The standard protractor you get in mathematical instrument sets doesn't cut it for me, it's designed to measure angles on paper, not a 3D object. So to the drawing board I went and an hour later I had a lovely design (I ALWAYS make a technical drawing of my projects before I start building them) that required only five parts.
SoOoOoO, the briefing is out of the way, let's start:
1) two sheets of alumin-i-um (not the American spelling). Mine are 3mm and 4mm thick.
2) a bolt with matching nut to hold it all together. Mine is brass because it looks cool.
3) a spring washer to keep all moving parts happy. A normal washer would also work.
The most precise tools you've got, all I had at hand was:
1) angle grinder with 1mm cutting disk and thick grinding disk.
2) bench grinder.
3) sanding machine with belt and disk.
3) wood chisels to make notches every 5 degrees.
5) drill press and assortment of drills.
6) spade drill-bits to make recesses for the washer.
7) steel ruler.
8) vernier calliper.
9) bench vice.
10) standard size steel file for shaping.
11) set of mini files for precision shaping.
12) scroll saw.
13) a couple of G-clamps.
14) a normal protractor.
I think that's about it . . . CONTINUE
Step 1: Step 1
Now shape the round side of the protractor-plate and the "bubble" of the guide-plate. Also remove the excess you left on the measuring edges BUT MAKE SURE THEY'RE 100% STRAIGHT, use a steel ruler and vernier caliper for this. All shaping was done with the sanding machine and standard file.
Please note that the edges on both sides of the "bubble" must line up with each other as closely as you can manage, the center of the hole for the bolt must also be on this line.
Also mark the protractor in whatever sized increments you wish to use, I opted for 5 degrees and used the wood chisels (naughty naughty) and hammer. Obviously you need to use a normal protractor to get these exact.
Also drill the holes for your bolt, This will naturally go through both plates and must be just larger in diameter than the bolt you're using. In my case: bolt = 4.8mm and hole in guide plate = a teeny bit smaller than 5mm. The hole in my protractor-plate is 4.5mm.
Step 2: Step 2
Next is making the recesses in them to allow for the washer. I used wood spade drill bits (naughty again) because aluminium is soft enough that it does little damage to these cheap bits. The reason I used spade bits is because they have a tapered point which conveniently centers the bit, and also because they have a flat cutting face as opposed to normal drill bits.
Remove the head of your bolt and shape that end so that you can press-fit it into the hole on the protractor-plate. Then cut a groove in the same end of the bolt almost as if you're making dumdum bullets. This will help you to keep the bolt secured in the hole later on.
Step 3: Step 3
Also cut a deep groove in the bottom of your protractor-plate so that when you pry the bolt open it has something to grip onto. I used the angle grinder for this.
Step 4: FINISHED - WOOT!
Pretty neat huh?
compare it to a normal protractor, if it's not precise enough I apologise. You'll have to start over.
Fortunately mine is only about half a degree out so I'm not complaining.
Side note: I used my sanding machine's belt sander to give the faces of both plates a nice brushed look, sort of visible in the picture.