Loosing Position while using Smoothing (g64)

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phazertwo
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Loosing Position while using Smoothing (g64)

Post by phazertwo »

My machine is a PM940, utilizing a 1700oz-in nema 34 stepper on the Z axis, and 1200oz-in nema 34 steppers on the X and Y. This setup has been VERY stable, and the only time I ever miss steps is if I simply crash. I have smoothing set to P1 or precision mill.

I have been really pushing the limts of High Speed Machining (adaptive in Fusion360) on this little machine and it's been eating it up (75ipm). I have been making this particular part for a while, but have always noticed that it is pretty "jerky" on the lead in and lead outs of the adaptive moves. So I decided to use G64 to run different smoothing settings while roughing, then just switch back to P1 for finishing (g code for tool in questions attached). I changed some settings around and ran it, and dang did it run SMOOTH! I mean it was a night and day difference! I really thought I was in good shape... then the finishing pass ran and I had lost about 0.080" in the Y and 0.030" in the X :shock: .

So I scrapped a part, and I figured I need to change the smoothing settings. I tried about 10 different smoothing setups re-running on the same part over and over and then probing to see if I lost position. Nothing works, if I get rid of the "jerk" I loose position... I'm stumped, I have searched the forum and read all the documentation I can find on smoothing including the mill operators manual, and the smoothing users manual:

https://www.centroidcnc.com/centroid_di ... manual.pdf

So why is it that I am loosing position while running smoother, and keeping position when it is "jerky"? The ONLY thing I can think of is that the smoothing users manual mentioned that you should turn backlash comp off while running more aggressive smoothing. However I can't find a way change my backlash comp with a G64.

My only other though is that I need to step up to servos...

Hopefully someone out there has some thoughts, or can tell me how to turn backlash comp off via G64.

Thanks,
PZ
Attachments
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swissi
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Re: Loosing Position while using Smoothing (g64)

Post by swissi »

phazertwo,

this is a hard nut to crack. The only way I know to turn off backlash comp is to set the values back to 0 but I just can't see how backlash comp could be responsible to lose 0.08" in the Y-Axis.

What I would do in your situation is to focus on the one axis with the bigger loss, in your case the Y-Axis. Install a 2x3 block or something similar on the table and a dial indicator mounted horizontally in the spindle and dial in the Y0 reference point. Now run the job with an air-cut and spindle off and check at the end how much Y0 is off. If there's no position loss, then the cutting moves are too aggressive and are causing the position losses. If there is still position loss even with an air-cut, re-calibrate Y0 with the dial indicator and run an air-cut again with the feed-overwrite at 50%. Check Y0 at the end and compare the two position losses. If the loss at 50% feed rate is significantly lower, than the federates are too aggressive and are causing the position losses.

Should the position losses stay the same I would do another air-cut with just 10% and if it still stays the same, I would need to think long and hard what to try next... :o

-swissi
If you are using Fusion 360, check out my CNC12 specific Post Processor
If you are using a Touch Probe, Tool Touch Off Device or a Triple Corner Finder Plate, check out my ProbeApp

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Sword
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Re: Loosing Position while using Smoothing (g64)

Post by Sword »

Just some thoughts.... Mixing two opposing smoothing settings on the same cut/part may not be such a good idea. If you use a set of "loose" smoothing values, the control will allow more deviation from the commanded path to allow the machine to make directional changes without jerk, and then following with "Precision" smoothing values, it will not deviate from the commanded path. While it's fun to take the corners fast, you either have to brake to make it or widen the curve and go off the road. ;)

Possibly try a higher/softer Accel/Decel to help alleviate some of the jerk when running "Precision" smoothing values.

One may say that loose settings would produce larger parts with rounded external corners, but that applies to inside corners as well, making smaller parts.

Just some thoughts on smoothing and the results of loose or tight values, for what it's worth.
Scott
phazertwo
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Re: Loosing Position while using Smoothing (g64)

Post by phazertwo »

swissi wrote: Mon Jul 27, 2020 12:18 pm phazertwo,

this is a hard nut to crack. The only way I know to turn off backlash comp is to set the values back to 0 but I just can't see how backlash comp could be responsible to lose 0.08" in the Y-Axis.

What I would do in your situation is to focus on the one axis with the bigger loss, in your case the Y-Axis. Install a 2x3 block or something similar on the table and a dial indicator mounted horizontally in the spindle and dial in the Y0 reference point. Now run the job with an air-cut and spindle off and check at the end how much Y0 is off. If there's no position loss, then the cutting moves are too aggressive and are causing the position losses. If there is still position loss even with an air-cut, re-calibrate Y0 with the dial indicator and run an air-cut again with the feed-overwrite at 50%. Check Y0 at the end and compare the two position losses. If the loss at 50% feed rate is significantly lower, than the federates are too aggressive and are causing the position losses.

Should the position losses stay the same I would do another air-cut with just 10% and if it still stays the same, I would need to think long and hard what to try next... :o

-swissi
Swissi, thanks for the reply.

The reason I wonder if backlash has something to do with it is that I am making a lot of passes... With the tool path I posted, I am guess there are 500ish direction changes for both the X and Y. I was just thinking that if I lost a little bit with each backlash comp move, that it could add up to 0.08"... I'm grabbing at straws here, I know. And again, it works with smoothing set to P1...

I have tired a little of what you suggested, but I used my probe to measure back to my reference point. What boggels my mind is that it works with regular P1 smoothing settings. Seems to me if I made the motion smoother the chances of losing position should go down.
Sword wrote: Mon Jul 27, 2020 2:04 pm Just some thoughts.... Mixing two opposing smoothing settings on the same cut/part may not be such a good idea. If you use a set of "loose" smoothing values, the control will allow more deviation from the commanded path to allow the machine to make directional changes without jerk, and then following with "Precision" smoothing values, it will not deviate from the commanded path. While it's fun to take the corners fast, you either have to brake to make it or widen the curve and go off the road. ;)

Possibly try a higher/softer Accel/Decel to help alleviate some of the jerk when running "Precision" smoothing values.

One may say that loose settings would produce larger parts with rounded external corners, but that applies to inside corners as well, making smaller parts.

Just some thoughts on smoothing and the results of loose or tight values, for what it's worth.
Sword, thank you also for your reply.

I NEVER change the smoothing settings during a cut, that would make NO sense, however I can't buy that changing smoothing settings in the NC program is a bad idea. Why else would they give us the ability to do it? Also, this is a somewhat common thing to do, even on industrial machines (look up Haas's G187, its basically the same things as Acorns/Centroids G64). Here is how I use it: I drop a G64 PXX (what ever smoothing type I want to use for this cut) at the beginning of the roughing section. Then, between the roughing section and the finish section, I drop a G64 P1 (precision mill). This should allow them mill to cut corners in the roughing op, and then come back and kiss it with the finish pass. The only thing to be careful of is that you MUST leave enough material to account for the "loose" tool path of the smoothing used for roughing.

Also, I have tried playing with the Acceleration settings via the smoothing parameters. I believe I have the precision mill and roughing smoothing set to 0.75 accel. No change.

PZ
swissi
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Re: Loosing Position while using Smoothing (g64)

Post by swissi »

This is worth a test:

Write down your backlash values and then reset them to 0. Run your job again with the higher smoothing values and see what difference it makes compared to running the same job with backlash comp.

At least this will point out what difference backlash comp makes when using higher smoothing values.

-swissi
If you are using Fusion 360, check out my CNC12 specific Post Processor
If you are using a Touch Probe, Tool Touch Off Device or a Triple Corner Finder Plate, check out my ProbeApp

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phazertwo
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Re: Loosing Position while using Smoothing (g64)

Post by phazertwo »

Well that is about as simple a test as there is. I will try to give it a shot tonight or tomorrow night and report back.

Thanks!

PZ
Sword
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Re: Loosing Position while using Smoothing (g64)

Post by Sword »

Perhaps I should have phrased it better, but I didn't imply that you were changing settings during the cut, or that changing settings in the NC program is a bad idea, I do that all the time. What I was trying to get at is that each set of smoothing values can yield a different sized/shaped part due to varying tolerances between the two and if that difference isn't accounted for somewhere......

But hey, I'm just a router guy....
Scott
phazertwo
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Re: Loosing Position while using Smoothing (g64)

Post by phazertwo »

I performed the backlash comp vs no backlash comp and I got no difference between the two results... Still losing position, but only in the Y. I turned backlash back on and ran the test again but this time running it with smoothing at P2 and then again at P1. Only 0.001" difference from P2 to P1, still only loosing position in the Y.

After playing with smoothing settings and speeds I was able to get down to about 0.005" position loss in the Y, and X was holding steady (±0.0002"). Also, I was probing the bore twice each time, and always using the same Z height.

I think what it comes down to is that I have a mechanical issue in the Y, and it just started rearing it's ugly head on this part since I was running so close to the edge of what the system could handle anyway.

I will have to dig into it a bit and figure out what is going on... It won't be for a while though, but I will try to remember to update this when I figure something out.

Thanks for all the help guys!
PZ
Gary Campbell
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Re: Loosing Position while using Smoothing (g64)

Post by Gary Campbell »

Are you sure that your computer meets the single core benchmark?
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