Xact measure changing nominals

[Richardbeyer87]

Good evening everyone,

I have an interesting issue that plagues me from time to time, I cannot upload the print due to company policy. In my program I am originally aligning to a 1,2,3 block that the part is clamped to, I then measure my features define my datums and run a Xact measure position check on 4 cylinders. My issue is that I set the nominal dimensions in the setup for Xact measure when I view the report my nominal dimensions have changed from what I set them as, as can be seen at the bottom of the program. The only thing that is unusual from what I normally do is that Datum b is a created plane as the datum is the center of the part. Any help would be appreciated, I feel like I'm missing something.

[RIDER]

You should do another alignment before measuring the cylinders. Align to the datum features (pln6 7 & 8), then when you measure the cylinders you can insert the nominal data into the feature code for theoreticals and targets. Exact is looking back up in the program and trying to assign nominals based on your measured features after your original alignment.

I see you're using a Romer, so there is no target, just theoretical and actuals.

[Richardbeyer87]

Thank you for your response,I will do that first thing and see if it helps. Its a copper part and its fairly thin .125" or so, bent twice, and about 5" tall so the angles could be off by 2 degrees or more as mentioned below. Another alignment never hurts .

[Rich P]

Nominal are probably changing because your planes in datum as well vectors on cylinders are at angles
cylinder 1 has a nominal vector <-0.9993889,0.0060154,-0.0344319> .034 in a vector trigs out to ~2 degrees
Your B datum mid plan has 2 opposing planes doing the same thing ( plane 4 K vector is -.039 is 2.235 degrees)
****
When you dimension a cylinder in position that is on an angle it will tell you where along the cylinder the "worst condition" results is so it is trying to report as angled cylinders on angled planes and give you the calculated coordinates along the angle.--this will change nominal on report
To correct this make your vectors Normal to the surfaces
0,0,1
0,1,0
0,-1,0 etc cylinder 1 vectors 1,0,0 and clean up your X,Y,Z to basics on drawing
I have never had good results with mid planes---B datum I would constructs midline or mid plane from mid points the are exactly opposing nominally
HTH

[Richardbeyer87]

Rich P, It does help a ton. The cylinder very well could be off that far and honestly I can probably explain it as such. The part being measured is made out of 1/8" bent copper so as you could imaI ine it can flex a great deal (+/-.100") easily. I normally dont completely clean up my vectors with my datum planes because after experimenting it can make my numbers appear different than physical measurements off the same datums on a granite surface that is calibrated, I believe this is due to the nature of the material I primarily measure copper (I dont have any issues ever when measuring steel or aluminum). The mid plane always gives me issues. I am using a romer have about 2 years experience, I have one more question if you have a minute. If I move to a system where I clean up the vectors on these parts my assumption is PC-DMIS makes the plane,cylinder square to eachother off of the centroid from all points taken is this the best way to show functionally how the part will fit? Sorry for the extended response and thanks again.
Richard Beyer