When to Use DMP Flatness

In this Question Line video, Brandon explains the application of derived median plane (DMP) Flatness and discusses its frequent misuse and potential issues when used for stacking parts. He also provides an alternative for controlling flatness for stacking or symmetrical parts.
Interested in learning more about Print Reading or GD&T? Check out more of our free GD&T resources here: download.gdand...
#gdandt #flatness #derivedmedianplane

[@chrmeiss67]

1) I‘m afraid, there is no reale feature function according to „flatness 0.005 M“ at datum A.
2) If this plate is srewed or stacked, a complete differend specification is required (please see: The plate can be screwed from two different sides).
2) If someone wants to use „flatness 0.005 M“ for whatever reason, then it is easier to write 1.000 -0.02 +0.025 without additional flatness. Kepp it simple!
If a fit is required, it‘s a very good idea to keep in mind the gauge, which explanes the simulation of the missing contact partner.
If you work with 3D-CAD it’s very helpful to work with assembly constrains to explane contacts.
If you create a nominal design where all features with fit function are at MMS, all parts in your assembly are representing the go gauge against each other! A specification is only necessary to control maximum of fit clearance.
Unfortunately with the modell of GD&T (or IS0 GPS) a specification uncertainty against nature function appears. Should we start to speak about this issue?

[@brettkrienke2109]

At 8:52 you say when using DMP flatness that those surfaces don’t have any form control. Certainly they are not uncontrolled. Although DMP Flatness breaks Rule #1, the limits of size are still controlling the form on the individual surfaces. There’s just no MMC boundary. Instead there is a virtual condition boundary that the surfaces cannot violate. So there’s still form control. Maybe you just meant it’s not as tight of a control as the “.005” flatness.