# GD&T Position

#### ckk33

##### Registered
Hi guys, I am new to GD&T, I hope someone can help me solve the below question. For the 4 highlighted holes (refer attachement), should I use Datum B or Datum C to calculate the Y Deviation? Many Thanks!

#### optomist1

##### A Sea of Statistics
Super Moderator
I do not find said attachment...pls share

Cheers Optomist1

#### ckk33

##### Registered
I did attach the photo in the post, did you see it?

#### EdenG

##### Involved In Discussions
Hi ckk33,

I believe you should level / origin to the face 'A' (Z), rotate to a line between 'B' and 'C' (Y+/- depending on line vector) before finally setting your origin to 'B'.

#### John Predmore

Trusted Information Resource
Hello @ckk33, welcome to the Cove. You say you are new to GD&T. There is a lot of information communicated with GD&T. It will be helpful for you to remember a single GD&T drawing communicates both information about physical features of the part (such as the size tolerance of the 4 yellow holes, 5.1+0.1/-0), and at the same time it communicates ideal geometry (the virtual location of each of the 4 yellow holes) which is not the same as the physical part. Conceptually, the ideal geometric location of a hole is represented by the centerline of a virtual hole. The tolerance zone which positions the yellow holes is a cylinder around the ideal centerline, of diameter 0.3.

I don’t see a Y dimension identified in the view, so I assume you mean Y as the vertical direction in the view as oriented here, that is, the direction between the 2 holes identified as datums B and C. I don’t see datum A labeled in this view, so most likely datum A is the large flat surface, perhaps the underside surface.

The feature control frame for the 4 holes (colored yellow) governs the position of 4 individual holes relative to datums A, B and C, with the MMC modifier (the M in a circle). More on that later. I found this website which explains the options for locating the 4 holes using the language of GD&T. The simple option, which you have here, controls the position of each of 4 individual holes relative to datums A, B and C, but the distance between the individual holes relative to each other is not constrained beyond the sum of individual position constraints.

So the answer to your question is the secondary datum, datum B, is the datum for measurements of the 4 physical yellow holes, in the Y direction in the plane parallel to datum A (the flat plate). Measurements in the X direction are relative to the centerline between datum B and datum C.

However, there is a complication. Tolerance of position with a MMC modifier (the M in a circle) provides a “bonus tolerance” to simple dimensions on the drawing. If the 4 yellow holes measure larger than 5.1 diameter (the lower limit of the hole size tolerance of +0.1/-0), half the difference between the measured diameter and the maximum material condition (MMC) of 5.1 becomes a bonus tolerance which is added to the position tolerance otherwise identified. Bonus tolerances simplifies the inspection of physical parts using physical gages. If you want to know more about GD&T, I encourage you to take training.

#### ckk33

##### Registered
Thanks Eden & John. Much appreciate!

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