Wednesday, April 4, 2012

straw chopper 4

Having become clear the mower was going to work the next thing to figure out was how. I imagined getting the blades razor sharp and the straw just falling apart upon contact. That sounded good, but what would actually happen? Well, the design of a reel mower is this- you have several long blades twisted around a central axle, as they spin they pass by a cutting bar located close to the ground, the action of the spinning sweeps the grass toward the bar and when the blades meet the bar it cuts them like a scissor. This seemed like good bet for chopping straw- get it sharp let it work they way it was designed too.

I looked up online how to sharpen reel mower blades and there were a number of sets of instructions and tutorial videos. Actually, the first video that pops up is from a saw shop right here in Eugene (i wanted to have the blades sharpened there, but there wasn't the money for it). The cheaper way to do it is to get rock dust, mix it with oil, paint it on the blades and run them backwards. I didn't know where i was going to get rock dust, let alone what that was. Turns out its powdered rock used, in this case, as a grit for polishing gems. Oh, okay, well then this should be easy to find, i mean, it is the pacific northwest. Even the homeless dudes have crystals.

Got some rock dust. Got some old motor oil. Mix. Paint. But, spin backwards? That took a minute to figure out.

I tried reversing the blades in the vise, but that did nothing, the gear spun without turning the blades at all. (I pretty sure this was the moment that i figured out that the driven gear would only engage the blades in one direction at a time). So i tried flipping the pin around, which got the blades to spin, but there wasn't enough power- the gear ratio was too high, so the belt just spun in place.

It occurred to me that i could use the original wheels in some way to spin the blades with more force. I had left the other, non-drive, side intact so the wheel fit right back on.

If you didn't notice, the teeth of the gear are on the inner circumference of the wheel. I had never seen this before, but it makes perfect sense for this application- it allows all of the gearing to stay protected inside of the wheel, no exposure to weather or grass clippings. Clever design.

I spun it with my hands, like a steering wheel, but that didn't do much. So i welded on a rod for a handle and then put a pipe around the rod so the rod could spin. (This might not make much sense to read, but you'll notice that all good cranks have a handle that spins, if it didn't your hand would get chaffed or burned).

It was sturdy, i had enough power to spin the blades backwards, made a lot of noise, i was excited. Then came the paper test; in the videos they measure the quality of their sharpening by cutting a piece of paper in half. The blades did a good job of getting the paper greasy, but that was it.

I touched the blades with my fingers, even got some cuts, but they were no match for the mighty eight and a half by eleven.

Admittedly i was pretty bummed about the rock dust fail. It looked pretty easy and highly effective in the videos. And learning a new process is always exciting, but only if it works.

The only thing left at my disposal was to do it the old fashioned way, with a file. Filing took a really long time and wasn't anywhere near as exciting as the other stuff. Boring, but effective, the blades were sharpened, probably the sharpest they'd been in years. Still wouldn't cut paper though.

In hindsight i think it may have been necessary to sharpen, actually i'm positive, it would have been necessary to have sharpened the cutting bar too. What's a scissor with one dull blade and one sharp blade going to cut? Nothing.

I had a fairly inaccurate vision in my mind that the blades were going to be so sharp and so fast that they would just shave the straw to shreds. I don't know why i thought this, considering that it bombed the paper test.

I threw together some pieces of scrap wood to make a tight straw holder. And if i just got the blades going fast enough i could slowly lower the holder down into them and the straw would come flying off, like a chipper or a snow blower. But that didn't happen. The straw acted as an excellent brake, stopping the blades from moving at all. What about less pressure? Didn't do anything, the straw just flopped out of the way of the blades. Some cutting, but not enough to be worth it.

The straw clearly needed to have something behind it so the blades could cut it rather than just pushing it out of the way. And, of course, this was how it was already designed- with a cutting bar. It would need to have some kind of container around it in order to keep the straw in, so i quickly fashioned one to see how it would work.

I had to add this piece of rebar in there to hold the sheet metal away from the blades.

The cover had an unexpected and hilarious result- all of the straw shot out in a giant ball and hit the wall. I wish i had it video taped, it was pretty great.

I covered the opening with an old napkin, obviously, making a huge difference. Now with the stuff circulating around in there i could get a sense of what it was doing.

The straw was being cut, or at least it was breaking, but it wasn't anywhere near the fineness that it needed to be for finish plasters.

I tried playing with the cutting bar, moving it closer in toward the blades and back out again. When it was close, all the straw got caught. When i backed it off it was too loose to cut anything. I couldn't find a balance, the cutting bar just wasn't working.

I left the cutting bar open and let the straw spin around in there for a while.

I was pleased to see that the straw, given some more time inside the machine, was getting closer to the ideal quality.

It was suggested to me that i try covering the blades with a closer fitting piece of metal, essentially put it into a tube. It just so happens that the shop used to be a wood stove testing facility, so there was plenty of scrap pipe hanging out outside.

I split a length of pipe and covered the blades with it. It didn't work at all. The distance between blades and the outer wall became so tight that it slowed the whole thing down massively.

Out of everything so far what worked the best was spinning it around inside of a closed container. It wasn't perfect, but it seemed like my best shot. I called it quits for the day and resolved to pick it up there next time.

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