Z Axis Worm Drive

Overvierw of new Worm Drive System
Bottom view of the holder for the worm drive

After some thought, it was determined that a previously infeasible Z axis drive solution, is now not only practical, but essential.

Originally, this method would require several expensive mechanical pieces to get working; driving up the cost and complexity of the solution. It was then determined that a simple electromagnetic brake would suffice; since these can be sourced from omc-stepperonline along with the necessary motors. The electromagnetic brake, will prevent the shaft of the motor from turning so long as it is not energized. Therefore, an electronic sensor would have been needed to detect when the stepper motor was no longer energized, so that the braking system could be engaged. A complex solution to say the least.

DC Electromagnetic Brake; available from
omc-stepper online

Now, with the new frame design, an old idea has become practical. Initially due to the size of the frame, a D-Shaft of approximately 27 inches in length would have been needed. This length cannot be obtained off the shelf. As a result, a custom length would have to be ordered, or a longer length cut down to size. Ordering custom length is cost prohibitive. Cutting the stainless steel rod down to size is no easy feat. Not only are they stainless steel but they are also hardened. Further, the cut piece would go to waste, as there would be no part of the machine that could be serviced by it.

D-Profile Rotary Shafts, available on Mc Master-Carr

Now that the frame’s size has gone down to a more standard 24-inch length, an off the shelf 24-inch D-Shaft rod can be used. To that end, a worm drive from ServoCity will be used to turn the driveshaft, that turns the timing pullies, that lifts the CoreXY subframe.

27:1 Worm Gear Set, available on ServoCity

This approach eliminates a stepper motor and it’s associated driver, replacing it with a 24 inch D Shaft, and a Worm Gear set and six 1/4 inch flanged bearings. Incidentally the total cost is only marginally higher, even negligible. This will take PolyNC from having a 4 stepper drive system to a 3 stepper one. in total this is a marked reduction from the original 7 stepper motor design.

Update: The parts do not fit together

Sadly, the parts do not fit together. The hub and worm gear from servo city do not fit the d-shaft from Mc-MasterCarr, nor any of the stepper motors in my collection. Here is why:

The flat width of the d-profile is 1/8 of an inch.

The flat width of the profile from McMasterCar (and most likely the stepper motors) is 1/8 of an inch. This means that the distance between the flat part of the shaft and opposite site of the wall is almost exactly 6mm.

ServoCity doesn’t specify the flat width

The parts from ServoCity, however, do not specify the flat width. As it turns out the distance from the flat to the opposite side of the hole is only 5.73mm; too small for the various d-shafts.

All in all its back to the drawing board.

Update: A new worm gear has been sourced, and the solution works perfectly.

Ebay listing of a suitable worm gear set.

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