【转帖】cmm and gd

yang686526

cmm and gd&t
when we use coordinate measuring machines (cmm) to check for tolerances how do we establish the datum? suppose your vendor gave you the cmm data on a manufactured part. you can't set up the datum plane through the three highest point on the datum feature, you can only ask for the coordinate of some points on the part. is there any established (probably statistical) method to use cmm with gd&t ?
if a large surface is to be used as a datum, it is good practice to put datum target callouts to show where the datum should be taken from.  sometimes a surface profile or flatness callout will state how many points to take or what spacing to use for sample points.
surface plates may be used to simulate the datums and cmm measurements taken from the simulated plates or surfaces.
the points can be placed on the drawing. you can ask the vendor for help where they are to be placed. i usually do this on castings.
chris
sr. mechanical designer, cad
solidworks 05 sp1.1 / pdmworks 05
alithepro,
   when you apply datums to your drawings, you should understand how your parts are going to be inspected.  presumably, you primary datum should be a large bottom surface that can sit on the cmm.  a second choice would be to design an inspection fixture that would sit on the cmm and pick up its reference points.
                     jhg
you can use a cmm holding fixture...
other than that any points on the datum surface will work to establish a datum. as long as the surface itself is within tolerance (i.e. flatness). the variance between the high and low points on the surface all fall in the tolerance range. the gd&t callout for features based on the datum structure should take into account the variance of the datum surface itself.
unless of course the datum consists of points and not surfaces鈥?.
regards,
ross evans