So I improved on my mechanism by building a ratcheting winch to tension the spring. I cut a wheel out of plywood with 12 ratchet teeth in it. I attached a small drum to the wheel for winding up rope with.
The drum is made of a piece of maple with the grain running side to side, and I cut it out to have a wide flange to glue to the plywood with just to make sure I'd make good contact. After I glued it in place, I drilled a 1/4" hole through the middle of the wheel and drum for a shaft.
I made a bracket for mounting this ratcheting wheel to the back of my bandsaw, and also made a wooden ratchet to hold the tension on the wheel.
I also changed my setup from one springs to two for extra tension.
I found that I was able to tighten the springs quite a lot just by turning the wheel with my hands. But releasing the tension was a bit dangerous, and while trying it out the wheel spun out while I had my fingers on it. One of the teeth put a small hole in the tip of my right index finger and I spilled a few drops of blood on my project. It's these little things that aren't even supposed to be dangerous that get me, because I'm not ready for it. Not a serious injury, but it made typing awkward for a few days.
To avoid further injury, I made a simple lever to operate the tensioner wheel with. The lever is loose on the bolt that it rotates on, so that it can be tilted into and away from the ratchet gear. It makes it really quick and easy to tension or de-tension the blade. And I love the sound the ratchet makes when tensioning the blade. One drawback is that it's so satisfying and easy to tension the blade that it's too tempting to just ratchet it up to maximum tension every time.
When I designed my second bandsaw, with the intention of making plans available, I came up with a much simpler design.
I joined the pieces with a double tenon joint. I cut two 1/4" (6mm) mortises separated by 5/8" (16 mm). This left 3/8" (10 mm) in between. I cut the tenons using my slot mortising machine as well, using a 3/8" router bit to clean up both cheeks and the space in between. I found that this resulted in an overly tight joint, so it's likely that my 3/8" or 1/4" bit cut away just a bit less than specified. So for each slot with the 3/8" bit, I'd cut it and then turn the vertical adjustment crank by 1/6'th of a turn to the next click stop and cut it again to widen my 3/8" slots by about .010" (.25 mm). The resulting tenons were a bit narrower and easily fit into the mortises.
Now that I had a table support and blade tensioning mechanism, it was time to do some more testing. This time, I propped the saw up on a milk crate and a bucket to bring it to a more comfortable height. I also did a coarse balance on the top wheel to cut down on shaking. The two big holes I drilled in the top wheel brought it more into balance. As a safety precaution, I also clamped a piece of wood to the post which I attached to the ceiling. My hope was that this would prevent the whole shaky and wobbly arrangement from tipping over.
With my more powerful blade tensioning mechanism, I experimented some more. I was able to make some cuts without any blade guides at all. The blade's tracking on the crowned pulleys was working well enough to resist the force of me pushing a piece of wood against the blade. This, even though my bottom wheel was just wood, without any sort of bandsaw tire on it. My thinking was that with the crowned wooden wheels, my bandsaw really didn't need any tires.
On regular bandsaws, the bar that moves the upper blade guides up and down is often fairly far back from the blade, with the blade guides extending forward from it. Wood isn't as stiff as metal, so I opted for a different design. The bar that holds the blade guide is actually mounted on the front of the top plywood, just behind the wheel. In fact, there is very little clearance between the back edge of the top wheel and this mechanism, and I had to remove the top wheel to take this photo.
The wedge towards the center is pulled back against the plywood by a screw. The piece on the left of the wedge is fixed, and the piece to the right is the bar for the blade guide. When the wedge is pulled back, it pushes the bar into the corner, locking it vertically and wedging it in place laterally.
I wanted the saw to have a lot of resaw depth. I made the top blade guide so that it can be moved all the way up and into the housing of the saw. Moving the blade guide all the way up is a bit awkward though, because the locking wedge needs to be repositioned, and the blade guard needs to be detached from the bar to keep it from hitting the wheel. With the guide bar all the way up, I can get 28 cm, or about 11" of cutting depth. I think 30 cm or 12" of resaw depth is more typical for a bandsaw with 18" (45 cm) wheels, but most of these bandsaws are a fair bit larger than this one and also use longer blades. Mine uses 116" (295 cm) long blades. Busy bee machine tools sells that length of blades to go with their 16" bandsaw.
I shot various video segments of my experiments while building this bandsaw and edited them together to post on YouTube.
Next: Bandsaw stand, and reinforcing the frame