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Parallelism Inspection Explanation Please

Markus
This is probably more of a GD&T question, but I am attempting to inspect the parallelism(s) shown in the attachment. There are two .020 parallelism callouts in the top right view. One that is checked to A, and another checked to A and B. There is third parallelism callout on the bottom left view. The questions are: 1) How do I check this? and 2) Why are the numbers I'm getting so different. If I just create two cylinders, and then check the long cylinder back to datum C (the small cylinder), I get a vastly different number than if I were to check the small cylinder back to the long cylinder. Should I be leveled to anything? Does that matter?

Keep in mind I've mostly self taught myself on both GD&T and PC-Dmis programming. I could be totally wrong on how I inspect something and there are at least a few things about GT&T that I don't understand in the slightest. I would not be offended if anyone responds as if I've never checked a part before. Any assistance or explanation is appreciated. Thank you.
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  • In very simple terms, think of the datums as your alignment.
    • For the first parallelism of the right hand face to A, datum A would be a plane on the left hand face and you are measuring another plane on the right hand face relative to it. To create the tolerance zone imagine adding another plane, identical to datum A but separated by a gap equal to your tolerance amount - in this case 0.020. If this tolerance zone (2 planes) was moved across to the right hand side, all of the points on your reported feature (the right hand face) should lie within that tolerance zone. If you levelled to the datum A plane and then looked at the range of deviations of the overall width between the two faces, that would be the parallelism and to be in tolerance, that range must be less than 0.020.
    • For the second parallelism - datum C cylinder in relation to A & B - Datum A is our plane on the left hand face, datum B is another plane on the back wall (as shown in your bottom, right view). Batum B is setting the specific orientation for the parallelism - essentially it is only looking at parallelism in 2D because it is a planar zone (no Ø symbol before the tolerance). This means that you would level to A, rotate to B and then look at the range of the distances between the left hand (datum A) face and the cylinder axis. Again, this range would need to be below 0.020 to be in tolerance.
    • For the third parallelism - Ø1.000 cylinder in relation to to C - datum C is the Ø1.750 cylinder. You would level to datum C and look at the range of the 11.073 distances which must again be within 0.020 to be in tolerance. The range of distances applies across the entire length of the Ø1.000 cylinder which is why you get a much smaller value for the parallelism if you reverse things. Then you are levelling to the Ø1.000 and looking at the range of distances applied across the entire length of datum C. Because datum C is a lot shorter you do not see as much deviation over it's length.
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  • In very simple terms, think of the datums as your alignment.
    • For the first parallelism of the right hand face to A, datum A would be a plane on the left hand face and you are measuring another plane on the right hand face relative to it. To create the tolerance zone imagine adding another plane, identical to datum A but separated by a gap equal to your tolerance amount - in this case 0.020. If this tolerance zone (2 planes) was moved across to the right hand side, all of the points on your reported feature (the right hand face) should lie within that tolerance zone. If you levelled to the datum A plane and then looked at the range of deviations of the overall width between the two faces, that would be the parallelism and to be in tolerance, that range must be less than 0.020.
    • For the second parallelism - datum C cylinder in relation to A & B - Datum A is our plane on the left hand face, datum B is another plane on the back wall (as shown in your bottom, right view). Batum B is setting the specific orientation for the parallelism - essentially it is only looking at parallelism in 2D because it is a planar zone (no Ø symbol before the tolerance). This means that you would level to A, rotate to B and then look at the range of the distances between the left hand (datum A) face and the cylinder axis. Again, this range would need to be below 0.020 to be in tolerance.
    • For the third parallelism - Ø1.000 cylinder in relation to to C - datum C is the Ø1.750 cylinder. You would level to datum C and look at the range of the 11.073 distances which must again be within 0.020 to be in tolerance. The range of distances applies across the entire length of the Ø1.000 cylinder which is why you get a much smaller value for the parallelism if you reverse things. Then you are levelling to the Ø1.000 and looking at the range of distances applied across the entire length of datum C. Because datum C is a lot shorter you do not see as much deviation over it's length.
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