Drawbar force on Van Norman C taper?
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    Default Drawbar force on Van Norman C taper?

    Is there a spec for drawbar torque for the VN mills with "C" taper?

    I have been putting the VN 12 in low speed, and using a 10" wrench to give the drawbar a good pull, without using a pin wrench to hold the spindle.

    The torque values for other systems, like the ER collets ,suggests I have been seriously under-torquing the drawbar.

    Thanks, Stoneaxe.

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    Good question, and I can't get at my VN docs for more than a week. My impression is that it should be equal or less than an R8 setup, which in practice is a lot less than ER.

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    Math would say the steeper the taper angle, the more torque required to obtain sufficient retention force.

    The VN "C" is 15 degrees, the R8 is 8 degrees. The Bridgeport documents say a "normal" tightening, whatever that is.

    Obviously tensioning to collapse a collet onto a shank may be different than retaining a solid endmill holder.

    Sure would be nice to have an actual torque spec, at least to ball-park the guess work. I was astounded to hear of the 80ft lb plus to tighten some of the ER collets- my motorcycle axle has a 70 ft lb value and I have to pull very hard on the wrench.

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    With ER as well as all double taper and similar collets, you apply compressive forces, whereas with 5C, R8, VN-C, etc, you are applying a tension to the body of the collet.
    Given that the goal is to hold the tool as tight as possible, the more force the better. However, with drawbar type collets, the limit is in the very structure of the collet and how much tension they could stand without damage. Definitely much less than the torque you can use on a much larger thread, like the nut of a ER chuck, etc.

    Paolo

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    Quote Originally Posted by Paolo_MD View Post
    With ER as well as all double taper and similar collets, you apply compressive forces, whereas with 5C, R8, VN-C, etc, you are applying a tension to the body of the collet.
    Given t hat the goal is to hold the tool as tight as possible, the more force the better. However, with drawbar type collets, the limit is in the very structure of the collet and how much tension they could stand without damage. Definitely much less than the torque you can use on a much larger thread, like the nut of a ER chuck, etc.

    Paolo
    The VN C has a .775 diameter thread- pretty sure it could take all the torque I could put on it with a 12" wrench. Standard rating on a grade 2 3/4" bolt is 125 ft lbs.


    The steep taper on the VN C would likely not be a problem with sticking.

    My concern is that the fretting on the taper was a result of insufficient drawbar force, and not solely because of an out of concentric spindle.

    Time will tell, I will probably get an inexpensive torque wrench to at least keep some control over it.


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