Position tolerance on an assembly 1

[Jacob Cheverie]

We have several blueprints that call out Position tolerances on the I.D. bore of a ball in the following way:

Primary datum reference A - Planar surface with normal vector along Z
Secondary datum reference B - Cylindrical feature with axis along X

There is a ball that is free to rotate within the assembly. On the drawing, the axis of the I.D. is along Z and the axis intersects the axis of B.

The Position tolerance on the ball I.D. is diametric and called out to A and B, both referenced RMB.

My misunderstanding is in the use of datum reference A. The Position error will increase as the bore axis rotates away from the normal vector of datum feature A. We use a squaring tool, or backer, to hold the bore perpendicular to datum feature A during inspection. My concern is that this is an artificial measurement, as we are really measuring the perpendicularity of the backer to A at that point.

Any thoughts on GD&T as applied to mobile part features? Again, it seems quite artificial to me. I will attach some sort of drawing tomorrow.

 

[Belanger]

I was unable to download it.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

 

[Jacob Cheverie]

Here is the image.

 

[chez311]

What does this backer look like? I assume it has variable position (allowed to translate in x/y while fixed in orientation to A) - does it have a pin of expandable or fixed size?

 

[Jacob Cheverie]

chez311,

You are correct in your assumption. The pin is of fixed size, though it should be expandable. Planar surface contacting A with a fixed size pin.

I also forgot to mention - When explaining the situation, I swapped the order of the datum letters for simplicity. The primary datum feature plane is actually B and the secondary datum feature cylinder is actually A.

 

[axym]

Jacob,

The position tolerance applies to a hole in a feature whose orientation is adjustable - I'm not sure I've ever seen that. It does seem like quite an artificial requirement - I don't think that the reference to the planar datum feature actually controls anything.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[Jacob Cheverie]

Axym,

When measuring this on a CMM, the software incorporates the Perpendicularity error to B (which is only as good as the backer and some form error in the bore) and the desired positional error to the secondary datum. I can ignore the Perpendicularity error by constraining the bore normal to the plane, but I believe there should be a geometric callout that can yield the desired error. I don’t see that in Y14.5.

Ideally, the stack-up would have been performed correctly and the I.D. sphere that the ball sits in would be positioned as shown while guaranteeing the ball is correctly positioned to the secondary datum. A customized DRF won’t help me here.

 

[pmarc]

The position tolerance says that the axis of the cylindrical tolerance zone needs to be perpendicular to B first and then needs to pass through the secondary datum axis A. Without referencing B, the former would not have to be met meaning that the tolerance zone would need to be perpendicular to and pass through A.

My take on that would be that as long as you find at least one candidate ball configuration for which the axis of the bore falls within the position tolerance zone, the bore is conforming.

 

[drawoh]

Jacob Cheverie,

There are sections in ASME Y14.5 that discuss flexible features, including datum features. If a pin is located to datum features[ ]A and[ ]B, your ball will rotate to accommodate it. Maybe you need a carefully written note.

--
JHG