Author Topic: Drawbar - different threads at each end ?  (Read 250 times)

Offline Boo

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Drawbar - different threads at each end ?
« on: January 16, 2023, 06:12:46 PM »
Hi,

I amd just about to make a drawbar for milling cutters etc in the lathe and remember reading that it can be beneficial to use a different thread at the nut end from the tool end in order to reduce the chance of the drawbar unscrewing itself in use.  My questions are:

Is this really a thing ?

Should the nut end be finer or coarser than the tool end ?

I have an M12x1.0 metric fine tap as well as the normal M12x1.75 taps for making the nut so could do either.  The nut is a casting from Hemingway.  Tool end must be M12x1.75 to suit the cutter holder.

I'm sure I'm overthinking this but if it makes a difference I might as well do it as not.

Thanks,

Boo


Offline awemawson

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Re: Drawbar - different threads at each end ?
« Reply #1 on: January 17, 2023, 03:18:45 AM »
I made the (7/16 UNF) drawbar for my Bridgeport probably 25 years ago. Nut end is (I think) a half inch whitworth  thread and itís never unscrewed in all that time.

I suspect I used Loctite on it. It must have been a decent bit of steel as itís still going strong.
Andrew Mawson
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Offline Sea.dog

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Re: Drawbar - different threads at each end ?
« Reply #2 on: January 17, 2023, 03:23:52 AM »
I turned mine from hex bar,m so not an issue. You could through drill and pin the nut.

Offline Boo

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Re: Drawbar - different threads at each end ?
« Reply #3 on: January 17, 2023, 06:46:59 AM »
Hi,

Thanks for the replies, I should have said that the tooling end is MT3 which has 2 different threads applicable and since the nut is a cast item from Hemingway, I wanted to use the same nut with two different bars having the same thread at the nut end and M12 or 3/8Whit on the other end.

So, really I just need to know whether thre is any advantage of using a finer thread at the nut end than at the tooling end, and specifically whether M12x1.0 is OK or should I stick to ISO M12 ?

As I said, I'm sure I'm over-thinking it :-)

Thanks,

Boo
« Last Edit: January 17, 2023, 03:15:31 PM by Boo »

Offline John Rudd

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Re: Drawbar - different threads at each end ?
« Reply #4 on: January 17, 2023, 07:06:38 AM »
The drawbar on my mill, a 626, has an M12 to secure the mt3 chuck, the tightening end has a nut on it ( threaded but not known what thread) and is cross drilled and pinnedÖa nip with a spanner sees it tight enough.

Personally, I wouldnít worry whether itís M12 fine or standardÖ
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Offline AdeV

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Re: Drawbar - different threads at each end ?
« Reply #5 on: January 17, 2023, 01:22:26 PM »
Hmm... I've had some ropey old collets in my time, would be pretty typical for me if the top nut unloosened (Joe Pie ref there :D) leaving the collet firmly attached to the drawbar...

Like Andrew, I have a Bridgeport, and I'm pretty sure the drawbar was turned out of a single piece of steel. Other than a mysterious incident where the drawbar grew by an inch one night (seriously - I had to turn a collar to make it work again, which I've had to use ever since), I've never had the top "nut" come adrift.
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