move? How do
I get the reverse or reverse trim numbers? Sorry for being a thickie.
If I do use a compensation table, ISTR that there's a limit of 256
entries and the table on my mill moves +-300mm. Is a table entry for
every 2.34mm what I need to generate?
Thanks in advance.
PS - If I do need a compensation table and work out how to generate one
I'll put something on the wiki for future newbies.
--
Regards,
Russell
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| Russell Brown | MAIL: [email protected] PHONE: 01780 471800 |
| Lady Lodge Systems | WWW Work: http://www.lls.com |
| Peterborough, England | WWW Play: http://www.ruffle.me.uk |
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