Lathe Chuck: Unstuck!

Witness marks
Witness marks

I managed to jam the 3-jaw chuck on my lathe by turning the lathe on without the formality of snugging the chuck against the spindle first; IIRC, there was maybe 1/8″ clearance. The resounding thunk when the irresistible force hit the immovable object was the prelude to about a year of increasingly desperate attempts to remove the chuck, punctuated by long periods of despair.

The absurd derring-do with clamping the 4-jaw Sherline chuck in the 3-jaw lathe chuck described there finally prompted me to ask my buddy Eks for advice, which is what I should have done in the first place. He suggested removing the chuck body from its backplate, building a lever that bolted to the backplate with the same six bolts as the chuck, blocking the spindle with wedges under the belt pulley, and wailing on the lever with a lead hammer.

We wondered if a hard hammer would be better than the lead hammer, perhaps because the impact would be less squishy, but that was in the nature of fine tuning.

The key idea is that removing the chuck body also removes a tremendous amount of rotational inertia, so that wailing on the lever arm actually transfers force / impact directly to the backplate, rather than trying to spin the body. The somewhat risky part is that there’s a pin connecting the spindle with the drive pulley (it’s disengaged when using the back gears), so that it’s entirely possible to break the pin rather than unstick the chuck. But it was still a better idea than any I’d had so far.

Stuck backplate
Stuck backplate

Making note of the witness marks on the backplate and chuck body, I removed the body. Fortunately, there was just enough clearance between the front bearing journal and the backplate that I could get the bolts out without dismantling anything else.

That left me with the rather grody and still firmly stuck backplate. The bolt disk was brazed onto the threaded cylinder with a keyway. Although the chuck body had a key slot, it looks like the matching key had been machined off of the cylinder, so the bolts were taking all the cutting torque. Worked OK for both the previous owner and for me, so I suspect it’ll continue to work just fine for the next owner, too.

Bicycle handlebar stem in spindle
Bicycle handlebar stem in spindle

Peering through the spindle reminded me of some recent bike repairs and it occurred to me that maybe, just maybe, a old-skool split-wedge handlebar stem would get enough traction inside the spindle to hold it in place. Some rummaging in the Bike Junk box produced just such a stem and it fit exactly into the spindle bore. Now the spindle is fixed in place by its ID and there’s no risk of breaking the locking pin or (shudder) the back gear.

Even better, the lumber pile emitted a chunk of 1×4 (actual dimensions!) wood that was precisely the correct length to reach from the floor to the stem. I like it when projects work like that; finding exactly the right stuff in the pile is sort of an omen that things are going well.

Fundamental rule: always start with a hunk of something that looks a lot like what you want to end up with.

Corollary: ya gotta have stuff!

Coordinate-drilling the lever arm
Coordinate-drilling the lever arm

Some rummaging in the parts heap turned up several feet of nice “angle iron”, so I bandsawed off a hunk. I should have realized something was wrong when a foot of teeth stripped right off the saw blade, but I ascribed that to, oh, maybe weakening a few teeth when I soldered up the blade.

I had our daughter run the trig to generate coordinates for the six holes, then lay out the center bore and bolt holes on the plate for practice. Drilling the first hole prompted me to resharpen the drill, but poking the remaining five holes into that plate produced vast clouds of wood smoke from the sacrificial plate underneath, despite boiling copious quantities of cutting fluid off the top.

I finally admitted defeat when the “angle iron” rubbed the teeth right off a 2-inch hole saw.

As nearly as I can tell, that plate is un-machinable stainless steel, hand-forged by the Devil himself specifically to taunt me, and is good for nothing. Obviously, I hadn’t used it for anything in the years it had been in my pile and, perhaps as an omen, it didn’t have any other holes in it from anybody else’s efforts. I’ll keep the pieces around just to sneer at them; won’t get fooled again.

So, at this point, I am out a bandsaw blade, a drill bit, and a hole saw. We won’t discuss the circle cutter or my abortive attempt to lash the damn thing down to the Sherline and perform helix-milling upon it.

Unstuck backplate with beating bolt
Unstuck backplate with beating bolt

While licking my wounds, I wondered if the bolt circle on the backplate would provide enough lever arm to make any difference. I tightened a sacrificial bolt & nut with one face of the bolt aligned along a radius from the spindle center, then deployed a big drift punch and the two-pound ball-peen hammer (a.k.a., The BFH).

A half-dozen good shots later, the backplate spun free. Notice the very small gap between spindle and backplate… that’s all it takes!

I added a closed-cell foam washer to fill the gap between the backplate nose and the butt end of the chuck; there was a remarkable amount of crud built up in there.

I am so happy that it even makes up for the death toll among the tools…

It’s worth noting that the headstock has two honkin’ big bronze spindle bearings, no delicate balls, and a few mighty thwacks didn’t do them a bit of harm.

11 thoughts on “Lathe Chuck: Unstuck!

  1. Congratulations on getting it loose! It’s galling to have broken or messed-up tooling sitting around waiting to be fixed.

    I built a lathe spindle driver, that’s basically the same thing as your bike stem wedge, only with a handle on it because I find it MUCH less stressful to thread short sections by hand-turning the spindle. The nice thing is it serves the same purpose, of removing a stuck chuck. (I found out the not-quite-as-hard way, by watching someone else do it, that locking the backgearing in an attempt to lock the spindle, is a good way to rip teeth off the bullgear, and that’s Very Very Sad.)

    It’s useful to mark “THIS IS REALLY FREAKING HARD STEEL” in Sharpie so you don’t grab that piece in a year and try again. I bought some apparently tungsten molybdenum steel shafting, that I’d intended to use for cutting a spindle, and watched in amazement as it rubbed a piece of carbide tooling smooth as a pool ball. Maybe some day I can figure out something to do with it but in the meantime it has lots of crabby notes about how worthless it is, written all over it. I got in this habit from stamping or writing on sheet silver solder so I’d know what temp range it melted at.

    1. Good suggestion! I’m just back from the basement with Sharpie in hand: although the gouges and hack marks should be sufficient reminder for me, the next person might need the hint. I’d recycle the damned thing, but it would probably bounce back.

      My buddy Eks taught me to write anything I had to look up on the inside of whatever it was that needed looking up. So, for example, the van’s engine compartment has “5qt w/filter” and “21 mm lugs” and suchlike scrawled all over it…


      1. The writing on the inside of the engine compartment is a great idea. I used to tape a note with the mileage every time I changed the oil, but then it’d fall off. Sharpie would be a much better idea. Sharpie everywhere!

        1. After a few years of figuring it out from first principle, I wrote the tire rotation pattern inside each wheel cover: in 2010, this tire goes on that corner of the van…

  2. I had a mysterious angle iron incident tonight. I grabbed a piece of 3mm thick sheet from the aluminum scrap pile, intending to make a motor mount with it, and then found it it was stainless. Not quite forged by Hephaestus himself, but close: I have some drillbits to resharpen now. Just thought you’d like to know.

    1. Maybe the stuff is transmuting on the shelf?

      Evil forces are at work in the world today …

  3. Ed-I tried your trick with the beating bolt to unstuck my chuck. I used a strap wrench on the other end. Worked like a charm. I think you save me a lot of time…and money!

    1. a strap wrench on the other end

      I thought of that, too, but it seemed entirely too rubbery. I suppose anything that prevents a spinning spindle is a Good Thing, as the impact ought to work even without locking the thing down.

      Now, repeat after me about a dozen times: “I will always snug the chuck before flipping the power switch!”

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