The incredible plastic mill gearbox

Believe it or not, but all the gears in the little (Chinese) hobby mill I bought this summer were of PLASTIC!

Doing some work in cast iron, the boring tool suddenly jammed, and I heard a grating sound, the mill motor suddenly started spinning wildly while the mill itself stopped dead.

Upon investigation, opening up the gearbox, I found that all the gears were mashed - the largest had lost one tooth, the smallest was in several pieces, see the photo above, which also shows some of the replacement gears I made of steel.. The cracking of another small gear had of course started at the key slot (photo at right).

I checked the original gears inside the gearbox - they appear to be modulus 2.25! Strange - or is it an imperial gear pitch?

Since I could measure the axle distances, I could plan a new gearbox with modulus 2 gears. Fortunately, such gears in different sizes are kept in stock by wholesalers and are thus readily available. So, I spent about 70 euros (same in $), buying six steel gears, and turned them to the right size, combining them two and two as seen in the picture above.

The hardest job was the keyslot in the steel wheels - I used a hardened piece of gauge plate clamped to the disassembled mill head, as seen on the left. Cranking the mill head up and down, and slowly advancing the table a thou at a time, I could use the disabled milling machine as a manual, vertical planer...

Now, my mill is operating again, with all steel wheels. I have installed a shear pin of tubular brass in the gearing (see picture below), so that there will be no extensive damage if a tool should happen to jam again. The pin is easy to exchange - I just have to remove the 4 screws that hold the motor in place to access the pin. The gearbox could be accessed only by disassembling the entire mill head - something I loathe do do again!

Here's the technical info for the gears I used, if you are contemplating a similar modification:

Motor axle gear: metric modulus 1.5 / 14 teeth
Outside gear (with shear pin): modulus1.5 / 30 teeth
Gearbox 1st axle: modulus 2 / 14 and 22 teeth
Gearbox spindle axle: modulus 2 / 32 and 24 teeth

With these gears, which are perfect for the existing axle distance, the gearing ratio is a little different to the original, but not more than 10%.

I was surprised that the noise level of the mill didn't increase too much - but then, it was very noisy to begin with!

Isn't it pretty surprising that a metalworking mill has all gearing made of plastic... or, perhaps these Chinese tools (er... toys) are only made to be sold, not used??!

Close this window when you are ready...

Any information presented on this website (especially any do-it-yourself instructions) is given without any acceptance of liability for damage or injury - so, always remember: SAFETY FIRST!

The material on this page and its related pages is Copyright © 2001-2007 by J-E Nystrom. You may NOT copy, transmit and/or publish any of my images or texts in print, electronically, on your own website or in any other way. The author retains all rights to this work, with this sole exception: Storing the pages on your own computer or printing out a paper copy, for your own, strictly personal use is allowed.

You may, however, freely link to the "Building Live Steam Locomotives" page at:, or to my Animation Home Page at:

You should NOT link directly to THIS page, since it's address may change in the future. Also, you may not put any of these pages or pictures into "frames" on your own website.

Thank you.