Dead centre vs. live centre

[Rufus RoughcutParticipant @rufusroughcut]

Hi james

That explains the astro phots in other threads, sometimes can't even see the sky in scunny, fab shots by the way, got a 6" dobsonian reflector but never really commited to doing the matching to a camera although I've fiddled about with it a bit

Rufus

[JamesFParticipant @jamesf]

Yes, my recollections of visits there do seem to involve the place being lit up like a christmas tree at night. The nice thing about planetary imaging is that you're looking at a tiny area of the sky so any light pollution has to be exceptionally bad to cause problems. I've even heard of people imaging the brighter planets such as Venus and Jupiter during daylight. It's tricky with a dob unless it's guided though. I know of one or two people who have produced good images with a manual dob, but it takes a fair bit of practice. You can't beat them for value for money though. I have a 10" dob that I cobbled together from parts on a home-built base and the views are astonishing.

James

[colin hawesParticipant @colinhawes85982]

With either type of centre the job can heat up.If a dead centre is being used a squeal is likely to develope indicating that a drop of oil is required and the tailstock pressure eased a bit because of heat induced expansion. With a rotating centre the expansion can still occur but ,as there is no obvious problem, a thinnish job will bend instead leading to chatter, unexplained lines or lack of parallism.So it is still advisable to check the tailstock pressure.The dead centre is also inherently more accurate. Colin

[JDEngParticipant @jdeng]

Posted by NJH on 17/04/2013 23:29:01:

Just so Gary

Safety is an attitude of mind. Even the smallest of machine tools can be dangerous and no hobby is worth losing an eye for.

Norman

Edited By NJH on 17/04/2013 23:29:48

I'm not being funny Norman when I say that I've never had a job that's worth losing an eye for either; or anything else come to that!

Safety has to come first.

John.

[BazyleParticipant @bazyle]

Off topic a bit James, There is a Tiverton club which might be nearer, and I have heard of an occasional gathering near Barnstaple. Don't forget to put the Taunton show in your diary. Even if it is 3/4 hour it is well worth joining a club as they often have sources of seond hand tools and anyway are just a great bunchof guys. I think it will be quicker if you try it as I can do Okey to Taunton in an hour.

Couple of people on here near Exeter too, so you might be able to sort yourselves out an occasional meeting. Pity Exeter never got itself a club as there used to be a bit of engineering around there cf Exe lathes.

[Sub MandrelParticipant @submandrel]

I spent a boring, uncomfortable few hours waiting the the emergency reception with a big, red, watery eye. Luckily hwever caused it scratched the white, not the cornea

Still not sure how I did it, but more care with safety glasses now. I wish there was some way of protecting fingers from splinters!

Neil

[Colin BarronParticipant @colinbarron94178]

The penalty at Thorn Spennymoor Durham training centre in 1970s for leaving a chuck key in a lathe was 20 pressups. After the first couple of lads doing pressups the lesson was learned.

[Pete RimmerParticipant @peterimmer30576]

A live centre turns with the spindle, a dead centre does not. A revolving centre is neither live nor dead in the traditional sense but in recent times the commonly-used definition seems to have changed to make revolving centres 'live' and solid ones 'dead'.

Anyway, the advantage of a dead centre in the tailstock is that it allows for a slightly longer workpiece if you're approaching your limit in bed length, and will hold the work more concentric all but the best quality revolving centres. Disadvantage is that it must be lubed and you're limited to about 300rpm then you risk burning up the tip. Carbide-tipped ones will suffer a bit more abuse without destroying the tip.

[Mike PooleParticipant @mikepoole82104]

Tallow was the stuff for lubing a dead centre, it stays in the hole until the centre is positioned and melts as the centre warms up to provide continuous lube until it is exhausted, the tiny reservoir at the tip from the centre drill also helps. The job will expand slightly as it warms up so the tailstock will need checking for tightness periodically.

Mike

[HopperParticipant @hopper]

Never-Seez or MolySlip grease works well too.

[Perko7Participant @perko7]

Further to the response from Norman (NGH) you will find when working close to the tailstock that it is easy to run your tool or toolholder into a rotating centre and either blunt your tool or turn a groove in the rotating part of the centre. I've done both. With a dead centre there's less potential for damage.

[Mick B1Participant @mickb1]

Posted by Hopper on 02/05/2020 11:53:35:

Never-Seez or MolySlip grease works well too.

Maybe it's the variant I use – Castrol LM3 – but I've found that moly/graphite grease will eventually disappear even on quite a slow-running between centres job, while tallow seems to last longer before evaporating. Got to admit I've gone over completely to a revolving tailstock centre, and a cheap one I bought 20 years ago at that.

I might stand corrected, but I suppose the main risk from the slight possible eccentricity of a revolving centre is a lobed polygon rather than a truly round diameter – and I suspect I've never made a component where that mattered critically.

[Howard LewisParticipant @howardlewis46836]

Based on my experience, be careful when you buy a rotating centre. I bought a cheap one and had problems because it was a rough runner, and introduced vibration!

You tend to get what you pay for.

Better to deal with a reputable importer, so that you have come back in case of problems, rather than some unknown on the other side of the world..

Howard

[old martParticipant @oldmart]

I normally use live centres, they are more convenient. But there are times where a dead centre has its uses and is more accurate than a live one.

I was setting up a new test bar to check out the tailstock of the museum's Smart & Brown model A. It was an Indian made test bar, MT2 one end and about 11 1/2" long. I set up a piece of 3/8" steel in a collet and turned the end to a 60 degree point. That made it true to the spindle, as long as it wasn't removed from the collet. The bar was set up between centres and checked. The results were disappointing, the tailstock end had 0.0005" wobble. My fault, as it turned out, I had a Rohm live centre in the tailstock which was in new condition, but when I replaced it with a dead centre, the wobble had gone. The test bar was better than my test equipment.

[Mick B1Participant @mickb1]

This thread got me out into the workshop to check my old Machine Mart revolving centre – ref. my earlier post above – that I use practically whenever I need tailstock support. It runs to about 0.0005" TIR, loaded or not.

I'm not sure what sort of work within the scope of a model engineer that wouldn't be good enough for. Clock shafts? Model aircraft engine crankshafts?

The only mod I've had to do to it is tap the tail end of the taper to put in an M4 capscrew. First time I used it on the WM250V it wouldn't eject because there's no tang, so I had to lever it out with a spanner (or something) under the revolving head. That maybe where the half-thou shimmy came from, but if so I reckon I got away lightly. The old Myford Speed 10 I had when I bought it had (IIRC) a bored-through tailstock quill and I could bop it out with a bit of bar.

Edited By Mick B1 on 02/05/2020 19:57:37

[old martParticipant @oldmart]

Its funny how a lot of live centres have a flattish end to the MT which could do with being a bit longer. Having milled extraction slots in the replacement tailstock barrel for the lathe (the dimensions were the same, but it was intended to be self extracting) and having encountered the short end problem, I just milled the slots longer, so the end of even the shortest taper now shows and the taper wedge can do its job.

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