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Thread: Profile of a line: implied 'symmetric' constraint

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    Profile of a line: implied 'symmetric' constraint

    Hello,

    I have attached a sketch that is a simplified version of a slot profile I'm trying to constrain. My question is: is it properly communicated that the '8' dimension on the top view is centered around the '2' dimension because of the feature control frame reference to datum B? I put 'symmetric' in quotes in the title because, although the slot length is centered on the square cut-out, it is not symmetric about datum B due to its tilt.

    Do I need to add another dimension from either end of the slot to datum B in order to constrain it in the Y-direction (relative to the top view)?
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    Administrator Kelly Bramble's Avatar
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    Quote Originally Posted by whknapper View Post
    Hello,

    I have attached a sketch that is a simplified version of a slot profile I'm trying to constrain. My question is: is it properly communicated that the '8' dimension on the top view is centered around the '2' dimension because of the feature control frame reference to datum B?
    Yes, the 8mm feature is drawn and dimensioned symmetrical about Datum B. Your 8 mm dimension should actually be placed parallel to the 8MM feature, or perpendicular to the end radius.
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    Kelly,

    Unfortunately, that is not the intent of the design. The intent is to have the slot tilted, but have an overall travel in the y-direction (relative to the top view) of 8 mm.

    The reason behind this is that there are multiple versions of this product with different angles, and I want to keep that 8 mm y-direction slot travel constant. That is why it is dimensioned the way it is, and not by the true length of the slot.

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    Administrator Kelly Bramble's Avatar
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    Quote Originally Posted by whknapper View Post
    Kelly,

    Unfortunately, that is not the intent of the design. The intent is to have the slot tilted, but have an overall travel in the y-direction (relative to the top view) of 8 mm.

    The reason behind this is that there are multiple versions of this product with different angles, and I want to keep that 8 mm y-direction slot travel constant. That is why it is dimensioned the way it is, and not by the true length of the slot.
    ? Placing the 8mm dimension parallel (perpendicular to radius) with the length does not change the intent nor the specification - it is most correct per. ASME and ISO.

    See ASME Y14.5-2009, Figure 1-29 and associated text.
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    Kelly,

    Dimensioning the slot parallel to its centerline is probably more acceptable per the standard, but it drifts farther from the intent of this design, which is to control the y-component length of the cut (i.e. the length perpendicular to datum B).

    The actual length of the slot is not critical to the design; what's critical is the angle, x-position, and the y-travel (making the length of the slot itself a driven dimension, rather than a driving one). If you imagine a right triangle, I want to dimension 'a' and 'b', plus another angle besides the 90, but not 'c' (the 'hypotenuse', or in this case the slot length). This fully defines the triangle.

    But I might be getting off point. Let's say the slot is straight up and down, making it symmetric about datum B. Is it understood from the sketch that the midpoint of the slot centerline is coincident with the datum B plane? Or do I need to add another dimension showing that the end of the slot is 4 mm from datum B in order to constrain the Y-location of the slot?

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    Administrator Kelly Bramble's Avatar
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    Quote Originally Posted by whknapper View Post
    Kelly,

    Dimensioning the slot parallel to its centerline is probably more acceptable per the standard, but it drifts farther from the intent of this design, which is to control the y-component length of the cut (i.e. the length perpendicular to datum B).
    As an ASME certified Senior level GDTP professional since 1997 (Google me), and having wrote and published eight books on dimensioning and tolerancing - NO... A parallel dimension does not change the orientation of the associated feature it gives a true length as would be measured.

    Look at the referenced ASME figure and paragraph.


    Quote Originally Posted by whknapper View Post

    But I might be getting off point. Let's say the slot is straight up and down, making it symmetric about datum B. Is it understood from the sketch that the midpoint of the slot centerline is coincident with the datum B plane? Or do I need to add another dimension showing that the end of the slot is 4 mm from datum B in order to constrain the Y-location of the slot?
    No, a center line simply defines that the drawing view is perpendicular to the eye. A "center line" does not indicate symmetry (common misconception). See ASME Y14.5-2009 paragraph 2.1.1.3 and 2.1.1.4.

    The symmetric orientation of the slot is captured in the drawing - you got it.. the as-built surfaces must fall within the tolerance lines given by the profile of line specification.
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    Quote Originally Posted by Kelly Bramble View Post
    As an ASME certified Senior level GDTP professional since 1997 (Google me), and having wrote and published eight books on dimensioning and tolerancing - NO... A parallel dimension does not change the orientation of the associated feature it gives a true length as would be measured.
    Okay! And I apologize if I came across as questioning your answers or credentials, I did not intend to do either. So it sounds like the original sketch I attached has the pocket fully defined and conforms to the ASME standard, if not in the best way. I just wanted to make sure that the slot ends couldn't be shifted up or down relative to datum B outside of the given tolerance.

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    For clarity on dimensioning a slot I was recommending that the op place the dimensions NORMAL to the slot feature not change the physical orientation of the slot feature. Though I do think the original dimensoning scheme can work for metrology though it could require additional work to properly position a mechanical stylus for the most correct measurement.

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