# TOS 3 jaw chuck



## Boris Ludwig (Jun 9, 2020)

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"?


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## mikey (Jun 9, 2020)

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.


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## Boris Ludwig (Jun 9, 2020)

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.


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## Mitch Alsup (Jun 9, 2020)

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.....


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## darkzero (Jun 9, 2020)

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.


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## Boris Ludwig (Jun 9, 2020)

Mitch Alsup said:


> Wish me luck.....



Good luck, Mitch.


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## Boris Ludwig (Jun 9, 2020)

Darkzero,

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


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## darkzero (Jun 9, 2020)

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


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## darkzero (Jun 9, 2020)

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!


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## mikey (Jun 10, 2020)

darkzero said:


> 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!


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## darkzero (Jun 10, 2020)

Yeah I suppose really only a concern on lathes big enough to fit our lathes between their centers?  

Edit: Big heavy interrupted cuts come to mind.


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## Tozguy (Jun 10, 2020)

I vote for trying the chuck first like DZ suggested.  You can make it a tap-tru at anytime later should the need arise. Bear in mind that independant jaw chucks are better suited to dialing in work than a scroll chuck with set-tru. 

The idea of making it a tap-tru would automatically apply if you overshoot the dimensions when cutting the taper. But it is an option that is not reversible and maybe not the most practical.


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## Boris Ludwig (Jun 10, 2020)

Next question: I don't have a mill or a DRO. But I do have a drill press. To find the position of the three holes on the backplate I'm thinking I'd use a set of calipers and measure/mark the position around from hole to hole. Is this the best way to do this or is there some practical machinist trick I don't know about that makes it simpler and more accurate?


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## darkzero (Jun 10, 2020)

Drill press is fine, the holes don't need to be precisionly located. Using dividers is fine too. I'm not sure if your chuck is a front or rear mount (mounting screws). If front mount you could use a transfer punch. I'm assuming it's a plain back chuck that's rear mount. In that case transfer screws would be a good method. Can also make them yourself.

In this case I personally would make the backplate register a close fit, or fairly close fit, make your mounting holes, then turn down for "tap-tru".


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## Boris Ludwig (Jun 10, 2020)

darkzero said:


> Drill press is fine, the holes don't need to be precisionly located. Using dividers is fine too. I'm not sure if your chuck is a front or rear mount (mounting screws). If front mount you could use a transfer punch. I'm assuming it's a plain back chuck that's rear mount. In that case transfer screws would be a good method. Can also make them yourself.
> 
> In this case I personally would make the backplate register a close fit, or fairly close fit, make your mounting holes, then turn down for "tap-tru".



Mounting holes are blind from the back. the Screws are M8 - 1.25 socket cap screws.
What are transfer screws?


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## Boris Ludwig (Jun 10, 2020)

Never mind, I just found some on the net


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## darkzero (Jun 10, 2020)

Transfer screw are like set screws which have points on them. You screw them into the blind holes with the points slight protruding. Then place the back plate on & tap the backplate (or chuck) lightly with a mallet so that the transfew screws will transfer punch marks onto the back plate.

If you search YT there are videos showing how to make them if you don't want to purchase them.


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## mikey (Jun 10, 2020)

I recommend you use transfer punches or transfer screws, depending on if the the bolts attach from the chuck side or plate side. 

Will and I were answering you at the same time. I'll elaborate.

Assuming your chuck bolts from the chuck side with bolts and also assuming you machine the back plate spigot for a loose fit, I would put a paper shim around the spigot and fit that into the chuck recess. That will center the chuck in the hole. Then I would clamp the backing plate to the chuck and use a transfer punch to mark one single hole. Then I would remove the chuck, spot drill that hole and tap it for the bolt. Then I would rejoin the plate and chuck and lock down that bolt you just tapped for and then clamp the plate and chuck again. This assures that nothing can move. Use the transfer punch to mark for the remaining two screws, then spot, drill and tap them. 

I would recommend you NOT use calipers to mark holes; it is not accurate enough - been there, done that. I also do not recommend you mark all three holes at once; mark one, clamp everything down firmly and then mark for the remaining two holes.

EDIT: I was just about to post my reply and saw that your bolts will pass through the back plate and into the chuck. In that case, use transfer screws instead of punches. The procedure is the same except that again, mark for one screw and drill the hole. Then insert the transfer screws for the other two holes so that their points just project above the surace. Bolt the plate and chuck together and snug the bolt down, then gently clamp the plate to the chuck and tap over the transfer screws with a dead blow or a regular hammer with a block of wood laid down on the plate. This marks your two holes and you can drill them and you're done.


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## Boris Ludwig (Jun 10, 2020)

Ok, cool. Thanks for the details


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## Boris Ludwig (Jul 8, 2020)

I took your advice and got myself some transfer screws and it was easy. Job completed. Run out 1" from the jaws is .00039"

I must say thanks to those who assisted.


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