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# Understanding and Measuring Flatness Spec

## Understanding and Measuring Flatness Spec

(OP)
Hello Everyone,

I recently started at a new plant and we seem to be having flatness issues with two of our parts. The two parts have a flatness spec that reads .0002/IN, while our other products are all .0005 for the entire surface. The quality department says the .0002/IN can just be multiplied by the width of the part for your entire surface spec (ie. a 3 inch part would have an entire flatness spec of .0006). The engineering department believes it is .0002 inches within a circle with a radius of .5 inches.

Two questions:
1. Which is the correct meaning of the spec (.0002/IN)
2. What is the best method of measuring this? We currently use and inverted gauge installed under a granite block. How would you measure a radius of .5 inches?

Thanks.

### RE: Understanding and Measuring Flatness Spec

I've always been of the mind that flatness is a callout regarding an /area/ of an object, therefore if you're going to callout a "flatness-per-unit" then the flatness should be a unit of area, which "inch" simply is not.

I would say that ".0002/in" is quite common in my world, though, and whenever conflict has come up, my experience is that the designer/engineer has almost universally intended it to be "1x1 in area" or "per square inch" or however you wish to phrase it.

"Legally" speaking, if you check out section 5.4.2.2 in the -2009 ASME standard (since you're talking inches, I assumed ASME Y14.5 of some flavor applied) They show two different examples of a per-unit basis, and both examples have a preceding shape for clarification.

An excerpt from that paragraph:

#### Quote (ASME Y14.5-2009 5.4.22)

Since flatness involves
surface area, the size of the unit area (e.g., a square area
25 x 25 or a circular area 25 in diameter) is specified to
the right of the flatness tolerance, separated by a slash.

One example shows [-flat- | 0.05 / 25 ] (for square area) and [ flat | 0.05 / (dia)25 ] (for diametric area)

### RE: Understanding and Measuring Flatness Spec

(OP)
Thank you for each of your replies.

CheckerHater, would it be correct in saying the entire surface flatness is the "Global Flatness" and flatness per unit of area is the "Local Flatness"? Is there a better way to measure the local flatness of a part than using an inverted gauge under a granite block?

### RE: Understanding and Measuring Flatness Spec

It doesn't have to be inverted. I was just showing both ways on the same piece, so whatever process works for you.

Also, as flatness may be specified "per square inch, or 10 x 10 mm, or 25 x 25 mm, I use "neutral" word "base"

"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

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