Parting off using a powered cross feed

[Mick B1Participant @mickb1]

Posted by Clive Foster on 25/03/2023 09:45:28:

…

Unfortunately angling the tip produces a side force on the tool so it rubs disturbing the finish. All too often hobsons choice. …

Clive

Edited By Clive Foster on 25/03/2023 09:55:16

In deep parting cuts, angling the tip can also bow the blade sufficiently to concave and convex the sides of the parting slot, even on softer materials like acetal/delrin. That may mean an extra facing cut on the bar end before machining the features of the next component. You have to balance that against any advantage you might gain, such as a clean cutoff of the parted component, if it's small enough and has a through hole. Horses for courses.

[Chris CrewParticipant @chriscrew66644]

"I regularly use a hacksaw on chucked-up pieces in the lathe. The cure to the problem you describe is a very simple 2-minute job to make – a piece of wood with 'feet' either side to straddle the lathe bed. I use the same board to protect the bed when changing chucks etc."

The kangaroo Courts Martial has found you guilty of conduct unbecoming, LOL.  

Despite your precautions, I think here will be quite a few volunteering for the firing squad!

And, before anyone points out the contradiction in my previous comments, if it works for you who is to criticise?

Edited By Chris Crew on 25/03/2023 17:03:09

Edited By Chris Crew on 25/03/2023 17:10:53

[JAParticipant @ja]

Posted by Chris Crew on 25/03/2023 16:58:09:

"I regularly use a hacksaw on chucked-up pieces in the lathe. The cure to the problem you describe is a very simple 2-minute job to make – a piece of wood with 'feet' either side to straddle the lathe bed. I use the same board to protect the bed when changing chucks etc."

The kangaroo Courts Martial has found you guilty of conduct unbecoming, LOL.

Despite your precautions, I think here will be quite a few volunteering for the firing squad!

And, before anyone points out the contradiction in my previous comments, if it works for you who is to criticise?

Edited By Chris Crew on 25/03/2023 17:03:09

Edited By Chris Crew on 25/03/2023 17:10:53

Sorry Chris. I will not be in the firing squad.

I will happily use a junior hacksaw for cutting through the small final diameter sometimes left from parting off. This is done with two blocks of wood over the lathe bed and with the lathe switched off. I do not consider this bad practice.

There are far worse things you can do on a lathe. At work, about 50 years ago, we used a rather big lathe for measuring the break-out torque of a large pipe coupling. This was only matched be using a tensile testing machine as a very large vice.

JA

Edited By JA on 25/03/2023 19:04:02

[Mick B1Participant @mickb1]

Posted by Jelly on 24/03/2023 23:09:19:

Posted by Mick B1 on 24/03/2023 20:01:02:

Posted by Jelly on 24/03/2023 18:50:29:

Posted by Mick B1 on 24/03/2023 14:23:48:

I actually quite enjoy feeling the tool cut under handwheel control, and like Nigel McBurney above, have never seen manual lathes or capstans in production using powerfed partoff – my suspicion is that it's done mainly for bragging rights by the exponents.

That seems like an entirely unnecessary snipe aimed at making yourself feel big by doing others down for their perceived motivations rather than adding anything useful to the discussion…

I'm not bothered by return fire . I can't see a practical role for powered partoff in the sort of quantities typical in model engineering, and where other parts of the cycle aren't also automated.

I'll give you a very practical example:

Next week a friend is coming over with a short length of 170mm Hastelloy C22 bar, because he wants me to turn him two DN50 blank PN40 flanges.

There's barely enough material to make them and even if I could fit it on my bandsaw it would not cut well or straight, if at all.

So I need to take an 82.5mm deep parting cut through a notoriously awkward nickel-chromium superalloy…

That will stress the lathe enough as is, there's zero chance of me maintaining sufficient or consistent tool pressure and feed rate through a cut that long feeding by hand; at which point Bang! Broken insert, if not broken parting tool, and quite possibly a £900 piece of material ruined.

Well, you've picked a considerable outlier to the normal range of model engineering components, in size, difficulty of material and cost . So, is powered partoff just another extreme sport?

My own view is that manual feed provides a flexibility, and an awareness of any issues developing during the parting cut, that I don't think you'd get from doing it under power.

The biggest, thinnest thing I can remember parting was some side flanges for toothed-belt wheels I was making for a subcontractor to Ford's toolroom in the late '70s. I had the dishing problem I mentioned above, and sorted it with light facing cuts in soft jaws. Can't remember whether I lost one through not having left enough excess, but if I did I had enough of the 5 or 6 inch bar to replace it.

But hey, as is often said on here, no-one can tell anyone else how to do summat, only how they think they'd do it.

And yes, with material like that and a tight quantity, sooner thee than me!

[old martParticipant @oldmart]

Nothing wrong with using a hacksaw to cut things chucked up in a lathe as long as you take care. The suggestion of a cover for the bed is excellent and easy to make.

Using a four jaw independent chuck to hold work that will be parted off helps with rigidity, a jaw every 90 degrees holds more securely than one with a jaw every 120 degrees, but some people are just too lazy or impatient to use one.The closer the parting off is to the jaws the better, if I needed to part off 2" from the jaws it wouldn't happen without tailstock support and then a hacksaw would be used for the final third of the diameter. Partial cutting with a parting tool helps with holding the final hacksawing square if you finish in a vise.

[Bdog507Participant @bdog507]

Good morning all.

When I was an apprentice we were told that one never uses power feed to part off. I got caught doing it & got a proper rollocking! (Misprint).

I've had lathes of my own for over 20 years now. Parting off with power or not depends on the material & the diameter. I always use a 3mm carbide insert parting blade. Steel invariably gets the hand treatment, especially on large diameter parts (pun intentional). On aluminium, brass or plastic I usually use power feed.

Cheers.

Stewart.

[SillyOldDufferModerator @sillyoldduffer]

Posted by Bdog507 on 11/04/2023 10:19:09:

…

When I was an apprentice we were told that one never uses power feed to part off. …

Was the rollocking for failure to obey orders or was there a technical reason. Men who train apprentices are often soured by the experience: coping with stupid boisterous know-it-all youths is hard work, so there's likely to be a lot of non-technical shouting. Industrial language!

I can see learning to part-off by hand as being useful for the same reason apprentices were trained to file cubes. Done to develop eye-hand coordination, and a patient disciplined approach to work. Jack-the-lad is brought to heel, and finishes the course as an effective grown-up. But do we always learn the right lessons!

After the apprenticeship, surely machinists need to work fast and efficiently, for which parting-off under power is best? Unlikely that parting-off is a purely manual skill. If it was industry wouldn't have replaced hoards of manual machine tools with multi-axis CNC machine centres. Most milling and turning these days is done under-power at high-speed inside a safety enclosure. Not entirely hands-off, but people rarely go inside, and the machine is operated from a control panel. I've even seen pictures where the control panel puts the operator's back to the machine so he can't see what happens when he presses buttons. Not sure why, possibly to reinforce a strict: 'Das machine is nicht fur gefingerpoken und mittengrabben.' Operators may be machine-minders rather than trained machinists.

Dave

[not done it yetParticipant @notdoneityet]

Nearly always is my reply to the OP.

‘Why have a dog and bark yourself’ comes to mind.

My response to those, that might decry the method, would be ‘Do you use the long travel power feed, or do you always feed by hand? I would assume they do, if it is available. There is a term for those that do one thing one way but decry the operation if carried out in the other direction.

[Bdog507Participant @bdog507]

Posted by SillyOldDuffer on 11/04/2023 13:05:08:

Posted by Bdog507 on 11/04/2023 10:19:09:

…

When I was an apprentice we were told that one never uses power feed to part off. …

Was the rollocking for failure to obey orders or was there a technical reason. Men who train apprentices are often soured by the experience: coping with stupid boisterous know-it-all youths is hard work, so there's likely to be a lot of non-technical shouting. Industrial language!

I can see learning to part-off by hand as being useful for the same reason apprentices were trained to file cubes. Done to develop eye-hand coordination, and a patient disciplined approach to work. Jack-the-lad is brought to heel, and finishes the course as an effective grown-up. But do we always learn the right lesson.

I suspect it was for not following orders, & yes indeed the language was industrial! As you said, parting off by hand does help get the feel for things.

Oh the good old cube. I saw it reduce some lads to tears! I did mine pretty easily, so I got the worse one. First file 2 inch piece of 1 1/2 inch a round bar into a hexagon, & then drill a hole in a piece of 1/2 inch plate & file it to a hexagonal hole that fits snugly over the bar. Also as a punishment for dicking around I had to cut a 1 inch slice from a 6 inch bar with a standard hacksaw!

Cheers.

Stewart.

[Martin KyteParticipant @martinkyte99762]

How do CNC lathes manage to part off ?

regards Martin

[Tony Pratt 1Participant @tonypratt1]

When I was on CNC lathes parting off was not my favourite occupation but high pressure flood coolant plus constant surface speed was a huge help.

Tony

[Anonymous]

Posted by Martin Kyte on 11/04/2023 14:06:30:

How do CNC lathes manage to part off ?

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

Andrew

[Martin KyteParticipant @martinkyte99762]

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

regards Martin

[John HaineParticipant @johnhaine32865]

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.

[Tony Pratt 1Participant @tonypratt1]

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

[Nick WheelerParticipant @nickwheeler]

Posted by Tony Pratt 1 on 11/04/2023 17:39:10:

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.

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.

[BazParticipant @baz89810]

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.

[FulmenParticipant @fulmen]

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.

[Anonymous]

Posted by Fulmen on 11/04/2023 19:10:52:

…there will be a loud and expensive bang.

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

Andrew

[JellyParticipant @jelly]

Posted by Fulmen on 11/04/2023 19:10:52:

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.

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.

[FulmenParticipant @fulmen]

@ 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.

[JAParticipant @ja]

Posted by Fulmen on 11/04/2023 19:10:52:

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.

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

[Nigel McBurney 1Participant @nigelmcburney1]

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.

[Martin KyteParticipant @martinkyte99762]

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

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