4-Jaw Chuck

4-Jaw Chuck

It rather sounds like a nickname you might give a person who can't keep his trap shut, doesn't it? Actually, a "chuck" is a device used to hold either a drill bit in place, or a workpiece on a lathe. Typically, 3-jaw chucks are self-centering; if you put something round or hexagonal in the jaws, and tighten the chuck, the workpiece gets centered. Not perfectly--but good enough for most lathe work.

A 4-jaw chuck is useful for holding rectangular, square, or irregular shaped objects. The 4-jaw chuck comes in both self-centering and non-self-centering forms. Self-centering is great for square workpieces, but not so useful for other shapes.

I have a 3.5" 3-jaw chuck for the Sherline lathe, and it works very well--but there are times that I need to turn bigger pieces--for example, when I wanted to very, very slightly enlarge the interior of a 6" diameter aluminum tube. And the 3.5" chuck just won't handle that. Strictly speaking, a Sherline isn't powerful enough to turn 5" diameter pieces of steel or aluminum; it's a bit insane to try it. But this will be an occasional use, and one where I will be turning pieces very slowly, taking very small strips at a time.

So, before I became utterly disenchanted with Sears, I saw that they were having a sale on 6" 3-jaw chucks. But it wasn't a thread that matches the Sherline (3/4"-16), and I wasn't sure that I wanted to spend a lot of time trying to find an adapter that let it fit. They did have a 6" 4-jaw, non-self-centering chuck at the discounted price of $53--which is an incredible bargain.

Okay, it's intended for their wood lathe--but I figured that for the relatively low precision stuff that I would use it for, it would probably work.

And it does. Of course, to get enough clearance between the chuck and the lathe bed on the Sherline, I had to put another spacer block under the headstock. Okay, got one of those that I use on the vertical mill.

Then I have to raise the tool holder up another 1.25". Okay, so I machined a spacer to fit between the cross-slide and tool holder. But I couldn't find a 10-32 3.5" long socket head screw--so I machined a stainless steel adapter 1.25" long that is 10-32 female on one end, and 10-32 male on the other--and now the cutting tool is in position.

Next issue: how do you center a workpiece? I found some instructions scattered about the web, but in essence, what you do is use a dial indicator on top of the workpiece, and slowly rotate the workpiece, watching how much motion there is. When you get to the lowest point, you loosen the jaw that is on top, rotate it 180 degrees, and tighten that jaw (and get progressively less vigorous each time). Repeat until you get the variation down low enough to make you happy.

So I used some scrap Delrin to make a holder that slides into the T-slot on the cross-slide and holds the dial indicator in position. (That means lots and lots of holes in the holder so that the dial indicator can be put at many different heights, depending on the diameter of the workpiece--or even if I use it with the standard chuck, and without the extra spacers.)


Click to enlarge

With one piece, I was able to adjust it to the point where there was only about .001" variation as I turned the workpiece. With another piece, I could never get it closer than about .004"--which probably means that the workpiece isn't perfectly round. (No surprise--they make these things oversize so that you can turn them to diameter.)

All in all, a pretty impressive purchase for $53 plus tax and shipping.

Don't tell Sherline. The owner would probably grin at my creativity in using a Sherline to turn stuff that is big but not very tough; their lawyers would probably file an injunction to get me to stop doing something that could cause the end of the world!