Rejuvenating a Pratt Burnerd LC-15?

[mcdanlj]

With everything frozen, I was going very gingerly to avoid doing any damage. I finally used a piece of soft 2x2 to drift the sliding cone a tiny bit forward from the back, which unlocked the sleeve enough that I could remove the 3/32" dog-point grub screw that retains the sliding cone, then drift the cone out further, and eventually removed the sleeve, release key (also frozen in its slot), and balls, and then drifted the sliding cone out.

The sleeve being frozen was from the sliding cone locking it in place by being frozen against the balls, pushing them out against the sleeve.

The mating surfaces on the cone and inside the body don't look too bad, but no amount of cleaning or light oiling has allowed me to mate them again.

I'll have to clean the sleeve and make sure it's in an acceptable state, and then I guess look at lapping the cone and/or body until I get a smooth sliding fit.

But the seller has also agreed to take the return if I can't get it working.

 

[mcdanlj]

I have stoned the outside ground surfaces of the sliding cone, and very carefully worked the body nose, with an arkansas stone and have not yet achieved a sliding fit. I have several grades of diamond lapping compound on order, and plan to work the surfaces carefully.

I don't have a great sense for how parts that still look more or less shiny and used to slide might no longer fit. My naive assumption is that slight oxidation (too thin to be visible as rust) can still make surfaces thicker and change a close sliding fit to an interference fit. Can components of the oil be incorporated into the metal? I know that high pressure oils can contain inorganic compounds (e.g. sulfur compounds including molybdenum disulfide, phosphides, etc.) — can some of these eventually be incorporated into the grain structure of the metal and change it dimensionally?

I see that in some strange coincidence, a youtuber hobby machinist had some similar timing on getting their hands on a lever-actuated collet chuck of a different design and from a different manufacturer that had some of the same problems with what should be sliding fits not sliding, though my experience is an even tighter fit than he shows, as far as I can tell.

I'll skip the "lubricating" with WD-40 part of that video, but making a shop-made mandrel for lapping makes sense to me. I think they will have to be aluminum; I don't have brass the right size.

 

[mcdanlj]

I mounted the body alone on the lathe. Since it's a D1-5 it has six possible orientations. In each orientation, the inner bearing surface of the body in which the sliding cone runs, which should be coaxial with the spindle, has about 1.5 thou of runout. Which, for a ground face, is unexpected. So my best guess is that this was hit pretty hard at some point (or perhaps dropped) and was damaged to be ever so slightly out of round, which made the cone no longer slide free, and that's likely also why it sat so long that it also had hardened oil in the cap threads. But that also means that lapping is definitely the wrong approach, and this collet chuck might just be dead.

 

[mcdanlj]

If I had the right equipment, I would love to have refurbished it.The cone merely slides in the body while setting the collet and is fixed in operation, so if I was set up for electroplating and had a toolpost grinder, I could have put a couple thou of metal onto the inside surface of the body, then ground it back off concentric with the register until the cone just barely fit.

But I'm not set up for that and don't plan to be in the near future, so I'm returning it.

Before discovering that the repair was beyond my skills, I had done a lot of work with an arkansas stone and the cap now screws on nicely and most of the surface corrosion is gone. So if anyone else with a D1-5 spindle nose, a penchant for repair, and the right tools would find this interesting, the dealer to whom I am returning it is Fram Fram in Savage, MN, and I'm sure they'd be delighted to get it off their hands again, this time to someone equipped for the necessary repairs. ☺ (I have no direct interest, just happy that they are accepting the return with 100% good grace and were good to work with while I was trying to make it work.)