Saturday, July 22, 2017

Finger Treadle, Part 4

At the end of Part 3, I stated that I wanted to finish the Finger Treadle in time for a library demo on July 22nd. That's today, and I had it finished with time to spare! No more need to show you a photo from a magazine - here's mine, and I'm quite pleased at how it turned out:

I even made a brief video showing the treadle in action:

Email subscribers, you'll probably need to click this link to see it:

The two remaining parts to make were the crankshaft and the connecting rod, and they were small. I used some brass stock, and cut and finished carefully to conserve material. In the photo below, you see the master block from which I cut in the background, and in the front the piece for the connecting rod to the right, the two small "webs" for the crankshaft, and a remainder that I finished to a precise half-inch (0.500") wide. You never know when you will need a precise half-inch brass shim.

From there, I needed to make some precise holes in the webs. Here I'm using a "chucking reamer" to do that. You drill it to a size just under, and the reamer makes the final hole exactly the right size, and very precisely round:

The rods for the crankshaft were made to size and slid into place. Of course, the middle of the long rod will be removed later - that's what turns it into a crankshaft:

Those webs were held in place with Loctite, a very strong glue, but they were reinforced with pins that go right through the center of the web from one side to the other, passing through the rod in the middle. Here I'm drilling for those pins:

I then cut out the center of the long rod between the webs, cleaned it up carefully on the mill, and used a belt sander to make the rounded webs match. That finished the crankshaft, and my attention could turn to the connecting rod. That required that I drill and tap the smallest holes I ever had done: 2-56. Phil Oles came through with the required itty-bitty tap:

In case you think I'm kidding about the itty-bitty screws, here's one of the 2-56 screws with a penny for scale:

I used a slitting saw to cut off the cap of the connecting rod. That was the first time I had used that saw, and I realized I bought a blade that was much too thick. It would have been much better to have one that was more like to half that thickness. But I persevered, taking very light cuts, and my little mill made it through just fine.

I then reassembled the connecting rod, using the tiny screws, and drilled the hole for the 1/4" crankshaft journal. Then I assembled the flywheels and connecting rod on the crankshaft and gave it a test. That was interesting, because it bound up immediately.

It was actually not a surprise, because the plan called out that hole to be "1/4+" without defining what that meant. Basically, it meant that it needed to be bigger, and it was up to me to figure out how much. I tried polishing the hole with part of the 1/4" dowel coated in rubbing compound, but that didn't help much.  Then Phil looked in his apparently endless stock of stuff, and came up with a reamer sized 0.253" - a bare three thousandths more than 1/4". He instructed me to use it by hand instead of in a machine, which took a while, but which ultimately gave a perfect rotating fit. Thanks, Phil!

There was a bit more work that was similar to things already pictured - shaping the connecting rod, slotting the treadle, and fitting the pins that hold everything together. That last bit gave me some heartburn, because I had initially flattened one end of the pins to make them stay put, but that made them bind. I removed that part of the rod and tried again, and was rewarded with the good performance you saw in the video above.

So... 25 or 30 hours to build this project. It was designed to teach multiple techniques, and therefore was perfect for my current level of development as a machinist. I'll go so far as to say I enjoyed it, even with the frustrations involved! 

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