Mini-Lathe 21 tooth change gear.

I received a 21 tooth gear in my mini-lathe upgrade gear stock shipment by error. At first I didn’t know where it could be used. None of the stock gears in a mini-lathe are 21 tooth. (See picture).

The upper 20 and 25  tooth gears are a part of the reversing system. They are small bore with no key way.

The lower two 20 tooth gears in the picture have a larger bore and a 3mm key way. So do the 21 tooth gears I accidentally received. I thought the 21 tooth gear may have something to do with metric threading.

I did some research and I found the identical 21 tooth gear recommended for replacing one of the 20 tooth gears to get closer to cutting certain metric thread sizes. Which one depends on the thread desired. Only one (1) 21 tooth gear is required. Both 20’s are used for some threads.

The mini-lathe because of it’s 16 TPI lead screw can never make all metric threads exactly on pitch.

So the mystery gear is no longer a mystery. The original 20 tooth gears are steel so they are not replaced in my upgrade kit.