Friday, June 29, 2007

AV - Midnight Turning

It’s twenty-three minutes past midnight, Thursday the 28th of June, 2007. (Better make that Friday the 29th.) I’m just out of the shower, got my pipe going, scotch & water in hand. My wife has already gone to bed, probably mad at me for losing track of time out in the shop. Long list of mail awaiting answers, mostly from kids who’ve just bumped heads with Reality. (‘My bus is on fire!!!! Should I put it out or what?’) But I’m too tired tonight.

I like making things but the enjoyment comes mostly from figuring out how to make them. I made my first coaxially mounted dynamo back in the early ‘70's using parts out of a Honda motorcycle. I’ve since made quite a few of them using different rotors, home-made rectifiers and other variations, figuring out how to mount them on either end of the crankshaft. In fact, if you’ll compare the drawing above to the one I posted a couple of years ago you’ll see that the dimensions have changed slightly. And even the drawing above isn’t carved in stone. Tonight I saw a way to save some time that will appear in the next one I make.

Time doesn’t count when you’re figuring out how to do something. Hours vanish in the blink of an eye. But making a copy of something you’ve already figured out is pretty boring, which is why you start thinking about ways to do the job faster.

The dynamo hub starts out as a three pound billet of aluminum, that lump on the right in Figure 1. It goes into the chuck and the work-face is cleaned up so you can poke a one-inch hole in the middle using a drill. Then you start shaving down the outer diameter, creating what will become the shaft of the hub. What will become the flange is being gripped by the chuck. The 12-inch lathe has a three-horse motor and can take a fair-sized bite but it takes time and you have to keep your attention focused on the job.

Once you’ve got the OD down to where you want it you swap tools, shift gears and cut a the oil-slinger thread into the OD. Then you change to a boring bar and open up the ID to match the nose of a Volkswagen crankshaft, using a broken crank as a gauge. At that stage the thing looks a bit like an aluminum toad-stool.

Now it comes out of the chuck, gets flipped end-for-end. With the shaft now gripped by the chuck you gotta dial the thing in. Since Permanent Magnet dynamos don’t seem to be very sensitive to alignment I’m happy with .001 or less.

I pick-off the overall length using another gauge and face-off the flange, leaving a little lip to index with the rotor. Then it’s back to the boring bar, opening up the forward face to accept a socket for the pulley-hub bolt. Which pretty much finishes the job, except for a few details.

From the lathe, the dynamo hub goes to the arbor press where I use a 6mm broach to cut the keyway. (No broach? Then rig your boring bar as a scraper and rack the carriage back & forth, adjusting the cross-slide to get the required depth.) Then it goes over to the milling machine or drill press where I’ve set up a rotary table, already centered and fitted with a spud to match the ID of the hub. Cranking the knobs of the rotary table through seventy-two degrees at a time, I drill the pilot holes for the five bolts that will secure the dynamo’s rotor to the hub. (If you don’t have a rotary table use the rotor itself as a guide to spot the location of the holes.) After drilling, the holes are tapped 1/4-28.

In Figure 2 you can see how much metal is removed when opening up the ID. In fact, the three pound billet has been reduced to a scant five ounces (leaving you with two and a half pounds of swarf to deal with :-)

There’s a few million minor details I haven’t mentioned, and depending on your tools & experience there’s dozens of different ways to do the job. So long as it fits the crankshaft and the rotor, and spins true around the stator coils, it will work. But I’ll tell you pard, after you’ve made a few, shaving that thing down gets damned boring.

I’ve been trying to find someone to make the hubs and mebbe laser-cut the mounting plate, leaving you to simply rivet the thing together and bolt it to the engine. But today I heard from the last (of three) local CNC shops and the lowest price was nearly three hundred bucks just for the hub... and that was in lots of ten.

I think that’s too much.

The whole idea here was to come up with a method of generating electricity that was inherently more reliable than a belt-driven system. If you can do that, you toss the magneto and use a lighter, less expensive more reliable electronic ignition system. Millions of motorcycles have confirmed the validity of this approach. But the system was also supposed to be more cost-effective than anything presently available. Having to pay nearly $300 just for the hub blows the idea right out of the water... unless everyone makes their own hubs.

Somebody drank my drink. And my pipe’s gone out.


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