Mendel: Prusa durable z-axis motor coupling
This is a design for a new, more durable flexible coupling (http://en.wikipedia.org/wiki/Coupling) for use on the z-axis motors in the Mendel: Prusa RepRap machine design. The design allows for 2-3 degrees of play, but can be adjusted depending on how it's machined.
Please see the following YouTube link that shows the an animated CAD simulation of the design:
Here is the design manufactured and in use:
The RepRap machine filmed in the above video is one of the two Mendel: Prusa machines located at the Digital Manufacturing Laboratory in Bath. (http://digitalmanufacturinglab.com/)
Where to get it
You can get it from
Start with a 25mm length of round brass or aluminum with minimum diameter 14mm . Hex brass is a good alternative, being easier to crossdrill and eliminating the filing step. In the USA, 1/2" hexagonal brass is available from Onlinemetals.com in 12" lengths for $8, in the UK, AJ Reeves in Brum has it in 24" lengths for £18.
On a lathe, face off one end making sure to use coolant where needed. Centre drill the recently machined face and drill a 5mm diameter through hole. Drill a 6.5mm diameter hole 15mm deep. Disengage the lathe clutch so the chuck can be turned by hand freely. With an M8 tap in the tailstock, using cutting lubricant, cut an M8 thread into the 6.5mm diameter hole you have just drilled. Turn the bar around in the chuck and face off the other end.
To ensure concentricity of the part, DO NOT REMOVE FROM THE CHUCK UNTIL THE THREAD HAS BEEN CUT.
If you used round bar, file a flat onto the end of the part which has the 5mm hole. Drill a 3mm hole through the diameter of the part approximately 5mm in from the end. Insert just enough of the motor shaft into the 5mm hole so that approximately 2mm of the shaft is past the 3mm through hole. If the motor shaft is a very tight fit in he 5mm hole, heat the part in the oven to expand it. Screw in an 8mm bolt as a stop to prevent the motor shaft intruding into the threaded section. Clamp the motor and part securely so they do not move independently. Drill into the 3mm hole again, this time through the motor shaft as well as the part.
Assembly: insert the motor shaft into the 5mm diameter hole, use an M3 hex bolt, M3 washers and an M3 nylock to attach it. Screw an M8 plain nut onto the z-axis stud, screw the stud into the M8 thread of the part and then tighten the M8 nut up to the part.