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Axis calibration precise but inconsistent
« on: September 08, 2016, 09:20:25 AM »
Hello,

I'm pretty new to Mach3 but coming from LinuxCNC, so there's "some" knowledge of the CNC stuff.
I set up my machine which is a pretty rigid construction and it is constructed for precise milling of soft materials (brass, etc.).

First I calibrated my motors via "Motor Tuning" with the following specs:

- NEMA 23 with TRIPLE BEAST board
- Microstepping 1/10 = 200*10 steps per rev = 2000 steps per rev = 400 steps per mm
- Ballcrew spindle (d=16mm) with 5mm pitch

Steps per: 400
Velocity: 850
Acceleration: 350

2us / 2us

Motors sound good with these specs. Backlash is set to 0.008mm but no difference to 0mm.

Axis problem No.1
I started the Axis Calibration. I told Mach3 to go for 1mm and adjusted the steps until precisely 1mm was reached. I tested this several times and Mach3 always reached the 1.000mm.
I then tried the same for 5mm. Surprisingly, the Axis would go 5.015mm. When I adjusted the Axis with these values and tried 1mm again, the 1mm would be off 0.018mm.
Any ideas where to problem might be?

Axis problem No.2
For my other axis I was able to get stable precise results for 1mm, 5mm, 7mm and so on. I then restarted, homed again and tested the axis at the same position with the same distances again and were off by almost 0.01mm. Any ideas on that miracle?

Thank you guys in advance!
Best regards,
Hancock

Offline Stuart

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Re: Axis calibration precise but inconsistent
« Reply #1 on: September 08, 2016, 10:33:20 AM »
What grade of precision are your ball screws , are they double nut or single nut

sounds like it's the ball nuts ,
Are they fixed tight , has the screws got any end float in them

All the about will cause the symptoms you describe

Have fun
Stuart
Re: Axis calibration precise but inconsistent
« Reply #2 on: September 08, 2016, 10:52:06 AM »
Hey Stuart,

thanks for your reply. The ball screws are grade C2 with a single nut.

They are pretty tight and I just disassembled the axis and tested it by using the my Mitutoyo (.000) and pressing against the nut. Max. 0.002mm movement of the nut on the spindle.

Best,
Hancock

Offline Davek0974

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Re: Axis calibration precise but inconsistent
« Reply #3 on: September 08, 2016, 10:56:49 AM »
One tip - when tuning go for a long travel - as much as you can measure, get this right and then the small steps should be good too.
1mm is not a good distance to tune on.
i used a 1000mm rule on my plasma and the mini-mill was calculated and checked over 400mm.
Bridgeport Mill, Mach3 V062, CSMIO-IP/A controller, AC Servo Drives

Offline Stuart

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Re: Axis calibration precise but inconsistent
« Reply #4 on: September 08, 2016, 10:59:19 AM »
well that only account for a bit of lost motion

just a thought ( you know its a dangerous thing to do  ???) try reducing the acceleration to see if it helps

on my small mill ( metric setup ) i use 100 acc with 4 mm pitch ball screws

just bored a 3 mm dia hole with a 2mm end mill just goes on a 3mm ground steel rod

Offline Davek0974

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Re: Axis calibration precise but inconsistent
« Reply #5 on: September 08, 2016, 03:14:10 PM »
Good point, try a rule i have used for a while and thats 1/10 for speed and accel.

So if your speed is 850 then set acceleration to 85 and so-on.

Even if its not a perfect target, it does seem a very good setting to start on.
Bridgeport Mill, Mach3 V062, CSMIO-IP/A controller, AC Servo Drives
Re: Axis calibration precise but inconsistent
« Reply #6 on: September 20, 2016, 10:04:06 PM »
Thanks for all the suggestions! It took me some time to try a few things out, recalibrate and test again.

I adjusted the steps via Mach3 and get repeatable results for 80mm, 100mm, 5mm, 1mm, 0.5mm - everything lies within a tolerance of 0.005mm.
The results are constant with speed/acc: 850/350 or 2400/600

But here comes the problem: When I mill circle, it's way too large. At this point I have absolutely no idea anymore why I can go a specific distance precisely but can't mill a circle with the correct size.

Did anyone had this problem before or still has an idea?
Thanks a lot in advance!!

Offline Davek0974

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Re: Axis calibration precise but inconsistent
« Reply #7 on: September 21, 2016, 02:11:47 AM »
If your movements are good, the difference is probably a code problem, put some code up and show what is wrong with it.

Also, what happens when you try a square? Is the issue related only to circles?
Bridgeport Mill, Mach3 V062, CSMIO-IP/A controller, AC Servo Drives
Re: Axis calibration precise but inconsistent
« Reply #8 on: September 21, 2016, 05:58:51 AM »
Here's the code for the circle. It's two pockets but only the second one is for measuring.

Thank you for your help!!

Code: [Select]
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« Last Edit: September 21, 2016, 06:00:23 AM by hancock »