Parting off using a powered cross feed

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.

it's a good thing if you have power Xfeed and it works better than manual otherwise there's no point and it's back to the hacksaw.

It's also a good thing if Xfeed backlash is minimal because if the tool digs in suddenly your 4thou per rev can be 25 thou and everything stops.

it's a good thing if you have power Xfeed and it works better than manual otherwise there's no point and it's back to the hacksaw.

I am assuming that you mean taking a hack saw to the work-piece whilst it is still in the lathe? That to me would be a very big 'no, no'. How many lathes have we seen with hacksaw marks gouged into the bed at the head end because a person was also unskilled in the use of a hacksaw and applied excessive downward pressure causing the blade to crash through the cut and into the lathe bed?

Apologies if my assumption is incorrect.

Edited By Chris Crew on 25/03/2023 08:38:37

Exactly Chris, there's lots of them out there. Don't know why a using the bench vice is so un appealing.

Provided you have a sharp tool and sensibly light infeed dig in isn't problem with power feed.

The 'bang, everything stops" experience with power feed is invariably due to chip jamming in the tool slot. Which is why lubrication is desirable and the narrowed chip produced by insert tools so beneficial. As Andrew says a properly set up and properly fed insert tool leaves an excellent finish showing that the chip doesn't significantly touch the sides of the slot. Any contact being so light that the surface is not marked.

In practice we generally have to grind a conventional HSS blade with a flat top. Geo.H Thomas put a Vee top and front on his (how?) but thats for advanced folk only.

Grinding the front at an angle effectively produces a tilted chip that is less likely to jam up as it's already trying to rotate away from the walls of the slot. The top of the tool also needs to be angled for best results as that re-enforces the twisting action. Get the two angles opposed and it will jam! Need to get the speeds and feeds right so a long chip comes out. Unfortunately angling the tip produces a side force on the tool so it rubs disturbing the finish. All too often hobsons choice. Pip on the end of the job or rub marks.

Tight spiral chips on top of the tool indicate incorrect speed and feed combination. High risk of jamming up.

As ever careful set-up and careful work practice will get round these issues but its important to be aware of what can go wrong.

Small lathes have slender feed screws.

Effectively the push force goes through a column the core diameter of the screw. Not very big.

The helix of the thread produces a side force so the unsupported length of screw between the handle bearing and nut has a bowing force imposed. Clearance between nut and thread doesn't help as this provides extra space for any bow.

If the tool stops cutting momentarily the feed forces causes the unsupported part of the screw to bow acting like a stiff spring. Further feed with the tool not cutting increases the bow until the spring force is sufficient to re-start cutting an action whereupon the tool jumps forward. It takes more force to start a cut than it does to keep it going so the restarted cut is even greater than pure geometry predicts. In the worst case the job climbs over the tool or breaks it during the climb process.

Naturally the deeper the parting off slot the longer the unsupported part of the feed screw so the greater the bow tendency. Deeper slot means more chip drag too and thinner part of job remaining makes the whole thing less stable so, if a hand feed job is going to go wrong it always seems to be about 2/3 rds through. Just when you start to relax thinking "Yup, it's going right this time!"

Hence Daves "4 thou cut going to 25 thou and bang" scenario. Most likely with materials that work harden or form built up edges.

Given maliciously careful hand feed its possible to demonstrate this spring action on a slightly loose machine with the slide visibly oscillating as the cut stops and starts. First happened to me with an alloy forming a built up edge leading to regular momentary stops in cutting after the built up edge formed before it broke away as the tool rubbed allowing cutting to resume. Spectacular marks on the cut faces.

Using a rear tool post with a conventional feed screw keeps the screw in tension so there is no bow effect. SouthBend Heavy 10 lathes fitted with a taper turning attachment have the cross feed screw effectively reversed so it pulls from the back operating in tension. Parting off with a taper turning equipped Heavy 10 is noticeably better behaved and generally less fraught than with the non taper turning version with its conventional push feed screw.

Clive

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

Sorry double post. Why?

Clive

Edited By Clive Foster on 25/03/2023 09:46:10

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.

Well the answer is obviously some do, some don't. Personally I'm in the do category, partly because I don't have a cross-feed handwheel any more but mainly because it works so much better (for me anyway).

If you are using HSS parting blades (front or rear TP), you can make a dish in the "top" surface using a round diamond rat-tail file in a few minutes. You can hold the blade in a vice or toolmakers clamp so the faces of the jaws will stop the file sliding off the surface, but once you've made a shallow depression in the right place it's self guiding. I'll try to get a photo of mine.

Like this.

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.

"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

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!

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.

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.

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

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.

Cheers.

Stewart.

How do CNC lathes manage to part off ?

regards Martin

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