Hole mobility and 90 degrees basic angle implied

[greenimi]

Standard used: Y14.5-2009 and “my shameless disclaimer”, assume drawing truly reflect the functionality of the product (just to get this out of the way)

The question is:

Do I need a tertiary datum feature B(MMC/MMB) to locate the Ø.375 hole (Ø.379/.371)? In other words without the tertiary datum feature B(MMC/MMB) the hole is allowed to move on a circle .750 radius around the center hole?

Again, the GD and T (when we don’t know it ) creates a lot of controversy. Some of engineers at my company said because 90° basic IMPLIED the hole CANNOT move around freely EVEN without tertiay datum feature B. Some said the opposite.

I have listed some paragraphs from the standard for your convenience.


2.1.1.4 Implied 90° or 0° Basic Angle. Where center
lines and surfaces are depicted on 2D orthographic engineering
drawings intersecting at right angles or parallel
to each other and basic dimensions or geometric tolerances
have been specified, implied 90° or 0° basic angles
are understood to apply. The tolerance on the feature
associated with these implied 90° or 0° basic angles is
provided by feature control frames that govern the
location, orientation, profile, or runout of features. See
paras. 1.4(j) and (k).
(j) A 90° basic angle applies where center lines of features
in a pattern or surfaces shown at right angles on a
2D orthographic drawing are located or defined by basic
dimensions and no angle is specified. See para. 2.1.1.4.
(k) A zero basic dimension applies where axes, center
planes, or surfaces are shown coincident on a drawing,
and geometric tolerances establish the relationship
among the features. See para. 2.1.1.4.

 

[CheckerHater]

Based on GD&T that I don’t know I would side with engineers saying your hole is constrained, but I would question using diameter symbol.
Now the question is: WHY? What would you gain by using this tolerance scheme?

 

[3DDave]

90 degrees will apply to the hole center depiction lines wherever the hole center is. It doesn't constrain the hole location. The axes are perpendicular to datum A and do not line up, so (k) doesn't apply.

The way you know is by looking at the feature control frame which says only to locate to A and D(M). The hole is free to orbit D within the other constraints. If there was some other reference, that is different.

The key to using feature control frames is to do exactly and only what they say to do, not to add additional constraints based on appearance.

In the case the hole is just an anti-rotation feature and it truly doesn't matter, why would one want to create additional work on the drawing to require less constraint on the part?

 

[axym]

greenimi,

I agree with the others. The implied basic 90 degree angle is present, but it does not affect the constraint. Final answer.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[greenimi]

Thank you. Thank you. Thank you.

You saved my day and my weekend!! Just wonderful to have you around.