bike blender blog: straw chopper 5

The next design challenge of the straw chopper became how to create a sturdy sealed container from which the blades could be removed without having to take the whole thing apart. Removable blades were necessary for two reasons, one, to be able to service the blades and, two, to be able to easily clean out the container. Coming up with some rough sketches of what that would look like was fairly easy. The design would be simple use, though, with limited resources, not necessarily simple to build.

If these drawings don't make sense- well, the first two are irrelevant (didn't end up building it that way)- then i'll explain a little what's going on. But not too much, cause you'll see how it works. My idea was that i build a frame for a rectangular container, the bottom of which would be semi-circular to account for the blades. Key to the removability of the blades would be two thick plates into which the blades could bolt. One plate is welded into the frame, and the other bolted. I'll explain more with the pictures.

I knew that there was a pretty thick piece of plate steel lying around somewhere, it would be perfect for making the two end plates for the container. I don't remember exactly, but i believe it was a quarter inch thick. Maybe 5/16".

To determine the size of the first plate, lets called it the fixed plate, i started by measuring the the diameter of the mower blades. I wanted the container to be relatively close to the blades, thinking it would help with cutting, but knew from my earlier tests that it couldn't be too close. I guessed an inch would be ideal clearance for that, and for other reasons i'll get to later.

So i drilled a small dimple into the plate metal, and utilizing my trusty make-shift compass, drew out 3/4 of a circle. I then drew a line along side the circle, touching the circle on only one point, then i drew a parallel line and cut out around what i'd drawn.

Then I grabbed another sheet of thinner metal, i think 1/8", and, after cleaning it up, drew and cut out the first piece of the removable plate.

For lack of a better word, the nipple on the removable plate is there to allow it to be bolted to the container. Eventually two more holes will get drilled in the top for more bolts.

I then drew and cut out two more pieces for the removable plate assembly.

The other nippled piece was similiarly cut out of the 1/8" metal, and the one with the smooth curve cut from the 1/4". Both of these pieces are only going to be thin arcs, their purpose is to extend the frame of the container downward in the exact dimensions of the removable plate.

It took a lot of grinding to get them to the right size.

Next I found some small angle iron to make the frame of the container with. And, of course, it needed to be cleaned up.

Once the angle iron was free of rust, it was time to start welding. I was fortunate enough to have a speed square and a square to use for aligning the frame.

To get everything all lined up I used a trick that i learned from when i was building houses- you attach a connecting piece to one of your posts, in this case angle iron, and then get everything square and attach it to the second piece. (That's what that perpendicular piece to the left of the speed square is). I tack welded that in place (a tack weld is a small non-structural weld) and then tacked on another. Then i started playing with the removable plate assembly trying to figure out where it should go.

The picture below is of everything set in its final place, pre-welding. The first thing i welded on was that little perpendicular piece, it would serve as the backing for where the bolts would get welded onto the frame.

Next i welded the smooth piece of the removable plate assembly to the frame.

I designed the frame so that as much of the welds would be hidden as possible. You can see why.

After welding on the first part of the removable plate assembly, i cut and prepared the second, nippled, piece to be welded on.

I wanted the second piece to lie flush with the frame so that the removable plate would fit without any gaps for the straw to get out. I happened to luck out that the angle iron and plate metal happened to be the same thickness. If they hadn't been i would have figured something else out, but fortunately it just worked.

Knowing that this weld was going to be exposed, i decided to hold off on it until i could get to a better welder. I tacked it in the meantime.

In the middle of working on the straw chopper i enrolled in a weekly night class- intro to welding. An overview, some of the science behind it, and an opportunity to bring in projects and work on them. And there it was, my opportunity to get the removable plate assembly welded all the way together.

The teacher was this guy. Dave Johnson. Nice fella.

It was pretty helpful to have someone who'd been welding for years to be able to give advice and constructive criticism. And though not beautiful, i was able make a much cleaner weld with a TIG welder than the stick welder* that belonged to Apro. *One of the constraints of a stick welder is that its meant for rough, thick metal- car chassis and things like that, getting a nice looking weld with stick is pretty tough, especially on thinner metal. TIG (Tungsten Inert Gas) makes a much finer weld, you have far more control over the heat and the precision of your weld. As far as i know its the most precise form there is.

(I know this now, but didn't then) The orange discoloration all around the weld is from the tungsten electrode burning away. Too much heat for too long. The other give away is the color of the weld itself, a dull grey. Ideally the weld would be "light straw," slightly shiny with a subtle rainbow coloration like motor oil in a puddle. But i digress.

To complete the first side of the frame i needed to weld a brace along the top. I cut out a piece and set it on top to see what it would look like. The joint looked bad, clunky. I knew this thing wasn't going to be beautiful but this was an opportunity to put into practice some ideas i had about how to make welded joints look better.

I cut down the top brace so that it could lie flush in between the two vertical supports. I also angled the top so that the analogous pieces for the contiguous parts of the frame would all tie in together as flush and seamlessly as possible.

And again, welds on the back.

And there it stands, more or less together.

Next i cut out the excess material on the lower part of the frame to accommodate for the blade removal.

Then traced the removable plate onto the face of the lower frame, specifically so i could drill two holes where the bolts will go.

Again, to hide the ugliness of the welds, but moreover to ensure a flat surface for the removable plate to bolt to i welded the bolts from the backside.

When i put the plate on everything lined up well, except the very bottom hole. So i used a bit that i had purchased specifically for filing out holes (it doesn't get used with fingers though, it'll show up later in full form).

The next thing to figure out was how the blades would need to be prepared to have bolts put on them.

I cut off all the excess material around the axles, trying to give myself as much of a flat surface as possible.

Once I had the area around the axle all cut down i then started preparing the fixed plate to be able to accept the gear pictured next to it.

Using what i remembered from high school geometry i found the center point of where i wanted the gear to go. I then measured the diameter of the gear (adding a small increment in size to account for error), and transferred that measurement into my super high tech home made compass, drew the circle and began to drill.

I chose to drill out the hole, at least to get it started, because i didn't really have any other way of opening it up without having to cut all the way into it from the side, thereby ruining it.

When the center was all the way out, i then took a sawzall (a reciprocating saw) and cut out as much of it as i could without crossing the outer line.

Then i got out the ridiculously large drill- this thing probably weighed eight pounds- and filed out the hole with that special little bit. This was the part that i saw coming and bought the bit for. It made a huge difference.

And voila! It fit great.

I then prepared the frame for this side just as i did the last.

Next i drilled two holes in the fixed plate where the bolts from the end of the blades would pass through. Then i traced the holes that i drilled onto two pieces of metal that had been welded onto to the end of the blades. I drilled out the holes i traced and welded the bolts to the blades.

With that done i could then weld the fixed plate to the frame. Next i gave it the same top piece that the other side got and cut it loose from its temporary support pieces.

The next thing to do was to drill out all of the holes on the removable plate. Similar to the fixed plate, but more holes.

Once finished, it would look something like this.

With the plate ready i could now put brackets with bolts onto the blades. I got the blades into the vice and welded on four pieces of metal for the bolts.