As you guys know when you are truing the three jaw chuck you need to put a washer or quarter in the very back of the soft jaws and tighten the chuck against it. This preloads the jaws and makes the jaws solid enough to dress.

A while back I posted about relieving about 1mm on the back of the jaws so that the 'truing washer' will sit on the steel part of the jaw rather than the aluminum soft jaw. This eliminates the much dreaded and very inaccurate filing of the tit left over if the washer is clamped in the soft jaw. This is shown below...




It still bothered me though that the jaws were being referenced against a really imperfect stamped washer. I suspected that if one were to use a more precise reference then you would wind up with more precisely turned jaws. I was messing around this week with a small piece of 1/2" round stock and stuck it in the center of the chuck. I could tighten the chuck against it but it was very hard to hold in place. My son has the idea of using a larger piece with a taper on the end so we whipped up one real quick out of some 5/8" aluminum round stock. A picture of the truing plug is below...




The extra length and taper help keep it more steady while closing the chuck. The backs of unrelieved jaws will slide right by the plug as well. Now the chuck can be tightened on the plug and very accurately trued. The downside is the small opening left won't clear the stock boring bar so you have to use a tiny one. (I think you could make the plug 0.7" dia. and gain a bit more working room.) The pictures below show the plug being tightened in the chuck. I was truing up the stepped side of the jaws here.






With a 2" piece of stock chucked in I was seeing about 0.0005" TIR. I need to find a larger piece of reference stock to get more reliable measurements though. Still, this is most accurate I've had the 3-jaw chuck. There are a couple of pics of what I was working on below. It's a 1.5" die holder in 2.5" dia. aluminum stock. I tapped the back of it to fit directly to the tail stock. I thought it might be more stable to turn the chuck while putting just a bit of pressure on the tail-stock handle rather than having the die-holder slide on it's own arbor. I still need to cross drill and tap it for the set screws to hold the die in place.

Thanks Jeff....Made it a Sticky

Cool Jeff - a half thou is pretty impressive for the stock 3 jaw scroll chuck.

BTW - I couldn't help noticing the bar holes on your scroll are nice and round. Most (including mine) are deformed slightly oblong from trying to get a good grip on the material in the chuck. What is your secret, or was this just a low time chuck?

Cheers,
Mark

The holes are a bit ovaled. I dressed the tops up with a file recently to make them look a bit better and the chuck is fairly new. I have found though that even when applying enough pressure to bend the bars/deform the holes the actual clamping pressure developed is not greatly increased. I think the trick with a soft jaw chuck is to design your machining process to keep the piece well supported for as long as possible. Using a steady-rest and/or live center can really make all the difference in the world.

I rough machined the die holder shown above last Friday on my Clausing lathe at work. Even with a six jaw chuck I was able to push the stock back into the chuck while drilling the initial hole to clearance a boring bar. I was just feeding too fast and overcame the static friction of all six jaws. (Yes, the six jaw chuck is very cool.) When I chucked the piece into the Taig I was taking very light ~0.001 finishing passes and had no slipping issues and my little Taig lathe produced a very, very fine finish.

I checked the run out again tonight with a ground reference borrowed from work (about 1.5" in diameter). It was indicating 0.001 TIR and I noticed the indicator was not quite square to the stock, so I adjusted it closer to square and was reading 0.0005 TIR. Not too bad!