This might be hard to explain without showing a print. Wish I could, but that's a no-no since they're confidential documents and all. Anyway, let's say I have an older print with holes that are dimensioned to each other, rather than to a datum or to a common origin point. They also have a true position call out. As I understand it, you can select either "current alignment" or "datum reference frame" when creating a position dimension for the holes. If I was required to report the holes with the nominal dimensions from the print, rather than from the datum reference frame, say for PPAP purposes, does that mean I would have to create an alignment for every single hole to get them to report exactly as the print dimensions them? Or is there a more efficient way to do that? I hope I'm making sense. I'm still pretty new to this.
do the position callouts have datums listed?
BASIC dimensions (distance between holes) don't get a tolerance and don't get reported (or they shouldn't). They simply tell you how to design a 'perfect' part. This one would require customer input as to what they want shown on the report.
Yes, the position callouts have a feature control frame with datum structure of ABC or AB or BAC, depending on the hole. But the way they are dimensioned on the print is to another hole, not a datum. I think this is a really old print that got true positions and datums added to it decades after it was first created. Some customers are extremely picky and require everything to be reported EXACTLY as shown on the print. They even want the basic dimensions and their deviations to be reported along with the true positions. I've even seen customers who demand reference dimensions to be measured and reported (the one with the signature green color and a forest animal in its name). But the only way I know of to get each hole to report as shown on the print is to create an alignment for each hole, recall the alignment I need for the hole I'm trying to dimension, and then select "current alignment." That can be a pain when you have 20+ holes.
without a napkin sketch, we can only help so much.
so you've got a pattern of holes, and some holes are defined as datums.
-Measure each datum hole.
-Construct an alignment for each feature control frame as you mentioned (AB, ABC or BAC),. and within each of the currently set alignments, measure & dimension output the specific holes that apply to that FCF.
--in my opinion if the pattern of holes is clear (like 12 holes about a set diameter, equidistant 30° spacing kind of pattern)... I would in addition to above, create a bestfit alignment to the 12 holes as a pattern, and check each hole's pass/fail result to the ABC FCF tolerance. it's probably a mount flange of some type and you'd be checking for functional fit this way, much better than the print calls out.
The FCF says ABC, then you report position to ABC.
Basic dimensions can be added or subtracted to find the PERFECT location from ABC to the hole.
One alignment, everything is from ABC. This is USUALLY what was always intended, it isn't common that an engineer actually wants things incrementally moving about hole by hole (the glaring excepting being bolt circles and patterns). If you are truly curious, you can ask if they meant it inspected True Position as today from ABC, or did they really mean incremental.
Report incremental moves as distances. They don't matter, they are BASIC and carry no tolerance.
Position is of zero value to a machinist or engineer (when a problem or adjustment is needed) as you say the position is .012. .012 what? Where? From where? three rotations? start of the feature or end of the feature?
Reporting the BASICs is required by some people, and I always do it out of habit. Not because they matter per standard, but because if something doesn't fit, a person can see I reported the "X" as being .002 off and the "Y" as being -.006 from perfect, and compare that to the mating part.
I also report all positions on any hole longer than twice it's own diameter twice, displaying the start and end. Then my machinist knows what he has and doesn't have to ask anything.
This is all extra to the fact that a FCF saying ABC means you are inspecting the feature with respect to A, B and C NOT to any incremental moves in and of themselves.
That is the only alignment you need if all FCF's say ABC. As Louis below mentions, if you have more than one datum structure in an FCF, that is when you need more alignments.
Incremental moves do not require a new alignment unless they are from a new datum that is called in an FCF. Or the secondary frame for pattern control, if the print is kind of sloppy. But if you are doing pattern control and all dims are from ABC, you have to "make" incremental moves (the software is going to do this for you) to insure the pattern is moving together, if you are inspecting manually.
The FCF controls the reporting, not the BASIC dimensions displayed on the print.
You are looking at the BASIC dimensions that are shown hole to hole. Thats not how basics work I am guessing that you dont have background in GD&T and you are applying " LINEAR DIMENSIONING " understanding to them.
Unfortunately some customers want to see the basic dimension measurements, in that case you usually need to create alignments or report hole to hole distances. I always add a note to the report, something like:
THE FOLLOWING BASIC DIMENSIONS AND TOLERANCES ARE FOR QC AND ENGINEERING REFERENCE ONLY, OR CUSTOMER REQUIEMENT.
No, I understand that basic dimensions are not meant to be reported and do not have a tolerance. Unfortunately, the customer wants what the customer wants, and if they want the holes reported with the basic dimensions, then I have to give it to them. I was just wondering if there was a better way to go about it.
I understand how basic dimensions work and that they don't have a direct tolerance. And I'm aware that I'm probably overthinking it. Unfortunately, some customers are picky and want everything to be reported as shown on the print. Often the people reviewing the reports have no knowledge of print reading or GD&T themselves and are just directly comparing numbers to see if they match or fall within range. The customer I hinted at above is an example. I was just wondering if there was a better way to go about that in this instance. But I'm gathering that if a hole is positioned to ABC for example, then that's how it should be reported. If they want the hole to hole distances, I'll just dimension those separately. Thanks for all the advice.
Personally if I had to report the basic dimensions I would title Dimension for customer ref only and remove the tolerance and deviation from the reporting in the preferences so as not to muddy the water with any potential out of tolerances as there is no tolerance to report to
