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Thread: Meaning of a radial tolerance

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    Meaning of a radial tolerance

    I am working on a series of very small parts that need to be laser cut, then verifying dimensions with an image dimension measuring system (Keyence IM6000). The part has an inside radius dimension of 3.47 +/-0.1. My understanding of this tolerance is that as long as the surface in question lies within the cylindrical shell with interior radius 3.37 and exterior radius 3.57, it fits the tolerance. However, since this part is not a full circle, just an arc, that means that arcs with radii anywhere between 1.5444 and 7.1940 can fit in this region (they just wouldn't have the same center).

    The Keyence system will recognize the edge and apply the best fit curve. So if I program it to find the arc that defines the edge and give me the radius, it could give me a radial dimension that is out of tolerance to a surface that is within tolerance.

    Is my understanding of the tolerance on the drawing wrong or should it really be a roundness/circularity tolerance? The Keyence system has the ability to measure roundness as well, I just want to make sure that I am interpreting the tolerance correctly. I've attached the image below to illustrate what I'm talking about.
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    Administrator Kelly Bramble's Avatar
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    Quote Originally Posted by hufeddk View Post
    I am working on a series of very small parts that need to be laser cut, then verifying dimensions with an image dimension measuring system (Keyence IM6000). The part has an inside radius dimension of 3.47 +/-0.1. My understanding of this tolerance is that as long as the surface in question lies within the cylindrical shell with interior radius 3.37 and exterior radius 3.57, it fits the tolerance. However, since this part is not a full circle, just an arc, that means that arcs with radii anywhere between 1.5444 and 7.1940 can fit in this region (they just wouldn't have the same center).

    The Keyence system will recognize the edge and apply the best fit curve. So if I program it to find the arc that defines the edge and give me the radius, it could give me a radial dimension that is out of tolerance to a surface that is within tolerance.

    Is my understanding of the tolerance on the drawing wrong or should it really be a roundness/circularity tolerance? The Keyence system has the ability to measure roundness as well, I just want to make sure that I am interpreting the tolerance correctly. I've attached the image below to illustrate what I'm talking about.
    First, if the dimension and tolerance is given as R 3.47 +/-0.1 - then it's a radius not diameter. If the dimension and tolerance is given as Dia. (symbol I mean) 3.47 +/-0.1 - then it's treated as a diameter.

    For a good old fashion radius (R) - the as-built radius surface can be any form as long as all of the surface elements fall between the outer and inner radius tolerance boundaries. - Not the same for a Controlled radius (CR).

    One could control the surface variations with Circularity, however the limits of size tolence technically includes a circularity tolerance control. So, in effect size with tolerance is controlling circularity.

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    With reference to the statement "any form as long as all surface elements fall between the outer and inner radius tolerance boundaries", if you look at the image I posted, you could theoretically fit a convex radius of 7.1940 in the tolerance boundary of a concave radius dimensioned as R3.47 +/-0.1 (arc length .97) and it would still pass?

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    Administrator Kelly Bramble's Avatar
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    Quote Originally Posted by hufeddk View Post
    With reference to the statement "any form as long as all surface elements fall between the outer and inner radius tolerance boundaries", if you look at the image I posted, you could theoretically fit a convex radius of 7.1940 in the tolerance boundary of a concave radius dimensioned as R3.47 +/-0.1 (arc length .97) and it would still pass?
    Here's what missing -

    The ends of the radius tolerance boundaries are to be blended tangent with the as-built surfaces intended (or drawn as such) to be tangent with the radius.

    Otherwise, without a center location or other location for the radius dimensionally and tolerance controlled - you could fit any number or arcs to the as-built radius. And for that matter locate the the radius tolerance boundaries at an infinite numbers of locations.

    Your tolerance boundaries are to be R3.47 +/-0.1 if specified on the engineering drawing as R3.47 +/-0.1 - no exceptions and if without tangent or blended surfaces the tolerance boundaries should share a common center point.
    Last edited by Kelly Bramble; 01-09-2013 at 11:05 AM.

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