Relationship between surface roughness and GD&T 2

[Sa-Ro]

If I want to control surface roughness for Ra max 0.4.

Is there any relationship between my GD&T tolerance applied on the same surface?

Should my GD&T tolerance less than that roughness value or vice versa?

 

[chez311]

Are you going for a record number of simultaneous posts or something?

Strictly speaking GD&T controls per Y14.5 applied to a surface controls the entire surface and all elements thereof. For example if a profile tolerance of 0.1mm is applied to a surface the ENTIRE SURFACE must fall within that range - this includes any micro-scale variations such as surface roughness. This might not be practical or necessary to verify in most cases as (1) the tools used to measure surface roughness may not be able to relate the measurements taken in location/orientation to your datum features and (2) surface roughness/micro form is usually an order of magnitude more precise than your macro form (profile) though if the width of the tolerance band on your surface approaches the allowable surface roughness tolerance it may be necessary to verify this.

 

[Sa-Ro]

I am not focusing on number of simultaneous records.

After I red the GD&T standard, I am applying this concepts to my parts for better understanding.

During implementation, I am getting lot of doubts. Those doubts are posted here for clarification.

 

[Sa-Ro]

I am not clear with your answer. So I am taking one example.

Flatness applied for a surface as 0.1.

Surface roughness required on that surface us Ra max 0.4 (0.4 micrometer).

As per flatness tolerance the surface can vary within 0.1 mm parallel plane.

But as per roughness value the surface can vary within 0.4 micrometer.

Is it right?

 

[SeasonLee]

Please see the new fundamental rule "s" added on Y14.5-2018 version as shown below:

That means all surface elements shall be within the geometric tolerance zone

Season

 

[3DDave]

It is not typical that surface texture limitations are so large as to affect the FCF tolerance verification. Typical roughness is evaluated over very short distances.

 

[chez311]

Since your question seems to suggest that you think the two contradict somehow, at the risk of stating the obvious:

0.1mm = 0.0001 m
0.4µm = 0.0000004 m

The surface roughness specification is several orders of magnitude tighter than the flatness specification.

To add to this is the specifics of surface roughness measurement. They are not equivalent controls:

1) Surface roughness is typically only sampled over a small portion of a feature. If it is a large feature it may be measured in several places, but almost never in one continuous measurement.
2) Most surface roughness specifications are for 2D measurement (stylus moved in a straight line over a small distance). There are 3D roughness specifications and measurement, but this is less common.
3) The most common measurement tool for surface roughness, a skidded profilometer, will not take into account macro-scale form error - it will actually filter it out. A skidless profilometer would be necessary to measure the combined effects of macro and micro scale form error, but this is less common and much more expensive.
4) Many surface roughness metrics are evaluations of some kind of averaging of the surface variations. For example the Ra you mention is the AVERAGE of the peaks and valleys of a surface. I'm not even sure if theres a perfect analog to "flatness" in surface roughness ie: even Rmax which measures the maximum distance from peak to valley doesn't require that said peaks/valleys must lie between two parallel planes (or lines).