2 1/2 Jacobs Taper
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  1. #1
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    Default 2 1/2 Jacobs Taper

    I'm rebuilding an old Drill Press that has a chuck which is labeled 2 1/2 Taper. It appears to be a Jacob Taper 2 1/2. Yes there is such a thing. Below is a table I found at Wikipedia.

    No one carries JT2.5 Drill Chucks. It looks like the JT6 is very close. Do you think it will be OK or do I need to modify the quill for the JT6?

    Thanks

    The Jacobs Taper (abbreviated JT) is commonly used to secure drill press chucks to an arbor.

    Taper Small End Big End Length
    mm inch mm inch mm inch
    0...........5.80.......0.2284.....6.35.....0.2500. ...11.11.....0.4375
    1...........8.47.......0.3334.....9.75.....0.3840. ...16.67.....0.6563
    2...........12.39......0.4876....14.20....0.5590.. ..22.23.....0.8750
    2 Short...12.39......0.4876....13.94....0.5488....19 .05.....0.7500
    2½........15.88.......0.625.....17.20....0.677.... ..26.80.....1.055
    3...........18.95......0.7461....20.60....0.8110.. ..30.96.....1.2188
    4...........26.34......1.0372....28.55....1.1240.. ..42.07.....1.6563
    5...........33.43......1.3161....35.89....1.4130.. ..47.63.....1.8750
    6...........15.85......0.6241....17.17....0.6760.. ..25.40.....1.0000
    33..........14.23......0.5604....15.85....0.6240.. ..25.40.....1.0000

  2. #2
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    If it was my restoration, go for it!

    You may have to lap in the two with a little lapping compound to get the tapers to match perfectly, but no big deal.

    Ken

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    interesting. how do you go about it to make it straight?

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    The quill and arbor are all one piece. Do you think I should risk trying to grind the arbor to be JT6?

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    I haven't done the math, but isn't the taper consistent across all sizes?

    Take the next size up, and have someone grind/turn it to fit?

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    Henry, I missed the length being almost .055" difference. But the taper may still match up. Still worth trying. If anything, change it to a No. 2 jacobs taper, or thread it for a threaded chuck mount.

    The No. 33 Jacobs taper was probably the replacement for the 2-1/2 taper they once used.

    Ken

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    If my maths is correct, the difference in taper corrected in the 2 and 1/2 one to 1" is 2.7 thousandths of an inch. Divided by 2 makes it 1.35 thou max on the surface of the thin end of the 2 and 1/2 taper on the quill.or you could say on the wall of thin end of the No6 taper in the chuck body.
    As Ken said , "a bit of lapping compound"

    Or maybe a bit of 'Fine' Valve grinding paste first .(With a cleanup before lapping)

    I would put a dab on the quill stub and hold the chuck body in a leather glove against the spinning quill . You will feel it when it's 'done' .

    Then its just a good clean up and an upward knock on the chuck with a leather hammer or block of wood (with the jaws fully withdrawn), and the job's a good'n.

    Davycrocket

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    I have hammers available for rental that will work from JT0 through JT5 and beyond. Or I'd be happy to mail you some abrasive paper free of charge.

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    Thanks everyone for the advice. I'm going to blue the taper and see how close the JT6 chuck I ordered fits. If it's off by a couple thousandths, I'll just lap it to fit. This will be the last chuck I put on this drill press, so a "custom" chuck is not a big issue.

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    I received the JT6 chuck from Enco and it does not fit on my drill press. It appears to be close but those few thousands of an inch matter. I can feel a slight rock if I push it side to side after mounting it. I'm debating whether to try something irreversable like lapping the nose of the spindle or grinding the chuck to be JT2 1/2. I'm also thinking of just keeping the old chuck. With new bearings in the drill press, the runout of the old chuck is 5 thousandths.

    I also checked if a JT 2 1/2 exists. It does not. It appears to be an odd ball taper used by Taiwan manufacturers in the 60s and 70s. No one makes a replacement.

    Thanks for all the advice.

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    Why not make a new spindle with the JT6 taper on it?
    If the chuck is too big for the spindle, I bet Loctite RC680 would hold it on quite nicely. You can get it off with pretty mild heat, like 350 DegF. Not something you'd do every day, of course.

    Whatever you do, don't weld the chuck to the spindle like the numbskulls where I used to work did.

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    Quote Originally Posted by Fasto View Post
    Why not make a new spindle with the JT6 taper on it?
    If the chuck is too big for the spindle, I bet Loctite RC680 would hold it on quite nicely. You can get it off with pretty mild heat, like 350 DegF. Not something you'd do every day, of course.

    Whatever you do, don't weld the chuck to the spindle like the numbskulls where I used to work did.
    I wish I had the skills to make a new spindle. I enjoy hanging out with you professionals but am in fact an amateur machinist. The spindle is about 14" long, has 6" of splined area, two precision bearing locations, and the JT2.5 nose. In addition to the skill it would take to make it, I would need to harden it properly and then precision grind the JT2.5 taper. This job is beyond my skills.

    Last night I cleaned up the old chuck, oiled it and, mounted it firmly on the rebuilt drill press. I checked the run out and it's only +/-3 thousandths. I think that is not bad for an old drill press so I have decided to leave well enough alone.

    I also found out that I had a 3MT/JT6 arbor for my lathe so the new chuck I bought will not go to waste. It's much nicer than the old chuck I had on my lathe.

    Thanks to all for your patience and help.

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    Henry did you ever solve the JT 2 1/2 issue? I'm encountering the same issue.

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    I used the green locktite on my Walker Turner drill press when the taper would not hold on tough drilling jobs. Its been on there 20 years or so, and runs true enough for a dp. If i need accuracy, i bore it on the mill. Just my .02.

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    Resurrecting a 7 year old thread here. OP has not posted here in almost 5 years...
    One thing that was not mentioned before, if the old and new chucks are Jacobs brand and bear the same base number like "6", "33", etc., there's a very good chance you can change out the old jaws and nut for a new set of jaws and nuts. Doing this may result in a almost good as new that would be much better in the long run instead of trying to find a exact replacement chuck. That's why they sell jaw replacement sets for these chucks. Ken


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