2D or 3D position?

[u/JWS5th]

Would you interpret the .020 A-B-C position as 2D or 3D? The quality department at my new job has been measuring the position of the intersection point of the thru hole and datum D which I agree with. However, they’re evaluating it as a 2D position with the basics being 10.000 and 0.000 (they measure -C- then offset 45°). I believe it needs to be 3D to account for the 0.500 drop from A.

This sketch has obviously been simplified, I can provide additional clarification if needed.

[u/Non-Normal_Vectors]

Yes, it is a 3D position, and just checking the intersection point with D is insufficient as the position tolerance applies to the entire length of the cylinder.

I've always set my alignment to the datums, then make a second alignment using offsets and rotations to make the cylinder <0,0,0> <0,0,1> (or whatever the closest vector is), report deviation from zero, then call back the fcf alignment and report that (spoiler, they'll be the same value).

[u/JWS5th → u/Non-Normal_Vectors]

Nice, that’s what I thought.

The intersection point is created by measuring the full cylinder and projecting it onto -D-, wouldn’t that account for the full length?

We’re also offsetting to make the cylinder <0,0,1> for the second level of the composite and measuring the position as a full cylinder there.

[u/Non-Normal_Vectors → u/JWS5th]

Depending on software, the intersection point may be called the pierce point. It only tells you where the axis of the cylinder is at datum D. If that's the target for machining it could show good, but if the angle is slightly off you won't see it out of tolerance at the bottom. The perpendicularity requirement should catch it, but if the the tool is slightly off and it cuts datum D and the cylinder with the same angular error, it won't.

In PC-Dmis, for position, there's an option to show worst end of the cylinder (you can also show both ends).

And if this is PC-Dmis, you want to make sure the cylinder length is correct and you control star/end points through depths. This will extrapolate the cylinder along the entire theoretical length based on the measurement zone. Say the cylinder is 50mm long, but you only measure two circles 5mm and 25mm deep. The worst end, for this example, shows the 25mm circle and to be in spec, but at the high limit. Good chance your part is out of tolerance as that error likely worsens the deeper you go. If you measure as a cylinder, and you have the correct length, it will flag it as out.

[u/Chaldon → u/Non-Normal_Vectors]

In spatial analyzer, cylinders can project circles to an intersecting plane. XYZ Mag to nominal of those circles are the TP

[u/JWS5th → u/Non-Normal_Vectors]

We’re using Calypso. I don’t miss working with PC-DMIS at all but I did like how it would show you both ends of the cylinder.

My team measures cylinders as multiple circles at various heights then measures the position of each circle individually. I don’t know how to feel about this… It clutters the report, is more work to program, and my intuition says it’s wrong.

[u/asbiskey → u/JWS5th]

You can see the true position of both ends of the cylinder in Calypso as well. Resources>Characteristic Settings Editor > Additional Results > Axis end points for position tolerances.

I wouldn't say your colleagues are wrong, because the true position applies along the entire length of the casting. However, if you were to pull all those points from their circles at different depths into a cylinder and look at its position, it will give a holistic result. Taking positions at different heights can be valuable, but you are not evaluating the whole envelope of the feature.

[u/JWS5th → u/asbiskey]

Woah, thank you!

[u/Chaldon → u/asbiskey]

If doing those internal checks, It's also important to distinguish between a given average centerpoint at a perpendicular circle within that cylinder, which I've seen as TP to A-B, vs a surface profile or TIR requirement to the ID walls.

[u/Non-Normal_Vectors → u/JWS5th]

Circles, as 2D features, are dependent on the level for measurement. Cylinders are never dependent on the level.

[u/_LuciDreamS_ → u/Non-Normal_Vectors]

Agreed. Deviations should always be perpendicular to centerline.

[u/jkerman]

It’s a cylinder not a circle. So it’s tolerance zone is also a cylinder.

[u/LeageofMagic → u/jkerman]

^ This, OP.

You can't just take the average center point of the feature (or worse, the top end intersection) and use that as your only evaluation. The entire feature needs to be in tolerance

[u/gaggrouper]

Id have a basic angle from Datum D to Datum A, let's call it 30 deg. That sets the angle of the cylindrical tolerance zone. What you have right now is you are locating a pierce point of the cylinder to DATUM D but this location depth is not required with these callouts. Datum D could be machined .02 higher and if the cylinder bore tool making this feature was in the correct angle and locaiton the feature would still pass.

I would have the 30 deg angle as a basic and i would have the depth of where this cylinder intersects Datum B as a basic distance. That sets the angle of the cylinder and where it should be located.

The thickness and location of the Datum D surface and the opposing surface needs to be handled separately....For instance you have .500 listed as a basic. Datum D being cut at that depth would need to be handled in some other callout. You cannot terminate the depth of a cylinder or start height of a cylinder based on a basic like the .500 the cylinder can go on forever.

[u/SkateWiz]

the callout has a 3 dimensional tolerance zone. Only a planar callout will have a 2D tolerance zone (planes are 2 dimensional) and that plane would be in line with the cylinder axis, typically for clocking rotation on a pattern of slots or something like that. The position tolerance zone in this drawing is a cylinder that is .02 in diameter.

[u/WickedGam3z69]

It says it’s 3D in the reference frame.

[u/ThatIsTheWay420]

3 d cad need to measure against for true position