Perpendicularity question

[Jason Munday]

I have a flat surface which is Datum A, there is a cylinder through it which is datum B. The print calls datum A perpendicular to B. (plane to cyl.). Does this make sense??? I dont remember ever seeing a perpenduclarity called out this way, its usually the cyl. to the plane (B to A). 2nd question if this is common & I have just never seen it before than why do I get two different results when I called up perpendicularity both ways??

[Kelly_Bramble]

Interesting question - nothing unusual about a secondary datum being a cylinder (axis). I have to wonder, what industry or application you are accustomed to.

There are two general approaches when defining datum’s on a part or assembly.
1) Datum’s for function: These are those features that facilitate alignment in terms of location, orientation or some combination thereof for a part or assembly in the target assembly.
2) Datum for manufacturing: These are those features that are defined as datum’s because manufacturing considerations outweigh functional requirements of the part or assembly.

In general, we should always define datum’s based on function unless otherwise required.

The order of precedence that we assign to datum’s is normally based on a features relative influence or functional contribution to the parts stability, location or alignment within the target assembly.

[Belanger]

The reason you get two different answers is that when a datum is "simulated" from the part, it is taken from only the high points. So when the datum is the plane (and the cylinder will be measured for perp) then any bumps or concavity in that surface is ignored; the datum is a perfect plane.

However, when the datum is taken from the cylinder, the flat surface is then measured for perp -- including ALL of the surface (bumps and all). Thus I suspect that the version measuring the flat surface back to the datum axis is the one that is showing a greater error.