Making Pulleys, Part 2

I left off a half a year or so ago with that little piece. I haven't had any need for pulleys so I just left it at that. But I've started building the Gingery Power Hacksaw project, and am not about to pay through the teeth for store-bought pulleys, with money that I don't have!

The project calls for 1 1/2" and 8" pulleys. My other pattern is about 2 5/8" (I don't have one here to measure) so I'll either have to make a really big (like...14") pulley, or make a smaller one closer to the spec. I'll get to that later. Right now I'll discuss the large one. The layout involves three spokes between the hub and rim, but you can use as many as you want.

9 Inch Pulley Pattern

This started out as a square scrap of 3/4" plywood. I started by marking a center point about 5" in from each edge (duh) then drew 4 1/2", 3 1/2" and 1/2" radius circles around it. The outermost defines the outside edge, the middle bounds the cutouts which save weight and the smallest defines the hub. Next is to draw a line through the center point - angle doesn't matter obviously, since all the markings are round at this point.

Pulley Diagram

After you draw the base line (black), set the compass to about 2" (it doesn't matter, as long as it's the exact same setting throughout this process). With the point on the center, mark a long arc starting a little across the baseline and extending to about above the center. (This is for the first two points just left of the center point.) Move the point to the intersection of the baseline and the new arc, and draw a short arc across the previous arc. If you draw a line between each of these points (the red lines), you get an equilateral triangle - simple geometry. Repeat this process once.

Now that we've got our three points 60° apart, draw lines through them (and erase the dashed part of the baseline) for where the spokes go. The observant will note here that if the lines are extended across the whole circle, you'll double the number of spokes (from 3 to 6). If you did it right, you should have three lines radiating from the center point as shown. Measure out 1/4" from each side of the lines and draw lines between these marks with a straightedge. The 1/4" figure is half the width of the spokes. Think "widening" the original layout lines.

This should (if it doesn't, you didn't do it right :P ) enclose three 120° pie sections between the hub and 'middle' circles. Mark these carefully so you don't cut anything extra! Drill some holes now to start the inside cuts between spokes. Take the marked blank over to the jigsaw, set the table for 5 or 10° and start cutting the outer edge. Remember to cut all lines so the draft is in the same direction, and always leaves more material out from the lines! When you're done with that, start on the inside holes. I'll assume you're already familiar with how to get the blade in there...

When you're done cutting, you'll see you have a beautiful spoked pattern with hub and rim, but one minor detail on its way to pulley-hood - no groove! The way I went about this is to drill a hole through the center point, nail the pulley (using a nail that just fits in the hole; 1/8" and an 8-penny nail are a good combination) to a backer board and clamp this to the tablesaw fence. Set the blade for um 15° (I don't know exactly what the belt's Vee angle is) and dangle the pulley blank over the blade. To cut the groove, crank the blade up until it cuts about 1/2" deep. Now start rotating the blank around the nail. Repeat on the other side so it's symmetrical. Using the same milling method you might also (with the blade set square!) flatten the bottom of the groove and the outer rim. Some sanding, filler and a coat of varnish or whatever your preference is (I use wax for both) and you're done.

Molding

I haven't gotten this far yet but I've thought it through. I'm going to clamp the pattern between two rounds of plywood and spread plaster (mixed half'n'half with sand I think, at a pasty consistency) in the groove. Three bits of sheetmetal evenly spaced will divy it up so I can get the core off the pattern. These cores will be removed, baked at 450F for at least half an hour, cooled and placed back on the pattern to mold normally in a 2-part sand mold. When done, the cores are placed in the mold (in the same place as when molding), mold closed and poured. Hopefully the uncalcined plaster won't cause problems! This guy has already done pretty much what I describe: http://isitcrunchy.com/fancy.htm

In a forum post, Crunchy adds:
The trick was to get the spokes to all have the same taper, and the inside of the outer ring to have the 'V'. So I used 8 pieces glued and bondo'ed together. The inner hub and outer ring I turned on my lathe. The spokes I made with a saw to cut the taper in two dimensions. A table saw to cut a long strip with a taper, and a chop saw to cut out the spokes with the other taper. I rounded the spokes with a belt/disk sander. The spokes also needed a notch in the outboard ends. I glued them all together and put on several coats of bondo to get the fillets. And lots of sanding.
It was molded in the drag. I coped down to the center for the spokes/ring triangle. I did not worry too much about any fallout from the outside as this was covered up by the groove pieces. When you get yours done, you gotta post pics.

Update (10-02-2004): I cast it!

Placing the pattern between two 10" discs of foam board (leaving 1/2" around the whole pattern), I spread 1:1 plaster/sand mixture (mixed thick, this stuff sets FAST!) into the groove all the way around, with sheetmetal shims to seperate it into three cores of course. After they set I removed them to make sure everything's okay (and it was, except for some sloppiness in some areas which can be filed out later), then proceeded to mold the thing. Since my foam disks aren't perfect I had to grab a screwdriver and carve some draft on the outer edge of the cores - no problem since they're still moist and soft.

Molding was uneventful, however, removing the pattern was a different matter. Seems the outer rim was a bit sticky, despite my slathering of parting dust (psst.. wood or paper ash works great). After a good bunch of pulling I managed to seperate the pattern and cores from the sand, but not without seperating two inside corners of sand from the face, and a bunch around the outside. (The stuff out there doesn't matter since the cores are in the way.) Some spit and gentle pressing got the corners safely back in place.

Now, I'd have gotten out the furnace then and there, if not for the fact that I haven't dried the cores yet! :-o So I took them out of the mold (all of 12 pieces now, broken up by the molding pressure... hint: get a sharpie before you take them out!), brought them to the kitchen and set them in the oven for 450°F for an hour. <Chemistry warning!> (The way plaster works is you start with partially dehydrated gypsum, CaSO4 1/2 H2O. Adding water reconstitutes it to CaSO4 2H2O, with a slight recrystallization or expansion or something, which in any case causes it to harden. Now, that 1 1/2 parts water comes out around 400°F, but leaves that half part. This *does* come out around molten aluminum temps, so you can see why a good red-hot calcining, or at least good porosity, is a necessity here!)

After lunch I took the cores out of the oven and placed them in the mold and closed it. Got out the reverberatory furnace and melted about 3 pounds of cast-alloy aluminum (I've been having bad luck lately so wanted to use something with good flowability). Uneventful pour so took everything back downstairs and shook out the mold. Started taking off the cope, and braced myself incase it didn't work... but... it did! HAZAAAHH!!! Chipping away more of the sand (shake-out, my ass) and the cores (actually the cores were pretty loose) I freed one (1) brand-new 9" pulley with attached sprue. Cleanup was straightforward if long and tiresome - simple job of hacksawing off the sprue (leaving some as a hub), filing *EVERYTHING* (ack), and that includes the groove, which because the cores broke into so many pieces had a good number of little webs across it. After that, I finally got some postable pics with the crap-era (camera...crap...crossed words...nevermind).

Big Pulley Side Shot

The ever-observant will note I haven't drilled the shaft hole yet. I did today (um, it's nearing the 12th now... sigh). Since our drill press doesn't have enough throat capacity I had to drill it with the cordless. So I took a piece of 2x4 and drilled it 1/2" on the 'press, clamped that over where the hole should be, and assumed it would keep the cordless straight.... riiiiight.... needless to say I have a hole about 1/8" out on one side, and the pulley doesn't exactly look happy when I run a belt around it and spin it with a motor. But that it runs at all means it must be working, since the belt isn't slipping off... yay. Now to make another one...



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