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basic dimensions

basic dimensions

basic dimensions

when doing FA inspection, you list every feature and enter actuals against it.
for those basic dimensions, you should not enter any, as any actual position tolerance (entered) covers it.
but our QA guy says he was trained by a large fortune 500 company to enter numbers against those basic.
take an example, a hole with position tolerance of .010 diameter, X, 1.000 and Y, .500.
the actual hole center on top is 1.005, .500; while at bottom 1.000, .505. which number should enter.
but I need some document/specification to show our QA.
is there any?

RE: basic dimensions

It is correct that basic dimensions don't get reported on a pass/fail basis simply because basic dimensions have no tolerance. The official standard ASME Y14.5-2009, paragraph 1.3.23, defines a basic dim as "a theoretically exact dimension." So there is nothing to report!

As you say, the real tolerance to be measured and reported is the position tolerance, shown in a feature control frame.

That said, I'm not sure I understand the numbers you give. If the position tolerance is diameter .010, then every portion of the feature's axis must fall within an imaginary diameter of .010, centered at the perfect, desired location. (In other words, no portion of the axis can go beyond .005 radius away from the perfect location.)

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

RE: basic dimensions

what i tried to say was, on the top, it (the numbers) meets .010 diameter position tol, at the bottom, it meets too. but the center XY locations are different from top to bottom. so, there is no way to enter numbers against the basic dimensions. this is to "support" why not entering any number against basic dimensions.

RE: basic dimensions

On an FAI, or any other inspection report, you enter the nominal dim and tolerance, then the actual value(s).
On Basic Dim's, you still enter them along with the modifier (tolerance), and then the actual value(s), either as the largest deviation (up to .xxx) or, the deviation range (+.xxx to -.xxx).
.625 Dia +/-.005, Basic Dim 4, Basic 6, TP DIA .010 A|B|C CK'S:
UP to .624 DIA, TP DIA UP to .006
.625 Dia +/-.005, Basic Dim 4, Basic 6, TP DIA .010 A|B|C CK'S:
.623 to .624 DIA, TP DIA .004 to .006

A Basic Dim has no tolerance (itself), but the feature has to have a tolerance. A Basic Dim without a tolerance on the feature is incorrect.
The Basic Dim is entered on the FAI because:
It defines the feature being checked.
It is called on the dwg or MBD, so it must be noted on the FAI.

Harold G. Morgan
CATIA, QA, CNC & CMM Programmer

RE: basic dimensions

i hardly agree with HGMrogan. like to know Belanger's view.
BASIC has no modifier, only position tolerance has.
largest deviation on X means nothing if not considered with on Y.
example, without modifier specified to the feature, where came the diff numbers on TP.
thanks for participating in this thread.

RE: basic dimensions

I can't say whether a specific FAI format "requires" a basic dimension to be mentioned, but I would find it confusing. So I still say leave it off. (Feel free to expand your thoughts, HGMorgan -- I think we agree in theory about basic dims, but in practice I'm being a little more strict than you.)

The biggest issue is the fact that most position tolerances are diametric, and most basic dims are in X and Y format. As mentioned in dho's previous post, it makes no sense to speak about variation on X only because the variation on Y is also intertwined with X (think Pythagorean Theorem).

A side note: I often see prints where a basic dim has been flagged as a "critical "dimension" (or whatever you want to call it) but that's just plain wrong, because there is nothing to statistically track regarding a basic dimension. Instead, the critical dimension flag should be tagged to the position tolerance (feature control frame).

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

RE: basic dimensions

I like to see the individual basic X & Y data just so I can check the math on the true position. Too often I have seen simple math errors such as forgetting that the tolerance applies to a diameter and not a radius. If they only put down the positional results you are depending on someone else to do the math which may not be their strongest suit. When we use a CMM or measuring microscope to do these measurements they print all the intermediate basic dimension measurements as well as the diameter(s) and the final positional calculation, these printouts are usually attached behind the summary sheet.


The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

RE: basic dimensions

my couple cents.
1) if someone can not do the math related to his/her required job function, he or she is not qualified to be an inspector.
2) go to an exaggerated case. if the center line of a hole is in a spiral shape, what X, Y to record?
thanks for joining in.

PS. i do not remember i ever saw a CMM report listing numbers against basic.

RE: basic dimensions

A little late, I know. When a hole is located by position, the entire length of its "axis" has to be within the tolerance zone. Reporting a 2D dimension from a basic dimension gives an incomplete picture anyway. I've never reported, required to be reported, or seen reported, a basic dimension.

John Acosta, GDTP Senior Level
Manufacturing Engineering Tech

SSG, U.S. Army
Taji, Iraq OIF II

RE: basic dimensions

Please review my post, and AS9102 (First Article Inspection Report).
The actual Basic Dimensions are not reported, the actual values per the tolerance modifier are.
An FAI or FAIR requires listing of all dimensions and the tolerance of each characteristic called out by engineering.
We have done 10,000's of FAI reports that are reviewed by customers and/or the FAA.

The engineering call out of:
.625 Dia +/-.005, Basic Dim 4, Basic Dim 6, TP DIA .010 A|B|C --- has to be noted on the report in the characteristic and tolerance column.

The actual:
UP to .624 DIA, TP DIA UP to .006 --- is noted in the actual column.

Please note that the Basic Dims are not noted in the actual column, because Basic Dim's do not have a tolerance.
Basic Dims are noted in the characteristic column because: (1) They are called out by engineering (2) They help define which feature is being reported on

Harold G. Morgan
CATIA, QA, CNC & CMM Programmer

RE: basic dimensions

I have never seen basic dimensions "bubbled" on an AS9102 FAI report. But I have sometimes seen reference dimensions "bubbled".

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