Cylinders in Base Alignment

[[Ji...]]

We were taught many years ago not to use Cylinders in the Base Alignment. We had someone in class last week and they were told to use a Cylinder instead of a Circle in the Base Alignment now. What changed and is a Cylinder in a standard Base Alignment really the industry standard now?

[[To...]]

I'm not afraid to use a cylinder for a Spatial Rotation, especially if it has good length.  I don't know if there is an industry standard on this but I think it comes from personal experience in certain scenarios. Just my opinion.

You could never use a (parallel) cylinder as the planar rotation feature unless you used it in an intersection to create a point. 

[[Ma...]]

I think there is no problem using cylinders in BA, but only for CNC mode - using manual scan can result into switched direction.

So if you use start alignment with circles into cylinder, then this way you can safely get position. If you will have same origin point in SA and BA, then you can run BA from SA

[[Dr...]]

Good points brought up by Tom & Martin, and I agree that you can generally use cylinders in a BA without a second thought.

To give another example where you wouldn't want to, consider the example below. The X/Y Origin could be taken as a circle or as a cylinder without significantly affecting its role in the BA. The Planar Rotation, however, should not be taken as a cylinder. Doing so would cause its "axis length" to be only the thickness of the part rather than the distance from the part center to the bore, significantly impacting repeatability in a way that might not be apparent to someone unfamiliar with how Calypso constrains degrees of freedom.

[[Ji...]]

Thanks for the answers. Did something change in the newer versions of Calypso or did people start using them and find they worked better than a circle. I only have one part that we have to use a cylinder in Planar rotation. That is a fairly long cylinder and controls the functionality of the part. We use circles on the hundreds of other parts for planar rotation. The X and Y assuming you are probing in Z direction is the one I was concerned with. Thank you all for clarifying this.

[[Ma...]]

Using cylinder as secondary will result in rotation of cylinder's axis.

Using one circle - same as point - will result in rotation from origin to this point and opens option to rotate BA to coordinate of that point in one of axis.

Nothing has changed over the years around cylinders and circles in alignments.

[[Ma...]]

I’m still old school. If a cylinder is to be used for Rotation in Space I will use a 3D Line generated from multiple circles I do not want the form of the cylinder to effect the axis line. Then I’ll go back and measure the cylinder for size and form.

[[Ow...]]

Put me in the old school bucket as well.  👴 

[[To...]]

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   Are you saying there is an actual difference between the calculated axes below? I added a 3rd option

A  - .5" Dia x 1.5" long cylinder with 3 circle paths strategy .1 from top, .1 from bottom and dead center.

B -  3d line from recalling 3 circles at same locations. 

C -   .5" Dia x 1.5" long cylinder with points recalled from 3 circle features at same locations. 

Seems like form error would affect all 3 of these the same, though I don't truly know how the cylinder axis is calculated.

Edited August 27, 2025

[[Da...]]

I agree with the old school guys. The the base alignment should be created with the simplest and most stable features.

[[Ma...]]

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I assume using ASME you will get different result from measured cylinder or 3d line with recalled centers of circles.

[[To...]]

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I am old school dude...lol   But this is the kind of folklore that floats around and nobody can seem to quote the science.  Why is a 3d line from circles more stable than a cyinder?

Edited August 27, 2025

[[Da...]]

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My experience: if a hole or shaft is not perfectly cylindrical, but slightly conical, a 3D line from circle centers is more stable.

[[Ma...]]

Here is an excerpt from The CMM Handbook v8

Datum Definition when the datum is a cylinder.

Datum Definition When the Datum Is a Cylinder.pdf

[[Ma...]]

You can use a cylinder with a lot of points and use maximum inscribe/ minimum circumscribe to simulate a datum simulator. Again, I like scanning sections of the cylinder(s) and using 3D Line, just the way I prefer it.

[[Ow...]]

I'll elaborate more on my reasoning, the way I see it anyway.

Solving a cylinder can mathematically be a tricky.

Solving a circle is not very challenging, given the orientation of the circle is controlled by the alignment and nominal A1/A2.

The ratio of Diameter to Length (I always use at the very least 3 scanning paths in cylinder features), as well as the measurement strategy, filters and form can cause cylinder calculation challenges.

Circles are relatively bulletproof, and creating a 3D line from the fit centers works very well to create a reliable and repeatable axis.

Edited August 28, 2025

[[To...]]

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Who is the author of this fine piece of literature?

[[Pa...]]

I've seen only one real problem using a cylinder as planar rotation.  Keep in mind that all of my parts are cylindrical, so here's an example of what I typically see:

Spatial rotation is often a long shaft constraining 4 DOF.  This also sets X and Z origin. 

Y origin is one face of the long cylinder constraining one more degree.

Planar rotation is often a plain bore in the face of the shaft which is parallel to (but offset from) the spatial, constraining the final DOF.

And I'm typically NOT in a rotary.

 - As an aside I see far too much guidance suggesting that the spatial rotation constrains 3 DOF.  Ignore this; it's entirely dependent upon the type of feature used.  Cylinders constrain 4 in the spatial AND as a primary datum. Every time.

Anyway, I once had a looped alignment go wonky on me.  Turns out the ORIENTATION (not form) of the planar was out to lunch (planar cylinder not even close to parallel to the spatial).  The first loop ran fine.  In the second loop the alignment had "re-clocked" as the CL of the planar shifted after probing the problematic planar.  CMM tried to bonk on the way in for the second loop.  It was only a few degrees but I didn't have enough clearance to absorb it.  

Good thing too.  Had it cleared and ran I might have burned up quite a bit of time trying to find the issue.

All of that said, I like to use circles where I can get away with it for just this reason.  My point here, however, is that there are excellent lessons to be learned in the old school.  When someone new comes along and asks why, if our only answer is "because that's the way I do it", perhaps both the FOG and the FNG have something to learn.  There's almost always an exceptional situation coming down the pike to take us outside of our comfort zone.  Be ready.

 

[[Pa...]]

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This is a good one.  There are some process engineers here who need to see this.  My napkin cartoons have never quite hit the mark.

[[Ma...]]

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I wrote this about 3 years ago. It is part of the CMM Handbook