Truning an MT4 down to an MT3

I'm posting this because the question seems to come around here pretty often about turning a MT taper for tail stock things.
A few weeks ago a friend gave me a couple of nice Jacobs 14N ball bearing chucks on MT4 arbors... He knew I needed a tailstock chuck for the Clausing, trouble is the Clausing tail stock is MT3, he figured I'd just go buy an MT3 to Jacobs...
Well today I had a bit of time and figured I'd see if I could turn em down from MT4 to MT3.. I figured I had nothing to loose... First thing I thought was I'd have to take them off the chucks, but didn't have a "pickle fork" for em.. but hey the BP hacked one out in about 5 minutes. I popped one out and as I pondered how I was going to set it up in the lathe I realized I don't have MT4.5 dead center for the head stock, and getting a dog on the already taperd part would have been a chalange, but I do have a dead center for the tail stock.... Hmm I thought well what now... I decided the best thing would be to stick a piece of 1/2 inch CRS in a collet, grab the 1/2 in the Jacobs chuck and see how close to on center I could get it, a minute of tighten, loosen, tighten, loosen on all three holes and I had the damn thing under .0005, hey close enough for Govmnt work.... Next I checked to make sure I had enough travel on the compound to do this in one pass, ahhh just barley (no I don't have a taper attachment) ... Then I checked to make sure that the #4 taper was the same as the #3 by holding a #3 backwards against the #4 and mic'ing it in several places, yup same size in all places... Q.E.D same taper... Took about 5 minutes to get the compound dead nuts on the #4, but the .0005 reading Starretts needle didn't move any I could see so... 4 passes with the compound taking .050 on 3 and .005 for a clean up, polish at 2000 rpm with 1000 grit, grind the tang down in thickness a bit and I'll be damned, it works just fine.. :-)
--.- Dave
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    [ ... ]

    Not quite, I'm afraid. According to _Machinery's Handbook_:
MT3    0.60325 inch/foot MT4    0.62326 inch/foot
The only two of the Morse tapers which have precisely the same taper are the MT4-1/2 and the MT7 (0.62400 inch/foot on each).

    Hmm ... maybe you were lucky. Do you have spotting blue? Blue up the arbor (as thin a layer as you can manage), fit it in the tailstock and gently twist it before pulling it out. That will give you an idea how close it is.
    Since the MT4 has a slightly steeper taper, I suspect that you will find that most of the contact is at the large end -- which is at least better than most of the contact being at the small end. :-)
    I would have expected that arbor to be hardened and ground on the OD of both the Morse and the Jacobs tapers -- you lucked out there, too.
    Good Luck,         DoN.
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DoN. Nichols wrote:

Don, as a matter of intrest, the 10 microinches per foot (an inch in 227+ miles) or <5micro in. / length of taper would be in comparison to how thick is the spotting blue? Also what is the surface finish? Curious. ...lew...
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    Did you miss that the second significant figure was different? That is 0.020010" difference per foot, or 0.001667" per inch, not merely 0.000010" per foot.
    Yes -- I was surprised by the difference there and double checked the table from _Machinery's Handbook_. This was the table which gave the ASA tapers, which mix B&S, Morse, and Jarno, IIRC, and which is the only place that I've seen the MT 4-1/2 listed (which is what is present in my lathe's spindle).
    Enjoy,         DoN.
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DoN. Nichols wrote:

WOOPS!!!!!! ...lw...
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Don,
I spotted it with a Sharpie, and it looked good the me, I see contact down the full length of my surface.
Guess I was lucky.. I'll take it :-) But geez .020 difference per foot on the 3 inchs I turned is probably good enough... that works out to .005 over the length I turned...
Honestly when I did my measurement with the 3 basackwards to the 4 I got about .001 difference down the 3 inchs I measured, I chalked that up to my test rig...
And yeah I gave it a quick file test to make sure it wasn't hardened.. FYI al the MT's I have seem to be un-hardened..
Oh and funny I never though of digging out one of the three Machinerys Hand books I have and checking the taper specs.. DUH.. :-)
Dave
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Hey Dave,
Ummmm.....you do know that MT3 to MT4 sockets (and other adapter sizes) are available. They are called "Extension Drill Sockets", and for instance from KBC Tools would be a KBC # 1-5161--045 .
<http://www.kbctools.com/can/main.cfm page 406
Using them will lose you some "between centres" distance.
Brian Lawson, Bothwell, Ontario. XXXXXXXXXXXXXXXXXXXXXXXXXX
On Sat, 6 Oct 2007 17:50:00 -0700, "Dave August"

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wrote:

A centre in the headstock is a _live_ centre. Honest!

The other trick is to mount a bit of stock in a collet and then turn a 60 degree taper on it. Only takes a few minutes more than taking a truing cut off a proper (soft) live centre, which you would do for precision work anyhow.
Regards Mark Rand RTFM
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Mark,
LOL.. Uh.. yeah... I'll guess it is... :-)

I thought about doing that, but figured getting a dog on the already tapered piece was gonna be a bitch...
I also thought about trying to get a good enough grip on the tang end in a collet (it is turned round), but it wasn't a nice size, IIRC it was like .970.. and I only have 1/16 collets... the .970 was just too small to "fudge" in a 1.00, and too big to "jam" in a 15/16.. now If I'd had a 21/32... I still probably wouldn't have done it.. :) Just didn't look like enough grip... which is probably wrong since I bet the surface area of contact was larger than what I had from the Jacobs jaws on the 1/2 CRS... I just didn't like how the contact would be on the collet, not enough surface on 2 of the sides... or worse only contacting 2 of the sides...

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    Though the terminology has drifted since the days when there were no ball bearing tailstock centers. :-)

    Or -- chuck some stock in the 3-jaw chuck and turn the 60 degree taper, and that gives you a surface (the side of the chuck jaw) to drive the dog.
    Since I've never found a 4-1/2 MT center for my lathe, I took the time to set up the taper attachment to match the spindle's taper, and turn a pair of adaptors -- one 4-1/2 MT to 3 MT (to match the proper tailstock), and one 4-1/2 MT to 2 MT (to match the earlier tailstock which I used when not using the bed turret until I got the proper tailstock. Both had the same bed fitting and the same center height, but the smaller one was made for an earlier Clausing with a smaller spindle thread. Mine came with a 2-1/4-8 spindle thread, and I later replaced it with an L-00 spindle, which is they one which has the 4-1/2 MT taper.
    Enjoy,         DoN.
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Brian,
Sure I know about those, I have several 2-3 and 3-2 an 2-1 from when I had a smaller lathe,
And as I said, my friend just thought I'd blow the 10 bucks and buy a MT3-Jacobs and replace it..
I said I just figured I had nothing to loose :-)
--.- Dave
wrote:

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you don't need forks to remove a Jacobs chuck from it's arbor. Simply open the jaws wide, drill a hole through the bottom of the chuck, 5/16 is about right, then place a 5/16 rod through the hole and 1. Hit it with a hammer, or 2. use an arbor press, or 3. use an air chisel with a long pointy chisel. out pops the arbor.
place arbor between centers and cut taper to suit.

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Naa using the BP to make a "pickle fork" was MUCH more fun...
'cides I like to make tools and now I have one, don't forget I said I have several of the 14N's, eayser to make the tool than keep drilling holes in chucks.
--.- Dave

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