Anatomy of a 1/4....Jack....

[pinkjimiphoton]

remember too, that a lot of times, they're manufactured not to 1/4", but 6.33mm. it's a minute difference, but it's there.

i'm jacked about this discussion...lol

[Jdansti]

^

You...Don't..Know...

[defaced]

All mechanical connections have tolerances.  It's just the nature of manufacturing.  Sometimes you can get tighter tolerances if you are willing to pay more.

Well-designed connectors take into account the force of the spring of the contact and make sure the tolerance stack-up is such that the parts still work if the parts are each on the far end of the dimensional tolerances.

Agreed.

It seems to me that the jack's ID has to consistently be slightly larger than the plug's OD.

Yes, but there are a few details in those drawing the casual observer will likely miss which make it not-so-obvious.  

The use of REF after a dimension means it's for reference only, and is not a controlled dimension.  Meaning, you can't manufacture or inspect to it, but it's a "nice to have" for one reason or another.  So that 1.4" REF could mean the part is 0.250 +/- 0.001 or 0.255 +/- 0.005 or 0.25 +/- 0.01, all of which are different dimensions mechanically but could easily be called out with "1/4" REF".  The real dimensions are probably contained on the manufacturing drawing, not the reference drawing.  

The other thing of note is the dimension on the jack, 0.25 dia.  Tolerancing is often called out using a global tolerance based on the precision of the dimension (places needing other tolerances are called out on a case by case basis, see the 1.015 +/- 0.015 dimension for instance).
For example:
all 0.X dimensions would be +/- 0.1
all 0.XX dimensions would be +/- 0.01
all 0.XXX dimensions would be +/- 0.001

So that 0.25 dia is really 0.25 dia +/- some tolerance, but without the rest of the drawing you're left hanging as to what that tolerance actually is.  

Now to the point, while looking at the drawings the 1/4" REF and 0.25 dia appear to be the same, but from a geometric tolerating and dimensioning perspective they are not, and further the drawings don't contain enough information to determine the type of fit (free, line to line, interference, etc).  So while intuitively the plug must be smaller than the hole in the jack, the drawings don't have enough info to support that.  When one accounts for the tolerance stack up of the parts, you could get something that doesn't work if that was all of the information provided to the shop for manufacturing.  

P.S., sorry to anyone reading this who actually does GD&T for a living, I hope I didn't butcher the terminology too bad.