a set true chuck?

[WarrenP]

I took a little bit off the back plate of my chuck which made it able to be adjusted down to within a 1/2 thous runout. I am thinking of taking another 1/2 off so it can be even closer... so far its worked for me.

 

[mikey]

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.

My 3 jaw is a Rohm chuck and is very accurate - maybe a thou or less of run out. However, my 4 jaw, also a Rohm, can be dialed in to be even more accurate. I actually bought my Yuasa set tru to see what all the fuss with set tru chucks was about. I used a precision gauge pin and ran my own tests and got the chuck to run with almost zero run out. But if I then changed to a pin of a different diameter, run out increased. I repeated my tests using a Buck set tru chuck and got the same results, only worse because that chuck had more wear. Many others have reported the same thing.

My point is that a set tru chuck can be adjusted to run really accurately on a specific diameter, and it will hold that accuracy on multiple parts of the same diameter, but if you change to a part of another diameter then I don't think you can expect the same degree of accuracy without adjusting the chuck again. It will likely be more accurate than a non-set tru chuck but not as good as when it is dialed in. This variability is thought to be due to variations in the scroll.

And again, this only applies to second operation work. Any 3 jaw chuck in good condition will be just as accurate as a set tru for first op work.

I'm getting off topic but wanted to respond. In Savarin's case, I think he just wants to improve the accuracy of the chuck mount to minimize run out and he can certainly do that.

 

[darkzero]

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.

I've done it to the cheap 3 jaw chuck that came with my lathe & the ER40 collet chuck I made. I call it Tap-Tu but I wouldn't do it to anything that will see moderate to heavy machining, can get knocked out of true, but easy enough to readjust again. That 3 jaw chuck I did it to barely gets used, I really only use it for polishing stuff. My main 2 scroll scroll chucks are actually real Set-Tru chucks, both Bison, so I didn't care about the cheap 3 jaw that I turned into a Tap-Tru. I first did this to the stock 3" or 4" chuck on my mini lathe which was fine cause it was only good for light duty use.

 

[savarin]

Heaps of info here guys, thanks.
As I said it was a passing thought because of repeatability issues when re-chucking a part.
I just thought it may save a little time instead of changing to a 4 jaw.

 

[mikey]

For the second op stuff you need to do, a real set tru chuck, a 4 jaw independent chuck or a collet chuck will work pretty well.

 

[mmcmdl]

All of the cylindrical grinders I've ever run had regular old 3 jaw chucks mounted on faceplates with NO registers at all . Faces ( on the parts ) were indicated within tenths to perpendicularity and then the chuck was bumped in for concentricity . The chuck nor the parts ever moved while working the part . After the face and shaft was brought in on the first part , the diameters on the following parts always repeated darn near perfect every time if they were machined in the same lathe set up . I think what savarin is saying is perfectly functional and smart and most definately will save some $$$$ over a set true .

Think of a magnetic lathe chuck !