My Carve King is cutting inconsistently on Y Axis

[johnl]

My Carve King suddenly started cutting parts incorrectly.
The issues always relate to the Y axis.  
After cutting 60 parts perfectly, I took a break, leaving the computer on and Carve King breakout board on.
After the break the parts were wrong; the problem seems to happen intermittently.
The discrepancies are small - about 1mm. 

Any ideas what could cause this issue?


I'm using Builds Controller on a Mac. It's worked well for months until today.


Thanks for any advice. 

[johnl]

I found this post from another forum:

When we see this issue, its usually mechanical, and then usually: setscrews slipping on motor shaft, leadscrew side, or pulleys shipping on motor shaft (you didn't mention what machine you have, so mentioning both) - for fear of stripping people undertighten the grubscrews.
If those are tight, gripping the endmill and giving it a wiggle will help you see what else is loose / badly adjusted
​
So yesterday I started tightening the setscrew in the motor coupler. As I was doing that, the coupler came apart - so that's clearly what caused the issue.

[txshopsales]

Check to make sure the gantry is square. I have had the issue where the distance from the back was different on the 2 y axis. Measure each with a block of wood to make sure they are aligned with each other.