metric thread cutting on old imperial (new visby) lathe??

[cba_melbourne]

It does not work so well when you need to thread a non native pitch, ie metric on an imp screw. You still need to position the start of the cut accurately, not easy whist running the lathe forwards. And you still need a run out groove to start in.
..............Ew

Yes, still need a "run out" groove to start. I leave the half nuts closed for non-native threads, so no difference in this regard.
Yes if it is a deep shoulder you need to stop before hitting it when reversing, then turn the chuck manually (or slow down with the VFD or use jog). Agree it is a bit a hassle with those lathes that do not have variable speed nor a back gear to slow below 100rpm. If its only a shallow shoulder, I retract the cross slide far enough to clear.

[PDW]

It does not work so well when you need to thread a non native pitch, ie metric on an imp screw. You still need to position the start of the cut accurately, not easy whist running the lathe forwards.

Dead easy - if your lathe has a clutch or foot brake or similar..... PDW

[RayG]

It does not work so well when you need to thread a non native pitch, ie metric on an imp screw. You still need to position the start of the cut accurately, not easy whist running the lathe forwards. And you still need a run out groove to start in.

Works a charm with a native pitch though....
https://www.youtube.com/watch?v=rjl8...5p4rkKjPAwVO8Q

Ew

Hi Ewan,

Enjoyed watching your video. Neat trick. I made up an adjustable stop, and run low speed to do the same job, but the advantage of running the other way is better finish due to higher speed.

Just to clarify something, you can do metric threads on an imperial lead screw and disengage the half nuts.... the trick is knowing when to re-engage them, in other words which time around on the thread dial is correct.. If I recall correctly I think Stuart did the maths on how many revolutions were required.... it got complicated real fast... in the end it was obvious that it was easier to leave the half nuts engaged and reverse back.

Ray