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Thread: Datum Features

  1. #1
    Associate Engineer
    Join Date
    Oct 2017

    Datum Features

    Hello All,

    I have had a fair bit of confusion recently regarding the use of Datum Features when making an engineering drawing. I do a lot of ordinate dimensions and know datum features are really helpful for machinists; the problem is I've never had it clearly explained to me how I choose my A, B, C, etc. datums and how to make sure my datums stay consistent from one drawing view to another. I assume there are datum feature conventions and I want to make sure I follow them.

    As an example here is a drawing I am still working on with a front and left side views shown. Where should the datums on this be and does every ordinate origin need datum features?

  2. #2
    Project Engineer
    Join Date
    May 2011
    You would benefit from a good introductory course to GD&T (check out the various options from Engineers Edge, even the online training). But to answer your general questions, datum features are the clearly identified physical features that an inspector would have to make contact with in order to create a "datum reference frame." Subsequent measurements (such as the position of the small holes) would be taken from that datum reference frame. In the GD&T system datum features are identified with letters such as A, B, C, and those letters are labeled in a datum feature symbol, which is the letter enclosed by a box and connected by a stem to a triangular base.

    The picture you've shown sort of identifies the datum features by using the 0, 0, 0 coordinate method. I say "sort of" because there is still uncertainty about which of those zero edges should be more flush with the gage or fixture. Imagine: in a real-world situation, that bottom-left corner might not be exactly 90. Suppose it's 89.5: that might still be within the allowable angular tolerance, but now it's impossible for the part to be fully flush against both of those zero lines when placed in a fixture! So it's not good enough to identify the zero lines (the datum features); you also need to assign an order of importance among the three zero lines.

    This is where the GD&T system comes in. Using the GD&T method, you would label the datum features with letters (probably A for the bottom plane in your top view, and B for the longer edge in the main view, and C for the left edge in the main view). Then... you'd have to use a special callout for "position" of those holes which invokes the three datums in the exact order of importance for fixturing. This special callout is known as a "feature control frame."

    Well, that's just the tip of the GD&T iceberg, and maybe it's more than you were asking for. But if you've never heard of GD&T, do a quick search about it, as well as the phrase "feature control frame," and also consider some training option for this important language of drawings.

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