a set true chuck?

savarin

savarin

Active User
H-M Supporter Gold Member
Joined
Aug 22, 2012
Messages
4,271
If I turn the register down a tad and drill the holding bolts out to a larger dia on the chuck back plate would this become a set true chuck?
Just thinking out loud here.
 
I did something similar to my little 7x12 years back. I skimmed a bit off the register and with the 'tolerances' I already had in the mounting holes through the back plate I could loosen the little nuts holding the chuck on, put a DTI on the workpiece and turn the chuck while gently tapping on the chuck body to get a decent reading. Snug the nuts up and go. Worked for me, but then I wasn't pushing the lathe/taking large cuts with it either.
 
Seems like you should have some way of locking the setting rather than tightening bolts, but if it works, fine. My Chinese collet chuck has a "set true" system that works well. The D1-4 back plate has a boss that sticks into the chuck body but is slightly smaller than the mating hole in the chuck. The chuck has 4 set screws that engage that boss and allow a bit of " adjustment. When the gage pin in a quality collet runs within a couple of tenths, all set screws are tightened and seems to hold well. Of course I have the indexing marks on the spindle and chuck so it always goes on the same.
Not sure how well it would work with a Chinese 3 jaw. Seems like the scroll always has some variations. Would the variations from a .200" part be the same when the chuck is holding a 5" part??
 
Larry$ said:
Not sure how well it would work with a Chinese 3 jaw. Seems like the scroll always has some variations. Would the variations from a .200" part be the same when the chuck is holding a 5" part??
Click to expand...

Set-tru chucks are useful when you need to dial in a part that has already been turned or has a ground surface that is accurate. Then you can dial it in and run multiples of that part. However, that accuracy is only good for that one diameter and does not hold for other diameters due, as you said, to variations in the scroll.

This is useful for second operation work but really doesn't help for first operation work. Any 3 jaw chuck will turn the part concentric on the first go. It is after that, when the part is already turned but needs to be re-chucked, that you sort of need something like this. Most of us just use a 4 jaw chuck instead and dial it in or go to a collet chuck. I have a set-tru Yuasa chuck and its good but I admit that I don't use it often because my 4 jaw is far more accurate and holds the work much more solidly.

Now, a set-tru collet chuck is another thing. I think that is very useful and allows you to dial out tiny variations in the accuracy of the collet system itself. But for 3 jaw, meh ...
 
I have many flavors of set-true chucks. They make life in my shop a happy place. They are very accurate and repeat throughout their range. I only use the four jaw when the part is a weird shape. And the collets I have collect dust. I would not buy a chuck unless it was a set-true. I do not skimp on the chucks I procure. The chuck is the heart of the lathe. A good chuck makes all the difference in the world. Savarin, YES, do it, modify your chuck, you will wonder why you didn’t do it long ago…Dave
 
Last edited:
Mikey: Maybe your 4-jaw is more accurate than your 3-jaw, but I can practically zero out a part in my PBA 3-jaw set-tru. How can I be more accurate than that? I chuck mostly pre-turned parts, like barrels, bushings, etc., which is why my set-tru seldom ever leaves the lathe.

Disclaimer: I have never used the 4-jaw chuck that came with my lathe, and have only used on once previously on a really weird project my boss and I were doing. I also don't do heavy turning where I 'need' the additional clamping power of a 4-jaw. But that's just me. :)

I do have a set-tru collet chuck and will use it if I have more than one small diameter part to turn, but it won't hold a very large part. So it's usefulness is somewhat limited for me.
 
I made a 3"diameter cheap three jaw chuck into a set-true, but it turned out to be a bigger job than I expected because of a number of things I hadn't thought of before starting. Here's a full illustrated article about the project:

Modifying a chuck to SET-TRUE
 
I am a fan of the 4-jaw, but I use it so much less now that I have a Pratt-Burnerd set-tru (can't remember who owns which trademarks). I don't know how I lived without it. The constructive part of my comment is for Savarin: On my chuck, the clearance for the back plate and the chuck bore is 1.0 mm all around (so 1 mm adjustability each way from center in both axes). The drive pins are 5 mm, and the adjusting plugs are 6mm. It doesn't have to be big (referencing a 6" chuck here). If you decide to do it, post pics!
 
savarin said:
If I turn the register down a tad and drill the holding bolts out to a larger dia on the chuck back plate would this become a set true chuck?
Just thinking out loud here.
Click to expand...

Easy to get off track with opinions about the value of a set-tru chuck so before doing that I wanted to respond to the original question. No, making the register and bolt holes bigger won't turn it into a set-tru. You need 3 or 4 bolts to adjust the chuck on the backplate before locking it down to turn it into a real set-tru. On the other hand, you can do what you're thinking and possibly improve the accuracy of the chuck overall. @darkzero does this and it seems to work for him. I do this, too, when I mount a new chuck to a backplate so I say go for it.
 
My cheap 3 jaw chuck and the original register were out, so I opened up the register about .010 and bumped the chuck to center. I'ts no big deal to repeat this if it's out, just takes a little more time, and that's what I've got, more time.
 
You must log in or register to reply here.
Back
Top