Features for Alignment used as Characteristics?

[[Si...]]

Hello again,

I don't know if this is a weird question but I was told way back when, that what I used for my Base Alignment.. to not use for a feature in my Characteristics. Example, If I make Plane 1 my Spatial and Z axis in my Alignment.. not to use it for a Flatness or // Characteristic. That I should create a new feature.

1: Is this correct?

2: Why?

My supplier does this and I do not remember the reasoning to why you should not.

Thanks in advance.

[[Ju...]]

Best practices says to not use alignment features for measurements, but as long as the alignment is looped, I personally don't see a difference. Alignment features are used to tell the software where the part is. The general idea is that the first probing of the alignment features is done before the part is fully set in the software space, so it may not get accurate data the first time the features are probed.

[[Je...]]

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Base Alignments are used primarily for navigation and only secondarily for evaluation.

As Justin said, it's best practice and not a legally binding rule. Base alignment features are the practice pancake that tell you the griddle is hot, but then you serve more carefully crafted pancakes to your guests.
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[[Ja...]]

I think it is a good question. I will be waiting to hear from someone that is confident with an their answer. I wonder myself. If plane one is Z origin in the base alignment then plane one Z actual is zero when you open the feature but it may have been off before the alignment was calculated. I have never had an issue using base alignment features later in the program.

[[Ja...]]

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I get it. The reason base alignments should be looped is the same reason you would want to remeasure the base alignment features if you are using them elsewhere. I always loop the base alignment.

[[Je...]]

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Typically, I will use single points or a less dense scan on the base alignment features. Then, I remeasure the same features with a more complete scan for evaluation. I will typically use different filter & outlier settings for the evaluation features than for the base alignment, which I often do not filter (although of course filters & outliers can be specified within characteristics). Also, you cannot use a secondary alignment for base alignment features or formulas that reference post base alignment features without causing a reciprocal dependency error.

For these reasons, I consider my base alignment features vaulted from evaluation. They locate the workpiece on the CMM.
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[[Ch...]]

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I have to agree with Jeff here, post of the day .. mmm.. pancakes ... 🤣

[[Th...]]

We apply the same measurement and evaluation settings to our base alignment features that we do to any other feature, so our programs don't have duplicate measurements for evaluation purposes. There are a number of reasons we do this (and I have Opinions on these) but for our particular requirements I don't believe there's value in separating the BA features from evaluation. We do loop our alignment to mitigate probing error.

In other words, our practice pancakes and finalized flapjacks are the same quality.

[[No...]]

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The simple answer is:
Because alignment features were measured in a non-aligned state.
Although Calypso updates the nominals after alignment, the measured points stored in the feature are still the same and so represent the non-aligned state of the part. Even if you use loops in alignment, the features will only represent the state after the second to last loop. Better than without loops, but still not exactly the same.

This is not necessarily an issue, but it might. A real-world example that happened to my recently:
I have a part where two circles in a stepped cylinder bore are used for secondary alignment as well as for diameter characteristics. The diameter on the outer ends of the bore is larger than the diameter in the middle. The circles are measured in the middle. The problem is that they need to be measured almost at the edge of the step.
Due to inaccuracies of the fixture, the part was able to move slightly along the cylinder axis. The circles measured were sufficient for alignment, but the part failed, because one of the diameters was slightly too big. The stylus had measured a bit too far outside.

In the end I decided not to create extra circles, but added a formula to the circles, making them relative to another cylinder on the part, so now they always measure at the correct height. Problem solved.

[[Ri...]]

Back in 2001 I was taught the same. Don't use base Alignment features for Characteristics.
I was taught, and teach to this day, the Base Alignment is only to create an envelope of where the part is in the working volume of the CMM.
All of my Base Alignment features are placed in a Group labeled "Base Alignment".
All of my Base Alignment features have a prefix of "ba".
Example:
Plane1ba
Circle1ba
2d Line1ba
Etc...
This makes identifying Base Alignment features from a drop down list easily noticed.
Keep your Base Alignment Features SIMPLE, 3-2-1 Alignments where ever possible!
They do not have to be relative to any Datums. If Datums keep the Base Alignment simple, by all means, use them.
In my programs I create a "CNC Alignment", sometimes its simply called CNC Alignment
CNC Alignment Features are placed in a group called: CNC Alignment Features. This group always follow's the Base Alignment group.
CNC Alignment and Other Alignments are named accordingly to Datums,
Example:
CNC Alignment A_B_C
Alignment C_B_E
Alignment A_B-C
ect...

Hope this is helpful.

[[Si...]]

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I do the same except for the CNC part. I always do BAs and then go an remeasure more in depth on those surfaces if I am using them for Characteristic call outs.

[[Er...]]

It depends 🙂

On fixturing, part form, tolerancing, time to complete BA, etc.

My preference is simple features to find the part. If the BA has anything but simple features Calypso 5 sometimes fails to run Subsequent Evaluations. This has saved me several times by being able to rerun parts from Saved Points to see what went on during a run a month ago that an engineer is "getting back to me" on.

Anything with tighter toleranceing gets its own feature because it would slow down the loops on the BA. One part has a plane as datum A, cylinder as datum B, and a slot as datum C. The density of points the engineers want slows down the XXT to a crawl. The BA has single points for the Z plane, circle as fast as 1400 points can be taken for finding the X and Y(cylinder), and a simplified slot.

Parts with "loose" forms get single points. Then scanned once found.

[[Je...]]

If your part fixturing, alignment structure and measurement strategies are sound, there is no reason you cannot use Base Alignment features in a characteristic.

My advice is to either use a Start Alignment and/or Looped Base Alignment with break condition. Unless your fixturing method is not repeatable or reproduceable then you may need to use more robust methods. If using a relatively robust fixturing design, a Start Alignment using the same features as Base Alignment but with a few touch points is typically sufficient to have your measurements repeat within your machine spec accuracy.

I can tell you that I use Base Alignment features for characteristics in nearly every program I have ever written. I have also remeasured BA features under more refined strategy for better reporting. It is a case-by-case scenario. If you are measuring a large bore diameter that is +/- 0.002 mm then you may want to rescan that at a slower speed than what would be usually done in a Base Alignment for location alone.

How does this differ from reporting of any other measured feature that may not be at near nominal geometry values? Do you loop features used in sub-alignments? No geometry is perfect but your CMM and Calypso are designed by its very intention, using the proper measurement strategies, to adapt to exactly that.

If you feel you cannot use the Base Alignment features in your characteristics, you may need to re-evaluate your methods of locating your work piece and fixturing. The fact that the feature is used in the Base Alignment is mostly moot. Your success depends on the inspection equipment, measurement environment, relevant measurement strategies, work piece handling and overall proper inspection methods.