concentricity error

[jcombs716]

when measuring concentricity I have a part that is large and is cone shaped ( like a road cone). i measure the diameter on top and then drop down to a diameter at the middle bottom and get the following.

(top diameter)
X -0.0001
Y -0.0001
D 15.9433
RN 0.0001

(middle bottom diameter)
X -0.0004
Y 0.0001
D 22.0128
RN 0.0001

however, when checking the concenticity between the two i get it out 0.0025

how can i be almost spot on in X & Y but be out that much in concenticity?
this is a very large part about 36.5" in over all length.
we put the part on the table level, level the top of the part and check to top OD then the bottom OD and construct a centerline then inspect the above mentioned areas and check concentricity between the two.Doc1.docx

[Wes Cisco]

"M - O - O - N" That speels concentricity, Laws Yes. Tom Cullin knows this.

Unless this is ISO GD&T you are doing it wrong.


ASME Y14.5 featured a unique, archaic, and insidious definition of "Concentricity" that REQUIRES the part to be rotated about the datum axis . . . (blah, blah, blah, ad nauseam). . . and Finally, Thankfully, with the release of ASME Y14.5-2018 both concentricity and symmetry have been dropped from the standard and are no longer supported.


Datum axis revolutionary nonsense aside, and presuming you really need to report position, ( which is what Pc-Dmis will report even though it lets you use the 'concentricity' symbol (see numerous rants by myself and others on this and other cmm machine machine forums spanning the last 20 years or so), You need to align to the DRF, not whatever portions of the part are most convenient. If the top of the part is the primary datum feature in the DRF, then, Ok, but if not . . .

. . . and he stopped me Right There. . . .