Securing workpiece for parting in lathe or ‘left feed’

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Securing workpiece for parting in lathe or ‘left feed’

This topic has 26 replies, 12 voices, and was last updated 16 June 2021 at 17:45 by Stuart Munro 1.

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15 June 2021 at 15:55 #16289
Stuart Munro 1
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@stuartmunro1
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15 June 2021 at 15:55 #549899
Stuart Munro 1
Participant
@stuartmunro1
Hi, Not sure if this should be a beginners post – but I've been at this couple of years yet continue to face a problem of work holding when parting off.

My issue is that I use a Sherline lathe which has a 10mm shaft in the spindle. This means that any larger diameter piece is held either:

1. Just by the chuck.

2. By the chuck and a rotating 'live' centre in the tailstock.

3. Assisted with a 'steady'

My problem occurs mainly when parting off; the live centre needs to be removed (I'm advised) for parting off and the piece becomes less stable. I've tried moving closer to the chuck by cutting a bit off of the end and re-inserting the work piece into the chuck but I still occasionally find the piece coming loose during parting.

I say it occurs mainly when parting; I also sometime experience problems when feeding left and pulling the piece from the chuck. The steady can not be used as it obstructs the crosslide.

So I've finally admitted defeat and decided to ask the experts. Any advice?

Stuart
15 June 2021 at 16:30 #549904
Martin Kyte
Participant
@martinkyte99762
You only need to remove the live centre when you actually get close to separating the parted off bit from the parent stock. Up to that point you are just turning a deep groove. Just keep an eye on what you are doing and don't go too deep. Remove the centre for the last bit.

regards Martin
15 June 2021 at 16:38 #549908
JasonB
Moderator
@jasonb
Also nothing wrong with using a hacksaw for all the cut or the last bit once the tailstock ctr has been removed.
15 June 2021 at 16:51 #549913
Stuart Munro 1
Participant
@stuartmunro1
Martin, Jason,

Thanks – experience and the blindingly obvious!. I imagine there is some complex design to secure the piece for cutting off and you both provide simple, no cost solutions. Thanks.

Reminds me of NASA spending $100,000s designing a pen that would work in zero gravity, only to learn that the Russians use a pencil.

Stuart
15 June 2021 at 16:59 #549915
not done it yet
Participant
@notdoneityet
If the workpiece is coming loose while making a cut away from the chuck it may mean the chuck is goosed. Dull cutters or with too much feed and/or depth of cut (there are limits!) might also be why?

Most items can be cut from both ends towards the chuck if mounted between centres. I expect extended narrow centres are available – or able to be sorted in-shop – if you set your mind to it.
15 June 2021 at 17:04 #549916
Stuart Munro 1
Participant
@stuartmunro1
not done it yet, goosed?

I like the idea of extended narrow centres but would they impair stability when they get too far from the tailstock?

Stuart
15 June 2021 at 17:27 #549917
Dave Halford
Participant
@davehalford22513
If you mount a piece of bar in the chuck tighten it as much as you normally do. Rotate the chuck till one of the jaws is at the top. Now shine a torch at the jaw from behind if you can see the light shining between the work and jaw the chuck is not gripping properly and therefore goosed, duff, mullered, knackered and all the other words the English invent to say broken.
15 June 2021 at 17:28 #549918
old mart
Participant
@oldmart
A four jaw independent chuck will give better support than a three jaw when parting off. If there is more than 1 diameter overhang from the jaws, the added support from the tailstock would be welcome, and finishing with a hacksaw can be much safer than parting all the way.
15 June 2021 at 17:38 #549919
Stuart Munro 1
Participant
@stuartmunro1
Dave, old mart, got it – goosed. No my chuck is not goosed but I do have a 4 jaw independent one that I thought about using.

My hacksaw problem – albeit small – is that the Sherline parting tool is 1mm (0.04&quot and the hacksaw not much less, so it takes care not to damage the finish.

Think I'll try getting almost there with a live centre then finish off with a jewellers saw, very fine blade,
15 June 2021 at 17:39 #549920
Stuart Munro 1
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@stuartmunro1
Arghhh. The smiley snuck in instead of a closing bracket. Sorry
15 June 2021 at 17:42 #549921
JA
Participant
@ja
If you are going to use a hack saw, I frequently do so but not under power, put a piece of wood below the job on the lathe bed to protect the bed from the saw blade.

JA
15 June 2021 at 17:42 #549922
larry phelan 1
Participant
@larryphelan1
My pennyworth would be that if the workpiece is coming loose in the chuck, the chuck is not much use to you, something wrong !

I decided many moons ago Never to use the hacksaw for parting off, that,s a last resort, try a rear toolpost.

PS I dont always have happy part -off,s, break the odd tip now and then, like everyone else.
15 June 2021 at 17:58 #549926
old mart
Participant
@oldmart
The four jaws support the work at 90 degree spacing, wheras a three jaw is 120 degrees. Have you tried a junior hacksaw with the sides of the blade rubbed with 400 wet and dry paper just a little to smooth the sides of the set? A lot depends whether you want the parted off faces to be a finished surface, of if you can take a bit more off by facing off.
15 June 2021 at 18:17 #549931
JasonB
Moderator
@jasonb
Allow sufficient length to reverse the work for a finish facing cut to clean up the sawn part as well as the parted surface as finish is seldom as good and if dia of work is large your 1mm tool may well wander to one side or the other
15 June 2021 at 19:34 #549950
Dave Halford
Participant
@davehalford22513
Posted by Stuart Munro 1 on 15/06/2021 17:38:52:

Dave, old mart, got it – goosed. No my chuck is not goosed but I do have a 4 jaw independent one that I thought about using.

My hacksaw problem – albeit small – is that the Sherline parting tool is 1mm (0.04" and the hacksaw not much less, so it takes care not to damage the finish.

Think I'll try getting almost there with a live centre then finish off with a jewellers saw, very fine blade,

In which case tighten the chuck properly
15 June 2021 at 21:42 #549965
Dave S
Participant
@daves59043
I use centre support all the time on the CVA.

This is mystery steel, about 2" diameter:

Just remember to loosen it of as you get towards the actual cut off.

Dave
15 June 2021 at 22:58 #549972
MadMike
Participant
@madmike
Stuart, where are you located. With luck somebody is close at hand and could help you with this problem. For instance if you are in the East Midlands you are invited to come and see me and I can show you the way in which parting off can be done. I do a lot with stainless steel which would possibly alarm some. Oh yes and I never cut off with a hacksaw.
16 June 2021 at 08:16 #549989
Stuart Munro 1
Participant
@stuartmunro1
Lots of thoughts overnight. Thanks guys.

I do use a rear post cut-off already. It is definitely better than a normal front one. And I do tighten the chuck properly which is not 'goosed' but will try the 4 ja that I have.

I'm convinced the problem is the ratio of overhang to the amount gripped by the chuck. Using the live centre until I'm almost through makes sense but I never thought of leaving the piece in the lathe to cut it with a hacksaw. I tried taking it out and putting it in a vice. Leaving it in place makes sense perhaps:

Smooth the edge of a junior hacksaw blade (old marts idea) but then clamp it in place and manually rotate the piece for the final cut. Needs some thought on clamping but it should ensure the the cut is perpendicular to the base – not damaging the finish.

I'll try it

Stuart
16 June 2021 at 08:44 #549996
Martin Connelly
Participant
@martinconnelly55370
Don't know if this is an option but consider replacing the live centre with a small rotating chuck in the tailstock to hold the workpiece without putting pressure on in the direction of the headstock.

Ideally for holding a small part such as the item you are cutting off from the longer stock a set of soft jaws cut to suit would be better but I suspect that is not an option for your Sherline. An alternative is a bored and split collet that fits in the ordinary chuck, this may be something worth considering since it will work for lots of other awkward to hold items as well. There are plenty of examples of how to do this on line.

What about an ER collet chuck on a parallel shank. You can cut the shank down to the maximum length that the chuck will hold for minimum overhang but you will get much better grip from the ER collet than from a standard set of chuck jaws when holding short parts. ER11 holds up to Ø10, ER 20 holds up to Ø13 and ER25 holds up to Ø16 for example.

Martin C
16 June 2021 at 10:09 #550011
John Haine
Participant
@johnhaine32865
When you say "10mm shaft in the spindle", do you mean that this is the spindle bore? I assume you do. If it was bigger, why would it make any difference to workholding, in the chuck, or otherwise?

Does it have the type of chuck that you tighten with a pair of tommy bars, or one that has a chuck key that you but in a square or hex hole in the side of the chuck? The Sherline website seems to show the former, and these I think are quite difficult to exert a good tightening torque on. Even so I'm surprised that it can't hold well enough for turning left-to-right. Mind you, in 20+ years of machining I can't recall a single occasion when I have needed to do that!

Their 4-jaw independent chucks seem to have the conventional one-screw-per-jaw, which would allow a better grip.
16 June 2021 at 13:11 #550033
Stuart Munro 1
Participant
@stuartmunro1
Martin, John. some good ideas. I have used a metal rod of the right diameter where the workpiece has a centre hole. The rod held in a drill chuck which works well but only if the piece has a centre hole!. I'll look at the ER collet idea.

The 10mm shaft helps because the I can pull the piece in tighter to cut off. Also, the 'tail' sticking through the spindle means that the piece is unlikely to come out of the chuck, though it might flex!

As to the left to right movement, I've avoided it recently by using the parting tool to get a neat 'right hand side' edge. I cut a lot of pulleys!

Stuart
16 June 2021 at 14:32 #550041
John Haine
Participant
@johnhaine32865
??? Sorry, this still makes no sense to me! I think the Sherline spindle BORE is 10mm diameter, it has an MT1 taper. Chuck fitting is 3/4 – 16 so the spindle OD must me significantly more than 10mm – in fact the bearings have a bore of 20mm so that's the OD in the bearing section. What's a metal rod of the right diameter and why is it held in a drill chuck and which piece has to have a centre hole to use it? Where do you put the 10mm shaft and why does it allow you to pull the piece in tighter? If you could be clearer in stating your problem, possibly posting a photo, it would be easier to give specific suggestions!
16 June 2021 at 16:10 #550054
Stuart Munro 1
Participant
@stuartmunro1
John,

Mia culpa if I've used poor descriptions but its actually quite straight forward.

The ID not OD of the spindle is 10mm and the Lathe chuck – 3 or 4 jaw is as you say 3/4 – 16tpi. This means that the spindle with the chuck on it has a 10mm hole right through is axis.

A stock piece to make say, a brass pulley, which is 10mm OD fits nicely through the hole and is clamped in place with the chuck. I then bore a hole in it with a 'drill' chuck fitted to the tailstock, say 3mm.

I can then place a 3mm rod into the tailstock drill chuck and use this as a dead centre poking it into the hole in the workpiece. When parting the workpiece can slide further onto the rod but is supported laterally.

Alternatively, if working from stock with a max diameter of 10mm I can position it so that it extrudes from the lathe chuck by say 25mm. Work on the end then loosen the chuck and slide it out another 10mm. This is not ideal as I lose indexing but with care this can be re-established. Ergo, the piece remains well supported laterally.
16 June 2021 at 16:19 #550055
Stuart Munro 1
Participant
@stuartmunro1
Following on from John's question above, from my answer you can see that I am not into heavy engineering; no steam engines. Mostly small components for radio controlled yachts/power boats which are made from Brass or Aluminium.

I've taken to using delrin rod for pulleys of late. I had managed to reduce the parting off problems with Brass but are finding them more acute with delrin (or sometimes nylon). I expect this is because these plastics, however rigid and machinable they are, are more slippery than brass.

I also make components on a sherline mill – cases, cogs, small steering boxes etc. I'd like to try using Delrin here but buying sheet delrin is very pricy. Any sources of reasonable prices materials or suitable 'plastic' substitutes?
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