Parting off using a powered cross feed

Without fuss so as to not get caught and receive a rollicking!

Andrew

Exactly, so if CNC machines part off under power do can you.

regards Martin

One advantage CNC machines have is that the feed rate can be selected to whatever you want within reason, whereas mechanical lathes are very limited. IIRC my S7 only gave two slow auto feed rates. With CNC I now generally use quite a low rate of 5mm/min but never bother with back gear, the cut is much better at high surface speed anyway. I'm sure I could go faster if I wanted to. I never understand why manual lathes still have mechanical auto-act when a little motor would be much cheaper and more flexible. Also why all the bother and expense of change wheels for threading, or worse a gearbox, when a bit of electronics would do the job better and cheaper these days. Doesn't have to be full CNC, but would result in a better and probably cheaper product.

John Haine, it's got to be cost hasn't it? My Warco lathe is so much better with an electronic leadscrew but the cost is way more than the few crappy change wheels supplied with the lathe as new.

Tony

I've often wondered why modern small lathes don't come with an ELS as standard or even as an option.

An aftermarket conversion is going to be far more expensive to buy than a manufacturer installing them by the thousand. The mechanical parts are hardly complicated after all.

CNC lathes not only part off under power but also manage to do it from a front toolpost, overhead toolposts set at 10 and 2 o clock positions and from a rear turret.

My problem with parting under power is that sooner or later there will be a slight growl. And unless you back off immediately there will be a loud and expensive bang.

Same thing seems to happen when parting off by hand, judging by some reports.

Andrew

Depends on the lathe, mine will make a gentle pop sound as the automatic clutch in the power feed disengages due to excessive tool pressure, which can be adjusted by a little knob on the carriage.

On the Warco I've used in the past, it would be an inexpensive but incredibly awkward to replace sheer pin.

@ Andrew: True, but I don't have to both stop the feed and reverse the slide at the same time. I don't know if my track record is better than with the automatic feed, but the illusion of control is worth it.

The quality of the tooling matters of course. I use cheap tooling in a cheap machine (Chester Crusader type), I suspect the aluminum headstock has too much flex.

On my Myford the belt slips (and it is in good condition).

Worse things can happen. When parting off a hollow cylindrical item I use a "catch" rod held in a Jacobs chuck in the tailstock. Once I failed to do so when parting off a cylinder about 38mm O.D and long. The part fell between the bed and a jaw of my large four jaw chuck. That produced a large bang but no damage. However I now do not run that chuck at high speed.

JA

Edited By JA on 11/04/2023 21:39:38

In answer to why do cnc Machines part off, industrial machines part off because they are designed to part off,centre lathes are designed to accurately turn parts from billets or bar pre cut to size,though of course the hobby lathe is expected to do many tasks.

Actually my point was that CNC lathes must by definition part off under power admittedly usually with flood cutting oil, sharp tooling and appropriate feeds and speeds. Therefore parting under power is eminently achievable. If you can’t get at least 2 out of 3 of these right you are going to struggle parting manually or under power.
regards Martin