Centralizing Acme Vs. General Acme
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  1. #1
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    What would be a good example for using a centralizing Acme thread versus a general Acme thread? The only thing I get from the handbook is that general Acme seems to be only class on class fit, where as a centralizing Acme is interchangable fit from one class to another. Is this assumption correct? If so I am very curious as to why one would use centralizing. Thanks

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    Since no one will reply I will take another stab at it. Centralizing has a tighter tolerance for pitch diameter in relation to major diameter?

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    If you read the handbook carefully and are lucky enough to have the right edition it explains the diff between Gdneral Purpose Acme and Centralizing Acme. GP Acme are "flank fit" threads intended for valve stems, piano stools etc where ever motion producing threads are requires but where side loading is not a significant factor. They may be precision grade as used for axis drives or cut with die heads as used for C clamp threads.

    A general purpose Acme develops high friction because of the wdeging action of the thread flanks were long heavy lead screws are employed.

    Centralizing Acme threads are used where side loading is a consideration as in long lead screws that sag between supports. In a centralizing Acme thread the diameter fits are controlled so the flamks don't bear the weight of the screw. Usiually this means major diameter fit but not always. The major ciameter of the nut is made to clear the major diameter of the thread by a few thousandths while the endplay is controlled by the class of fit. The endplay is specificed indirectly by the difference between pitch diameters of the screw and nut and the algebraic resultant from the flan angle. The weight bearing and axial force are born by separate elements of trhe thread/nut design.

    There are 3 classes of fir for General Purpose Acem and 5 for centralizing with the tolerances and allowances closing from generous to tight with increasing numbers. If you like shop mathematics you'll love developing thread manufacturing data from the Acem thread foemulae.

    So when cutting centralizing Acme screw threads you'll need to control the thread depth in the nut to provied the correce amount of cleatance. This means ythe handy tool tip width on tha Acem thread pitch gage won't do you a lot of goo.

    Also you need to determine the internal hteas major diameter. For this you need to bore the minor diameter and take an accurate reading from it. Then when you couch off with the tool you know how much radial depth to give the internal thread and you;l have something accurate to measure from when the minor diameter is in range. Once you get the major diametes plus clearance you can side **** the tool to get the flank fit.

    A shrewd machinist makes short gages as he goes that duplicate the as-manufactured features of the screww and nut for immediate as well as future reference. There's nothing more lame and awkward looking than watching two or three guys trying to fit a long leavy lead screw in a nut still chucked in the lathe. It's a scene sure to make the boss foam at the mouth and come out of his office howling like a pack of wolves.

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    K,
    One application A Wormgear that is the nut on certain jackscrew assemblies, not having bearings, the major dia's provide location and bearing, (centralizing). Of course a Wormgear with bearings at the ends would not be required to be of the centralizing type.
    Les H.

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    Forrest,
    Again and again your knowledge just pours forth. Thank you for another highly enlightening post.
    Joe

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    Thanks for the detailed response. This certainly gives me enough to work with.

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    Laymen's terms...
    A GP Acme is held to a tolerance pertaining to the pitch dia... The faces of the thread are what takes the load, and only the faces. (Major and minor are usually held to like .01 plus/minus thread depth per side, giving .02 +/- tolerances on the major and minor dia) Once that PD starts to wear, nothing stops it but the major or minor diameters, which then begin to wear.
    In a Centralized Acme, you hold the major, minor, AND pitch dia to a tight tolerance, giving the screw "more" contact (and load resistance)on 3 surfaces instead of just the PD.
    Contact on 3 surfaces rather than just one, is mostly beneficial where the screw and nut will get regular use... all day,every day. A long screw (only supported on the ends)or a bent one, will quickly wear through a pitch dia in a nut, and "slop" or "backlash" will be created. If that is undesirable, use a centralized acme.
    Think of the lead screws and nuts in your machines.


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