This is a tool you can use to check if your Y axis is perpendicular to the X axis.
Some people, myself included, have printed out http://www.thingiverse.com/thing:11978 and noticed that the cable chain arcs to one side. Investigating the problem I have discovered that my Thing-O-Matic's Y stage is not perpendicular to the X axis. This may be the cause of the arcing problem.
Measure the diagonals.
Share your results!
If your Y axis is perpendicular to the X axis, both measurements will be identical.
Please leave a comment with your results. If the error is consistent among many machines we can create a new part to fix it.
On my machine the measurement from lower left to upper right (/) is about .65mm longer than lower right to upper left ("backslash")*
/ = 79.85mm
"backslash"* = 79.21mm
*The backslash will not display here but it will in the comments.
** Thanks to those who share their results.