Tightening a Rectangular Positional Tolerance

[ceekack]

I've got a question regarding the use of a Composite Positional Tolerance to refine a Positional Tolerance defined by the Rectangular Coordinate Method.

The goal is to allow ±.3mm linear shift of a hole pattern relative to an edge Datum in one direction while controlling the hole-to-hole position more tightly. I've been reading through ASME Y14.5-2009 and think I've found the relevant sections (7.4.4.1 Rectangular Coordinate Method and 7.5.1 Composite Positional Tolerancing), but I'm not sure if I am combining the principles correctly. I've attached a mock-up of what I am currently thinking of putting on the Print, but would like some feedback regarding whether it accomplishes the intent.

I've omitted the basic dimensions for clarity.

 

[drawoh]

ceekack,

Your drawing complies with Fig[ ]7[‑]28, and I can make sense of it.

What is to stop me from going...

4[×] 6.8/6.2
[box][⌖][/box][box].06[/box][box]A[/box][box]C[/box]
[box][⌖][/box][box].015[/box][box]A[/box][box]B[/box]
[box][⌖][/box][box][⌀].015[/box][box]A[/box][ ]?

One of the things I like about GD&T is that I can collect the entire hole specification into one block on the drawing.

--
JHG

 

[3DDave]

Grouping like that means there is not a simultaneous requirement between the first two, which means that if datum feature B and datum feature C are not perfectly perpendicular the location relationship will not be rectangular.

This is why in Figure 7-28 the entire DRF is duplicated and the directions explicitly shown. I don't know that even using custom datum references will fix the problem as they won't be identical.

 

[drawoh]

3DDave,

Usually, datums are perpendicular by definition. The actual datum[ ]B feature is two points on the surface. The datum[ ]C feature is a single point on its surface. Being in a pocket makes everything harder to fixture to, but that is not what we are discussing. Check out Fig[ ]4[‑]7 in ASME Y14.5[‑]2009.

--
JHG

 

[ceekack]

Oops, it appears I forgot to click the composite tolerance checkbox for the first. Changes things a little bit. Updated drawing attached.

I guess I'm unclear about whether I should be duplicating the Ø.015 TP requirement on both callouts or if just including it on one of the callouts captures the intent adequately.

 

[3DDave]

Datum feature simulators are put against the datum features causing the part to be oriented by that contact. If the datum features are not perpendicular then the two separate DRFs will not be coincident.

 

[3DDave]

In the attached the two orientations - with [A|B] vs [A|C] are shown. The rotated actual part doesn't have to touch datum feature simulator [C] but it's got to be somewhere. The rotation is shown in dashed font. The datums do remain perpendicular, but the datum features don't and, separated into individual DRFs, they can drive rotation.

Which is why Figure 4-7, '2009, uses simultaneous requirements and explicitly depicts the direction of the tolerance zones.

https://files.engineering.com/getfi...ff5-4aaa-b5bc-728c4a45416e&file=bcexample.pdf