Can't figure this one out!

[avp]

Ok, so here is the situation. It may be hard to explain. I have a 5x8 table on my cnc router. Acorn upgrade from mach 3 was a lot of work alone with other machine upgrades at the same time ATC as well.
All is working good on a pretty consistent basis. Here is the issue I am having.

This problem seems to be limited to the X axis.
I have just over 61" of travel in the X direction. There seems to be a discrepancy in correct positioning within the first 24 or so inches from X0
If I command 12" in MDI the machine overshoots the 12" mark by about .01 This error is cumulative up to about 24" with the most error in the 16-23" range.
After 26" or so the machine is dead accurate. Ex: G0 X30 on up is right on. So from X0 on up it is more off the further it moves toward 24"
Very little error from X0 to say 4-6"

The machine runs with rack & pinion with and avid cnc pro R & P drive plate. I had first thought it could be a rack that wasn't machined properly but I really don't think that is the issue.

Does anyone have any thoughts on this. I recently upgraded to cnc12 4.80 but this problem was the same in 4.64 as well.
Current report attached.
Thank you
Bill

report_E062346A1C67-0716214939_2022-09-21_18-36-17.zip

(860.64 KiB) Downloaded 7 times

[Gary Campbell]

Check for backlash at 6" intervals
Drop out pinion and check for binds across complete X travel
Use a precision straightedge and make sure that the X linear rails are perfectly straight

[ShawnM]

It seems like a mechanical issue in the system to me also. Make sure you have smooth, consistent travel the entire length of travel without the motor(s) attached. Post some pics of the X axis, what type of linear rails? Round or square? Helical or straight cut rack and pinion? How are you precisely measuring 30"of travel to be "right on"? With a tape measure or ruler?

[Sword]

Does it always return to exact X0? Along with the backlash check at the problem locations, check to see if the rack itself is dead flat, meaning, is there a dip away from the pinion in that range. I mention this because Avid uses rack clamps along the extrusion to clamp the rack in place and perhaps it has a dip there. Just a thought.

[avp]

Thanks guys for the suggestions, pretty much all of that has been checked already. All solid and straight. So say if I cut two panels say 12-15" or so wide, the panel cut on the left side of the machine will always be larger than the one cut on the right side of the machine.
The machine does always go back to X0

[cnckeith]

sweep indicator along x rack to see if is parallel to the axis rails in the trouble zone.

[avp]

Checked that with indicator, it is within 4-5 thousandths. I would not think that would effect it since the pinion is spring loaded into rack.

[Nigelo]

Checked that with indicator, it is within 4-5 thousandths. I would not think that would effect it since the pinion is spring loaded into rack.

If your linear rails are not parallel by as much as 4 to 5 thou and in the "trouble zone" this may well be your problem. Suggest placing a dial indicator on the spring assembly itself to see if it is actually doing its job properly without being compressed whilst traversing the trouble zone. Also double check for binding in this area as Gary suggested above

[avp]

All been checked already, over and over again. Carriage is smooth throughout the whole travel. Rails are perfectly in line. I used a dial indicator to position them when I installed them. Like I said, this is one issue I can't figure out. At this point I have an another rack from an older machine. It is a different brand, the same ones I currently have on my Y axis. I will install that piece and see if the problem is still there.
Bill

[avp]

Ok, so did some more testing as was suggested. I think what happened was when I used a dial indicator to align the linear rails I was going off the extrusion. So toward the left side of the machine toward X 0 the top rail was just a little off. This was probably mimicking the extrusion since the dial indicator showed it perfect when in reality it was not. And obviously this is much exaggerated down at the tool level.
After a slight adjustment of the top rail it is much better, Still needs a little fine tweaking, have to check the bottom rail and hopefully I will get it just about perfect. Thank all of you for your suggestions. I knew it was some mechanical issue just couldn't figure it out.
Bill