Thread: Cheap Live Centres

Thanks Chris - Thats helped me to understand things a bit better.

Bill

Originally Posted by cba_melbourne

a life center is used to turn between centers. For me, turning between centers in most cases means that I am making a part that I want to be as accurate and round and parallel as I can make it.

You can use a dead centers turning between centers. In fact it can be better as you dont have to worry about run out. But this is where is can get confusing as a dead center turning in the headstock can be called a live center. If you really want the best possible repeatability you want 2 dead centers, though I haven't seen that on anything but small lathes or grinders.

Stuart

Originally Posted by Stustoys

You can use a dead centers turning between centers. In fact it can be better as you dont have to worry about run out. But this is where is can get confusing as a dead center turning in the headstock can be called a live center. If you really want the best possible repeatability you want 2 dead centers, though I haven't seen that on anything but small lathes or grinders.

Stuart

That is good advice but be careful when using a dead centre in the tailstock . The centering hole in your work piece needs to hold some lubrication in order to minimize the friction . I was using a dead centre in the tailstock at one time, the work piece began turning a blue colour and, as it heats up, the workpiece expands and the problem is made even worse Mike

Yes that would be correct.

Hi Mike,

yeap. I vaguely remember a saying about resting your thumb on the dead center, you'll know soon enough when things start to get hot. Though its easy to forget and as I cannot tell a lie I'll fess up and admit I've cooked one dead center bad enough to need regrinding. Though it was only a carbon steel one, I assume a HSS or carbide one would likely have survived but as I've never used one I wouldn't really know.


Stuart

Yes that would be correct.

Originally Posted by Stustoys

Hi Mike,

admit I've cooked one dead center bad enough to need regrinding.

Stuart
Sorry mate, but if you haven't friction welded a dead centre into the end of the job, shearing the point of the centre off entirely and causing the job and centre to be scrapped, you just haven't been trying hard enough.

I did that a couple of times as an apprentice. Not too bad if you are using a chuck but, aaah, "entertaining" if turning between centres.

regards
bollie7

I've seen the result, but wasnt to blame

Stuart

Originally Posted by Stustoys

You can use a dead centers turning between centers. In fact it can be better as you dont have to worry about run out. But this is where is can get confusing as a dead center turning in the headstock can be called a live center. If you really want the best possible repeatability you want 2 dead centers, though I haven't seen that on anything but small lathes or grinders.

Stuart

Stuart, yes I heard before that a dead center in the TS has better repeatability. I am not sure if it is true though. I suspect it may have been true long ago, before good quality life centers became affordable. It is certainly still true if comparing to a low cost life center (like those made with only a row of radial ball bearings in tandem).

There are several types of "dead" centers for use the tailstock. There is the plain hardened center and the center with a brazed-on carbide tip. And both types can be made as a full center, or a half center to ease access with the cutting tool. I do not like using any of these except for static supporting or very slow jobs such as threading:
- Dead TS centers need to be lubricated (and frequently re-lubed during work), else they soon run hot and friction weld to the workpiece, thereby ruining the workpieces center hole (and with it any repetition accuracy the workpiece had, which sometimes means a scrapped workpiece).
- One also has to be careful with TS pressure, too much and the center will bind (problem is that a longish workpiece can grow quite a bit in length as it gets warm, thereby increasing pressure, and a dead center has no give at all.... whereas life centers are much more forgiving regarding pressure... so with dead centers you need to frequently re-check the pressure whilst re-lubing, it can really be a pain).
- More so a pain, if you consider that for every re-lubing you have first to clean out the center hole..... else you risk squashing some chips or metal dust between the center and the center-hole....thereby ruining the hole and loosing the repetition accuracy....
- Dead TS centers can only be run at slow speeds to avoid the friction welding problem above. Centers with carbide tip may run a little faster, but still less than 1krpm. Not ideal for small diameter workpieces. Not ideal for say turning over the commutator of an electric motor.
- BTW, in the old days they used Lead to lubricate centers (a paste made from lead powder and oil) as this prevented friction welding. That stuff is naturally not palatable anymore. Nowdays there are products specifically for center lubrication not based on lead. And these are said to need less re-lubing than just oil.

Yes some people like to call a dead center installed in the headstock a life center. In any case, unlike dead centers used in the tailstock, the headstock center is usually left soft. The idea is that it can be trued up by a light finishing cut after it is installed into the headstock spindle (I mean after it is installed AND one expects to do some exceptionally accurate work). This will take care of any minute out of center runout of the spindle taper which can be due to either spindle tolerance, or a damaged/scored/dented taper, or simply a grain of dirt. Some larger lathes may use hardened centers in the headstock too, reason being 1) the larger the workpiece diameter the less important (relative to diameter) is a minute error in the center position, and 2) larger lathes are usually in larger workshops that if needed would have access to a toolpost grinder to true the center. Chris

Have I been missing something all these years.

I have always referred to them as either a LIVE or DEAD centre but not a LIFE centre.

A life centre is a live centre that murmurs positive thoughts.

Originally Posted by pipeclay

Have I been missing something all these years.

I have always referred to them as either a LIVE or DEAD centre but not a LIFE centre.

I have to admit pipeclay, I was wondering the same thing. It's not just a play on words is it Chris.
I don't think dead centres are a problem in fact in years gone by they were very successful.
The problem ( I think) with using dead centres nowadays is the tricks of the trade and the procedures have been lost with the advent of live centres.
I have a small container of centre grease kicking around somewhere that (at the time) the local engineering supplier I used to frequent gave me.
With dead centres, too much grease will kill it quicker than not enough grease. The pilot hole created by the centre drill is the reservoir and should only be about half full of grease, that is, the proper grease if you can still get it.
Constant attention to tailstock adjustment and centre temperature were second nature years ago but barely given a thought now.
Gotta admit though, a live centre is a set and forget thing and I am finding it hard to concentrate on more than one thing at a time lately, and even that's a struggle
Live centres for me but get a quality one or you may as well use a dead centre.

Phil

People whose first language is German tend to pronounce their consonants one notch harder than in English, eg V sounds like our F, and sometimes this creeps into their spelling. I used to go out with a German girl. She'd lived here for many years and didn't think she had a German accent, but the consonants gave her away.

Originally Posted by Steamwhisperer

With dead centres, too much grease will kill it quicker than not enough grease. The pilot hole created by the centre drill is the reservoir and should only be about half full of grease, that is, the proper grease if you can still get it.

Phil, I'm guessing the proper stuff is leaded? What would be the next best thing in a modern grease? I've used HTB and moly, because I have them but neither seemed quite right.

I don't get the half full thing. Can you expand a little?

Also what is the method for setting correct pressure? My feeling is you want the absolute minimum that removes all play. Is that right?