GD&T Gurus

[AliThePro]

This might seem like a simple question but I asked some GD&T experts and I got different answers.

Suppose you have a ring with two (cylindrical) surfaces. There are two surface profile on each surface with the tolerance .06

The way the ring is dimensioned is that the basic radius for the inner surface (call it Surface U) and the basic dimension for the thickness t are given. This means that the radius of the outer surface (call it surface P) is inferred from the radius of U and thickness t.

Now does the profile on surface t establishes the tolerance for t or radius of surface P?

Stated differntly, which combination of the tolerances do we have:

A: Surface U +/-.03
Thiscness t +/- .03
Surcace P +/- .06

Or

B: Surface U +/-.03
Thiscness t +/- .06
Surcace P +/- .03

 

[ringman]

Ok,
Lets back up and try again. It appears that you have 2 parallel surfaces from the last input. The sentence defining Datum B requires some clarification. 'makes' does not quite work somehow.

 

[monkeydog]

I wish I started reading this at the start, I am getting confused.
Mr. AliThePro,
Is there a figure example in the ANSI or other spec of the part you are looking at?

 

[btrueblood]

Hmmm. Let's review:

Datum "A" is the flat side of a turbine wheel.

Datum "B" is a cylindrical surface on the wheel. Datum "B" is perpendicular to datum "A", and has a basic dimension (radius "U") and a profile tolerance of 0.06. (editorial note here, I hate the idea of a basic radius dimension for this purpose; realistically the part will be inspected based on the diameter)

A new surface (flowpath outside diameter) is dimensioned by a basic dimension "t" extending radially outward from surface "B". Essentially, this gives a basic radius dimension of P = U + t for the new surface. The new surface has its own profile tolerance of 0.06 with respect to datum A and B.

We can ignore datum "C" for your example, since we are concerned only with the concentric cylinder surfaces.

You gave us values of U and thickness t as 14 and 1 respectively. Thus an equivalent basic radius dimension for the outer cylinder is 15.

"Now suppose you inspect a part that has U of 14.02, t of .96 and therefore a P of 14.98 do you reject this part or accept it? "

As I read your example, the part is acceptable, provided there is no point on the outer surface where the measured radius "P" is less than 15 - .03 = 14.97, or more than 15 + .03 = 15.03. The radius is taken from the "true" or best-fit centerline of surface "B".

 

[AliThePro]

Thank you btrueblood for your answer. This is the same as my interpretation. But I work with a reputable customer in the industry and their interpretation is different. To clarify let me first say that the way they inspect the part is thru CMM (Coordinate measuring machine). The customer says that first the CMM measures the location of surface U which should be 14+/-.03, then it goes to the point right above it on surface P and it measures the amount travelled (actually it measure the location of the corresponding point on the surface P but then, one can be calculated from the other). This is because the basic dimension are given only for t and U. Now, the profile on the surface P stablishes a tolerance of +/-.03 for the dimension t and not for P for which we do not have a basic dimension on the drawing. Now since U can be any where between 13.97 and 14.03 and t between .97 and 1.03 then P can be any where between 14.96 and 15.06 which is wider than the range you have calculated. I spoke with couple of people that are gurus in GD&T (instructors) and once they heard the customer opinion they all agreed with that. Now, I am not sure if the fact that the customer favors this last interpretation has anything to do with their opinion or not. But I definitly think these people are more knowledgable than me and unless I find a definite proof, I cannot be sure which interpretation is right.

 

[ringman]

Ali,

I am still curious as to how the point C on your drawing is determined. What feature is it associated with? Is it used in the locating of the points for datum A?