Thursday, July 16, 2015

Machining Question

The three jaw self-centering chucks that I have for my lathe won't hold a fairly long heavy workpiece without sagging a liitle, so I can't get the workpiece to stay centered.  I have tried centering a hole in the far end and suspending it between chuckm and live center in the tailstock, but it is never all that well centered because it droops.  I suspect a chuck with deeper jaws might do the trick.  Is there a name for such a chuck?

6 comments:

PhaseMargin said...

I've never really tried it with a self-centering chuck. The way I've done it in the past has involved using a hollow chuck and clamping what will be the end in the live center and piloting there, then running the bar and live center out down the bed. That works very well if you've got very stiff materials (I was typically using fairly long, thick solid stainless). It all depends on your requirements, of course, but if you take your time you can do some pretty accurate machining that way.

james albinson said...

Usually one uses a fixed or moving steady to hold long cylindrical pieces in the lathe.
E.G. http://www.homews.co.uk/page83a.html.
Longer lathe jaws wont help that much.

Mr. Engineering Johnson said...

I think you're looking for a through hole chuck. The jaws aren't necessarily any longer but you can position the workpiece deeper in the chuck so the unsupported length is shorter. Failing that, you should probably be using a steady rest or follower.

Will said...

Why chuck up a long piece? Are you working along the length, or doing some end work, and require the length for the finished part?

I have a suspicion you need a gunsmithing lathe, which typically have a decent size bore through the headstock, to enable long pieces to extend out the backside, and allow you to work on a short part of an extended tube or bar. They tend to run 2"+ in diameter, but are not cheap, or small. Some are set up with a collet system like a vertical mill would use.

Clayton Cramer said...

I am trying to reduce a long rod to a smaller diameter.

Will said...

Feeding it through the headstock, and only working it close to the jaws is the more optimum method. You would need at least one roller support just beyond your working zone, to control deflection.

If the rod is too flexible, you might try adding a sleeve(s) over the area that you are not working, to increase rigidity.

You may be able to find a collet setup for the tail end, to simplify locating/centering it, instead of trying to use a live point. That, or a drill chuck on a live mount.

Have you done any searching on:

http://www.practicalmachinist.com/vb/

Lots of info on that site, and some very helpful people.

If this is a regular/common thing you need to accomplish, you might want to think about making some dedicated fixturing, or buying lathe tooling that would help with accuracy and/or speed.

I expect a working shop would just have it centerless ground to size. It's when you don't have the shop or money, that you have to get creative. That's when it gets to be fun!