Position of an unintentially skewed hole

[[Me...]]

How does Calypso handle the orientation component of Position for an axis skewed cylinder such as a drilled hole where the drill wandered, which is most drilled holes. The control frame is TP diametral MMC to two datums, neither datums are MMB. Does Calypso Position characeristic automatically create a perpendicular RAME from the UAME when selecting MMC or is Normal Vector selection necessary?

[[Aa...]]

I don't believe Calypso typically uses the RAME for MMC position FCFs. Nor should it. The "additional tolerance" (axis interpretation) is calculated as the difference between the UAME and MMC size. (Y14.5-2009 7.3.3.1) And the axis being controlled is the axis of the UAME.

[[To...]]

I love watching Aaron work.... 🤣

[[Me...]]

Thanks Aaron. Hi Tom, Jason from IDM, you provided initial training to "we three amigos" 10+ years back in our shop in NH.

Aaron, what happens when: Apply Normal Vector Constraint with Max Inscribed fit to the skewed cylinder, prior to using in the position calculation?

[[Ri...]]

When you apply "Normal Vector Constraint" you are telling Calypso that the axis of this cylinder is perfect. Calypso will then use that to calculate size and location. If the axis is actually perfect, there will be no difference. If there is a significant difference in the axis orientation actual to nominal, then there could be a significant difference in size and location if you use Normal Vector Constraint.

[[Jo...]]

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Speaking in terms of how the tolerance zone is located, if you have ISO 5459 turned on, the tolerance zone will be oriented perpendicular to your primary datum (providing the feature is nominally perpendicular to the primary). If you have ISO 5459 turned off (this is the default setting) then the tolerance zone will be aligned with the feature (not perpendicular to the primary). Is that what you were asking?

[[An...]]

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[[Me...]]

John, yes that is what I'm asking, thank you. I saw this to be the case when toggling the Normal Vector on/off and watching the X,Y locations and reported diameter of the feature change. I'm trying to drill down to the "ultimate truth" on how to measure the position of a hole (per strict interpretation of Y14.5) where the drill has wandered off coarse. The most succinct answer would address whether or not it is perfectly correct to apply Normal Vector to the skewed hole prior to measuring it's position, True to the datums.

OR is it NOT necessary to apply NV as the Position calculation does this "best fit with normalized axis" automatically when MMC is applied? Trying it both ways yields much different results; NOT using NV reporting much worse position than with NV.
Again, the control frame calls for TP Diametral, MMC, A, B where A & B are perpendicular to one another and parallel to the hole in there respective planes. In other words, the hole axis runs smack down along the intersection of A & B so it's my assumption the skewed hole must first be orientated before it's position measured.
Thank you as well Richard.

[[Me...]]

Correction: The hole axis is coincident with A and offset from B, parallel to both.

[[An...]]

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[[De...]]

I would expect that it will always yield better results to measure to the 'actual' position of the hole, subsequent evaluation may be made relative to a DRF in order to obtain results per print, but measuring the hole is done in the physical world.

The CMM uses the radius value of the probe, and the calculated normal vector from the probe center to the surface to be measured in order to determine the measurement. The greater the error between the actual normal vector and the calculated normal vector the greater the error in the measurement.

In most cases the amount of error created by small deviations from the nominal hole position will cause minimal amounts of error, however if you absolutely need the greatest accuracy you can align to the hole itself prior to measuring it.

To do this, you would first measure the hole in its nominal location you could call this feature 'TestHole1'

Then you would create an alignment using that hole 'TestHole1' as the spatial alignment and locating feature.

If desired you could loop the alignment to force it to find the best alignment possible.

Then create a new feature 'Hole1' to measure that uses the alignment you created, this hole will use the alignment during measurement so that the probe is kept at the proper vector to the hole axis based on the axis of the hole.

Now when you do your characteristic, you can use whatever constraints you want, but the underlying data will have been measured based on the actual position of the hole and will therefore be the best representation of the physical part.

[[Aa...]]

Derek, I like your creative approach at measuring the hole twice. But is it even necessary to measure the hole the second time, when you could simply recall the points from the first cylinder into the second? (What is recalled is ball-center points.)

Jason, to directly answer your question, it is not only not necessary to apply NV constraint to the controlled feature, it is incorrect. What is controlled is the UAME. That's according to Y14.5. (That is, of course, speaking in terms of axis interpretation.)

Mention has been made here of the ISO 5459 switch. If you turn it on, it will not affect the controlled features. That switch only affects the non-primary datum features as they are used in DRFs. And yes, if you're shooting for Y14.5 compliance, you should have it turned on.

You've noted that you get better results when constraining the NV. I would expect that to be the case, for two reasons.

First, there's the simple fact that the position characteristic reports the worst location result of the two ends. When you constrain NV, the two ends will have the exact same result (which will essentially be the root-average-square of the two ends' results, though this could be affected by how much of the length you cover with measured points).

The second reason (which I suspect isn't really having an appreciable impact in Jason's situation, with drills leading off), is that if the hole has a significant amount of taper in it, the Outer Tangential or (for holes) Maximum Inscribed cylinder result could be unstable--the resulting cylinder is typically "shoved all the way to one side of the wider end". If you're looking to center it at that end (what ISO calls for, but not exactly what Y14.5 calls for), one way to accomplish this is to evaluate the cylinder at Outer Tangential, but constrain the normal vector to to itself evaluated Least Squares.