Re: 3D Contouring - Loss of Accuracy

Hi Peter,

As a test, have you run the same contouring program with 2 axes
stripped out, leaving only 1 axis, but with a bunch of little
forward/backward moves, (normal accel, linears and rapids) and if so
what results did you get from this?

One more question, can you supply more information on one of your
setups? For example, using the Gecko 210, how many microsteps per
full step did you have(1,5,10)? How many inches per rev are your lead
(ball)screws (5,10,15,20) and how many full steps per revolution are
your stepper motors(180,200), (or in other words what value does the
parameter that has steps/inch hold), finally, what is the smallest
move the G-code program will allow (.0001"?)?

Jeff
http://www.xylotex.com

--- In CAD_CAM_EDM_DRO@yahoogroups.com, Peter <prenolds@s...> wrote:
> Hi all,
>
> I'm stumped! On my BP-clone knee mill, I've tried everything I can
> think of and nothing seems to produce the results I'm looking for.
What
> I find is that running a large 3D file I gradually accumulate a
small
> error typically in one of the most active axes which often exceeds
0.3mm
> or .012" after half an hour to an hour of running time. It is
usually
> in the same direction, but not always. Below are some of the
things
> I've tried:
>
> Large Steppers couple directly via heli-cal couplings running G210
> Geckos and Flaschcut CNC - 650VA PSU
> Large Steppers couple directly via heli-cal couplings running G210
> Geckos and Indexer LPT & G-Code Controller - 650VA PSU
> Large Servos running G320 Geckos via 5mm HTD belts & reduction
pulleys -
> both Flashcut and G-Code Controller - 1500VA PSU.
> Large Servos running Rutex R990's via 5mm HTD belts & reduction
pulleys
> - both Flashcut and G-Code Controller - 1500VA PSU
>
> Even if I remove the couplings or belts (so the motors aren't
loaded)
> the error remains. The only thing that seems to work is slowing
things
> down to snail pace rapids, with very low acceleration rates. I
don't
> think it related to a lack of power in the motor - the stepper was
an
> 1870 oz-in and the servo motors are 880 oz-in at 17amps (which my
PSU
> can supply) through 2:1 or 1.75:1 belt reductions. All axes have
zero
> backlash ball screws fitted with dual thrust bearings at one end.
>
> I'm an EE and while I don't claim to know everything about noise
and
> driving optos etc. I do have a 100Mhz scope and have observed
clean,
> full level step & direction pulses going into the Gecko's/Rutex's
etc.
> The power supply wiring is neat and organised, with 36,000uF of
caps
> fitted to the output. All cables are shielded and grounded at one
end.
> All terminations are made either with soldered DB-25 connectors or
> AMP/TYCO CPC connectors with crimped pins using the correct tool
etc.
> All servo encoders are running in differential mode.
>
> I've tried both old and new versions of Flashcut and the current
version
> of G-Code Controller. These tests have been run over the last
(almost a
> year) and in each case, a similar magnitude error occurs. I know
one
> would then start to suspect my CAM program - Visual Mill but I run
in
> absolute mode and I can see the positions at the end of the test
file
> specifiying either machine zero or program zero and then using my
limit
> switches or long nails hotmelt glued to the pulley as a zero
reference
> when the controller zero's the table, the error is there...
>
> If I run relatively simple 2D or 2.5D files the error doesn't seem
to
> occur - it is only a function of large multi-axis contouring files.
> Some of these problem files have 10,000+ lines of g-code code -
> typically creating a "point cloud" that the software then contours.
> BTW, I'm not criticizing the abovementioned products - they work
great
> in everything except these large 3D files. I seeking suggestions
for
> what nugget of wisdom I'm still missing.
>
> Is it possible that even a servo system can occasionally lose a
step or
> two per million? If so, could that be why commercial CNC machines
use
> glass scales mounted on each axis to provide an absolute position
> feedback loop in addition to a (motor) shaft mounted encoder? I'm
not
> quite sure how to incorporate such a thing, but I think it may be
> possible with G-Code Controller and Indexer.
>
> Anyone with experience in CNC contouring of large 3D g-code files
or how
> commercial CNC machines are made - can you help with any
suggestions?
> Thanks.
>
> Cheers, Peter