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Path not correct
« on: June 11, 2008, 05:15:38 AM »
Dear Support,

I write this for the following problem on Mach 3 - Release 2.63 on Win XP.

We use our CNC for milling lots of equal pieces.
We fix the start panel on work table and go with the program on Mach 3 for the entire day ( we start the program every 30-32 minutes )
The software and the CNC work very well, with good precision and a good speed.

But after some pieces ( 1, 2, 3, 5, 10 .... ) the program has a strange behavior :
the path is wrong of a lot of millimeters ( sometimes 100, sometimes 30, sometimes 10 ) , the tool breakes and I must restart the machine.
But the path is not wrong when the program starts ... is wrong after 2000/3000 step of program !!!!

Some days, with the same program, the CNC work well for the entire day !!!!!!

I have checked the position of Mach 3 axes and it tell me that the position is OK but the axis is wrong of a lot of millimeters !!!

Is like sometimes Mach 3 think to have made 100 mm of travel but are made only 80 mm.
If I restart the program the machine work good.

How is possible ?

Bye
Maurizio









Offline da21

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Re: Path not correct
« Reply #1 on: June 11, 2008, 06:14:56 AM »
check your timing settings you may have them slightly too high , steppers can and do miss steps .
also check for any binding on your slides or ballscrews etc , any tightness can in some cases make the steppers lose steps

As the steppers only move then presume they have reached the correct point , if they have missed then their is no closed loop feedback to tell the controller it's wrong
servo motors fitted with encoders are the best as they can send any error ( or counts ) back to the controller , and then the controller can adjust it's settings to allow for change

Dave

Offline Hood

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Re: Path not correct
« Reply #2 on: June 11, 2008, 06:43:49 AM »
Is it always the same axis that is out? What hardware do you have(drives/motors etc)?
 Have you tried lowering the acceleration slightly as it sounds like your motor tuning may just be on the edge and sometimes it just misses steps on a rapid move.

Hood

Edit, sorry Dave see you already mentioned about missed steps, was halfway through my reply when a customer came in and didnt check if anyone had replied before I posted.
« Last Edit: June 11, 2008, 06:46:26 AM by Hood »
Re: Path not correct
« Reply #3 on: June 11, 2008, 07:43:17 AM »
Thanks for your help.

I produce about 22 piece per day ( always the same pieces, always the same material, always the same program, always the same CNC setup )

If it is a problem of missing step, acceleration/deceleration time, mechanical parts....... why some days I produce for the entire day a good products without problems ?

When the machine work good all the particulars have the same accuracy and the same dimensions ( tollerance +/- 0.01 mm !!! )

When the machine work no good I have error of 10/20/30 mm ...... Is possible that there is an error on calculation of path when I repeat 20-30 times the same program ?
I make this question because there is also another strange thing ...... for 3 times in last 2 month the program stop the execution when the spindle is turned on ( M3 command ) . I don't know what it is waiting ..... If I don't make a restart the CNC stay with spindle on for hours ..... I restart the machine and the machine go good for days.

Thanks
Maurizio




Offline Chaoticone

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Re: Path not correct
« Reply #4 on: June 11, 2008, 09:01:37 AM »
Lots of things could cause good days and bad if it is skipping steps. Heat, resistance of mechanical components, lack of lubricant, dirty slides, ways, dull cutters, ect. Do us a favor and tune your motors down a little and see if it fixes it. Lower the accel. and vel. by 10% and let us know how it does.

The spindle issue sounds like it may be noise to me. If it is running the same code and nothing has changed except sometimes it doesn't start as it should noise is the only thing I can think of that would make it random. Ferrite chokes and/or cable shielding may help this.

Brett
;D If you could see the things I have in my head, you would be laughing too. ;D

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Offline Hood

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Re: Path not correct
« Reply #5 on: June 11, 2008, 09:06:54 AM »
For the spindle problem, do you ever see a ticker Error message down near the bottom of the screen?

Hood
Re: Path not correct
« Reply #6 on: June 11, 2008, 09:32:47 AM »
No, for the spindle problem I don't see nothing .... the Pc is operative, the timer if elapsed time work ok, the other commands are operative, I can make all I want but the program is freezed.

But this is not a big problem because the machine has not start to work and I must only stop with F5 the spindle, I close the program, restart the program and all is OK ..... the other problem is very hard because I broke the tool and I jump off the piece ( and money ! )

Thanks.

Offline Hood

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Re: Path not correct
« Reply #7 on: June 11, 2008, 01:21:50 PM »
Please reduce your velocity and acceleration slightly and see if it solves the problem, If your motors are tuned on the edge of their performance then the slightest thing can cause problems.
Hood
Re: Path not correct
« Reply #8 on: June 11, 2008, 01:39:28 PM »
I have tried also this test.

The problem is not the acceleration or the speed because I work at 30% of jog speed.
The jog speed is 60 % of maximum speed of CNC.

The problem is always present when I move at work speed ( very lower the maximun speed ) and never when I move at jog speed.

I have also tried to make a demo program with 1000 step of moving back and forward at maximum speed.
The start and the stop position after 1000 step is exactly the same ( each step has 800 mm of travel path for a total of 800 meter of travel )

Another strange thing is that when I program 1000 step of 800 mm forward and back the stop position is 0.000 exactly.
If I load a G code program for a work after 300000 step of program the start and the stop position is different of 0.006 mm ( is non importat for me but is strnage because the start and the stop position must be the same for simple path without complex path ).

Thanks Hood for your help.


Offline Hood

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Re: Path not correct
« Reply #9 on: June 11, 2008, 01:51:18 PM »
0.006mm? thats quite a small amount, would even be hard to measure, or are you meaning the DROs read that amount?
 If you have a G0 move in your code you will run at the maximum that you have set in the motor tuning and if you have an earlier version of Mach and use thr FRO at all you will also override your G0 moves.
 What is your kernel speed set to? Have you run the driver test to make sure you have a clean pulse?

Hood