TOS 3 jaw chuck

[Boris Ludwig]

I was lucky enough to be given an unused IU 160mm diameter TOS chuck that needs a D1-4 backing plate to fit my lathe. According to the signed inspectors specifications the chuck has a TIR of ~ .03MM - approx .001".

So, my question. The chuck has a register that will need the backing plate to be turned down to fit the register. Should I turn the register to fit with no play. Or should I make it say .001" smaller so that I have to ability to possibly eliminate the TIR of ~ 0.03mm or .001"?

 

[mikey]

I vote to turn the snout of the backplate down 0.002" smaller than the recess in back of the chuck. You may be able to align the chuck a bit more accurately by tapping it into alignment this way. Just know that even if you align it perfectly with a precision pin, once you chuck something other than that pin the alignment will no longer be as accurate.

A 3 jaw scroll chuck, even when not perfectly aligned, will still be accurate on a first operation turning (first op = the first time you turn something). Likewise, a scroll chuck will almost always be inaccurate when using it for a second operation. Just the nature of the beast.

 

[Boris Ludwig]

Thanks, Mikey. I was just curious, in case I didn't envision some other detail that wasn't obvious to me, if it was better to have a tight fit in the backplate or some small amount could be allowed to centre the chuck.

Yes, I understand, but I'd just like to have it as concentric as I can get it. For some of my work a small variation from first to second is ok as long as it's small. When I need better I can use 4 jaw chuck or 5C collets for smaller things.

 

[Mitch Alsup]

I am heading into a similar micro-project with a 4" ER-40 collet chuck on a D1-5 backplate.

I am going to turn a face and register on the D1-5 backplate.
I am going to thread 3 set screws on the 4" ER-40 collet chuck.
Then after the machining, mount the collet on the backplate onto the lathe.
Then get out the dial indicators and make the tapered bore of the ER-40 chuck concentric with the bore on both axes.
{tapping for concentricity errors, using the set screws for angular wobble.}

Wish me luck.....

 

[darkzero]

Mitch, that's pretty much how I made mine. Except I put the mounting bolts on the face of the chuck even though the chuck was threaded for mounting bolts. Makes it much easier to access the bolts on the front face for the "tap-tru" feature.

 

[Boris Ludwig]

Wish me luck.....

Good luck, Mitch.

 

[Boris Ludwig]

Darkzero,

Is your backplate cast iron or some steel alloy? You got a great looking finish on that plate.

 

[darkzero]

The backplate is cast iron. The ER plate is some kind of hardened steel.

 

[darkzero]

Boris, here's a thought. Whenever I'm making a chuck backplate & my intention is to make it a "tap-tru" regardless, I make the register a close fit first. The practice is good but I then fit the chuck to see what kind of runout it has just for curiosity. If the chuck has excellent run out then I'll leave it as is (which has never really happened to me anyway with the few I made with "tap-tru" in mind). But ya never know! Then you can quickly proceed to turn down undersize as planned.

One disadvantage of turning down the register smaller is that the chuck no longer has something to register against. So in theory with heavy machining the chuck can get knocked loose out of concentricity. It's never happened to me though & doubt my hobby lathe is even powerful enough to cause that to happen. I suppose this would be more of a concern on big lathes with big chucks. Not sure why I'm even mentioning it as I doubt it would be a concern for you either but there, I already typed it.

Carry on & don't forget to update us with pics, we like pics!

 

[mikey]

Boris, here's a thought. Whenever I'm making a chuck backplate & my intention is to make it a "tap-tru" regardless, I make the register a close fit first. The practice is good but I then fit the chuck to see what kind of runout it has just for curiosity. If the chuck has excellent run out then I'll leave it as is (which has never really happened to me anyway with the few I made with "tap-tru" in mind). But ya never know! Then you can quickly proceed to turn down undersize as planned.

One disadvantage of turning down the register smaller is that the chuck no longer has something to register against. So in theory with heavy machining the chuck can get knocked loose out of concentricity. It's never happened to me though & doubt my hobby lathe is even powerful enough to cause that to happen. I suppose this would be more of a concern on big lathes with big chucks. Not sure why I'm even mentioning it as I doubt it would be a concern for you either but there, I already typed it.

Carry on & don't forget to update us with pics, we like pics!

I've never seen a chuck come loose and move in use, either. I know it is possible but when two presumably precision flat surfaces are held together with hardened screws under torque, movement is sort of not that likely. If you have a crash or tool dig in that can move that chuck then you have other problems bigger than the chuck moving!