Tolerances on Morse taper? Effect of error?
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    Default Tolerances on Morse taper? Effect of error?

    I see from the Machinerey's Handbook, that the actual, given, tolerance on an MT is 0.002 per foot on the rate of taper.

    For a shank, it must increase taper rate, (making large end bigger) and for sockets the opposite, making large end smaller. In that way the small end will always be the loose point.

    That seems like a lot..... it suggests that for a 3" long taper, about an MT3, there might be a half-thou looseness.

    I have some MT2 blank arbors that I obtained for making T/S tooling. They are some unknown amount small at the small end, but are small enough there that they won't rub off a stripe of marker ink all around.

    My suspicion is that that is probably at least an error of the max amount, since for an MT2 that would be only about 0.4 thou of error. Maybe a little more, since there is no rub-off at the end, but I don't have a way of measuring it aside from a complex setup of gage blocks and sine plate, which I would rather not do. Besides, teh exact error isn't really the issue.

    I cannot FEEL any looseness, but obviously it is there.

    In practical terms, what is the effect of a maximum (0.002 per foot) error in taper rate?

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    "In practical terms, what is the effect of a maximum (0.002 per foot) error in taper rate?"

    I think it would depend upon the application. The MT was originally designed as a drill bit shank with a tanged end. The loading on a drill bit is straight down the axis, with no side load. In that application, I think a slight looseness at the small end would not cause any problems.

    But looseness when a side load is applied will probably cause problems on a tanged end tool. In the case of a tool in a vertical socket, like a drill press with a chuck on a MT arbor, a rotating side load can cause the tool to fall down out of the socket. That is one of the reasons milling in a drill press is not a good idea.

    In a lathe tailstock, the side load on a center is not rotating, and there is a high axial force keeping the center tight, so there is no problem from a slight looseness at the small end.

    There could be a problem using MT tailstock tooling that can exert a reverse axial load. For example, I used a three-roll Quick knurling tool with a straight shank inserted in a 3MT endmill holder on my lathe a couple of days ago. I did not pound the endmill holder into the tailstock, which has a lever feed, and just pushed and twisted it in hand tight as I do with a drill chuck. I found that pulling the knurling tool back off the bar exerted enough force to loosen the holder and make it spin in the tailstock. As far as I know, the tapers fit well and it was just a problem caused by not driving the tapered parts together hard enough.

    Milling machines with MT spindles, like the old Bridgeport M head, use drawbar tooling instead of tanged tooling. In that case, a slight looseness should not cause a problem. Side or reverse axial loading can not loosen the tool. The axial force of the drawbar should prevent the tool from perceptible wobble.

    Larry

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    I made some tapers and rigged a simple way to measure them. Using a small surface plate with a post mounted DI. I set a stop at a point where the big end of the work was under the point, zero the DI, insert a 2" jo bloc, measure again, that was the taper in 2". Obviously move the work to be sure of the high point.

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    Well, I simplified the measurement, to get a "close rough estimate", and found that if a taper is way out of spec, it won't fit well..... Kind of obvious, I guess.......

    Not having a gage, I measured the large end, and the small end, and the distance along the taper, then did the trig (based on a nominal taper) to get the axial distance, then converted to a "per foot" taper.

    0.7148 large end, 0.5764 small end, 2.741 along taper, Result I got is 0.6066 inch/ft taper. Too bad these are an MT2 with a 0.59941 taper per foot spec......... and a limit of 0.60141 per foot.

    Sighting them against straight edge, they don't appear "barreled" or "wasp waisted",

    Since they pretty much "flop out" of the taper pretty easily at any reasonable seating, the answer seems to be that a taper error 3X the max tolerance will not give an acceptable fit.


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