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Thread: Die Set Hole Position Tolerance

  1. #1
    Associate Engineer
    Join Date
    Jan 2017

    Die Set Hole Position Tolerance

    For a manufacturing tool, we're designing what is essentially a die set using off-the-shelf shaft, bushing, & roller element components and pressing those into in-house designed plates. Two shafts are press fit into the base and are quite long. The two mating bushings are press fit into a plate. A cage with captive ball bearings fits between the OD of the shaft and ID of the bushing, providing a rolling contact. Die sets like this are commonly available off-the-shelf (e.g., here: xxxxx, we’re just mimicking with custom-shaped plates.

    I've been tasked with assigning the shaft & bushing holes' positional tolerances & perpendicularities. The dilemma arises since the die bushing vendor indicates that the shaft-ball-bushing fit is preloaded, ie a “rolling press fit”. As I understan it, there’s actually miniscule interference between each of the dozens of ball bearings and the shaft & bushing. So this is a very rigid fit with no play (clearance). This poses a theoretical problem for pairs of these bushings in terms of shaft-to-shaft spacing & perpendicularity. You’re basically trying to tolerance corresponding press fits, so fixed fastener type approaches are out the window (zero clearance in the “clearance hole”).

    A vendor competitor’s design guide recommends shaft-to-shaft spacing tol of +/- .0005” which I’m guessing they plucked out of the air as a tight but achievable machining tolerance. Mating two pairs of zero clearance interfaces needs perfect location and perfect orientation though. Like I said, these sorts of assemblies are available off-the-shelf lots of places so this is achievable. Commercial die sets don’t use slotted components or flanges w clearance holes – they’re exactly as we’re attempting. Thoughts on how to approach tolerancing the location & orientation of these holes?

  2. #2
    Technical Fellow Kelly_Bramble's Avatar
    Join Date
    Feb 2011
    Bold Springs, GA
    Tell me and I forget. Teach me and I remember. Involve me and I learn.

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