Reverse thread cutting

Gentlemen,

Having read the post regarding the Mini lathe repair there was mention of "reverse thread cutting" can anybody, Jason , please explain this method as I think it would be of use to me particularly as I cant disengage the lead screw and I'm constantly in fear of running into the chuck, are there any pictures to help the little grey cells as well please.

Martin P

Edited By martin perman on 21/12/2018 12:21:20

This vid as a demo of cutting an internal thread away from chuck.. same sort of process for external- reverse the lathe and cut from the back or with an inverted tool from the front.

https://www.youtube.com/watch?v=k1DHKjMtdQw

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.

So do I assume the chuck runs backward and the tool is upside down or is the tool mounted on the other side of the work piece. Not so easy to imagine.

Martin P

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

If your lathe can't run backwards or you are worried about the chuck coming loose you really should fit a simple electrical stop switch to the motor that is pushed by the carriage. If you have a lathe that has interlocks like for a chuck guard then check it uses low voltage and wire a plain micro switch into that circuit. If you only have a mains on off control get a sealed switch. google 'limit switch'

The Boxford had this as a standard accessory to protect the school lathes from careless children.

Thank you Jason, a picture speaks etc, Bazle, my lathe can run in reverse and the chuck bolts to the spindle back plate, this would make thread cutting less panicky as I dont have split nuts and no thread dial.

Martin P

Edited By not done it yet on 21/12/2018 14:43:00

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

That is already planned as the first second mod to the SC4!

Neil

Which lathe do you have Martin

It should be noted that external and internal full form threading inserts do not necessarily produce the same form.

Of course only a dedicated follower of over-complication would know about that sort of thing.

Andrew

I have a Clarke CL500M which does nicely for me,

Martin P

Andrew, if you happen to know any people like that you may want to let them know that if you put a full form external LEFT hand insert into a RIGHT hand internal holder you can use it at the back of the machine and get the correct profile, though I expect they know that already

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

I used to have serious trouble unscrewing my old ML7 chucks, always needed spindle locking down and a hard sharp smack on spanner or gripped bar in the jaws.

I often turned in reverse, machine cut better that way.

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 Bob,

As far as parting off with a rear mounted tool post, just invert the tool and run in the conventional manner.

I do it that way on my Boxford.

Steve.

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.

Hi Bob,

I never have to tighten the rear saddle clap screws when parting off with HSS..

I purchased a parting tool from JB cutting tools at the midlands show this year. My Boxford can easily part off in the conventional manner when using it. I have yet to try it in the rear tool post. I think this would probably be the best set up though.

Steve.

Edited By fishy-steve on 22/12/2018 17:31:31

Edited By fishy-steve on 22/12/2018 17:31:48