I have an old Jacobs keyed drill chuck in my box-o-spares, and I need a drill chuck for my hobby lathe. I have a suitable tailstock arbour, but the drill chuck has something already installed, and I can't remove it.

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The label reads:


and further around reads

3/8 (10mm) CAP
3/8 24 THD KG

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Here's the shot up through the wide-open jaws. Inside is a large-flanged bolt with a shallow 1/8" hex socket. A 3mm tool spins in the hole, and a 3.5mm tool does not fit. Being a metric country, old imperial/SAE/standard/american sizes are less common. I have exactly one allen key that fits. As per photos, I've managed to pretzel the shaft by using a vice and pliers to apply torque in either direction.

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  • Is the little bolt going to be conventional right-hand thread or backwards left-hand thread? It doesn't move in either direction so far.
  • Will applying heat damage the chuck? If there's thread-lock, heat may loosen it.
  • Which two areas should I be applying torque between? The old arbour, the outer ring, or the hex tool in the small bolt?
  • Is the old arbour threadded or some kind of taper? The Jacobs chuck references like http://www.michiganpneumatic.com/customer/mipnto/extrahtm/jacobsref.htm and http://www.jacobschuck.com/drill-chuck-removal-guide do not show how this model chuck is mounted.
  • LAST RESORT - should I drill out the little bolt ? No idea what kind of steel its made from, but it didn't distort when I put a 90 degree twist into the allen key, so its likely to be hardened.

I do not need to save the old arbor. There's plenty of apprentice marks on it now from my vice.

I hope to mount it on this morse-taper0:threadded arbour from my Sherline lathe. The largest flange is 16.48mm in diameter on the old arbour and the MT0 from the lathe has 15.90mm

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The chuck moves smoothly so I'm happy it will be good. I have not bothered cleaning it yet - unless I can remove the old arbour it will be pointless.

Certainly the usual solution would be to order parts, but given the C19 lockdown, I can't even leave my home, so everything needs to be in-stock already.

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    Similar to but not a dupe of diy.stackexchange.com/questions/160778 and diy.stackexchange.com/questions/108337 – Criggie Apr 1 at 1:49
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    Thread lock requiring heat to remove is pretty normal, yes. Apply heat to the back - try to avoid heating the movable jaws, which are hardened. – Ecnerwal Apr 1 at 3:30
  • @Ecnerwal thanks - I have just brought the chuck to ~70 degrees C using a small butane torch, measured with an IR thermometer, and that made no difference to the bolt. Chuck is now in the freezer chilling down to see if shrinkage helps. Once that idea fails, I'll try to file the old arbour and see exactly how hardened it is - one suggestion from coworker was to simply file the old arbour to a MT0 taper and use it as-is, but that accuracy level could be difficult at home. – Criggie Apr 1 at 3:53
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    If you can mount it on the lathe (arbor supported by tailstock center) so it comes out properly concentric, filing/grinding to fit a Morse taper could even be upgraded to scraping it to final fit - a very classy machining operation more tedious than difficult if you have some paint product that will emulate the role of Prussian Blue in most of the tutorials you'll find, and a great Quarantine project (learning to do scraping to fit) IMHO – Ecnerwal Apr 1 at 4:03
  • @Ecnerwal I'm working on this approach right now. Downside, have blown the main drive belt and my home-brew solutions there aren't much good. Will update this later. – Criggie Apr 3 at 3:29

I agree with heat if that is a screw but that doesn't look like any screw I have seen on a Jacobs Chuck (and mine were left hand threads) .

Is the model number on the chuck visible? If it has a 3jt or a 4jt it is a Jacobs taper different than Morris taper but to remove will require force. I normally use a mechanics pickle fork for separating ball joints , lock the tail in a vice and a few good wacks and the chuck will come off the tapered shaft if it is tapered or JT3 or 4 . There might be a 2 but I have only worked with 3 & 4.

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  • Its very small but the code "3/8 24 THD KG" is engraved on the side. My current plan is to reduce the arbour enough to use it in the morse taper 0 tailstock. I'm using a whiteboard dry-erase marker to show what's touching and still too high. I have low expectations of alignment quality on this job. – Criggie Apr 3 at 3:32
  • I missed that the first time it is a 3/8-24 the center screw is normally left handed and lock tight thread locker is normally used getting it hot will soften the lock tight once the screw is out then you should be able to remove the chuck. I have used a heat gun many times to soften both red and green locktight where I was ruining a screw once the metal got hot the screws usually come out easily. There is grease in the jaws but if it smokes you can keep it working smooth with oil or grease adding at the inside base of the jaws and cycling closed and open so don’t worry about the lube. – Ed Beal Apr 3 at 15:42

Final update - I managed to machine something close to a MT0 taper into the old arbour. I also cleaned up the surface rust from the outside.

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This lathe has a head stock that can rotate for tapers, which was a little sketchy given how much stick-out there was.

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This shows the rotated headstock, so the taper is very approximately straight. I didn't have a good solution for calculating the 1.49 degree rotation - will have to do better. Perhaps a really big protractor?

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And here's the newly finished drill chuck in use, drilling a hole in the top of a schrader valve cap... for reasons.

Only downsides - 1) its not stunningly accurate, but has already proven way better than "aim and pray" with a hand drill. 2) the tail is quite long, I did not want to remove the center hole on the end, so it only gives 20mm of travel on the tailstock.

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    ...but teacher, when are we ever going to use trigonometry in real life? - Machining a 1.49 degree angle during a quarantine, for one thing. – Ecnerwal Apr 5 at 11:00

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