Re: [CAD_CAM_EDM_DRO] Re: 3D Contouring - Loss of Accuracy

Hi Jeff,

No I haven't tried stripping out 2 axes from the test file - but I
ceratinly can try that as opposed to running a single axis back and
forth a set amount. I've tried a range of different step pulse widths,
acceleration rates, rapid rates etc. Only mega slow everything reduces
the error.

With the G210's, I tried 1, 5 and 10 microsteps per step. Ballscrews
are 5 turns per inch (Rockford). Steppers were 200 step per inch
Bi-Polar Sanyo Denkis. My servos have 250 line encoder on Z and 500
line encoder on X & Y and currently 1.75:1 belt reduction on all 3 axes.
With the servos, I can resolve about .0002". With the steppers it was
about .0005" (These nums are approx - I don't have the exact #'s right
at hand.

Visual Mill, my CAM software can specify however many digits to output
for each position - I've tried up to 8 digits.

Cheers, Peter

jeffalanp wrote:

>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
>>
>
>
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