I came across some Armstrong student and intermediate model flutes that have .018" bodies and .014" headjoints....

How does the headjoint fit on to the body??? If I took that Armstrong flute and put a .014" headjoint from a different brand on to the flute would the head be far too loose?

This thickness refers to the thickness of the metal and not the diameter of the tube. Flute companies will tell you that the thinner metal will have a brighter sound. But the fact is that the player controls these things. The thinner flute material will usually dent easier when you bump it and the temperature of the flute's metal will change more quickly. This can be an issue if the flute is being played in a colder airspace because of increased tuning volatility. The difference in weight between a thinwall and heavywall flute isn't a great deal (About the weight of 2 US quarters) but a less physically strong player may have issue with the weight of a heavywall flute or the psychological effect of the idea of it.

http://iwk.mdw.ac.at/Forschung/english/linortner/linortner_e.htm

"CONCLUSION:
Tests with experienced professional flutists and listeners and one model of a flute made by Muramatsu from 7 different materials showed no evidence that the wall material has any appreciable effect on the sound color or dynamic range of the instrument. The common stereotypes used by flutists and flute makers are exposed as “stereotypes”."

~bilbo
N.E. Ohio

Also,
"If I took that Armstrong flute and put a .014" headjoint from a different brand on to the flute, would the head be far too loose?"

Again to display something, The diameter of the flute's tube may be somewhere around -0.750" or about 3/4" while the thickness of the metal may be 0.014" or about 1/100".
The diameter is the most important factor for fit.
Another brand flute body diameter may be larger or smaller by a very small amount from the Armstrong one and you will notice this when you try to fit the two together. So the headjoint would probably have to be resized by a repair technician.

You can even take the same brand of flutes and the same model and depending on when it was made, the diameter of the body will be different and the tubing thickness can be different too. At one time, I had an Armstrong 104 flute from the 1960's and an Armstrong 104 flute from the late 1990's. The tubing on the older flute was thicker. The diameter of the body on the older flute was smaller.

Neat stuff.

The bore is pretty consistent on flutes. What differs for headjoint fit is it's tubing thickness. As the tubing thickness increases, so does the outer diameter of the tube. Barrels are made to fit the tubing thickness so that the bore profiles always line up.

Even slight deviations in thickness can make a fit too tight or too loose given the "same" size.

If you have a .014" head tube, other .014" headjoint thicknesses "should" fit, or at least be the closest option.

Joe B

"Barrels are made to fit the tubing thickness so that the bore profiles always line up."

Interesting concept if one regards that any value in having this measurement down to the precisest ~0.001" +or - would be offset by the fact that one will generally have their head joint pulled out thus causing their finely honed oscillating air pressure waves to be disrupted by such an imperfection.

I would say that it still doesn't change the fact that one can more than likely expect to have any different head re-sized to fit the barrel of their flute.

<Added>

Something to peruse regarding this. Not that it's going to help the issue much. The two pieces still have to fit nicely.

http://web.fmarion.edu/goferjoe/headsize.htm

Notice that there may be somewhat of a relationship between diameter and tube thickness but not always. As a matter of fact, there isn't even a relationship always between tube thickness of the head and body tubes. Go figure.