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Friday, November 08, 2013

CNC mill conversion - Part 7

Stepper-motor couplings

The stepper motors are in! 

390 oz/in (2.75 Newton/meters) stepper motors

They are a little longer than I expected, and that means I will need to make more room around the mill, but at least I can continue with the project.

Stepper motor specs

Looking at these powerful motors, I decided the couplings I had purchased earlier are wholly inadequate. 

Small coupling

For one thing, I was expecting the motors to have 1/4” (6 mm) shafts, while these beasts have 5/16” (8 mm) ones, then these coupling only have one tiny set-screw with which to hang on to the shafts while transferring all that torque, and finally they are way too short, translating into very little surface in contact between shaft and coupling, ergo not enough friction, and this last condition is ripe for slipping.

Slipping is BAD! Slipping translates into table movement that hasn’t really happened, but that the computer controlling the cutting thinks has happened. This means the table would be in the wrong spot for making the cut, and that accuracy would go out the window, meaning the part being worked just turned into scrap.

Since accuracy is one of the main purposes for this conversion, slipping cannot be tolerated. For this reason, and also because it saved money, I decided to stick with the plans and make my own.

Coupling plans

Obviously, given that both the stepper-motor shafts, and the ball-screw shaft ends are both 5/16", the design had to be slightly altered with a constant diameter through hole, and while at it I decided to increase the gripping power by doubling the number of set-screws.

One thing I’d be giving up with this design is the ability to compensate for a slightly offset motor shaft, but upon test fitting they seemed spot on to me, so I decided not to worry about it unless and until there is a problem.

Looking in my parts bin, I found just enough 2024 aluminum left over from making the heated pitot tubes noses.

Repurposed Pitot tube nose stock

I parted it in two, and drilled the center 5/16” hole.

Drilling the coupling on the lathe

In order to drill and tap the set-screw holes, I had to use the half-converted mill in the “poor man's CNC” configuration, with two drills powering the X and Y axes.

"Poor man's CNC"

Believe it or not, this actually worked very nicely, and fine motion control was still possible with light trigger action.

Drilling the coupling set-screw holes

Tapping #8-32

The coupling came out better than expected, and just one set-screw felt like it had plenty of grip, so doubling their numbers was probably an overkill, but it was free.

Before and after

"This ought to hold!"
Old coupling next to the new ones

Coupling installed on the stepper motor

I had left the ball-screw long on purpose a few months ago, but now that I had the motor and coupling in hand, I could decide how much to cut.

Ball-screw needing to be shortened 

Shortening the ball-screw on the lathe

All better now!

And finally, I could install the motors for the first time.

Y axis stepper in place

X and Y axes completed

Wires need to be hooked up to the motor drivers

The Z axis is still part of the “to be done” category, but I’ll focus on the electronics and software/hardware integration next, in order to get this bitch moving!


  1. Be carefull with rigid couplers. If the shaft alignment isn’t perfect damage to either the ball screw or stepper will happen

    1. Thanks for the comment Kent. I ran the mill with a set of non rigid couplers for a while, but one of them actually got damaged (I forget what happened). So I put these ones back on, and never had any more troubles for the past five years. Perhaps the alignment is spot on.