Help me explain this alignment

[[Ni...]]

Good afternoon,

I was working on an old program that used a circle and a point as a base alignment and had a lot of secondary alignments to pretty much find different features on the part. Made things confusing and was not even repeating in the simulation.

Tried explaining how the program has a bad alignment and it needs to define a rotation in space but the CMM operator said that it doesn't need it because it is using the center of the OD to establish it. And the point on the face isn't important.

Basically said the alignment of the older program was fine.

I believe it still needs a plane or cylinder to stablish a rotation in space and a planar rotation since it has slots on both sides.

I would like to hear everyone's thoughts.

Thank You,

alignment2.pngalignment1.png

[[Ma...]]

I think if Calypso will allow that, then it will use missing from machine alignment. It's rotary, so if you don't want to stop rotation, then you don't need to use even primary plane.

Perhaps it's similar to using theoretical features for primary and secondary.

But it's odd.

[[Ke...]]

As it stands, your rotation about X, Y, and Z are all uncontrolled by the part, as the spatial and planar are left blank, so the Roll, Tilt, and Yaw of your base alignment will be established from the CMM. This means that any measurements that include any form of projection will have error, because they are not being projected along the axis of the datums(the part), but are instead being projected along the direction of the CMM axis. It also means your measurements will not adjust to any slight tilt or rotation of the part, so any circles you measure on the part are actually likely ovals as they are at a slight angle to the part (but square to the CMM), and it may mis-measure the slots if they are rotated slightly out of position due to the alignment not following them. Additionally you will have issues with repeatability because the CMM is unable to correct for any slight tilt or rotation in the part, meaning if you load it perfectly vertical once, tilted left once, and tilted right once, you will get different answers.

In more basic terms, you're not measuring the part to itself. You are measuring the part to the location of the part(as you have defined X, y, and Z origins by measuring the part), but the orientation of the CMM(because nothing is controlling spatial or planar rotation).

The Z point is also necessary, though a plane on that surface would perhaps be better, for the simple reason of "how would the CMM know where the Z origin was if it didn't touch it?"

Also, if this alignment is used in any characteristics such as Position, then it would violate the Can, May, Must principle.

[[No...]]

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Is this even true when the program uses secondary alignments (as he mentioned) for all important measurements? That would be (bad) news to me.

[[Ke...]]

Yeah, I guess it depends on what and how you are evaluating the part.
If you are only checking diameters, form, feature to feature evaluations, etc, then it's "probably fine"
I would measure the same part 3-5 times without unfixturing the part, and then remeasure the asme part 3-5 times with refixturing each time, just to make sure it is repeatable (probably faster & easier than digging into an overly convoluted program...

[[Ke...]]

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If the secondary alignments are fully defined, then no it is not true. However any degree of freedom not defined in a secondary alignment is taken from the parent alignment (you can make secondary alignments with other secondary alignments as their parent), so if his secondary alignments do not have Spatial or Planar features, then that would cause issues as it would still follow back to the CMM orientation. I apologize for any confusion, I was only speaking about the alignment shown as a Base Alignment in isolation. I do assume based on his description of poor repeatability (which can easily be caused by poor alignments) and seeing this Base Alignment that the secondary alignments are likely similar, but without seeing them or the program I have no way of knowing or speaking to that.

It is certainly possible to use Secondary Alignments to make good measurements, they just need to be properly defined. Typically I recommend fully filling in every secondary alignment, even if it is just a refinement of another alignment. Calypso will take the undefined values from the parent alignment, but it does not hurt to fill in those spaces that would be blank in an alignment with the features in those spaces from the parent alignment, while making it easier to read and troubleshoot for anyone who may follow behind you.

[[Ow...]]

Everybody has a preference with the amount of detail and constraints being set in the base alignment or using post-secondary alignments ( I do it both ways depending on the complexity of the part) but as a standard practice, I like to have my base alignment control all 6 degrees of freedom to start with to check the parts fixturing stability with loops for repeatability and in case a feature or characteristics assigned secondary alignment gets inadvertently missed, lost or unassigned.

If you’re always having to do a manual alignment every time the part is set-up to define the parts location to the CMM and don’t want to have to manually probe every feature in the base alignment required, use a start alignment.

[[Ch...]]

I've seen this before. As many people have pointed out, it will pull from the machine coordinates if left blank. But this is not always the case. if you use a start alignment, the it will pull values form that too. So, you don't necessarily get project error, depending on what other alignments are used. Us rotary table guys end up doing some weird stuff.

I attached some interesting reading on this topic. It's been years ago, but I believe this was originally posted by Owen Long.

Iterative Alignment.pdf

[[Ke...]]

Chad is 100% correct the Start Alignment will serve as a pseudo-parent alignment for the base alignment, I did not even think to mention the effect Start Alignments have in that process.

In order of hierarchy it would go CMM System->Start Alignment->Base Alignment->Secondary Alignment with Base as parent->Secondary alignment with Secondary as parent. Calypso will work its way up that chain referencing back until all degrees of freedom are fully defined.

[[Ow...]]

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Hard to believe it's been almost 20 years (2005) since that document was created on the very part I was working on (the part pictured) with Mr. Mark Busha himself when he came to our facility to give me Curve training.
It was one of the last topics we covered when he was there because I was having trouble getting repeatable 3D best fits checking the part on a rotary table. We went through several strategies in about an hour right before he left when he come up with the solution that worked.
I'll never forget coming in the next morning to see an e-mail from him received around midnight (at least 8 hours after he left) with the instruction document attached. That guy must've never slept but, was such a pleasure to work with.

[[Ch...]]

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Variations of this strategy have helped us put a lot of superchargers on the road. When you get passed by a Hellcat going 3x the speed limit, you can blame Mark Busha, Owen, and myself.