I have tested rotary drives with both gear reducers and belt reducers. And while the gear reducers had sufficient holding torque the backlash was not acceptable. The belt driven reducers had very little backlash, but has very little holding torque.


I recently purchased a 50:1 harmonic rotary drive kit from Amazon for $430.

50:1 Harmonic Rotary Kit

The kit consisted of the following:

  • Sanou K11-100 3 Jaw Chuck

  • Narrow Jaw Set

  • Wide Jaw Set

  • Chuck Key

  • Live Center Tail-stock

  • NEMA 23 3Amp Stepper Motor

Checking Runout

Since the drive is not yet connected to any of my CNC machines, I chose to use my stepper motor tester for the runout tests.

Th first thing I did was to check the runout.

Shown in this video I made, the base plate has a TIR of .0006”.

I could probably improve on this if I was willing to tear down the drive, but this is acceptable for my needs.

I added the three jaw chuck and then chucked up a router calibration blank. This produced a runout of about .009”. Something to note. The same chuck mounted on my lathe has about .002” runout.

The base plate on the harmonic drive does fit a little too tight for my tastes. You have to use the mounting bolts to pull it into place, and use a hammer to tap it back off. All my chucks have the same issue when mounted on this harmonic drive.

I plan on using the drive to mill four sides of a piece of square stock, and on my plasma cutter to cut features on both sides of a piece of steel angle.

I needed a 4 jaw self centering chuck for my fixtures, so I ordered a K12-100 from Amazon.

Self Centering 4 Jaw Chuck

It came with both narrow and wide jaws and a chuck key.

I paid a little more for this drive ($150) as the seller was located in the states and had them in stock. I would have preferred a Sanou K12-100, but there was about a month lead time for them to arrive from China.

The drive is a little rough as the chuck key can be a little stiff as the jaws are tightened. It is loosening with use though, and It fits the harmonic drive. (like the others a little tight)

I will primarily be using the the wide jaws to hold fixtures and square stock so I checked the runout on them first.

Here I chucked up a machined collet nut used for my collet chuck.

It gave me .005” TIR which is acceptable for my uses.

I installed the narrow jaws and chucked up the blank.

The narrow jaws were much more accurate with about .001” TIR.


Is this $430 harmonic drive as good as a $2000 harmonic drive? No it is not. But I would not be willing to put a $2000 drive near my plasma torch. Once I get the drive connected to one of my CNC machines, I will run some repeatability and backlash tests.

The drive does exceed my expectations. I like that the drive has a generic 4” back plate with both 3 and 4 mounting holes. It is going to be fun trying out some new operations that I was not able to perform until now.

As I continue my tests and experiments, I will post them here.