He means starting where you would normally stop the carriage. Nothing more difficult than that to imagine….

Cutting the thread with the lead screw running in the opposite direction. Try it. Your only fear then will be running into the tailstock – if you bother to use it to support the workpiece.

You set everything up as per cutting conventionally but just mount the tool at the rear of the work (right way up) and run the machine backwards. It can be done with a conventional HSS bit or if you use insert tooling and don't need ctr support then the internal threading tool makes it easy to reach around the back without having to move the topslide rearwards. Cut the run out (in) groove as you would so that you can put the cut on and simply switch the lathe on with nuts engaged or engage the nut once upto speed if using the dial. For internal you either need a cack handed bar to cut at the back or mount it upside down cutting at the front.

Here is one I prepared earlier.

You set the leadscrew up for a standard right hand thread don't reverse it for left hand cutting. Basically by running in reverse you are following the same helix by unscrewing the thread rather than screwing it on.

Edited By JasonB on 21/12/2018 13:17:55

What ideas do the team have to lock the chuck onto the spindle nose when cutting in reverse or rear tool post parting off. Typically on a Boxford with screw on chuck.
BobH

Which lathe do you have Martin

Some of the later Eastern lathes (examples are:Warco BH600, BH900, Chester craftsman, Engineers ToolRoom BL12 – 24. And probably others) with screw on chucks have dogs which hook behind the flange on the mandrel, do that the machine can be run in reverse without fear of the chuck unscrewing.

Possibly, it may be possible to devise something similar for lathes without this feature.

Maybe even a small grubscrew and brass pad to bear on the thread, if space allows?

At times, I used to run my ML7 in reverse, having made sure that the chuck was firmly in place (not so firmly that it could not be removed without undue force) . But slowly, and without taking big cuts!

Howard

Nope, don't personally know anyone like that.

Surely a LH insert may not have the correct clearances? Not that I've got any LH inserts as thus far I've never needed to cut a LH thread.

Think I'll stick to the conventional method of screwcutting.

Andrew

hi Steve, yes, but you are "lifting"the saddle off the bed. On the Boxford, there is a rear saddle clamp, and a front saddle/ bed lock, but its a fiddle trying to tighten the rear cap screws when they are inverted and out of sight.

You would need to be threading very near to the chuck to be in any danger of running into it,no thread goes right up to the chuck.

I leave the leadscrew/halfnuts engaged all the time,while threading,using the angled top slide to set the dept and the cross slide to put on the cut. When you reach the end of your cut,wind out the cross slide and reverse the lathe back to the starting point,while bringing in the top slide a bit deeper. You get into the habit of using both of them together,and it works a dream.

Sparey was using this approach back in his day,and it still works.

The only time I have to watch it,is when I,m cutting large threads like 21/4"x 8 as on my lathe spindle,since the carrage is really moving and while I might not hit the chuck,I could run out of road,so to speak. Result is much the same. Chips might not be blue but the air sure is !

Lathes do not like things like that.

Another option, if you are afraid of running into the chuck (or shoulder on shaft) while screw cutting is to fit a crank handle to the spindle and turn it by hand.

Paul.