Stamping on the brake - will I destroy the (lathe) gearbox?

I'm trying to cut some imperial threads on a lathe with a metric leadscrew. AFAIK there is no way of doing this without keeping the the leadscrew nut engaged throughout the process. Because I have limited space for relief on either side of the thread I've been stomping on the emergency footbrake at the end of each pass, which brings the chuck to pretty much a dead stop. I think I've read somewhere that doing this puts great strain on the gearbox though. Is this really a problem, or (given that the chuck is only doing 65rpm) is it one of those things that is theoretically inadvisable, but can be gotten away with without dire consequences? Lathe is a generic Far-Eastern 12x36, and I have no idea where in the transmission the brake acts.

Regards, Robin

Brake acts on the input pulley and at 65 revs you won't harm anything.

Stomp away.

Thanks, that's what I hoped to hear. My foot-to-eye coordination is improving daily!

Robin

If it has a thread dial on the lead screw, the half nuts can be released. They must then be re-engaged as the lathe is started in reverse, as the dial passes the number it was cutting at. A s

imple alternative that avoids any drastic braking.

Whether or not the brake acts before, within or after the gearbox is bound to increase momentary loadings, which cannot be seen as the most ideal mode of operation. As an extreme, a gear could fail, but unlikely at that speed. Won't do the brake any good, either, I would think.

Not when he is cutting imperial threads on a metric lathe, surely? The leadscrew could be one or more full revolution out of sync and the dial would read the same number it was cutting at, but the tool would be one metric leadscrew pitch too far in either direction, which is not going to pick up the imperial thread.

Edited By Hopper on 02/11/2016 06:42:08

if the lathe has reverse and a rear toolpost you can cut a thread in the "unscrewing" direction which saves the panic to stop the lathe. The tool needs to be upside down and even if between centre turning there is usually more space at the right hand end to bring things to a gentle halt.

(Edit: sorry missed the bit about limited space either side of the thread. I'll get my coat)

John

Edited By Journeyman on 02/11/2016 09:02:41

Whatever threads I am cutting I always reverse the lathe with half nuts engaged. I know I should really fix the hesitant to engage half nuts but it works for me. Threading up to a stop/shoulder you need to be quick to wind out and stop the lathe.

Thats the official way of cutting non-native threads on a Smart & Brown 1024 VSL. Albeit via the big red button rather than a foot bar. So nowt wrong with the principle unless serious engineering cock-up has been made so the brake engages before the motor drive drops. On my 1024 there is a very short, but distinct, pause after the motor power is cut before the brake hits. About 1/2 turn of the chuck at the 55 (ish) rpm I was using to do 14 tip on my metric 1024 yesterday.

Motor may be a bigger issue. Single phase ones don't like too many starts in quick succession as the starter winding doesn't get a chance to cool down and can eventually overheat. Even high quality ones tend to be specified for around 10 or less starts per hour. In practice how bad too many starts is probably depends on loading. I guess the book refers to the motor being used at its rated start load capacity so the starter winding would be working hard. Lathe spins up under pretty light load viz:- chuck, work, drive and gearbox innards. Not insignificant but nothing like full motor capacity.

This is the one time where VFD driven three phase motor is not an advantage as you need a finite time to spool down the motor unless you want to trespass on the overload safety margin by simply dropping the input contactor out. Can be OK if you have a decently oversize VFD with a inherently good overload capability, like the Whole Shop set-ups Drives Direct sell, but pretty much a no-no for a matched VFD & motor combination.

Clutch is nicer. I keep wondering if there is a way to get one on my 1024.

Clive.

Not sure I'd want to use the VFD input contactor to stop the motor. But logically, how would this be quicker than decelerating the motor via the VFD? If it will produce an overvoltage internally during decel, then achieving an even quicker stop by opening the contactor would risk damaging the VFD? And you wouldn't want to disconnect the motor under load - and even if you did, it would take ages to spool down.

With a VFD, optimising the decel ramp parameters, enabling DC injection and possibly fitting a braking resistor (if the option is available in the VFD) would be the way to go.

TNot when he is cutting imperial threads on a metric lathe, surely?

It does not make a jot of difference whether metric to imperial or vice versa. One is only reversing the screw until the same point on the dial, the rest of the reversing is with the half nuts engaged. It is simple and it works. One does not leave the lathe running for an unlimited time, so reversing the screw after stopping the lathe immediately simply drops those half nuts in exactly the same place as a few seconds before. Try it. I think doubleboost has demo'd it as he was as sceptical as I was (had always been told you could not disengage the half nuts, even for an instant, during a threading operation such as this). Think of actually stopping the lathe, disengaging the half nuts and re-engaging. Not a lot of difference, really, but an extension of just a few turns of the chuck past and back.

How to disengage and re-engage the half-nuts when thread cutting metric on imperial (and vice-versa)

https://youtu.be/HXt4TWa382Q

https://www.youtube.com/watch?v=HXt4TWa382Q

(I still can't add a youtube video to a post, despite following the instructions about using the embed code. I think some things are made deliberately obtuse by spotty geeks)

Edited By Steve Pavey on 02/11/2016 15:17:44

Edited By Steve Pavey on 02/11/2016 15:19:12

Since fitting a variable speed drive to my Warco BH600 I find that I never bother to disengage the half nuts when screw cutting. Just slow down and stop the spindle when the cutter reaches the desired point then retract the cutting tool to clear the threads then reverse the lathe before advancing the cutting tool again.

> With a VFD, optimising the decel ramp parameters, enabling DC injection and possibly fitting a braking resistor (if the option is available in the VFD) would be the way to go.

Probably. A braking resistor to suit a hobby sized lathe won't 'brake' the bank, one for my 0.4kW inverter is 100R 200W and costs lest than £15.

Neil

Thanks, Steve Pavey. Clearly more than one u-toob to demo this. Now, even more of us can see that a brake is simply not required when threading to a shoulder - as long as the lathe has a threading dial. My original lathe (for about twenty years) didn't have a thread dial so I got so used to not releasing the power feed and didn't give it any thought until I was enlightened by u-toob.

Thanks for all further comments, but especially to not done it yet for pointing out that it is in fact possible to make use of the thread dial indicator thing in theses circs, and to those who further elaborated on the technique - I had believed that it was necessary to keep the half nuts engaged all the time, but obviously not.

This is to do with my (so far abortive) attempts to cut a (LH) 5/8" x 4tpi (so non-standard) ACME form external thread - because it's such a coarse thread in slenderish material, the work need all the support it can get at both ends, hence the limited space for the chuck to wind down in it's own time.

Robin.