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

 

[ewh]

How can t be both a thickness and a surface?

 

[AliThePro]

t is the thickness. I aplogize for the type the correct sentence should be:

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

 

[btrueblood]

Neither A nor B. If "t" is a basic dimension, it has no tolerance. Surfaces U and P each have a profile tolerance of 0.06 relative to their respective basic dimensions.

 

[AliThePro]

Suppose that basic dimensions, which are only given for Surface U and thickness t is 14 and 1 respectively. Now suppose you inspect a part that has U of 14.02, t of .96 and therefore a P of 14.8 do you reject this part or accept it? What I meant by the tolerance for t in this case is:
1-.96 = .04

 

[AliThePro]

Sorry for another typo, U of 14.02 and t of .96 makes P of 14.98 and not 14.8

 

[ewh]

I agree with btrueblood. Surface P would have an inferred basic dimension of 15 with a profile tolerance of .06 (14.97/15.03).

 

[TheTick]

Answer: Neither A nor B. The thickness actually has no tolerance associated with it.

Basic dimensions are added with no tolerance, always. They establish a virtual ideal condition to which actual conditions are compared.

The profile tolerance on surface "U" does not affect any tolerance frame placed on outer surface "P", unless surface "U" as used as a datum reference to measure "P".

That's the core of GDT. Not just dimensions and tolerances, but the references used to measure.

If outer surface "P" has a profile tolerance that does not relate to inside surface "U", it is possible for "P" to nearly intersect "U" (zero thickness) and still be in spec as long as its radius is correct. There is nothing relating the position of "P" to the position of "U".

If a single profile callout has two arrows pointing to both surfaces, the profiles are measured simultaneously from the virtual condition. This is better, but does not guarantee thickness or concentricity.

Chances are, you want to measure profile of outer surface "P" with respect to "U". You may also wish to spec runout of "P" w.r.t. "U". This certainly demands clarification from whoever originated the drawing.

AliThePro
My question to you:
What are the datum references for each of the profile callouts?

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[AliThePro]

The datum references for the profile is A,B,C were A is the side surface of the ring (the surface where the ring lies when you leave it on a table), B is center line of the ring, and C is a point on the ring used for Clocking control. By the way this is not realy a ring. It is a gas turbine disc. The point to remember is how this part is dimensioned. The radius of the inner surface (U)is given and the tickness t (both basic dimension), and profiles for U and P. P's radius is inferred from U and t. Now when we inspect the part do we add the basic t to the actual value of the U and then apply the tolerance (that is +/- .06 tolerance for P, but +/-.03 for t), or do we add basic U and t together, find out the basic value for t and then apply the profile tolerance (wich makes the tolerance for P +/-.03 while tolerance for t will become +/-.06).

 

[ewh]

It is not correct for B to be the centerline of the ring without relating it to a surface or feature. What would establish it for inspection purposes? In this case, it needs to be the centerline of either surface P or surface U. It is incorrect to label a centerline as a datum, but it could be labeled "CL A" (centerline of datum A).
I realize that this may be beside the point, as the drawing probably already exists, but to conform to Y14.5, it needs to be corrected.

 

[TheTick]

ewh is correct. The GDT scheme is not valid. "B" must be the centerline of a physical feature, not a virtual wishful non-feature.

The fact is that your customer has put you in the position of interpreting incomplete and incorrect information. Anyone's best guess is likely to be wrong, as there is no true correct answer based on this information.

 

[AliThePro]

B is related to a feature. It is the center line of the circle made by the tips of the blades. In my simplified example it is the center line of the surface U which makes a cylinder (ring).

 

[ewh]

The centerline of the circle made by the tips of the blades is not a feature you should refer to in the inspection of this part. Consider the part by itself, no other assembies to reference. If it is the centerline of surface U, then surface U should be datum B.

 

[AliThePro]

Well, the blades are part of the ring. There is only one cast part. But how does this issue affects the answer to the tolerance question? I still don't know that my example part should be rejected or accepted.

 

[ewh]

It affects the answer in that we are trying to determine if this is a valid tolerancing scheme. Once it is determined valid, then it can be determined if the parts pass/fail.

 

[AliThePro]

Well as I said earlier this is a turbine disk with more features than a ring. The thickness t is the flowpath height (blade height) surface U is the inner surface for the flow path and surface P is the upper surface for the flow path. Datum A is specified through three datum target points that basically make a surface that is parallel to the disc (side of the ring in my example).Datum B is a surface on the part which is actually neither surface U nor P, but all the three surfaces are theoretically concentric. datum C is a datum target, which is a point on the disc used for clocking control (the datums make a cylindrical coordinate frame).

 

[TheTick]

The scheme is not valid for checking individual components. It may be valid for checking parts once they are assembled.

 

[AliThePro]

What do you mean by that, there is one part here, no assembly, and we are inspecting this part. The datums are features on the part. Why is it not valid?

 

[ringman]

Ali,
Could you clarify datum B please? You state 3 points.. I believe it is not quite proper to state that datums are features of the part. Datums are theoretical, points planes and lines, associated with part features.

 

[AliThePro]

the three points define a surface which is datum A not B. Datum B is a surface the makes a feature that is a leading edge for the gas flow. In our ring example, suppose that the outer surface of the ring steps down a bit near the end and makes another smaller cylindrical surface that is of course concentric with other surfaces of the ring. This surface is datum B.