Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here


Understanding and Measuring Flatness Spec

Understanding and Measuring Flatness Spec

Understanding and Measuring Flatness Spec

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?


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 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

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

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close