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?
[IMG]https://www.engineersedge.com/engineering-forum/attachment.php?attachmentid=2016&stc=1[/IMG]

You would benefit from a good introductory course to GD&T (check out the various options from Engineers Edge, even the [URL="https://www.engineersedge.com/training_engineering/gdt-online-training.htm"]online training[/URL]). 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 90º. Suppose it's 89.5º: 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 [I]exact order of importance for fixturing[/I]. 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.