You're inspecting the surface of a machined steel plate to make sure it is within 0.005 inches of flatness across its surface. What does flatness refer to, and how do you measure it?

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Your interviewer will likely ask you situation- and knowledge-based questions to understand how well you are able to explain concepts to others. Be prepared for questions like this one by practicing a few possibilities ahead of time. In this case, flatness refers to a type of Geometric Dimensioning and Tolerancing (GD&T) callout. This is a specific parameter that refers to the distance between two parallel planes that define a surface profile. It's a measurement of how flat a surface is relative to its own shape, and it doesn't require a reference plane or datum (unlike the GD&T parallelism callout). Therefore, the part must be secured such that the measuring tool is parallel to the nominal surface measurement. Measuring profiles that are not nominally flat can usually be done with a computer-controlled probe on a machine called a CMM, which compares the probe measurements to a 3D Model to measure the variation of the surface profile.

"Flatness is a measurement of the surface deviation relative to its own profile. In order to measure it, I would secure it to a work surface and use a height gauge to measure the total variation. However, the measured surface would need to be secured such that it is parallel to the measuring tool's reference plane, otherwise the data points will need to be adjusted. A CMM is a great way to do this because it uses a computer-controlled probe and a 3D Model as reference, but a height gauge could be used as well if clamped correctly."

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How to Answer: You're inspecting the surface of a machined steel plate to make sure it is within 0.005 inches of flatness across its surface. What does flatness refer to, and how do you measure it?

Advice and answer examples written specifically for a Mechanical Engineer job interview.

8. You're inspecting the surface of a machined steel plate to make sure it is within 0.005 inches of flatness across its surface. What does flatness refer to, and how do you measure it?

How to Answer

Your interviewer will likely ask you situation- and knowledge-based questions to understand how well you are able to explain concepts to others. Be prepared for questions like this one by practicing a few possibilities ahead of time. In this case, flatness refers to a type of Geometric Dimensioning and Tolerancing (GD&T) callout. This is a specific parameter that refers to the distance between two parallel planes that define a surface profile. It's a measurement of how flat a surface is relative to its own shape, and it doesn't require a reference plane or datum (unlike the GD&T parallelism callout). Therefore, the part must be secured such that the measuring tool is parallel to the nominal surface measurement. Measuring profiles that are not nominally flat can usually be done with a computer-controlled probe on a machine called a CMM, which compares the probe measurements to a 3D Model to measure the variation of the surface profile.

Written by Jason Toby on October 12th, 2020

Answer Example

"Flatness is a measurement of the surface deviation relative to its own profile. In order to measure it, I would secure it to a work surface and use a height gauge to measure the total variation. However, the measured surface would need to be secured such that it is parallel to the measuring tool's reference plane, otherwise the data points will need to be adjusted. A CMM is a great way to do this because it uses a computer-controlled probe and a 3D Model as reference, but a height gauge could be used as well if clamped correctly."

Written by Jason Toby on October 12th, 2020

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