A way to measure top thickness of a closed box

[Dave Bagwill]

I thought that I would approach this using angles - thus my trig questions from yesterday.
So using some aluminum bars, an angle gauge ($12), and a length of carbon fiber, I made a very sturdy structure that ain't going to move. Except of course for the 'hypotenuse' arm. :-)
When it was competed, I clamped it to a bench, lowered the arm to the pivot at the end of the lower jaw, and zeroed out the gauge. Then, using feeler gauges, I started with .070" and worked up .005" at a time to .135", writing down the given angle from the gauge for each increment.
It's stable, and the measurements are repeatable.
It fits into the soundhole of the guitar easily - once you have zeroed the gauge, the arm swings up and out of the way until you've inserted the jig.
One problem to overcome is this: I've attached a drawing showing the area of the top that I can reach with the jig, and in order to reach the farthest parts, I sacrifice areas closer to the waist. I am going to shorten the jig about 1" tomorrow and retest; this will allow me to cover the lower bout edges in 00 up to jumbo sizes.
Not a candidate for the Museum of Modern Art, but it works and puts my mind at ease as I sand down the top around the lower bout.
I'm sure there are possible improvements that you will see and I hope you will let me know what they are.

[John Link]

Interesting tool Dave.

[Dave Bagwill]

I of course avoided the calculating by simply separating the jaws a given amount and manually reading the angles; and of course I do the reverse when I am checking thickness: I read the angle and check my table to get the correlated thickness.

[John Parchem]

That is pretty cool.

[Dave Bagwill]

I also tried the dial-gauge at the end of a fixed aluminum bar; zero-out against the pivot at the end of the bottom 'jaw', pull up the gauge stem, insert the jig, lower the stem, read the thickness.
That worked ok - you have to make it rock-solid, of course, but the problem I ran into was getting the thing into the guitar, because of the short throw on the gauge - since the gauge arm is fixed, there is barely enough room, even after pulling up the gauge stem, to insert the jig without some acrobatics involved, which raised the chances of a nick or scratch quite a bit.
If I can come up with a way to use dial gauge by making the upper 'jaw' so that it can swing up during the insertion, and return to its exact zero point when I swing it down, that would be a useful jig. I'm working on that today.

[Dave Bagwill]

I plotted out the points that can be accessed with the standard configuration (everything below the red linw), with the pivot point moved shorter by 1" (everything below the green line (includes the area below the red line) and with the pivot point moved shorter by 2" from the original point (everything between the curved blue lines).

For those of you that do think your perimeters - which looks most useful to you, or is the entire enterprise without value? Yeah, I want that too if that's what you think!

[Dave Bagwill]

I could generate another 1" or so of reach by reducing the area in the red circle by sawing out the area in yellow. That would mean cutting out a portion of the aluminum upright bar, but the thing is so sturdy that I don't think it would affect anything. Hmmm...