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Thread: 1/2-12 3mt threaded drawbar, convert to 3/8-16?

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    Supporting Member suther51's Avatar
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    1/2-12 3mt threaded drawbar, convert to 3/8-16?

    An old pre war British or European thread was 1/2-12. Some machines carried this over for some time such as my new to me 50's steinel sv4 mill. I thought that I could find some 3mt collets with this thread on the British ebbay, no luck so far. I am beginning to think that my best bet is to re thread the draw bar to the standard 3/8-16 thread. ANY input would be most appreciated. My initial feeling was not to modify the mill and modify a set of collets as the taps for 1/2-12 are readily available. All of the collets I have seen for sale are listed as hardened. With out a specific hardness listed I question whether I can re thread them in a simple manner with out anealing and rehardening. By using Tony Foale's mt adapter adaptation for reworking mt's i think that i can work the back side of the collets. Will just need a positive way to keep the collets enguaged in the adapter. Thinking a pointed set screw that just catches the edge of the collet will work. Thanks for reading.
    Eric

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    Supporting Member suther51's Avatar
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    Well right (I hope correct) turn in approach to this problem. Took the draw bar out of the machine to day. Looks like the right thing to do is to just make a new draw bar. Mabey eaven with an improvement. The bar is 1/2 by 19 3/4 long with a collar just down from the top. I'm thinking a piece of 4140 prehard with a machined grade 8 nut to make the collar welded on and threaded to 3/8-16 for the collets. As for the improvement there is just a 3/8 square up top for a wrench. I'm thinking a nut welded on may be more convenient, not so much for more torque but just may work better than a little square.
    Just have to see what time n doin will do.
    Eric

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    Supporting Member suther51's Avatar
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    First the nut on top is a no go, the spindle retainer nut would not fit over a larger diameter. Bought 12' of cold rolled bar, full length with a free transport cut was cheaper that a 24" price, go figure. Will be a good way to learn threading on my lathe. I did find a t handle female square wrench in the toolbox that will work good on the draw bar. Just need shop time and to get the mill home from bosses shop.
    Eric


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