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Thread: New Engineer learning GD&T with some questions

  1. #1
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    New Engineer learning GD&T with some questions

    Hi fellow engineers,

    I'm fresh out of school and still coming to terms with Y14.5 2009 standards. While I'm learning, I have some questions I hope some experienced engineers here can help me with:

    DATUMS: I often see drawings with defined primary (A), secondary (B), and tertiary (C) datums, and then a hole that has a datum reference frame with a location tolerance applied at A B C. But a book I am studying from tells me that we can switch the datum order to set a part up at B C A. Is that allowed if the drawing already specified A to be primary, B to be secondary (relative to A), and C to be tertiary (relative to A and B)?

    The other thing I'm seeing is often a drawing would define a datum D or E that is a feature based on A B C. Then another feature would be defined with a location tolerance with datums A D B. Is that allowed? Shouldn't B be before D?

    STRAIGHTNESS:

    When it's straightness of a size dimension like a diameter, what is the derived median line? How do you find the derived median line realistically?

    PROFILE:

    This book says we can use profile for everything, but if we can use a size dimension refined by orientation, form, or location, is that a better way of defining things than putting a profile tolerance and using some basic dimensions?

    Any and all help would be greatly appreciated! Thank you guys so much!

  2. #2
    Administrator Kelly Bramble's Avatar
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    Gd&t

    Quote Originally Posted by mingyichen95 View Post
    DATUMS: I often see drawings with defined primary (A), secondary (B), and tertiary (C) datums, and then a hole that has a datum reference frame with a location tolerance applied at A B C. But a book I am studying from tells me that we can switch the datum order to set a part up at B C A. Is that allowed if the drawing already specified A to be primary, B to be secondary (relative to A), and C to be tertiary (relative to A and B)?
    Datum features are specified with identifying letters and every letter can be used except I, O and Q. Depending on the functional or manufacturing requirements of a part or feature the identified datums are defined in an order of precedence within the feature control frame or using other approved methods. The order of precedence defined for a feature is most correctly specified to meet the functional requirements of that feature. The Datum order of precedence can be different for separate features on a particular part.


    The other thing I'm seeing is often a drawing would define a datum D or E that is a feature based on A B C. Then another feature would be defined with a location tolerance with datums A D B. Is that allowed? Shouldn't B be before D?

    The order of the letters is irrelevant as the order of precedence defines inspection and manufacturing setup requirements. Seriously, a feature control frame can have a primary datum Z, secondary P and tertiary K.


    When it's straightness of a size dimension like a diameter, what is the derived median line? How do you find the derived median line realistically?
    Assuming there is not a material modifier, measuring opposite or opposed surface elements on the diameter and deriving a line along the length of the cylinder and comparing to the specified tolerance. Many surface lengths (cross sections) are measured


    This book says we can use profile for everything, but if we can use a size dimension refined by orientation, form, or location, is that a better way of defining things than putting a profile tolerance and using some basic dimensions?
    This is a more of a trivia concept in that Profile can define an inner and outer tolerance boundary for all geometries present and possible on a part.


    However, this profile all features approach rarely provides the dimensional and tolerance specification required for proper functional and manufacturing requirements of features and parts.


    Donít blindly put Profile of a Surface on your end item design parts without verifying that this meets your requirements.
    Tell me and I forget. Teach me and I remember. Involve me and I learn.

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