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Author Topic: Arcs not cutting smoothly  (Read 14807 times)

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Re: Arcs not cutting smoothly
« Reply #20 on: February 16, 2012, 06:47:23 PM »
http://www.ebay.com/itm/CNC-Stepper-Motor-Lovejoy-Coupling-3-8-X-1-4-spider-/330548952567

Basically two hubs that squeeze a rubber spider, which allows some misalignment while transmitting torque. The squeeze of the spider and any space between it and the coupler ends results in backlash.

Offline RICH

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Re: Arcs not cutting smoothly
« Reply #21 on: February 16, 2012, 07:36:25 PM »
If you can get the alignment right get rid of those suckers and hard couple.
Also watch out for the Flexible Beam Couplings ( all metal by the way). They are not cheap. I used them on the lathe and then all of a sudden
it was experiencing backlash / inaccuracies ( .001 to .003" and not repeatable). Short story, the coupling over time just became week and with rather little torque ( say 40 in oz )
applied to it it allowed twisting with out motion. Drove me nuts until I found it and would never even have thought that it was the culprit.

RICH
Re: Arcs not cutting smoothly
« Reply #22 on: February 16, 2012, 08:11:27 PM »
What material coupler did you use? Aluminum helical couplers are total crap, and wear out rather quickly if used out of spec. The ones I have on the way are stainless, and rated far beyond what I'll need for torque. I worked in a manufacturing plant where we used these for a number of different operations - some with good results, and others with horrid results. The stainless couplers are indeed not cheap, but it's a lot easier to spend the money once than to worry about grinding your bearings to bits over misalignment that you can't even effectively measure. They're plenty stiff, and enjoy a fatigue limit that aluminum can't achieve.

Offline BR549

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Re: Arcs not cutting smoothly
« Reply #23 on: February 16, 2012, 09:23:05 PM »
The coupler is not the factor here the SPIDER is what makes the coupling good or bad. We always had good luck with the ORANGE spiders.

I have studied this over and over again. I have never seen Mach3 cut an arc in this fashion whne it was not line segmented code. BUT the error is TOO consistant in nature to be mechanical or so it would seem. It repeats itself exactly over ALL the stepdowns in Z.

It would be interesting to see it DRAW a circle based on ARC as large as you can(no cutting loads)  and see IF the error is still there.

I do know IF you ZOOM in on the toolpath you will "see"  segments BUT that is just the display. OR is it????

(;-) TP
« Last Edit: February 16, 2012, 09:28:49 PM by BR549 »
Re: Arcs not cutting smoothly
« Reply #24 on: February 17, 2012, 02:01:29 PM »
Got the new couplers installed this morning, x backlash is down to 0.0012", y down to 0.0024". Definitely an improvement. With BC on, I see no variation in position from Mach3 to DRO. I'll hopefully get to cut some curves this evening, and we'll see what happens.
Re: Arcs not cutting smoothly
« Reply #25 on: February 23, 2012, 01:57:11 PM »
I have had this problem. It was nothing to do with the machine, the machine was a Bridgeport VMC500 withe a Heidenhein TNC530. it was caused by the tolerance used in the Cam package. I reduced the tolerance then it was fine.

Backlash will show up as mismatch on direction change.
Re: Arcs not cutting smoothly
« Reply #26 on: February 23, 2012, 02:03:30 PM »
That makes perfect (symmetrical) sense !
Thanks for posting the suggestion.
Russ

Offline Dan13

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Re: Arcs not cutting smoothly
« Reply #27 on: February 24, 2012, 02:08:01 AM »
It's different in this case, I am afraid, as he has arcs programmed in the Gcode and not segments.

Dan
Re: Arcs not cutting smoothly
« Reply #28 on: July 24, 2012, 07:02:58 PM »
Got the new couplers installed this morning, x backlash is down to 0.0012", y down to 0.0024". Definitely an improvement. With BC on, I see no variation in position from Mach3 to DRO. I'll hopefully get to cut some curves this evening, and we'll see what happens.
I know this is an old thread, but did this ever resolve?
I am experiencing the same exact problem.
Re: Arcs not cutting smoothly
« Reply #29 on: July 25, 2012, 03:08:41 AM »
If you are cutting an arc say from 12 o'clock to 3 o'clock is it smooth in any position? i have noticed on my machine its better in some places than other. Between the 1 & 2 position is the worst.