Concentricity/Coaxility

[[Da...]]

I getting what I feel are unrealistic results when measuring the concentricity between holes on two plates on a weldment. Callout on one hole is .5mm to the other hole. No other datums.

Functional gage passes thru both holes on all pieces, but the CMM is giving a result of 1.2mm to 6mm (wow). I need help figuring out of there is something I am doing that is causing it to skew. 

Don't judge my terrible drawing skills. I don't have the parts anymore and will not post the drawing, but you get the idea. Basically there is a bottom plate, with a plate perpendicular to the bottom plate on each end. There is a hole in each of the side plates. 

For my alignment, I used the top side of the bottom plate as my Z and spatial. I made a line across the front side of the bottom plate for Y and planar. I took a point on the outside of the left plate for my X. 

Information that may or may not be helpful:

The left and right plates are perpendicular to each other within .1mm.

I first probed them as circles and asked for concentricity. 

I then probed them as cylinders and asked for coaxiality.

The holes are laser cut and sort of rough. Every time I probe them as a cylinder, it thinks its a cone or sphere, and I have to change it to a cylinder. 

I tried changing from the default ISO to Loose and in some cases, that made it worse. 

I tried changing the alignment to use the left plate instead of just the single point. 

I tried plugging additional datums into the characteristics. 

I tried using a straight down probe with the RDS XXT at 90 and I also tried using a star probe. 

I cannot get any of the pieces to give a concentricity or coaxiality results below 1.2mm. 

 

I don't have the parts anymore, as my manager stated we would send them on the basis that the functional gage passes. I would just like to know what I should do differently in the future. 

 

[[Ma...]]

If you are using gage, then scan it like one cylinder and use outer tangential.

Due to undefined drawing can not tell more.

[[Ch...]]

You're almost certainly getting projection error caused by trying to measure concentricity across a distance.  A small error on one side is projecting to a much larger error on the other side of the part.  I've posted this several times on the forum, but its worth a read if you haven't read it yet.  Concentricity across a distance starts about halfway down the PDF.

Something like this.  

Profile_Concentricity_CALYPSO.pdf

[[Da...]]

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Thank you! I figured it had something to do with it skewing one of the holes and projecting it over the distance between them. I tried what was suggested and got .2mm one direction and .1mm the other direction. 

[[Je...]]

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, thanks for sharing this PDF and the root cause of projection error in Calypso's concentricity characteristic.

Since this PDF was presented during a live Lunch-and-Learn, there are some gaps in the how-to. How do you deal with concentricity in Calypso?

Have you created a macro similar to the one described in the PDF?

[[Ch...]]

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I have created the macro, but I didn't use it much.  But that's not entirely true.  I did a bunch of testing years ago and the difference between the macro and default concentricity wasn't very much.  Not for my parts at least.  I'm sure there's some parts where it would make a significant difference, but I'd do the testing before I committed to doing the crazy macro all the time.  

On the bright side, ASME 14.5 did me a favor and obsoleted concentricity.