Zone for surface finish requirement.

I frequently see engineering drawings that call out a specific surface finish in a specific area on the part. This area is often called something like "zone A" and is denoted by arrows with a dimension in parenthesis. Do these zones have a tolerance? Why or why not?

 

Hello. A small sketch showing the tolerance callout would help.

As a general thinking my take on this would be as below.

Tolerance callouts are at times defined for specific marked-out regions on parts. It’s not just surface roughness but form tolerances like flatness or cylindricity as well as profile tolerances are defined for specific regions or along specific segments. Even Datum targets which are marked out regions on which the workpiece makes contact with the datum, are specified with such untoleranced dimensions. A dimension written in parenthesis is a Reference dimension. Reference dimensions are always expressed without a tolerance.

So to put it simply, there is no tolerance on the dimension that mark out the region for a tolerance callout. In the case that you talk about, where a specific region is assigned a surface roughness tolerance, the design is made such that the mating component surface or region is significantly smaller in area than the surface that has been given the roughness tolerance. Variation in the untoleranced dimensions is not going to impact the proper seating or bearing of the two surfaces even with a significant variation in the untoleranced dimensions. The smaller size of the mating surface will also make the assembly robust to any position variation (mismatch) between the two surfaces.

As a manufacturing engineer, I would verify whether what is discussed above is indeed the design intent, and go with the General/ Unspecified Tolerances scheme that the drawing is governed by.

Hope this helps.

 

I think the Zone is defining the minimum requirements. At a minimum, all of that area needs to meet that surface roughness requirements; areas outside the specified zone may or may not meet the surface roughness requirements but everything inside needs to meet. I think this works 99% of the time as roughness is usually a one-sided requirment so you can always make something smoother and still be conforming.

In an example like a casting with a machined surfaces specified as a "Zone A" there would probably be a default surface roughness for the majority of the part and then a smoother area with a specified finish (for a gasket or a mating part). In that case, all of Zone A would have the smoother surface finish and if a bit more of the part outside of Zone A was also smoothed to ensure all of A was smoothed it would be OK as it would still meet the rougher requirement in that area.

In places where you get mutually exclusive maximum and minimum roughness in adjoining areas it obviously becomes more complex and I'd be really concerned about the person who designed it and how they'd envision it being producable.

David

 

Considerations of performance and appearance are criteria in determining these zones. Light, distance, viewing angle and time are fixed for visual inspection in appearance cases. They do have tolerance in the number of point defects, line defects and area defects in such visual checks. Criteria is from the visibility from outside, from inside, from top, from bottom, inside finish not visible without opening etc etc ...

 

There are a number of ways

(1)Define the area with dimensions and tolerances.
(2) Mark the area with a box defining the finish requirements
(3) Use datum's A-B. Mark on drawing A then B were you want the finish.

 

No
They are references. No tolerance applies.