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Neil,
Almost all cutting on the lathe is a tearing action !
The trick is to have the right cutting geometry on the turning tool to match the material being used and the speeds and feeds to suit.
Every material and tool combination will have its own best range of surface speeds for cutting and different tools will have different feed rates that work better than others.
The use of lubrication (or not) will make a difference.
So rake angle, clearance angle, cutting edge sharpness, type of chip breaker, type of tooling (HSS, carbide) and significantly the coating type etc etc all make a huge difference.
The difference can border from impossible to easy, super smooth to full of chatter and rough torn finish.
You would not use the same geometry and tooling by choice on aluminium to stainless or plastic to cast iron !
Some materials are 'sticky' and some are not, so the tool you are using totally makes all the difference.
Ensuring that any swarf does not get caught back up into the cut will make a big improvement.
It should be possible to cut a high quality thread using the right machines tools and technique.
Taking off too much in a cut can result in a rough surface finish, but likewise, taking off too little may result in a poor cut.
Most sintered carbide insert tooling likes to take a heavy cut whereas a sharper tool will work fine in a fine cut.
The rigidity of your machine and the tool setup as well as backlash in your slideways and thread cutting lead screw can make all the difference.
If your machine has lots of backlash and isnt stable, then the heavier cut may cause chatter and result in a poor finish.
Try to ensure that the tool is as short as possible.
So I am wondering if the technique you describe is more about trying to overcome some machine deficiencies than a method to get a good thread cut.
Another technique is thread rolling, this deforms the material under pressure rather than cutting.
You can also mill a thread with a rotating milling cutter - but you'll need a CNC mill to do that effectively.
Very high quality threads are actually ground.
Hope that helps rather than confuses, but sometimes its a non trivial affair, that requires a bit of experimenting to get the best finish.
Cheers
Rally
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