Profile of a surface/point instead of flatness?

[raginipaul24]

So, a customer has asked for the flatness of some of their products to be checked after they were found to be quite bent. Unfortunately their drawing only calls for surface profile. It's a relatively straight edge, so I was thinking of either creating a plane from already programmed points to create flatness, or using the already called out profile for the points to work out the flatness.

[Conrad Hoffman]

Right forum but probably not a lot of people into dimensional metrology! Flatness is always a problem because you need a bunch of points, and you need them in the problem areas. A simple warp isn't too bad, but things like edge roll-off are harder. I think profile means the same thing as flatness, if the profile in question is a plane. Collect your points and see what the spread is. Also, does the surface have any relationship to some other surface that further limits how far out it can be?

[rubidium]

Get yourself an optical flat and a cheap 532nm green laser pointer and insert the pointer into a hole made in a ping-pong ball as a diffuser. Observe the fringe pattern and draw qualitative and semi-quantitative conclusions regarding the surface flatness. Photograph the fringe pattern, take the 2D Fourier transform (FFT) of the digital image, and work out the scaling details to get a more quantitative estimate 2D surface profile. The latter is, of course, much more work.