Threading...plunge straight in

[winklmj]

So I'm working on a replacment axle for a bicycle hub.

Broken:


Turning between centers with a ball-bearing center. It gets in the way of angling the compound over typically recommended for thread cutting. So...for the 12x1mm thread I just plunged straight in 2-3thou per pass. It turned out OK I think. Is there anything wrong with doing it that way all the time?

Start of new one:

[doubleboost]

Hi
No problem on fine threads
It is on heavy threads you need a angled top slide
John

[andyf]

I think one objection to going straight in is that a chip is peeled off by each flank of the tool, both chips try to occupy the same space and wedge up. But it obviously worked for you.

A half-way house, with the tool set at right angles to the job can be achieved, if the topslide is set parallel to the bed. Each time the cross slide is advanced to deepen the cut, the topslide is advanced towards the chuck by half the amount by which the cross slide was advanced. That makes the leading flank of the tool do most of the cutting, as with the angled approach. It does need a bit of concentration (for me, anyway) to keep track, and if the cross slide dial is marked up in "reduction in diameter" rather than actual depth of cut, then the top slide advance should be a quarter, not a half, of what the cross slide dial shows.

Andy

[AdeV]

I've always threaded straight in, usually because I've started before I remember you're supposed to set the cross-slide over... The main thing is to take ever shallower cuts, as you approach the final dimensions; and I mean  0.001" or less; the amount of material removed even on such a shallow cut is quite remarkable, as you're approaching the thread's depth.

If you go too deep on the final cuts, you'll break your cutter. DAMHIKT.

[arnoldb]

Mike, there's nothing wrong with plunging straight in; just take smaller cuts as the groove deepens.
I've cut from 0.5mm pitch trough to 8tpi ACME form like this; my lathe's topslide can't set over enough to use the set-over method.

Andy's suggestion is good as well - but the 1/2 feed method on the parallel top slide is only correct for metric and US threads - for Whitworth you need to feed less with the topslide.

 , Arnold

[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.