Flatness: what is the best method?

[John Riggins]

We have a call-out like this:

Flat|.0050/4x4

I understand that to mean "flat within .005 in a 4"x4" area."

The full area is relatively large and there is a related call-out: profile|.0250|A|B|C

My plan was to fill in the area with points 1" apart and then create planes with those points roughly 4" square and tolerance each plane at .005. All of the planes would overlap by 1".

Then use the points to create a set for the profile.

Does this sound like a reasonable approach?

[uber user]

you need 3 points for a plane, but you need 4 points to measure flatness on a plane. atleast 4, anymore then that are at your discretion.

[Jan d.]

I think what it means is that they want the complete surface to follow the profile. However, any 4x4 area can not exceed the 0.005 flatness. This is done many time for surfaces that have an object moving over it (I'll bet the foot print of the object you are looking at will be 4x4). On you CMM, you see this call-out many times when dealing with the air bearing surfaces: with volcomp you can allow a large long term flatness variation, but you can not have it all in a locallized area (that would be disasterous for your air bearings). Other times they use this call-out to prevent lines to show up on molding tools. It is basically something of a 4x4 template that you move over the whole surface, and anwhere where you put the template, the flatness must be within 0.005.

So I think you did understand the call-out right. However, I think your method is a good try, but fundamentally flawed (it is too limitting, you are not covering every position). But I can not come up with a better one because I have no idea how to program this correctly in PC-DMIS. I am a novice, so I am not of much help. I am sure there are some jocks out there that have figured this out.


Jan.