Nov 17 2019 new design
This is a simple overhang calibrator for printing more accurate threads. With it you can measure the thread clearance and sag of the overhanging threads and tune your printer parameters and print better threads.
These threads are M10x1. In Fusion 360 the thread tolerance is class 6g for the bolt and 6H for the nut. I measured in the CAD model the nut to bolt axial thread clearance at 0.13mm.
The nut has 20 notches and one notch has a pointer. The bolt also has a pointer and the angular position difference allows measuring of the thread clearance.
Print the nut and flip over the bolt before printing. Print with no supports on a raft then screw them together (the raft removes bed leveling accuracy from the measurements).
If you have printed perfect threads then when they are fully screwed together the pointers should go past each other by the thread tolerance clearance or about 2.5 notches (0.125mm). In this model each notch is (1mm thread pitch)/(20 notches) or 0.05mm of clearance per notch of rotation.
Next you flip the nut over, screw them together again and remeasure the clearance. If the threads are perfect the pointers should show the same thread clearance of about 2.5 notches.
If both numbers match and are close to 2.5 notches or about 0.13mm then you have printed great threads.
If the two clearance numbers do not match then the overhanging threads are sagging, try to improve cooling or tweak the PLA temperature.
If the numbers match but the clearance is not 2.5 notches then you are over extruding or under extruding (or less likely the slicer you are using has issues).
Hobby King PLA
Designed in Fusion 360 with high STL refinement and printed on a cetus3D MK2 sliced using the UP studio software
0.4 mm nozzle with 0.1 mm layers
Fine (best print)
Very small surface blemishes can be knocked off by screwing together and apart a few times before making the measurements.