What is the best method for checking the flatness of a flexible part?



Good morning thought I’d throw this out there for every one to chew on.
What is the best method for checking the flatness of a flexible part?

Proud Liberal

Quite Involved in Discussions
Need a little more information. What tolerance are you trying to check? Also, explain more about the actual part (material, geometry).

Finally, what is the application? Often times, a designer will specify flatness but really has something else in mind. BTW, true flatness is one of the more difficult to actually measure although there are several good approximations that will do the job.


Sorry for being vague. The part in question is best described as a extruded rubber about 1.5mm thick, 6mm wide and 29mm long. Flatness of .1mm has been called out on the thickness. As far as the application I’m not sure.The customer’s drawing doesn’t deal with Free- State Variation. If I’m correct the part can’t be restrained in any way to alter its Free-State condition. That is where my problem lies.

[This message has been edited by Stymie (edited 12 December 2000).]

Steven Truchon

Flatness is a geometry that is in reference to itself, thus dependant on itself. If the item in question cannot support itself in a geometrically stable state, then the initial reference cannot be established. Enter restraint conditions. Is restraint feasible? No? What about Datum Targets? No?
If the part cannot be supported to establish the primary plane and then measure deviation from that plane, due to flexibility based on gravity then I submit:

1) You most likely cannot measure flatness on this part. Even the most sophisticated optical technology would fail in reliability because the surface form would be dependant on the form of the part support .

2) Your customer will also not be able to measure flatness on this part as it is defined in geometry.

Have you contacted your customer with this problem? Sometimes a customer will have a method they use, whether correct or incorrect in the ideal sense. If they do, you can match their method, you'll be on the same page and you live happily ever-after.

Short of all that, as much as I despise hearing these words;
If they cant check it, they cant reject it!.

Seriously, I would contact them for clarification. Maybe the part used to be aluminum or..., and they never took that control off the print. Just a thought.



Too measure flatness of the tolerance you have specified can be accomplished by a 3 axis video inspection system with Z axis scales and a focus repeatability method. The Flexbar / Optiflex system can provide this capability. While the System is in excess of $20,000.00, some comercial calibration labs, including ours, will perform contract inspection for you. There is a little more to it than just throwing the part on the stage and measuring in the Z axis, you need also a fine leveling plate to fully establish a referance plane, otherwize you are really only measuring parrallelism.
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