Problem- Axis not repeating

[Aircraftman]

Check this problem out.....

My shop recently purchased this very used Mighty bridge type vmc. It has a Mitsubishi Meldas 520AMR control. I got it up and running and ran a few simple programs to check for axis position repeatability. To my amazement when it tried to do a basic square tool path, with cutter comp on, every time it rounded a corner with the G02 or G03 command it would be way off in X and Y on the next depth cut pass from .005 all the way up to .100!! Each time it went around the square the location would be random on the next pass, no pattern at all. Program does the same thing whether it's Radius or I and J called out. However if I remove cutter comp so the tool path does 90 degrees on the corners with no radii it's perfect. Is it backlash or parameters?
Any ideas?

[cogsman1]

Check that you have selected the correct Plane. G17, G18 or G19.

[xavierdemoura]

Aircraftman,

I think thats maybe, its possible backlash. I never see somthing like this, for example, like you said, with cutter comp on, square no good, if you use cutter comp off the square is good. Check the backlash and see the nuts in the ball screw if is loose.

Regards,

Daniel

[Aircraftman]

Hi guys,

Thanks for your replies.

Here's a little bit more info....

Positioning for drilling cycles and point to point movements are always good. I do call out a G17 in the safety line of every program I run to make sure I'm working in the correct plane. This problem only shows up when the machine attempts to cut an arc. When I single block the tool path I can see the tool starting at the correct position but when it finishes the arc it's out of position. The DRO displays exactly how far out I am. If the end point of the tool was supposed to be X6.0000, and it's .030 out, the DRO will show X6.0300. In other words it displays the correct position, but that's not where it's supposed to be.

I'm going back to work tomorrow and investigate this problem further. I'll check out some of your advice and let you know what I discover.

[xavierdemoura]

Aircraftman,

Try do an complete interpolation and check the diameter. See if the interpolation will be problem in the diagonals or in the quadrants.

Daniel

[Aircraftman]

We have answers gentlemen!

After fearing I had ball screw problems, or backlash issues with this machine, I was able to look at the manual for my Mitsubishi Meldas 520AMR control on line at meau.com (Mitsubishi Electric Automation). After finding the control parameters page I spotted parameter 41 (R Compensation).

When this parameter is set to on it says:
In circular cutting, an inward move caused by a servo delay against the command is corrected.

When this parameter is set to off it says:
In circular cutting, an inward move occurs because of a servo delay against the command resulting in a smaller arc than that specified by the command

Anyway, I don't really understand what either one of those means but when I looked at the control parameters page on my machine, parameter 41 was set to on. So I set it to off, and ran my basic cut a square program with cutter comp on and walaaa! the tool path ran perfectly including all the arcs.

Everything is working fine now, but I will look into the parameter 41 thing so I can understand what the purpose of it is.

Thanks for all of your suggestions and ideas.

[M.BAVYA]

In our machine shop one Tal -v500 machine has been done laser calibration.that person corrected backlash & positional errors. but he told repetability was much in x axis so i can't controlled that please correct your mechanical side.but that LM guide version .could you please any one answer me how can we controlled x axis repeatability. is this correct or not?In my knowledge we can change new bearings that time repeatabilty will come?