I use the angled top slide method until the thread is at full depth. This might be a good point to mention the trick set out in G H Thomas's Model Engineers Workshop Manual (ISBN 1-85761-000- 8) for setting up to so the slides do the trigonometry to get the thread to the correct depth when using an angled top slide to deepen it. You work to a point where the dials read ZERO in the end.
1. Set top slide to almost one half of thread angle, usually to 29.5° (US/metric) or 27° (Whit/BSF). Zero the top slide dial collar. Feed cross slide forward until the tip of the tool just touches the work (nip a cigarette paper or bit of kitchen foil) and zero the cross slide dial collar. Now, the tool is touching the work and both dials read zero.
2. Move carriage to the right so that the tool is in clear air, feed the cross slide forward by the true depth of thread (but see A below) and set its dial back to zero again.
3. Leave the cross slide untouched, and withdraw by means of the top slide until the tool just clears the outside diameter of the work.
4. Move the carriage back to the left, and feed the top slide forward until the tool touches the work again. This determines the starting point.
5. Move the carriage back to the right to clear the work, advance the top slide slightly and make the first cut. Before traversing the tool back between cuts, use the cross slide to withdraw it in the usual way, then return the cross slide to zero and apply further depth of cut with the top slide. Once both dials read zero at the end of a cut, the correct depth of thread has been reached.
GHT doesn't mention the complications which arise:
A. Where the cross slide dial shows the amount by which the diameter will be reduced, rather than the actual depth of cut. Then, in step 2 above, the cross slide must be advanced by double the dial indication . For example, for a thread depth of 0.030”, advance the slide 0.060 as shown on its dial.
B. When the thread depth is greater than the pitch of the cross slide feedscrew (only 1mm on many small Asian lathes) so the dial has to go round more than once to withdraw when the thread is getting deep. It can get difficult to keep track, particularly if A above also applies. Consider a positive stop on the cross slide, or at least a temporary witness mark on its side at the correct zero point. Likewise, if the depth of thread is more than about 80% of the pitch of the top slide feedscrew, that may have to be turned more than once. But as the top slide will only be advanced, and in small increments, the thread will be obviously incomplete when an “intermediate” zero is reached, and almost fully formed as the desired zero is approached.
Andy