Does Translating The Trihedron Ruin "True Zero"?

Technically, as written, the Y14.5 standard is written so that you align to the datum structure in your Feature Control Frame (lets say you have POS | Ø .014 MMC | A | B | C |).
You constrain all degrees of freedom that A will constrain (lets say it is a plane, that is two rotations and a translation).
If you are measuring two dimensional features (not points for itterative but circles or the like) you need to align that plane now prior to measuring the circles.
Then you measure B, lets say it is a plane also, so it constrains the final of three rotations and another translation.
This leaves one translation for C, it can be whatever you want. Doesn't matter.

You now have zero zero zero. Name it ABC so you remember what it is and can recall it whenever you want.

Now, you translate your origin (from your example) zero on X and 14.5 on Y.

This moves your trihedron.

You then inspect the actual position of your feature compared to this "TRUE" position you translated to.

This doesn't help a machinist move the feature to fix it, but this is what you are to do, technically.

Then you recall ABC and continue measuring.

You do NOT re-measure the features of A B C and redo the alignment.

Basic dimensions are just that, basic moves from zero zero zero to get to the feature. That is why they can be added and subtracted.

Part says this hole has a .010 position to A B C, but the basic is from some random hole in a bolt pattern? YOU add/subtract as necessary to measure the hole compared to true position FROM A B C. The distance from that other hole means nothing. Only the distances and angles from A B C.

It concerns me that you say you can't get back to zero zero zero.

I measure 1500 dimensions on a manifold, zero A B C and recall it, bouncing all over the place for compound angle holes, and I get back to zero every single time.

I'm more worried that your part is so far from perfect that it doesn't match the model enough to reconcile on the first alignment anyway (which might explain you doing an iterative to begin with), of you are re-measuring the datums but not using the same features/points as the first time. maybe?

If you don't have angle holes, you don't have to move the alignment to check the hole.
If you use ExactMeasure or GeoTol you don't have to move the alignment to check the hole.
In both cases, though, the software IS moving the alignment to check the hole in the background and you don't see it.

Make a ExactMeasure or GeoTol position and then click into it. You'll see a yellow trihedron pop on the screen.
That is the alignment the software is using to measure the hole's position against true position.

If you can't move it yourself and get back to where you were, you need to look at your alignments and see where you went astray.

I said I do it on large, complex manifolds.
I didn't say I got it perfectly right the first time, every time.
It is very easy to hit the enter key before you clicked on the button to apply something in an alignment, or do an angle in the wrong order with a translation.
If you are having a repetition problem, it's almost always in the alignment.

Technically, as written, the Y14.5 standard is written so that you align to the datum structure in your Feature Control Frame (lets say you have POS | Ø .014 MMC | A | B | C |).
You constrain all degrees of freedom that A will constrain (lets say it is a plane, that is two rotations and a translation).
If you are measuring two dimensional features (not points for itterative but circles or the like) you need to align that plane now prior to measuring the circles.
Then you measure B, lets say it is a plane also, so it constrains the final of three rotations and another translation.
This leaves one translation for C, it can be whatever you want. Doesn't matter.

You now have zero zero zero. Name it ABC so you remember what it is and can recall it whenever you want.

Now, you translate your origin (from your example) zero on X and 14.5 on Y.

This moves your trihedron.

You then inspect the actual position of your feature compared to this "TRUE" position you translated to.

This doesn't help a machinist move the feature to fix it, but this is what you are to do, technically.

Then you recall ABC and continue measuring.

You do NOT re-measure the features of A B C and redo the alignment.

Basic dimensions are just that, basic moves from zero zero zero to get to the feature. That is why they can be added and subtracted.

Part says this hole has a .010 position to A B C, but the basic is from some random hole in a bolt pattern? YOU add/subtract as necessary to measure the hole compared to true position FROM A B C. The distance from that other hole means nothing. Only the distances and angles from A B C.

It concerns me that you say you can't get back to zero zero zero.

I measure 1500 dimensions on a manifold, zero A B C and recall it, bouncing all over the place for compound angle holes, and I get back to zero every single time.

I'm more worried that your part is so far from perfect that it doesn't match the model enough to reconcile on the first alignment anyway (which might explain you doing an iterative to begin with), of you are re-measuring the datums but not using the same features/points as the first time. maybe?

If you don't have angle holes, you don't have to move the alignment to check the hole.
If you use ExactMeasure or GeoTol you don't have to move the alignment to check the hole.
In both cases, though, the software IS moving the alignment to check the hole in the background and you don't see it.

Make a ExactMeasure or GeoTol position and then click into it. You'll see a yellow trihedron pop on the screen.
That is the alignment the software is using to measure the hole's position against true position.

If you can't move it yourself and get back to where you were, you need to look at your alignments and see where you went astray.

I said I do it on large, complex manifolds.
I didn't say I got it perfectly right the first time, every time.
It is very easy to hit the enter key before you clicked on the button to apply something in an alignment, or do an angle in the wrong order with a translation.
If you are having a repetition problem, it's almost always in the alignment.