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What makes them so accurate is that you cut the gripping surface right on the spinning lathe. You are cutting a pocket for the part, using the machine they’ll be spinning on, right into the jaws, “en situ”. It HAS to be concentric. Hence the elegance.

There’s a key step to this. You’re probably thinking “but the jaws wiggle in the chuck body on the scroll”, and that’s correct. That’s what the back jaws are for. More on that below. First things first, close or open the uncut jaws to where you want them such that they’ll have the most grabbing surface once your pocket is cut. To do that, make a slug the exact (sic) diameter of the parts you intend to grab. Or, as often is the case for me, I’m working a new prototype part through the steps to make it, and I come to a situation where I need to grab a diameter I just machined; you could use THAT instead of a slug. But you need the slug at the desired finished diameter; the slug will be your gauge to test-fit the pocket you are cutting into the jaws. You are cutting that pocket the exact (sic) size of the diameter you’ll be grabbing.

So you’ve got your blank jaws positioned where you want them, no pocket cut yet, but they are loose and floppy. Cannot cut them like that, don’t try. That’s where you need a second slug, and it needs to be the correct diameter (ish) such that the back jaws are grabbing it when the soft jaws are where you want them. That tensions the master jaws against the scroll. A thousand words. See the slug deep in the chuck with the red X I put on it? That’s the purpose of those back jaws!



I reverse those aluminum jaws to grab two different diameters on these parts I’m running. A huge plus I forgot to mention- note the built-in work stop you create via the depth of the pocket! If you made the jaws correctly so that they have that light snap-fit onto the master jaws, you can reverse them with impunity, without even needing to adjust concentricity, once you’ve adjusted for the first side….. (that comment is assuming now that you have a Set-Tru or Adjust-Tru type chuck), because, at least in this case, I cut both sides of those jaws with the chuck body in the same position, adjustment-wise.

On a non-adjust-tru chuck, IE one mounted directly to the spindle, it *should* repeat its basic position on the spindle quite well. So when you put the jaws back on the chuck, they’ll be in the same place on the master jaws- because of your careful snap fit-, and the chuck body will be in the same place on the spindle nose, because of the taper……. And so there your pocket will be, in your soft jaws, running damn near perfectly (sic) true.

(Or, if you are an Adjust-Tru geek like me, put the soft jaws on then use your slug, or a part, to set everything running true at that gripping diameter. I won’t own a scroll chuck without this feature. A chuck that is out by even just a few thou as far as concentricity is useless to me. It just creates downstream headaches…)

It’s the same idea as 5c “emergency collets”, which are another thing, if you aren’t using them, you should! Obviously just buy those. They are CHEAP.