have you not set the top/compound slide to 29.5 on the scale? Surely it should be 29.5 to the lathe axis, which should read 60.5 on the scale?
You are quite right Ned, but there are actually two ways of doing it, the way John has shown, which gives much better control on the depth of cut, and to me, if doing super fine threads, I would use, but only with 60 deg thread forms, the sidewards feed is a little too much for 55 deg or lower threads, and you risk actually cutting the threads away and so going undersize on the OD.
Or the normally accepted way, which you mentioned, and gives a more aggressive depth of cut, but less sideways feed, and is the way I would use most of the time....
John
John, I'm feeling a bit stupid, because I don't understand that
. I can see that it gives finer control over the depth of cut. But I should have thought that increasing the depth of cut using a topslide set at just over 30 deg to the lathe axis (rather than just over 60 deg) would result in a thread with its LH flank correct but the RH flank wrong - a lopsided 90 degree thread. If it's not too much trouble, would you explain a bit further so I can grasp how it works?
On the subject of threading, it might be worth mentioning G H Thomas's method of getting to the right depth when using an angled topslide, for anyone who doesn't already know it:
1. Feed cross-slide forward until tool just touches the work (nip cigarette paper if desired). Set both the cross- and topslide dials to zero.
2. Move saddle to the right, so tool is clear of the work, advance the cross-slide by the depth of thread, and reset its dial to zero.
3. Withdraw the tool using the topslide, move saddle to the left, advance topslide until tool just touches the work, and then start cutting the thread in the usual way, applying depth of cut using the topslide. Unless you have a swing-up toolholder, pull the tool clear of the work with the cross-slide when traversing back, and re-advance the cross-slide so its dial reads zero before applying more depth of cut with the topslide for the next cut.
4. The thread depth is correct when the topslide dial has got back to zero again. Of course, if the thread is a deep one, the topslide dial may need to go round more than once before you get there, but when passing through intermediate zeros it will be obvious that the thread is only part formed.
This avoids resorting to things like trig or Pythagoras; I always was a bit of a duffer at sums.
Andy
