[JMay]

Twofer’ or Two for one in English, 3d printed motor magnet attraction and torque tester jigs.

Motor magnet attraction jig
   

Motor torque jig
   

For racing use to find which motors have greater magnet traction on plastic tracks I created a version of the motor magnet attraction tester, permutations of which have been around for some years. This technique although not delivering industry specification accuracy is sufficient to compare motors and motors mounted in cars.

My version uses a set of digital scales from Amazon (Criacr Digital Pocket Scales, 500g High-Precision Kitchen Scales) with a claimed accuracy of 0.01grms which in reality is probably more like 0.1grms but in practice looking for say 0.5grm differences then not a problem. The jig is sized to take this set of scales at 106 x 127 x 19mm.

The scales sit on the base plate over which is a bridge piece that takes a steel shim to act as the attractor plate and over that but resting its 4 feet on the scales are the motor pods or the car cradle. The pods, cradle, cars and motors are placed on the scales away from the bridge to zero or Tare, then the motor is placed in the cradle over the attractor plate such that the magnet attraction can be directly measured as the motor magnets pull the car/cradle downwards applying force to the scales.

Included is 2 off motor holders one for the boxer type (or S can) motor the other for the Flat 6 type so both are presented to the scales at the same height from the scales to underside of motor. There is also a tray to present a car as well for club racing checking use, and has the car sitting on its tyres so ground clearance and magnet traction can be checked against a set limit. Putting wheel cut outs in this would take out the track clearance aspect.

Built this up as shown and thought why not add a motor torque test function as well, and decided it was easier to design a new bridge piece than try and dual function the existing one, hence the 2nd set of files using the same base. The motor sits in the bridge and is clamped down, then the disc is fixed to the motor shaft and held with 1-2 grub screws. Depending on filament used this may or may not take a thread that lasts so have included a 2nd disc which can be glued to a cog that has grub screw fixing for more permanence. The disc takes the 3 pusher pieces at 120 degrees at 1cm from the motor axis so that the motor torque can be checked in 3 positions then averaged.

Methodology for testing is to clamp the motor, align one pusher to the scales, zero the scales then apply 4 volts momentarily to the motor. This is done 3 times at 120 degrees and then averaged and the result multiplied by 3 to equate to 12 volts operation. The disc can be rotated 60 degrees or the pushers repositioned around the disc, then the testing carried again to get 6 readings for greater accuracy, but found in practice taking the 3 readings was sufficient. This setup gives a good equation to manufacturers figures once calculated.

Note: this will need a minimum of a 5A power supply for testing hard bodied motors otherwise the PSU will not supply sufficient power to allow the motor to generate maximum torque.

STL files content

Printed in PLA+ at 0.16mm layer and 40% fill.

   

   


Cheers 
             John