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View Full Version : Problem cutting circles - arc shifting issue



pkuhns
08-14-2010, 05:03 PM
Hello - I just built a router (4'x8') and it looks great but it won't cut circles accurately.

Please see the att'd image:

http://www.kobbleit.com/blog/wp-content/uploads/2010/08/circle_problem_14Aug2010.jpg

I traced a circle with a Sharpie - please notice that at the end of each arc, the router shifts the Y axis.

I have no idea why it is doing this. Have you seen this problem before?

ger21
08-14-2010, 09:37 PM
Backlash. Is that pic rotated a little?

pkuhns
08-14-2010, 10:36 PM
Thank you for the fast reply. I don't think the photo is rotated. I tried to keep the camera straight when I took the photo.

Backlash might be the culprit then. Now to figure out how to account for that within Mach3. Still new at this stuff.

My system is a kit from http://www.buildyourcnc.com

X axis: NEMA 34 HIGH TORQUE STEPPER MOTOR 1200 oz-in
KL34H2120-42-8B (1/2" Dual shaft with flat)

Y & Z axis: Keling NEMA 23 BIPOLAR STEPPER MOTOR 425 oz-in
KL23H286-20-8B (1/4 shaft with a flat)

Breakout boards and electronics from Keling as well.

Mach3 v2.63

ger21
08-14-2010, 11:13 PM
It's better to try to remove the backlash from the machine, rather than try to let the software compensate for it.

Also, why are you using such an old version of Mach3?

stk2008
08-15-2010, 06:18 AM
Hi there do you have a DTI you cauld actually measure the amount of backlash with?.

And yes do not compensate for backlash in software especially if that's all
backlash its to much.

Could you show us some pics of the machine and the actual shafts and nuts and all so how they are fixed in the machine etc.

CarveOne
08-15-2010, 08:12 AM
As the direction of travel is constantly changing, something mechanical is slipping when it gets around to these two points. Tension builds up and at these two points the tension is released and the router moves over a little.

Software or electronics won't normally cause that.

CarveOne

pkuhns
08-15-2010, 01:04 PM
Problem fixed for now it seems (sorry the pix are so large!):




Thank you so much for the fast reply - your suggestions led me to the cause (and got me off my butt).





My Y-axis stepper motor sprocket was loose. Two set-screws attach it to the stepper motor. They were both loose. Loc-Tite! Somehow the problem only manifested itself at exactly the same location while drawing circles (I must have cut/drawn 10 circles and always the same location). Anyway, I'll monitor that from now on. The setup was pretty crude but was very helpful:





This newbie really appreciates all your help. You can see more pix of my router (and my buddy's router) at our site:


http://www.kobbleit.com/blog/

Khalid
08-15-2010, 02:28 PM
Good troubleshooting CarveOne.. I thought the same yesterday... The backlash things only gives the signs at the Quadrants of the circle...

I think the Mach3 software backlash compensation is good.. I nowadays milling metal and i have 0.32mm backlash at X/Y axes and 0.21mm backlash at Z-axis.. But enabling backlash at Mach3 software excellently work for me...

CarveOne
08-15-2010, 08:45 PM
Good troubleshooting CarveOne.. I thought the same yesterday... The backlash things only gives the signs at the Quadrants of the circle...

I think the Mach3 software backlash compensation is good.. I nowadays milling metal and i have 0.32mm backlash at X/Y axes and 0.21mm backlash at Z-axis.. But enabling backlash at Mach3 software excellently work for me...

Thank you Khalid. ger21 and stk2008 identified it as backlash and I just provided a little more description of how it may be happening. It was enough to help pkuhns identify his problem. Sometime the simple things are the most bewildering of all. :) I have had something similar to this problem on my first machine in the past, so I also knew it was a mechanical issue.

CarveOne