NEMA 17 Shaft Adapter

The NEMA standards for stepper motors don’t specify the shaft dimensions, alas. While most NEMA 17 steppers have 5 mm shafts, the X and Y axis motors in a Thing-O-Matic have 3/16 inch shafts: MBI belt pulleys with 4.76-ish mm ID won’t fit on 5 mm OD shafts.

(Note: the “17” in NEMA 17 means the mounting holes are on a more-or-less 1.7 inch circle. The side of the motor frame will be close, but that’s not the controlled dimension. Some relevant diagrams live there.)

I plan to replace the Y axis stepper with a better motor (I got a set of three, one of which is now driving the stepper extruder), which means either buying a new pulley or having some Quality Shop Time. Plus, a bit more length on the Y axis shaft than what comes standard would be a Good Thing, too.

[Update: From the motor label, not that you’ll ever find one like it…

• 38 mm case
• Minebea-Matsushita 17PM-K150-P1V
• No T6824-02

]

So I built an adapter from 5/16 and 3/16 rod with a setscrew to grab a flat on the stepper shaft and a pin for the torque. The larger rod turned out to be La Salle Fatigue-Proof steel, not that it matters, and the smaller rod is plain old W-1 Water Hardening Drill Rod, both from Brownell’s, a long time ago in a universe far away. You could turn and drill the adapter from a single length of 5/16 rod if you prefer, but take some care to maintain the alignment.

A bit of lathe & Sherline CNC work:

• Face one end of the 5/16 rod
• Drill half an inch with a #9 drill (0.196 + runout = 5 mm)
• Drill another quarter inch with a #12 drill (0.189 = 4.8 mm)
• Saw off 3/4 inch, face the raw end
• Saw & face an inch of 3/16 rod
• Epoxy little rod in big rod, set upright, wait overnight
• Cross drill #43 and tap 4-40 near big end
• Cross drill #56 for 0.045 music wire pin
• Chamfer pin hole, clean, epoxy pin in place, wait overnight
• File two flats on 3/16 shaft for MBI pulley setscrews

I grabbed the small rod in the vise with the large rod resting on the top of the jaws while the epoxy cured, figuring that it’d be pretty much self-aligning. Not that a few mils one way or the other will matter, as it’s driving a timing belt in a flexy machine anyway.

Cross-drilling the pin hole required eyeballing the center of the length of 3/16 rod within the 5/16 rod. It’s not critical, but avoid missing the poor thing entirely. You want to minimize the nested length, so as to keep the adapter as short as possible, but keep at least one diameter (3/16 inch) so as to maintain alignment.

Tapping should involve a bottoming tap, but I used what I had and it worked out OK.

Now, one reason I was willing to do this is that the stock Y axis motor shaft was already too short. As nearly as I can tell, the TOM dimensions were set before MBI started shipping those cork sound-deadening plates, because the shaft is recessed into the pulley by about the thickness of that plate.

The MBI pulleys are an extremely tight fit on a 3/16 inch rod, so, rather than forcing the pulley, I enlarged the hub with a #12 drill (same as in the adapter) to get another 1.5 mil of clearance; it’s now an easy slip fit on the rod.

Anyhow, the bottom flange of the pulley is 17 mm above the ridge on the motor and this one worked out to a bit over 20. No problem, I can just lower the motor a little bit, flip the pulley over to get the setscrew end of the hub on the top, and it’ll have plenty of room. A bit more shaft is much better than not enough, sez I.

The motors came from the usual eBay seller complete with a squishy silicone sound deadening panel that turned out to be exactly the right thickness, when stacked atop a cork sheet, to put the pulley where it needed to be. I cut a second cork sheet, so as to isolate the bolt heads from that acrylic body panel, and it’s all good.

Now, to print a suitable test object…