Dead centre vs. live centre

From my reading so far it's not at all clear to be when one would use a dead centre in preference to a live centre and vice-versa, though at the moment I'd always use a dead centre because it's all I have.

Can anyone enlighten me?

Thanks,

James

James,

Strictly; a "Live Centre" is the one in the Headstock Spindle, when you are turning "between centres" in an engineering or woodturning lathe. Watchmakers also turn between a pair of dead centres, and drive the work directly with a bow. [This is a simlar idea to the old "Bodger's" Pole Lathe.]

.... Several new terms to search for on Google, I suspect.

However: You are probably thinking of using either a dead centre or a rotating centre, in the tailstock. ... In which case the big practical difference is that the dead centre needs to be kept lubricated; whereas the rotating centre should have a nice ballrace to take the load.

Hope that helps

MichaelG.

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These are very good ... and priced accordingly  

Edited By Michael Gilligan on 16/04/2013 23:17:11

Make sure your centre is a hardened one if using it in the tailstock. There are also some that are soft which are the ones you would use in the headstock spindle because you would then (in the good old days) turn it true before use..

Aha! I didn't realise a live centre went at the headstock end. You are quite correct Michael that I am thinking of a centre that rotates on bearings. I was perhaps confused by the fact that arc sell "live centres" with an MT2 taper that would fit the tailstock quill of my Conquest Super. (In fact I think there's at least one "live centre" in their C3 tailstock accessories section.)

Would a rotating centre generally be preferred over a dead centre given the choice, or are there times when a dead centre is more desirable?

James

James,

Yes, the nomenclature has got very sloppy in recent years.

Rotating Centres are great

1. because they don't need lubricating whilst you work
2. because they take heavier loads without overheating
3. because you can vary the design [e.g. to support tubes]

... but they can never be 100% accurate; and the very best "watch" work is done between dead centres.

Buy or make a good Rotating Centre when you can ... but until then, just use the [hard] Dead Centre carefully.

Welcome aboard ... and let us know how you get on.

MichaelG.

Thank you, Michael.

I think it will be a while before I consider working to watch scales and accuracy. I'm still very much a beginner, trying to develop a "feel" for the lathe by doing simpler things and not getting excessively ambitious.

James

You'll use a rotating centre 99.99% of the time and it'll do everything a dead centre will do and more. You don't have to pay top dollar but don't go too cheap either as that's a false economy. It'll get a lot of use so get the best you can afford.

... and don't get one that's overly large unless you're turning propellor shafts for battleships. It'll just get in the way.

.

James,

That's exactly the right approach ... well said, sir!

MichaelG.

Hi

I always turn the dead centre in the head stock before use. That way I [know] it is perfectly concentric.

Just a light trim cut.

Soft dead centres are easy to make; often I just pop a small end of parallel sided steel I use as a centre in the three jaw. skim the point to make it concentric and use.

It saves time compared to removing the chuck and fitting the nose plug and a centre, although that method has less overhang.

Cheers

John

Edited By John McNamara on 17/04/2013 08:08:35

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My point was just that the very finest work is still done with the simplest of equipment.

MichaelG.

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For a nice overview, have a look through this

... and here is an excellent watchmaker's video [he has lots more]

Edited By Michael Gilligan on 17/04/2013 08:35:31

Hi James

All the above is true and, when I can, I use a rotating centre in the tailstock. The "when I can't" is if work needs to be done near to the tailstock. Rotating centres tend to be biggish and it's sometimes difficult to get the tool in to the work but OK with the fixed centre or, exceptionally, a half centre. Michael mentions watchmakers lathes and mine has a double ended dead centre - full one end and half centre the other.

James, you say you are a beginner, so welcome.! I've been a beginner for over 60 years! Always something new, always something to learn.

(James is a good name too - my father's third name, my second name and my eldest son's first name!)

Regards

Norman

Hi
Drifting away from the thread a bit . You can use 2 dead centres on a cyclical grinder. Also as john says you can turn a centre in a 3 jaw chuck and save it all it needs next time is a light trim
Nobby

Edited By Nobby on 17/04/2013 11:26:17

You can think of live centre as one that does not use the actual centre-drilled hole as a rubbing/sliding bearing surface, but instead relies on either in-built ball bearings or the headstock of the lathe.

So any centre that 'rubs' on the centre hole and thus needs lubricating, is a dead centre.

Martin.

Always worth having a hard dead half-centre. It lets you turn down tor really small diameters. The other way to do this is to grind away the back of an HSS tool bit.

Neil

Hi James

I use one of these, it has a two stage taper to the point to give more tool room to the end of the work piece and also still gives a good sized bit for in the end of tube work.

It's RDG £29.40

Barry

That looks quite handy Barry. I shall bear that in mind next time I place an order.

James

Hi JamesF

Have you learnt not to leave the chuck key in the chuck yet, I've used lathes for nearly 40 yrs and still do it, and the old adage of learning from your mistakes is now just referred to as frustrating inefficiencies in progress.

Enjoy

Barry

I've not yet managed to leave the chuck key in the chuck. Surprisingly given the paucity of my knowledge of metalwork my dad worked in engineering and had a Boxford lathe in the workshop at home when I was growing up. He was pretty good about workshop safety generally and some of it must have rubbed off. He began his working life at the steelworks in Scunthorpe when it employed almost every person of working age in the town and it wasn't too uncommon when visiting my grandparents to see people in the shops with bits misisng or fingers that pointed in strange directions/didn't work and so on, so I guess the lessons didn't need much more reinforcing.

It does help that my particular Conquest Super came with a sprung chuck key on the three-jaw chuck. It's actually a pain in the neck and I've been tempted to remove the spring, but haven't given in to it yet. The four jaw independent chuck doesn't have a sprung key, but because I usually have the dial gauge on top of the workpiece for centering I have to use the chuck key horizontally and in that position it falls out if I let go of it. I have a feeling that if the chuck key were left in at the top then the safety interlock on the guard wouldn't operate so the motor wouldn't turn, but I've not checked.

For someone such as myself who is not blessed with "delicately-made" hands the tailstock and quill area actually the major liability thanks to the short bed. I've often taken to removing the tailstock from the bed altogether when I don't need it.

James

Hi James,

Welcome to the forum.

My mini-lathe came with a spring on the chuck key. After it had jumped out of the chuck for the third time I took the spring off. I just got into the habit of making sure the key is on the bench before I start the lathe and also not standing in line with the chuck is an idea. I also removed the guard which was another major inconvenience and prefer to just be careful.

Gary