cnc gantry runout

[waterjetman08]

I have an overhead gantry rack and pinion and am running into an odd problem. I can set home on the machine the pierce a hole in the material to mark my home point. Then run my program which basically cuts 5 holes @ .3 dia on about 12" spacing and then a 4' line, stop the program before the end and abort the program which sends the machine back to the home point. Only problem is it doesnt go back to the correct spot, it goes about .05" to the left of the original home yet thinks it is in the right spot. I have already checked for slop at the gear to rail interface and seem to find none. I tightened the tensioner 3/4 turn 3 times now and never saw a change, What am i missing?

[One of Many]

Is the cutting offset still on before you make the call for it to go back to that point?

DC

[waterjetman08]

I actually preprogram the offset for the jet in autocad before it is ever posted to the machine. I have a guy combing the rail completely clean and relubing now, hoping a build up of garnet possibly throwing ratio off.
thanks
waterjetman08

[Med-Pac]

If it always comes back to .05 left off of home, it has to be in the programming. If it comes back home in variable positions (no consistency), it's the table.

Mike

[waterjetman08]

It is either the physical machine parts or the encoder counts and flash settings. It is not the cad program itself. If I set the home with the machine at say x 10.0 y 10.0 and run 90% of the program and abort it it goes back to x 10.0 y 10.00 but it is actually not at the same real world point as it started. it is an avg of .05 out to the left in the x but it does vary some. would wear on the gear rail make sense ? I have already put a new pinion gear on.
thanks,
st

[One of Many]

If the machine is servo driven with feedback from encoders, I wouldn't expect it to get .05 off from its commanded verses actual position without some error message. If it is stepper motor without feedback, there is more than likely some sort of bind that contributes to lost steps and the control would never know it. You could run a simple program to have each axis travel alone and return to see if it returns and repeats to an indicator position. If there is an accumulation in offset then you can get an idea where the bind may be localized.

If the pinion on the rack is keyed to the motor, and well meshed with the rack, I doubt that would be the cause with that large of a difference. Could it be that the tensioner is too tight or the linear rails tweaked somewhere along the X axis travel?

DC

[waterjetman08]

It is servo driven with an encoder feedback. It has a 6' rail and upon inspection I think the problem is in fact wear. the middle 3 to 4' of the rail is worn quite a bit and the ends are not really worn. We have run a few too many programs in the middle of the travel i do believe. The only wierd thing is that until a month ago I have had no problems with the machine. Then I did a job with about 1500 circles per panel and was seeing what I thought to be cumulative error of almost 1" sometimes by the end of the program. . turns out i have the problem in general now regardless of amount of holes only less substantial.
Thank you for your thoughts.