Maybe the original way was best.

[DavidA]

Here is an 'oops' with a difference. The difference being that a useful idea came out of it.

As some of you may know,  I retired about four years ago but occasionally get hauled back in to work if some one is off.  Well,  one of the lads managed to break his leg.  so I'm back for a couple of month.  I only work half days (mornings) and this suits me fine.  It also means that I can use the 'big' machines during the lunch break before I leave to do any jobs I have for myself.
After all the discussion on half-angle thread cutting over on the other thread I decided to have a go at it. On Monday I had made a dummy 8 tpi mandrel nose to assist in making bits to fit my lathe. So today (Tuesday) I thought it would be interesting to make another but this time set over the top  slide to half the angle and do it the approved way.
So, it's all set up. Lathe running,  just touch the tip to the job (30 mm EN8 bar), zero the cross slide.  Zero the top slide.  So far so good.
Set a slow speed (45 rpm) engage the feed and away we go.
I'm only cutting about 3/4 " long,  and have machined a run-out groove for the tip to stop in.
I keep the feed engaged all the time,  reversing to get back to the start.
All goes well.  But there is a potential problem. The lathe,  and old Colchester Triumph 2000,  has a lot of backlash in the cross slide wheel.Almost half a turn.
After about eight cuts I am into a rhythm. return to the start,  wind in to zero on the cross slide, add a few thou on the top slide, make the cut, wind out the cross slide a full turn and reverse back for the next cut.
Then somehow I lost concentration. On the next cut there was a bang and the tip went to meet it's maker.
I worked out  that,  due to the excess play in the cross slide wheel,  I had wound it in two turns.  I actually realised that the tip was too far in just as it was about to touch,  but it was too late to stop it.

So, my own fault.  But how to avoid this happening again ?
In future I would fix a stop on the cross slide once I had zeroed it so that I could not wind in any further than the one required turn. I could remove the stop for the last 'cleaning up' cut using the cross slide.

Of course,  I could simply go back to the old 'straight in' method that I have always used.

Dave. 

[andyf]

I sympathise, having almost done the same myself, tjough I stopped just in time.  I haven't made a proper stop yet, but I do put a spirit pen mark on the cross slide and saddle at the correct zero.
As the feedscrew on my Chinese lathe is 1mm pitch, it can be a bit hard to see if the marks are lined up, but it does help, especially if the thread depth is so great that I have to wind the cross slide back more than one turn to clear the thread on the traverse back.

Query: Is there any reason, apart from the greater number of turns involved, why I can't leave the cross slide alone, and just wind the angled top slide back far enough to clear the thread? Having only one slide to  about might be easier than two.

Andy

[hopefuldave]

Ouch - but we've all done it!

I guess you could retract with the,topslide, might be slower though?

My current lathe is blessed with a quick-withdraw on the cross-feed that repeats to well under a thou", that makes it much easier! Maybe one of the quick-withdraw threading toolholders would be good and save on tool edges? A stop works well, too, and isn't hard to add to a lathe and can even have a micrometer dial if you're feeling flashy

[DavidA]

Andy,

Retracting the top slide every time is probably going to be more prone to errors.  More turns equals more chanced to screw up.

In my case,  what I should have done is turn on the DRO and set it to zero when the cross slide was at zero. And used that to get back instead of the dial. It would then have been obvious if I had wound in past the original point.

Isn't hindsight  truly useless.

Hopeful,
I do have a micrometer set up on my own lathe at home as it has no dials.  Probably the stop will be better for this application.

Dave.

[andyf]

Good point, David - there'a a fine feedscrew on my top slide, too, so I'd probably lose count. What I really MUST do is make a swing-up toolholder, so I can leave the half nuts on and run back under power without touching either slide, then just advance the angled top slide (or the cross slide if I'm going in at 90°) for the next cut.

Memo to self: get on with it 

Andy

[DavidA]

I like that idea.  Seems like the ideal answer.

Dave.

[Jonny]

Just use the DRO, works for me.

Dro or no dro internal threading have clouted opposite wall with tool, too true you get in to a rhythm.
Or mistook say 78.9mm and put 79.9mm on watching digits come down, works both ways.