Live Centers vs. Dead Centers? I'm Confused

[JWB_Machining]

Hey,

I haven't been machining for that long so I've constantly got questions since I've got no mentor to prod all day with questions, so today I was pondering when would I ever use a dead center? I know I use a dead center when I'm turning wood on a wood lathe but when I was in college our metal shop only had live centers so I always thought that is just the way it is, live center for metal, dead center and a lil wax for wood. Then I got this gig here and my boss has a carbide-tipped 60 degree dead center for our Haas TL-1. So I'm curious when should I use a live center and when should I use a dead center? My intuition tells me that a Dead Center is useless and would only create unwanted friction and heat and probably a really annoying noise, but my intuition isn't always my best friend.

[Crevice Reamer]

Hi JWB!

If you are turning work too large (or irregular shaped or for any other reason) to fit into chuck, you will need to remove chuck and use a face plate and lathe dog to turn it, and support it between centers. The center in the tail stock would be a live center. The Center in the Head stock would turn WITH the work, (no friction) and so could be a dead center.

http://jjjtrain.kanabco.com/vms/lath...ctions_11.html

CR.

[JWB_Machining]

Ahhh Thank you. What if I don't have a Face Plate n Lathe Dog? Could I just Chuck it up in a 3 jaw chuck or 4 jaw to make it even more precise or would that not be good since the dead center is tapered?

[Crevice Reamer]

No! You don't USE a center in a chuck. In fact, The center installed into the morse taper spindle may run more true than the chuck. The faceplate and lathe dog or dog clamp are used to turn the work. You could just install a center into the spindle, but it will not drive the work very well and it can slip when you start to cut.

Irregular work can also be bolted or clamped TO the face plate and then turned.

AND of course, you can always grease up the dead center and use it in the tail stock in a pinch. It has a hardened point for that very reason.

CR.

[Geof]

The carbide tip dead center is intended for use in the tailstock and when you are working with a very precise manual machine you can sometimes get better precision using a dead center.

If you think about it a little you realise that the center on a live center has bearings that have a finite level of rotational precision so their in-accuracy transfers to the work piece.

Also a dead center can be more rigid than a live center because you do not have bearings and a long skinny shaft inside the center that can flex.

Also a dead center takes up less space so it may be more useful if you are threading especially on a manual lathe when you will be running slow

Naturally you have to limit your rpm with a dead center and it can cause a lot of frictional heating. I think these days with the quality and precision available in live centers dead centers are a dead issue.

[flick]

Originally Posted by Crevice Reamer No! You don't USE a center in a chuck. In fact, The center installed into the morse taper spindle may run more true than the chuck. The faceplate and lathe dog or dog clamp are used to turn the work. You could just install a center into the spindle, but it will not drive the work very well and it can slip when you start to cut. Irregular work can also be bolted or clamped TO the face plate and then turned. AND of course, you can always grease up the dead center and use it in the tail stock in a pinch. It has a hardened point for that very reason. CR.

When I want a center in the chuck, I generally just make one in the chuck (provided I don't need it to be terribly hard). Then it's as accurate as your spindle bearings. File a flat for your drive dogs' set screw and you're off to the races.

[Mcgyver]

dead centres are used in the headstock and as Geof says in the tailstock when you want that extra bit of accuracy.

Ahhh Thank you. What if I don't have a Face Plate n Lathe Dog? Could I just Chuck it up in a 3 jaw chuck or 4 jaw to make it even more precise or would that not be good since the dead center is tapered?

i rarely use the drive plate or headstock centre, just chuck anyold piece of steel in the three jaw or whatever, and turn a 60 degree point in situ. lathe dog englages on one of the jaws. This is the quickest and most accurate way to set up a centre in the headstock end

[JWB_Machining]

Ahh I like that. I'll have to look into one of these lathe dogs. I Guess using the set up with one dead center and one live center allows you to turn down a bar to the same diameter without having to use a 4 jaw chuck n perfectly align it after you flipped it? I've been having a lot of trouble with thin parts being pushed away by my tools and not cutting to the right diameter across the length, is this common? I kinda asked about it in another thread where I would compensate along the length but I've been having random effects and can't seem to nail down my compensation. I'll probably have a big post about this later.

[USMCPOP]

Technically speaking, a live center is a solid center that is inserted into the headstock and revolves with the spindle. A dead center is one that goes in the tailstock and does not move with the work. Then years later, ball-bearing centers were introduced to confuse the terminology. Old references to "ball bearing dead center" and live center can be found.

[Geof]

Originally Posted by JWB_Machining ...I've been having a lot of trouble with thin parts being pushed away by my tools and not cutting to the right diameter across the length, is this common? ...

To prevent this you need a Travelling Steady.

[flick]

Originally Posted by USMCPOP Technically speaking, a live center is a solid center that is inserted into the headstock and revolves with the spindle. A dead center is one that goes in the tailstock and does not move with the work. Then years later, ball-bearing centers were introduced to confuse the terminology. Old references to "ball bearing dead center" and live center can be found.

One instructor I had beat this terminology to death, in complete agreement with what you state. It seems the "wrong" way of saying it is much more common, so I've learned to just go with it.

[mc-motorsports]

I've only used dead centers in OD grinding opps. I've always used a ball bearing center on a lathe, cnc or manual. The little bit of precision and rigidity that you lose is null on a lathe. Grinding, diffrent story. When you work with 1um tolerances, a bearing center can make life difficult. But the work piece rotates at 150 RPM on grinding opps, and a dead center would wear on the material if you tried spinning 600-3000 rpm leaving your material loose on a lathe opp.