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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!


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