When documenting hardware how close is "close enough"?

[crashwg]

Let's say that I've got my calipers out and I'm taking measurements of a D.O.T. spinner.  I measure one of the pieces and it comes to 1.505".  Should I just assume that the design called for 1.5" and 1.505" was "within spec"?

Also, a couple of the measurements like the hole spacing are close to standard imperial measurements such as 3.63" which is close to 3.625".  I again am assuming (since I don't know for sure) that these parts were designed and made in the U.S. of A. and therefore I should go with the standard imperial sizes right?  3.63" = 92.202 mm so it's definitely not metric...

[Minwah]

What are you documenting it for??  I would use the most accurate figure you have, then if anybody else can check their spinner/whatever, you have figures to compare and can make a judgement.

[u_rebelscum]

Let's say that I've got my calipers out and I'm taking measurements of a D.O.T. spinner.  I measure one of the pieces and it comes to 1.505".  Should I just assume that the design called for 1.5" and 1.505" was "within spec"?

Also, a couple of the measurements like the hole spacing are close to standard imperial measurements such as 3.63" which is close to 3.625"....

These could be a case of your measuring precision being higher than the accuracy of construction.  The first instance lends that way.  The second one, OTOH, could be intentional rounding; many dimensions were spec'd at hundredths of an inch when written in decimal instead of fractions.  You'll see this rounding in some of happs drawnings (example trackball: see 0.12" (~1/8) & 2.57" (~2 9/16)).  Five thousandths of an inch is not very much off, and accuracy of than quality is sometimes not needed.

But as Minwah said, you should check other's measurements and then judge.  Heck, take the same example from above.  That 0.12" (~1/8) really is 0.0625" (1/16, old school model) or 0.1875" (3/16", highlip model), both less than seven thousandths of an inch off from the number listed.

[rockin_rick]

I think that (if documenting it) you should keep as many accurate significant figures as possible, and let whomever is using the data determine what to discard or round to.  The accuracy of your caliper or mic would determine this.  Say you have calipers that are only accurate to 0.01", then the part measured as 1.505", should be documented as 1.51".  But if your tool is accurate to 0.001", then document 1.505".

The point of 'accuracy of construction' is valid and needs to be taken into consideration.  If this value is unknown (most likely), then you should not assume or guess what it might have been and rather document it as "measured to be 1.505" not "the original part was 1.505".

Another consideration is wear of the part.  Some materials will wear over time and this needs to also be taken into account.  Probably doesn't apply here, but I do not know what the DOT spinner is made of...  In that case you could document it as "measured used part to be 1.505".

I don't think that you can assume that since 3.63" is close to 3 5/8", that (while most likely) "the engineer intended it to be 3 5/8".  Who knows what the engineer was thinking?  Perhaps he moved them until they felt/looked right and 3.63" is what it happened to be.  Or maybe his birthday is march 1963 - OK that may be a stretch but the point is that you don't know so you can't assume.

Yeah, this is all pretty anal on my part, but without knowing the use of the data, then I say keep as much accurate data as possible.  I guess when you said "documenting", I think of accurate data to be used by unknown people for unknown reasons.  Now if you were simply "documenting" for your own use to build a CP, then there is nothing wrong with calling it 1 1/2" and 3 5/8"....

Rick

[PoDunkMoFo]

Don't forget as mentioned a certain level of accuracy is not possible in real world manufacturing.  Especially mass produced products.  If you had two spinners there is a good possibility you would not see exactly the same measurements.

[Havok]

Plus, these parts most likely aren't brand new. Depending on where the measurement is taken, there may be some wear involved.