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Thread: Feature of size question

  1. #1
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    Feature of size question

    Hello, my question is:
    Can a flat taper feature (consisted of 2 non-parallel faces) be considered a feature of size? The purpose is control that tapered feature with a position tolerance, attaching the feature control frame to the size dimension on the wide end of the taper.
    Thank you.

  2. #2
    Administrator Kelly Bramble's Avatar
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    That would be an Irregular Feature of Size. If the are surface elements opposite of each other then it is a feature of size.
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    Kelly, thank you very much for the reply.
    In continuation to that, another question if possible: according to ASME Y14.5-2009,
    Is it acceptable to control the taper for position and use it as a datum for the hole position, in the way shown in the attachment. The datum derived from the tapered feature should be understood as a centerplane.

    taper.jpg

    Thank you

  4. #4
    Administrator Kelly Bramble's Avatar
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    Yes... that works though without functional fit, and form specifications of the actual installation my response is only cursory. As you aware datum B is only two line elements.
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    Thank you very much for the reply, Kelly.
    I was concerned that since datum feature B symbol is attached to the width dimension given between the fillets, it might not represent the center of the tapered feature in all cross sections. Thus the orientation of the centerplane derived for datum B might not be clear. However this sketch represents a real case where the location of the wide end (3mm dimension) of the taper is the most crucial, and that is the reason why the position feature control frame and the datum feature symbol are related to it. Should I conclude that there is no such issue? If so, what makes the orientation of center plane B unambiguous? Again thank you very much for your ongoing help with this subject.

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    Administrator Kelly Bramble's Avatar
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    Quote Originally Posted by Semiond View Post
    Should I conclude that there is no such issue? If so, what makes the orientation of center plane B unambiguous?
    I'm seeing a very small detail of an end item were I have zero knowledge of end item design intent and functionality. The datums are adequate to define a DRF.

    Should you require the entirety of the taper contact surfaces, review ASME Y14.5 - 2009 Figure 8-24 and the associated text.

    Since this is a public and free forum, I only engage general answers and will involve myself with deep design GD&T or DFM/DFA analysis.
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    Thank you very much, Kelly.
    By your advice, I reviewed fig. 8-24. The combined control of profile of a surface and position can certainly be relevant, thank you for the reference.
    I'd like to ask one more question, If possible, probably the last one on this matter
    In case that the tapered feature is part of the stock material, and detailing the taper with a profile tolernace is not needed, would it be appropriate to attach 2 datum feature symbols, for example D and E to the two angeled faces of the taper, and call it out as E-D to establish the center plane to be referenced for the hole? My doubts are because I've seen Multiple Datum applied only with parallel or coaxial datum features (figs 4-22, 4-25 in Y14.5) if not proper, is there any other method to define a tapered feature as a datum feature other than specifying a profile control and attaching the datum feature symbol beneath the feature control frame of the profile?

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    Administrator Kelly Bramble's Avatar
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    Figure 4-29 and 4-47 as relative templates, identify the two surfaces as single datum entity. This should resolve into an equalizing datum feature as shown in the second referenced figure.

    And, yes you can create a single datum entity using the E-D specification. Just be sure you understand the resultant datum element (plane, axis of some combination).
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    Thank you for turning my attention to the resultant datum issue. I suppose that something like the "complex" datum feature (fig. 4-3, illustration g) is a good reference.

    I'm a little confused about the figure numbers you mentioned in your last reply. My copy of Y14.5-2009 has:

    "Fig. 4-29 Contoured Datum Feature Constraining a Rotational Degree of Freedom"

    And:
    "Fig. 4-47 Applications of Movable Datums"

    I can somehow see how fig. 4-47 might be dealing with two surfaces as a single datum (datum A ?) but I can't see it in fig. 4-29. Just want to make sure I'm not missing something.

    Thank you very much for all the help.

  10. #10
    Administrator Kelly Bramble's Avatar
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    A datum surface can be any geometry you can dream up - which is clear to me however the point I intended with Figure 4-29 was that you could find a method that indicates to the surface(s) which you intend to be a datum feature. If the dovetail surfaces were controlled with a profile tolerance (2X) you could simply add a datum callout. Obviously, 4-47 figure, left side are two datum lines. If you extend two lines from your dovetail feature surfaces and indicate both similar as shown you can define a single datum (equalizing).
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  11. #11
    Administrator Kelly Bramble's Avatar
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    So now, this is turning into high level training. I'm going to cut you off as training and consulting with folks on the internet does not put food on the table.

    Feel free to ask simple questions in the future unless you're already a customer - then email me.

    If your wondering about qualifications, google me "Kelly Bramble, GDTP, A&P"

    See:

    https://www.engineersedge.com/GDT_Training.htm
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  12. #12
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    Thank you very much for the help.
    I got all my questions answered, and I appreciate your effort very much.

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