Stabilizing +Z plane during alignment

[[Za...]]

Hello all, I'm pretty sure I made a similar post regarding these type of parts a couple of years ago, but I couldn't find the thread to continue the conversation.

This is an aluminum part that's relatively thin (~7mm). It is also machined and then cut away from where the machine clamps onto the aluminum block (the cut line is right at the halfway point of the 7mm thickness so this tends to mess with my -X and -Y planes). The parallelism from the +Z plane to the -Z plane measures out of tolerance because the part seems to be bowed (to the naked eye and by the flatness graph of the +Z Plane). I trust that the results are accurate, because I'm having issues from one side of the part to the other. I'm almost positive I'm having alignment issues, but I'm looking for recommendations on a fix, because I think I've tried everything that I know how to do and I'm not having much luck. My goal is to sure up the alignment so that the cmm measures in the correct spot. Any help is appreciated!

All of the planes on the left side (-X) of the part measure correctly, but once the cmm gets to my +X planes, it's trying to measure where the dotted line is instead of the nominal yellow line.
Here is a picture of the part with the arrows pointing to features used for my base alignment:
Here is the actual base alignment:

[[Cl...]]

Switch planes for Rotation in space and Planar Rotation.

+Z Plane_BaseAlignment = Rotation is space

-X Plane_BaseAlignment = Planar

[[Th...]]

Oh yeah, projection error off that plane is gonna blast your alignment into next week. Clarke's already covered the solution, though.

[[Za...]]

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I tried switching them around and the CMM did the same thing as far as missing the plane on the +X side of the part. As of right now, I have scans on the -X plane and +Z plane, would taking points help me out?

[[Er...]]

When I have parts that easily bow it is always a difficult to determine what alignment, how many points, if the customer allows the part to be restrained, etc. When I am able, I will select points where I know the part will be stable (usually 4-6 points near the posts and clamps are that I know will repeat). To probe surface correctly where the Bow in the part is, I will add a Z height point to determine where the bow is and use this in the formula for the surface you want to check. Where ever the bow is, your probing surface will follow.

[[Za...]]

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Eric, thank you for the response. This is something a little different than I've ever had to do, but I want to understand what this is doing behind the scenes.

So for my +Z plane (Plane1 in your example), I'll give it points that I know are stable, then take a point in the bow (Point1 in your example), and use that to offset the value of the Z plane?

Taking all of these points in the strategy for the +Z plane would just take the average right? Whereas this is actually taking the Z plane and subtracting or adding a value as it scans?

[[Er...]]

So for the measurement of Plane1 in your picture, you would take a Space Point on the Planar surface close to plane 1, that you call +Z Plane_Base Alignment and use this point in the formula of Plane 1 Z nominal. So that however much the surface drops from the base alignment, it will shift up/down and measure this plane1 surface correctly.

[[Za...]]

That's pretty cool, I will give it a whirl and see if I can get this going in the morning. Thanks again for the input, I will update in the morning after I've had a chance to make the changes and run the program again.

[[Za...]]

Using that method didn't do anything for me (that I noticed anyways), but I did figure out the root of most of the issues. The planes that I'm having issues with are set to a secondary alignment that allows me to report the X and Y locations as if the part were flipped over. The bottom plane is actually what's causing my secondary alignment to be unstable which is causing the CMM to miss on some of these features it goes to measure. I may go in and try messing with the secondary alignment to see if I can get this any better, but for now, it looks like the part itself is causing most of my issues. Maybe they'll machine me a better one next time 🤣

[[Er...]]

If the holes are put in at the same time as the pockets, I would use the Z plane and two holes for the base alignment.