I'm no expert at screwcutting, but I would leave the tool as normal rather than invert it, run the lathe in the conventional manner and cut to a relief at the end of the thread……back to basics as they say…Set the tool at right angles to the work, with the compound slideoffset to 30 deg if its metric, adjust DoC with compound and back tool out using crossfeed..

So,

Rather than blame your technique, what material are you practising on?

Are you threading using an imperial or metric lathe, is the leadscrew engaged all the time or are you disengaging it? Using a thread dial indicator? ( I cant remember if the leadscrew remains engaged for m or imp….) Tool on centre height?

Edited By John Rudd on 05/03/2019 16:49:10

two thoughts

1. If it's an imperial lathe and you are cutting metric threads don't disengage the nuts.

2. Make sure you have the topslide at 30 deg (or 29.5) to the face of the lathe not the central axis

Edited By JasonB on 05/03/2019 17:34:46

Sounds like the thread dial is being misread, or it needs a different pitch wheel to be fitted. (Not all lathes support this.)

Thread dials work best on imperial lathes doing turns per inch. Metric threads are based on pitch, which isn't quite so user friendly.

On an imperial lathe doing metric the thread dial is meaningless, so always maintain thread/cutter alignment by keeping the leadscrew engaged at all times.

On a metric lathe, the thread-dial instructions need careful study and the dial may not cope with all pitches. Missing pitches are made by leaving the leadscrew permanently engaged. This is the general method that always works.

On a metric lathe the pitch wheel under the thread-dial engages with the leadscrew to show alignment as on an imperial lathe. Unfortunately some pitches need a different dial to leadscrew ratio, which may not be available. The thread dial on my metric WM280 has only one pitch wheel, and its thread dial can't be used to make some common threads, boo hoo. From memory my metric mini-lathe had 3 pitch wheels and did more pitches. Provided you remembered to set it up right…

Usually I'll tell anyone not already dead from boredom that metric is immeasurably better than imperial. Cutting metric pitch threads on a metric lathe is so awkward it must be the kludge that proves the rule.

Dave

Torn threads and multiple starts are probably caused by different issues. First we need to sort out the multiple starts. More information is needed:

What is the OD and pitch of the metric thread to be cut?

What lathe is being used, and is it imperial or metric?

Can it cut the required pitch?

If it can, be aware that higher spec lathes have multiple gears on the thread dial indicator – you need to use the correct gear for the required pitch

Note to SoD – hooray for imperial and tpi

For the torn threads, again more information is needed:

Is the tool HSS or carbide threading insert?

If an insert where did it come from? Hint: bang good is the wrong answer.

What is the material, and where did it come from, ie, known specification or gawdonlynose scrap?

A picture of the setup would be helpful.

Andrew

Are you using any cutting oil?

Martin

One item not mentioned here – I am no expert and you probably know this – is depth of cut which for Metric is 0.6134 * pitch with 60 degree tool

One draw back of the inverted tool is that the cutting force is upwards. This would be lifting the saddle instead of pushing it down. You would need a robust machine and well adjusted gibs.

That could explain the poor cutting as your tool bit will be jumping up and down.

Think most are trying to baffle him, start logically keep compound 90 degrees on and do it the way the pros do.
Will then iron out and or eliminate that way of doings things.

Rest as Andrew said yesterday.

Err just checking….when you inverse tool…..did you check tool height…. don't ask

Strange how everyone and their dog seems to suggest inverted parting tools when the subject comes up but then says it won't work for threading?

I' quite often turn large flywheels with an inverted boring bar where load is far greater and get perfectly good results so don't think that's an issue unless the lathe is really worn

On behalf of my dog I can categorically state that he has said nothing of the sort. Parting off right way up from the front works fine. However, I run tools inverted on the hydraulic copy unit since it is mounted behind the work.

Andrew

41 x 1.5mm threads from the same bar under (more or less) the same conditions:

No prize for guessing which was the bang good insert. I was surprised because the bang good looked perfectly OK…..

I know many people have had good value from bang good, but in this case it didn't work – a bit of a lottery, which isn't what you want when you're trying a new technique and unsure if it's you or the tool.

Robin