Variant on Threading – More Detail Please

[DuncParticipant @dunc]

I was reading Mr Harold Hall's article "Machining Internal & External Fittings for a Lathe's threaded Mandrel" (MEW 155, Oct 2009) & noted under "The Thread" (page 14, centre column) that: For an internal chaser I use a tap having the required pitch…sets the cutting edge at centre height…diameter of the tap is unimportant…

The author continues with a discussion of the angle of the tap to the lathe axis & the tap's height in the toolholder relative to the centre line.

Being rather new at this I am not disputing the author's skills. While I can visualize that the tap's teeth (or more accurately, perhaps, the first full, non-relieved tooth) initially would have clearance this must become pretty dicey as the depth of the thread is increased. How is this avoided?

Finally, is the lathe carriage/tool bit combo driven under power by the leadscrew (change gears set to the desired pitch) or is the tool engaged in the work and the thread then allowed to drag the chaser/tap into the work? (leadscrew not engaged)

Thanks

[DuncParticipant @dunc]

[JasonBModerator @jasonb]

Yes you drive it as you would if single point cutting with the correct feed for the pitch. If you are cutting a thread that is very much larget than the tap then the shallower helix angle can start to cause problems but if you keep within reason then its not a problem.

The internal part of this 1/2"x40 was cut using a 5/16x40tap, click to see it full size

Edited By JasonB on 25/07/2012 18:26:48

[David LittlewoodParticipant @davidlittlewood51847]

Dunc,

I think Harold made these things clear in the article, with one slight exceptioon Yes, he says diameter of the tap is unimportant, but he goes on to say he uses the largest one which fits for maximum rigidity, and later on he explains that this gives a better match of helix angle.

There will not be a front rake clearance issue; the helix clearance issue is – as Harold points out – only an issue if the tap is much smaller than the bore, and is helped (if necessary) by setting the tap slightly high, as Jason said.

Yes, the cutting is done using leadscrew drive. The main point of using a tap is to get the correct thread form; this is more important for threads like Whitworth and BSF, which have rounded crests, less important for ISO metric threads which have truncated crests. As an alternative, you can use the right full-form screwcutting tip, but these are not cheap, and you are more likely to have the thread in your workshop already.

David

[Author]

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