Compact T square.

Uses 2 x 3mm heat inserts.  2 x M3 - 6mm capscrews / screws

Printed in pla - 20% infill.  2 hr print.

To test/calibrate - put t square on a straight edge of a board. Draw a line then turn over as per the pictures, and draw another line. It is correct when the two lines are parallel. 

Middle lines in the picture show that the calibration are not correct.