Now I'm counting every minute, Every single minute.

by Ivan Hamilton 5/2/2008 4:44:00 PM

I'd mentioned previously about my efforts to fit a rotary encoder to my DC motor. It was my first attempt at this, and also my first attempt at any real machining with my mill.

I've since done this on another motor (or three), and with the benefit of hindsight (and some measuring equipment), it was much easier, quicker and less painful. I thought I'd detail my experiences here, for the benefit or anyone trying this themselves.

Thanks must go to Richard J. Kinch for his original article.

This is your single shaft DC motor.
We want to fit an encoder (and the extra shaft it needs).

  You'll need a nice flat spot to mount the encoder.
This motor has nice circular raised area, but there is some logo & writing in the way.

Most motors have 2 longitudinal bolts. Remove these and get ready to take the end plate off.

HELPFUL HINT!! Be ready for parts to fly everywhere. The brushes are spring loaded, and once free of the commutator, they will want to spring out.

After removing this, you'll see the end of the shaft that you want to extend.

Notice the counter sunk hole in the end? This is where the shaft is held when it was initially made. This is what will guide our new hole.


You're going to put a hole in the end plate for the new extension shaft to exit.

Find the center of the bearing mount within the end plate.

HELPFUL HINT!! Don't guess the center position. Find the center with accurate measuring tools.


Put the hole in the end plate.

I went for 1mm larger (5mm) than my shaft (4mm).


The mounting face needs a clean up.

Mount the end plate, and prepare to mill.

  After a quick face mill, you should have a lovely clean mounting area.

Simply slipping the end plate on (with the springs removed and brushes just tucked out of the way), makes for a good sanity check.

You should see the shaft's center hole neatly in the middle of your new hole.


To drill the hole in the shaft, we're going to rotate the shaft and hold the drill bit stationary. This will cause a "self-centering" action, and help keep our hole straight and centered.

I used the motor's own power (you'll need the motor reassembled to do this).

HELPFUL HINT!! During the conversion you'll need to reassemble the motor (to a working state) a couple of times (you'll might even convert a couple of similar motors). Do yourself a favor and find a quick way to refit the end plate. I bent up a few paper clips to make brush retainers, which made fitting the end plate a breeze. Find a good way at the start, and you'll save lots of time and frustration.


Center your drill bit over the motor. With the motor stationary, drop the bit in and see if it bends toward the shaft's center hole. Adjust until you're in the center.

HELPFUL HINT!! Check the rotation direction of the motor. Made sure it's spinning anti-clockwise from the bit's perspective.

HELPFUL HINT!! Use a good quality sharp bit. It will be easier on the motor, and make the job quicker.

HELPFUL HINT!! Use a variable supply so you can adjust the motor speed. My first attempt was with a fixed supply at the motor's rated voltage. The very high speed simply cooked the drill bit.

Drill with short "pecks", as there is limited area between the shaft, bearing and end plate, and you'll want to bring out as much swarf as possible.

HELPFUL HINT!! Use a powerful magnet to help remove swarf. Put the magnet against the bit, or a nail and dunk it in the shaft hole to drag swarf out.

I drilled a hole about 10mm deep into the shaft.


With the shaft drilled, clear the swarf out of the motor.

HELPFUL HINT!! When removing the drilled end plate, hold the motor with the drilled end plate on the bottom. This way the swarf will fall out, and not in to your motor. With care (and luck), you won't need to remove the armature to clean out the swarf.


HELPFUL HINT!! That strong magnet you used to remove swarf when drilling, can help clean up the end plate afterward.

HELPFUL HINT on the HELPFUL HINT!! A plastic bag over the magnet will make removing the tiny filings from the magnet very easy.


Get your extension shaft ready.

If turning your own, made sure you're very accurate. Especially if the encoder wheel is press-fit.

You need to be smaller than the hole you drilled, but not smaller than the bore needed for the encoder.

I found that at 0.05mm above or below 4.0mm, the shaft would jam in the drilled hole, or be too loose for push-on encoder.


Clean the new shaft & hole. Put a drop of super-glue in the shaft's drilled hole and insert the shaft.

If a little glue doesn't ooze out, remove the shaft, and add another drop.

Refit the end plate (brushes not needed) to help judge the concentricity of the new shaft.

Spin the motor (manually is fine) and watch the new shaft. If it's off center, give it a slight nudge.

Let the glue dry a little (not fully), and re-align the shaft.

Allow the glue to cure.


Center the encoder's PCB.

The US Digital encoders have a placement tool, a tube which has an internal diameter equal to the shaft size. The tool's ends have a slight taper to center the board.

  Mark the centers of the screw holes on the end plate with a punch.

Drill and tap the mounting holes.

In my case, I drilled 2mm holes and tapped in M2.5 threads.

  Mount the base & PCB. Center with the centering tool.
  Insert and tighten the screws, and then remove the centering tool.

Mount the encoder wheel.

My encoders are the E4P push-on type. The US Digital encoders have a tool to push the encoder wheel on just the right amount. (The tool stops on the base tabs)


The encoder wheel is now fitted. Time for a final inspection.

My shaft was a little short in this case, but the wheel doesn't carry any load, and has a good grip on over most of its length.


Install the top cover, and you're done.

E4P - US Digital E4P encoder (US$20)
300 - cycles per revolution - 1200 pulses
157 - 0.157" (4mm) shaft
M - metric screws



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CNC | Electronics | Mechanics

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Name of author Ivan Hamilton
"My inner nerd can beat up your inner nerd."

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