inconsistant positioning

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Ozyrob
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inconsistant positioning

Post by Ozyrob »

Hi all,
I have been using a knee mill that I retrofitted for about the past six months. Till recently it has all been running fine. When doing a prototype in timber I noticed that it seemed to not do what it was supposed. I thought at the time that maybe it was just a posting problem from Fusion.
For a while I have been going to increase the amperage for the servos which I did yesterday so I then went about adjusting the acceleration and max feed. This all went ok.
I then probed a part that I have machined in the past to open some holes up. This was a bolt circle pattern done on my manual mill with a readout. When I went to open the holes up it didn't seem to align with the original holes. So I set-up to check the accuracy of the axis and found them to be out up to 0.3mm over 150mm so I fixed this.
I then went and probed the centre hole of my part and the hole measure 0.5 undersize with the probe. So I went through the process of resetting the prob runout and pre travel. This was pretty much fine. I then went back and checked the axis travel again and had to readjust it, it end up close to the original set-up. I then went and re probed the hole. I ended up with the same result so I just ran the cycle again and the centre potion was 0.25 out on both x and y and the hole size was right. So I ran it again after zeroing both x and y and it was right on with the correct diameter. I did this over and over and now and again it would be out 0.25 again.
So this morning after leaving the machine I probed the hole again after homing the machine and it was out again 0.25 and the hole diameter was wrong. You can see in the images one is out in one axis the other is fine on both and the hole size is right.
I'm not sure if this problem that developed just before I made the adjustments or if it's a result of something I've done?
Any ideas?

Rob
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cncsnw
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Re: inconsistant positioning

Post by cncsnw »

Don't chase your tail, trying to repeatedly adjust turns ratios based on an 80mm probing cycle. 80mm is too short a distance, and the probing cycle has probe pre-travel, stylus size, and backlash all rolled into it.

Measure your turns ratios based on the longest distance you can reliably measure, preferably using a method with no changes of direction. Then leave them alone. Any inconsistency you see in bore probing results is probably due to mechanical issues: backlash, probe travel, drive train slippage, etc..

Probe the bore several times, to see how consistent the results are. Then, without changing anything in the software, push and pull and shake on all of the mechanical machine parts. Then probe the bore several more times. How much variation do you see?

Your rapid rates and acceleration times are not too unreasonable, though 8500 mm/min is probably approaching the limits of what the motor can do. You might try backing off to, say, 7000 mm/min, and 0.35s accel time, just to see if starting and stopping more gently makes the probing more repeatable.
Ozyrob
Posts: 16
Joined: Wed Sep 02, 2020 4:31 pm
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Re: inconsistant positioning

Post by Ozyrob »

Thanks for the reply.
When I said I was chasing my tail I meant that I had already been through the process of setting turn ratios and backlash by the book. The probe was just to show that it's not consistent. Usually the repeat ability is really good maybe 0.01mm. I'm seeing about 0.25mm difference and it's not every time. The probe is still feeding at it normal feeds so it's no faster than before. The motors are 2000rpm with a 5mm pitch ball screw so 8500mm/m should be ok and the acceleration is smooth on the stop start. I will try and slow it down.
It nearly seems like the encoders are loosing or gaining.

Rob
tblough
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Re: inconsistant positioning

Post by tblough »

Has your backlash changed?
Cheers,

Tom
Confidence is the feeling you have before you fully understand the situation.
I have CDO. It's like OCD, but the letters are where they should be.
Ozyrob
Posts: 16
Joined: Wed Sep 02, 2020 4:31 pm
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Re: inconsistant positioning

Post by Ozyrob »

No the backlash is the same.
cnckeith
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Re: inconsistant positioning

Post by cnckeith »

hi, what probe are you using?
also check out Rolands Probe App, may be of interest to you.
viewtopic.php?f=60&t=6149
Need support? READ THIS POST first. http://centroidcncforum.com/viewtopic.php?f=60&t=1043
All Acorn Documentation is located here: viewtopic.php?f=60&t=3397
Answers to common questions: viewforum.php?f=63
and here viewforum.php?f=61
Gear we use but don't sell. https://www.centroidcnc.com/centroid_di ... _gear.html
Ozyrob
Posts: 16
Joined: Wed Sep 02, 2020 4:31 pm
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CNC Control System Serial Number: A900880

Re: inconsistant positioning

Post by Ozyrob »

The probe is a DP-4
Ozyrob
Posts: 16
Joined: Wed Sep 02, 2020 4:31 pm
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Re: inconsistant positioning

Post by Ozyrob »

If there was a loose encoder wire would this always give a following error or could this show up as not going to the right position?
tblough
Posts: 1706
Joined: Tue Mar 22, 2016 10:03 am
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CNC Control System Serial Number: 100505
100327
102696
103432
7804732B977B-0624192192
Location: Boston, MA
Contact:

Re: inconsistant positioning

Post by tblough »

A loose encoder wire would show up as a quadrature error. A loose or broken coupling on the encoder would show up as a positioning error.
Cheers,

Tom
Confidence is the feeling you have before you fully understand the situation.
I have CDO. It's like OCD, but the letters are where they should be.
polaraligned
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Re: inconsistant positioning

Post by polaraligned »

If your ways are not moving freely, your drive train will "wind up". You need to make sure your mechanicals are in good order first. Your ballscrews need to be reasonably linear and the backlash needs to be set with high accuracy. If you have wear on the ways and/or ballscrew it generally shows up in the center of travel. Try measuring backlash at each end of table and see if you get a different result. The control can't compensate (without laser measurement) for poor mechanicals. When you measure for turns ratio, you need to use a very precision reference and very careful measurements. The shorter the reference, the greater the error over a long distance.
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