I found that this simple drill bit saw will work to strip stubborn exterior grade wires.
Step 1: Another Poor Person's Wire Stripper for Stubborn Cases
I have a whole pile of tough exterior copper containing wire that I wanted to scrap and was having a hard time figuring a cheap way to do so without having to spend hundreds of dollars on a motorized stripping machine. The casing on this wire is extremely hard to remove. I tried whittling it off with a utility knife while holding one end in a vise--a method I found useful occasionally with solid wire cores, but after a few minutes I got so tired my hand began to ache. I also tried pulling this stuff through several home made jigs, but it was just too hard, so I decided I needed a power assisted solution. It took me a while to come up with this simple rotary saw bit I could mount in a power drill. Sometimes simple solutions work best.
I used an ancient Thor brand machine my neighbor had thrown out. It works perfectly because it has a button that locks it's running motor on, but you could also use a snap tie or perhaps duct tape(?) to keep any other unit's trigger on.
All you need to make this saw bit is some 3/4 " thick wood, a bolt, a washer, a nut, a miniature saw blade (many are available on eBay), and some patience (not to mention a vice and a few other standard shop tools).
Step 2: Get the Materials Ready
First find a piece of solid 3/4 inch wood and scribe a circle on it that's about a half inch or so larger than the saw blade in diameter. Then cut it out using a band saw or scroll saw. I happened to have this old mini circular saw blade gathering rust in my parts drawer. You can find similar ones for cheap on eBay. Unfortunately I can't even afford to buy a new one now, but this will do. Someday I might even try it with a mini fine-toothed carbide tipped one.
Step 3: Cut the Disk in Half
This is pretty easy, but be careful and don't rush it. First I drew a line down the center edge of my disk edge, then gradually cut it in half, using my band saw and a few hand saws. After that I sanded it a bit to smoothen out the saw marks.
One could also use a thinner material and simply cut two disks, or just cut two thicker ones.
Step 4: Scribe an Inside Circumference Line.
Scribe another line about 1/8" smaller than the circumference of the saw blade itself, indicating the final inside edge, which will allow exposure of the cutting teeth.
Step 5: Start Sanding Down the Bevel.
Now this part requires patience, especially if, like me, you don't have use of a belt sander. I ran out of belts a while ago and can't afford new ones yet. First I got a general shape with my grinder (not so effective on wood though). Then I used a rasp while holding the piece in a vise, being careful not to hit the line or take down any of the outside edge. Finally I had a small drum sanding bit I put in my drill press, and slowly ground a slightly contoured bevel all the way to my inside line. This took some time, but was worth the effort. Perhaps you could use something already profiled in that way, like a sawed in half pulley, but it would have to be the right size of course...Now that I think of it I'm pretty sure you could find a router bit to cut that bevel--should've tried it.
I also took time to sand out rust on the blade. The last picture shows how the inside bevel, being smaller than the blade, exposes the cutting teeth.
Step 6: Assemble Onto Bolt.
Find a bolt that fits snugly in the blade arbor hole, and is about 2-3" long, so it can fit into the drill jaws, drill out your disk center holes to match the arbor hole, sandwich the saw blade between the disks, using two washers against the outside wood, tighten a nut on the bolt, and voila: a wire stripping saw drill bit. Very simple but effective.
Step 7: Mount Drill, Plug Into Speed Control Box, and Strip Wire.
Simply mount the drill onto a vice, plug it into a rheostat control device, like a router speed control unit, set the drill switch to on, and tighten the drill chuck onto the bit's bolt, and when you turn on the box you can control speed. You actually don't really need the speed controller and could just plug into any power output that has a switch to simply turn it on and off too.
A higher speed is required to power through thick plastic coverings. I straighten out lengths before stripping, and hold each piece onto the blade using both hands (couldn't take a picture of that since I need a hand to hold the camera), or a pair of pliers. Cutting slows down rotation and the blade plows a thick groove that exposes copper inside, which you can later pull apart from remaining plastic. I cut down end sections with end nippers and use a box to catch ground up plastic.
You can use any size blade of course. My first attempt actually had a 28mm rotary cutting blade, which worked too, but was not as aggressive as the saw, and might work better for smaller wire if less cutting edge were exposed. This jig is experimental and sure to be improved for other situations.