【转帖】flatness relative to part thickness

yang686526

flatness relative to part thickness?
i have a composite structure or panel that is somewhat large in size.  design engineering is trying to control the bow or warp of these parts with a flatness callout.  how can i use a cmm to measure the flatness or warp of the part with respect to a surface plate?  if i take points on top of the part then my numbers are dependent of the part thickness tolerance at each point location.  how do i take measurements to control flatness and take into account the part thickness tolerance?
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joejack7
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joe:
flatness does not reference anything but itself.
there are a couple of ways to perform flatness on a cmm and i will give you the better way.
take 3 points on the surface that are 120 degrees apart and close to the edge. create a plane. hopefully your software will do this. now take at least 24 points one the surface usually in a circular motion from the outside of the surface towards the centre of the surface relative to the plane that you just created.
the software should give you the actual flatness which is the fim or tir of all your readings.
hope this helps.
  
dave d.
as dave says, there's a number of different ways to inspect flatness on a cmm.  if you can visually see highs & lows, probing those areas alone is useful as it is a max to min height difference that you are looking for.  when you can't find the highs & lows visually, another option is to do a mapping of the surface by tracing (hard on the equipment and some errors that accumulate) or by point sampling in a pre-defined x-y pattern where each subsequent row is offset from the previous row to miimize the risk of missing a transition.  i've had to do this to verify / counter supplier claims, and it yielded amazingly graphical results.  the software should be able to generate the plane of the surface and provide the flatness for you.
jim sykes, p.eng, gdtp-s
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