This is a practice block and fit tester for heat set inserts. I got this set of heat set inserts on Amazon and before using them on projects I wanted to test to see the best hole size and chamfer size, and practice with through holes as well as blind holes.

This is for M3, M4, and M5 heat set inserts, with hole sizes +/- 0.1mm from the outside diameter of the smooth part of the cylinder.

The first two holes in each set have chamfers of 0.25mm (basically none), the middle holes have 0.5mm chamfers, and the right holes have 0.75mm

The depth of the M3 and M4 blind holes is 8mm, the depth of the M5 blind holes are 10mm