Can't figure this one out!

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avp
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Can't figure this one out!

Post by 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
Gary Campbell
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Re: Can't figure this one out!

Post by 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
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ShawnM
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Re: Can't figure this one out!

Post by 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
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Re: Can't figure this one out!

Post by 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.
Scott
avp
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Re: Can't figure this one out!

Post by 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
IMG_6944.jpg
IMG_6943.jpg
IMG_6942.jpg
IMG_6941.jpg
cnckeith
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Re: Can't figure this one out!

Post by cnckeith »

sweep indicator along x rack to see if is parallel to the axis rails in the trouble zone.
Need support? READ THIS POST first. http://centroidcncforum.com/viewtopic.php?f=60&t=1043
All Acorn Documentation is located here: viewtopic.php?f=60&t=3397
Answers to common questions: viewforum.php?f=63
and here viewforum.php?f=61
Gear we use but don't sell. https://www.centroidcnc.com/centroid_di ... _gear.html
avp
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CNC Control System Serial Number: E062346A1C67-0716214939

Re: Can't figure this one out!

Post by 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
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Re: Can't figure this one out!

Post by Nigelo »

avp wrote: Thu Sep 22, 2022 5:07 pm 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
Posts: 42
Joined: Fri Aug 13, 2021 12:25 pm
Acorn CNC Controller: Yes
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CNC12: Yes
CNC11: No
CPU10 or CPU7: No
CNC Control System Serial Number: E062346A1C67-0716214939

Re: Can't figure this one out!

Post by 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
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