Flatness of a Vibration Plate

[frusso110]

Hello everyone,

I have a 21.56" x 23.50" x .75" plate with a bolt pattern in it to be mounted to a vibration table for some mil-std-810 testing. I am welding to this plate, the mounting structure for my equipment (a ruggedized display). I may want to put a flatness call out on the bottom surface of the .75" plate to ensure that it sits "flat" enough to the vibration table and that it is easy enough to bolt down. Is flatness the right GD&T callout? How does one determine how flat my plate needs to be? I have a feeling that I can tolerate relatively large flatness values such that I may not even need a flatness callout.

Thanks,
FMR

 

[pmarc]

A bunch of initial questions:
1. Which GD&T standard are you working to - ASME or ISO?
2. What is the size tolerance that you are going to apply to .75'' thickness?

And a bunch of initial answers:
1. It seems that flatness tolerance - if really needed - is the correct geometric characteristic to apply.
2. The maximum allowable flatness error should be determined by what you require from functional standpoint. I know, it sounds vaguely, but this is how it should be determined. If you are OK for example with a gap between the plate and the table, the flatness tolerance can be large, if not omitted. If you need to have nice contact surface between the plate and the table, the flatness must be controlled rather tight.

 

[frusso110]

ASME standard.

The thickness dimension is .75 +/- .010

How many thousandths of an inch is a "nice contact surface" ?

 

[pmarc]

Will I surprise you if I say that I am not able to tell exactly how many thousandths of an inch is enough. The plate may be in nice contact with the table (that is not wobble) even with current .020 of flatness tolerance available (if the bottom surface is concave). On the other hand in case of maximum possible convexity the part may rock when placed freely on the table. But then the plate is bolted down to the table, so I would not expect really serious problems with such amount of possible flatness error.