Lost chain length, now bad cuts?

Using Ground Control 1.26. While I was in the settings I must have suddenly lost connectivity, because I received the infamous “Unable to find valid machine position” error. I marked my chain lengths during my initial calibration, so I followed Madgrizzle’s instructions to reset the chains, and everything seemed okay from a distance, until I took some measurements. The whole part is skewed.

For reference, one I had cut the day before losing my chain lengths measured at 4.66", 4.65", 4.65". (unsure of orientation).

I was quite happy with the accuracy of the machine before this happened. Looking in the settings I don’t think anything else was accidentally changed (I wish I made a backup!).

Any ideas what might have happened or how to fix?

I looks as if one chain may have been longer than the other. Like it’s off by a link. You can try to reset the chains again and then, painstakingly, count the number of links to make sure the two chains are equal.

If you run a string through every 10 holes, its easy to count every five outer links. It’ not too bad, I have pics somewhere.

Edit to add pic. Alternate the string so it runs 10 holes on top, then ten on bottom. This wasn’t attached to a sled so it bunched up funny.

could you extend again and verify your mark is correct?

I counted links on the chains and they are equal.

(Thanks for the tip @kayaker37. I ended up putting a sharpie mark on every fifth link, which made it easy to count).

I guess next I’ll try feeding the chain lengths to verify it matches, or redoing the entire calibration.

I wanted to change over to web contol anyways. Should I just start fresh and calibrate in web control?

also make sure the that cotter pin at the sled passes through the same chain link ‘hole’ on both chains.

Checked that also. Used that as the start to my count.

There really is no reason you should need to calibrate. I assume you set both sprockets vertical and then pressed ‘define home’? Then placed the marked link on the top tooth… and then did the set chain thing? (can’t remember the button names well). Maybe reset the chains again?

I went to Set Chain Length Automatic, oriented both sprockets to vertical, quit back to the Action menu, placed the marked links on top tooth on each sprocket, then went to Set Chain Length Manual.

Seems you followed the wiki correctly (forgot what was there… been long time since I used ground control). I still say give it another try to see what happens. One thing you can also do is when the sled is attached and chains are extended is to mark where the router is on your spoilboard. That way you can verify everything is correct when/if you reset the chains again.

Did the “unable to find valid machine position” occur while you were still on the setting screen? @bar, do the setting changes get sent immediately upon a change of value or after leaving the screen? (Been too long since I’ve looked at ground control) .

Yes, it happened while I was in the setting screen. And not cutting anything in the background. It was when I was scrolling (which is annoying to do on a touch screen), and I think I entered and exited the change box for some setting (not sure which one, it opened and closed fast).

Go through and check all the settings… make sure its still triangular kinematics and not quadrilateral.

Thanks! I owe you a beer. That seems to be it. I was looking mostly at numerical values, but that setting must have been switched. And since it was the default I didn’t think much of it.

I made another cut which visually looks ok. Now to measure.


Well, its visually better but still off and parts that should fit together do not. Any other things to check?

Did they fit before you ran into the chain issue?

Yes. Though unfortunately I have given the pieces away so I can’t take precise measurements to see how far off I am.But certainly I have lost accuracy somewhere.

You might have made other changes inadvertently. Perhaps redo calibration and when satisfied, make a backup copy of the groundcontrol.ini file and keep it safe.