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Internal expanding collet for watchmaker's lathe

In a previous post/page I detailed the setup for face grinding gears and referenced the custom internal expanding chuck I made to accomplish holding all the gears for the operations. Below are some of the details of making this collet.
Internal expanding collet for watchmakers lathe

Because I've covered the machining of a basic collet to fit the watchmakers lathe elsewhere on this blog I won't repost the steps to create that shape. Here is the making of the expanding portion of the collet.
Once the basic collet shape was formed with a lengthy portion to extend beyond the headstock within which I would cut the four fingers and bore out the expanding chamber for the mini-drawbar to pull into.
boring out internal expanding collet

I shaped the mini-drawbar with a 30 degree taper on the end as I wanted it to cinch tight quickly. Since it has to slide through an 8MM collet, the threads on the end are not large (8-32).
Matching the draw bar for the internal expanding collet

I cut the fingers on my mill drill using a slitting saw and a spare headstock for the lathe that I could simply install the collet into and use the basic dividing holes in the pulley to rotate the cuts for the four quadrants.
Cutting the expanding fingers for the collet

The drawbar fits nicely into the collet and there is a smooth nut to draw the bar into the finger taper that is slightly smaller in diameter than the minor diameter on the collet. The main drawbar has to fit over it with no interference.
Smooth nut on draw bar clears collet threads

Here you can see it doing its job mounted in place on the lathe. The smooth nut and aggressive drawbar taper don't need to be registered in any way as the pressure from the taper quickly binds it fast and a half turn of the draw bar exerts a great deal of pressure to grip the inside of the 3/8" ID on the gears. Success!
Internal expanding collet at work for grinding gears

Off to face grinding a lot of gears!
Lots of gears to face grind

A very consistent grind!


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