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
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
RE: GD&T Gurus
Now does the profile on surface P establishes the tolerance for t or tolerance for radius of surface P?
RE: GD&T Gurus
RE: GD&T Gurus
1-.96 = .04
RE: GD&T Gurus
RE: GD&T Gurus
RE: GD&T Gurus
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.
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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RE: GD&T Gurus
RE: GD&T Gurus
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
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
RE: GD&T Gurus
RE: GD&T Gurus
RE: GD&T Gurus
RE: GD&T Gurus
RE: GD&T Gurus
RE: GD&T Gurus
RE: GD&T Gurus
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
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
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
Mr. AliThePro,
Is there a figure example in the ANSI or other spec of the part you are looking at?
RE: GD&T Gurus
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
RE: GD&T Gurus
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?