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# 2 Datum Features 1 Datum - Dimensioning an irregular feature of size

## 2 Datum Features 1 Datum - Dimensioning an irregular feature of size

(OP)
Hia,

So I wish to define an irregular feature of size - the 0.6/0.7 dimension in the pocket:

So I want to measure a line between the two aligned faces and then measure the distance to a point on the top face. I feel doing a x2 0.7/0.6 would suggest I want 2 measurements from each of the aligned faces to the top face which would mean constructing a tiny line on each face. but this isn't really the design intent. How do I imply a link between the two aligned faces?

Now I've put a centerline to connect the two side faces to try help explain what I want to do. Essentially I'm looking to probe the 3 points "x" and calculate the specified dimension.

I thought maybe you could define a datum between the 2 aligned faces (but then how is that done when there is a gap? - a bit of a tangent) and spec the top face to the datum. To be clear the pocket isn't very deep, but you could probably get a plane if you wanted.

Thoughts?

(to BS8888)

### RE: 2 Datum Features 1 Datum - Dimensioning an irregular feature of size

(OP)
Could this be the way? Is it okay to take a few points for a profile (aka a low res scan)? Or maybe a position instead?

### RE: 2 Datum Features 1 Datum - Dimensioning an irregular feature of size

For clarity, datum H ought to be two separate datum targets. "Hx2" is unsat.

### RE: 2 Datum Features 1 Datum - Dimensioning an irregular feature of size

(OP)
I actually have the datum on here now (I've changed it to a D) where I think it's a little clearer. it's also being used to define these other faces as you can see in the drawing below.

### RE: 2 Datum Features 1 Datum - Dimensioning an irregular feature of size

TopPocket,

Do you have access to a copy of the ASME Y14.5 standard? There are tools in ASME Y14.5-2009 and Y14.5-2018 that would accomplish a lot of what you're trying to do. There is a figure illustrating an irregular feature of size that includes 2 coplanar surfaces. There is also an example showing profile tolerancing on coplanar surfaces - Y14.5 does not support position tolerancing on surfaces (ISO 1101 does - are you using ISO GPS standards and not ASME? )

It sounds like you have a particular CMM point configuration in mind for defining the datum. This isn't how Y14.5 works - the default is that the entire surface of the datum feature is used. The datum would be defined by high-point contact on flat plates (or the tangent plane of the points measured by the CMM). As TheTick mentioned, if you want to define the datum using particular points then you will need to specify datum targets.

Also, the profile tolerance doesn't need to be Profile of a Line. Even though the pocket is very shallow and you might only be able to measure points at one depth, it's still a surface and Profile of a Surface is appropriate.

Evan Janeshewski

Axymetrix Quality Engineering Inc.
www.axymetrix.ca

### RE: 2 Datum Features 1 Datum - Dimensioning an irregular feature of size

(OP)
I'm using ISO GPS standards.

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