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Metric Thread On Imperial Lathe

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RichardShute:

--- Quote from: BillTodd on December 19, 2010, 12:52:38 PM ---<....>


If the angle is greater than the thread angle then the trailing edge is cut by the tip of the tool, (as the back edge clears the face) and is thus defined by the angle of the slide.
<....>
Bill

--- End quote ---

Bill,
your first drawing is essentially the same a mine. I showed the saw toothing as a result of each successive cut taken - rather coarse I accept, but it was intended to accentuate the effect. I agree with your other two drawings, I was just too lazy to bother....

Richard

BillTodd:

--- Quote ---your first drawing is essentially the same a mine.
--- End quote ---
Yes it is, Richard. Sorry I missed that first time through :)

Of course, all this means is that me and every other Hardinge owner (the ones that follow the manual anyway) have been cutting slightly incorrect threads  (albeit with the greatest of ease).

Bill

Ned Ludd:
It is regrettable that so few books show that what they mean by "1/2 the thread angle" does not in fact relate to the markings on the cross slide's quadrant. The half angle must be subtracted from 90 and that figure set on the quadrant. This is one of the problems of self taught machinists working in their sheds alone. They have no one to point out elementary errors, whereas those lucky enough to have mentors are blessed.

I must also point out it is a bit of a miss-perception that going straight in (not the set over top slide method) loads the tool on both sides. It does no such thing, once the cut is under way. Cutting a thread is no different to an ordinary sliding cut, just deeper, think about it! For an ordinary RH thread, the tool will only cut on the left side. The only time a threading tool would cut on both sides is when you do a plunge cut. As the old saying goes "unless you know different".
Ned

Pete.:
Actually now I think of it what you produce using a 30°-off-parallel angle for the compound is a saw-tooth pattern on the right flank of the thread because the angle you're advancing at is shallower than the angle of the cutter. My CAD skillz are lacking so I'll knock-up a quick pencil-sketch.

Pete.:
Ned I don't agree, if the tool is not loaded both sides what is cutting the right flank of the thread? I see feeding directly in exactly the same as plunge cutting except that you are continuing the plunge cut along the length of the thread, a 'running plunge cut' for want of a better term.

Anyway, here is my crummy sketch depicting the result of feeding in at 60° off perpendicular.

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