Gallery, Projects and General > How do I??

Threading...plunge straight in

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andyf:
It works for Whitworth, too, Arnold, according to Martin Cleeve's book. I don't think it would work for BA, though.

Cleeve also says advancing the topslide by 1/4 the advance of the cross slide will work for Acme and trapeziodal threads.

Andy

Lew_Merrick_PE:
Mike (& All);

The cutter forms the thread, not the feed.  The reason you want the cutter to (almost) follow one side of the thread is to minimize your cutting forces and maximize your chip flow.  The reason you angle your compound feed 1/2° less than the theoretically perfect "single side feed" is to assure that you did not go to far and leave an imperfect thread face on the "back-side" of the thread.  (In point of fact, there used to be an accepted technique to leave the "back-side" of the thread rough to create better locking forces.)  With a good protractor collar on a compound, you are still "eyeballing" the 1/2° increment.  As most people can "eyeball" a protractor reading to (about) 1/4°, this pretty well assures that your feed angle is not going to be greater than the desired included 1/2-included-angle of the thread.

Back in the days of high carbon steel cutting tools, another issue was maximizing heat flow away from the tip of the cutter.  That is not really an issue with modern tool steel and carbide cutters.  Additionally modern industrial lathes are a great deal more rigid than even industrial lathes made in the 60's & 70's -- another factor often overlooked.

However, when all is said and done, I would not recommend attempting to "plunge straight in" on any thread with a lower pitch than (say) 16 TPI (call it 1.5 mm pitch) for a single lead...

winklmj:
Thanks for all the comments. I think I will stick with the "straight in" approach for the finer threads. I use a hand crank as well on the short lengths so I can easily get a feel for if it gets too hard to cut.

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