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GD&T Gurus

GD&T Gurus

GD&T Gurus

(OP)
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

RE: GD&T Gurus

How can t be both a thickness and a surface?

RE: GD&T Gurus

(OP)
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?

RE: GD&T Gurus

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.

RE: GD&T Gurus

(OP)
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

RE: GD&T Gurus

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

RE: GD&T Gurus

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

RE: GD&T Gurus

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?

I could be the world's greatest underachiever, if I could just learn to apply myself.
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RE: GD&T Gurus

(OP)
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).

RE: GD&T Gurus

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.

RE: GD&T Gurus

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.

RE: GD&T Gurus

(OP)
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).

RE: GD&T Gurus

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.

RE: GD&T Gurus

(OP)
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.

RE: GD&T Gurus

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.

RE: GD&T Gurus

(OP)
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).

RE: GD&T Gurus

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

RE: GD&T Gurus

(OP)
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?

RE: GD&T Gurus

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.

RE: GD&T Gurus

(OP)

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.

RE: GD&T Gurus

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.

RE: GD&T Gurus

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?

RE: GD&T Gurus

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".

RE: GD&T Gurus

(OP)
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.

RE: GD&T Gurus

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?  

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