Finger Treadle, Part 1

You know what a treadle is, right? It's usually foot-powered, and converts an up-and-down motion to a rotary motion. The classic example is a treadle sewing machine:

That same mechanism was used in early lathes and other machines, and it worked well until we invented motors to do the hard work. But what is a finger treadle? It's a toy, one designed as an exercise for early-intermediate machinists like me. I found the plans in a 1991 issue of Projects in Metal magazine, and decided to give it a try. Here's a picture of the finished project, from the cover of that magazine:

There are a number of parts on this that will stretch my abilities, and I decided to start with the treadle itself - the part to the right that you exercise with your finger. Here's the plan for that one part from the magazine:

The first problem was that I didn't have any aluminum angle stock in 1/4" thickness, but Phil Oles told me exactly where to find a likely bit of scrap at Make717. It was perfect! I used my vertical bandsaw to slice off a piece. I actually made two, because I inevitably made a boo-boo on the first one.

I cleaned up that cut on the mill, and then marked the "ears" for the proper length plus a bit. I was having trouble seeing my line, but then I remembered my "sticky light" I had just finished (link) - it did a fine job of bringing light to the subject. I used one on the saw and the other on the mill for the remainder of this work.

This project has a good bit of tolerance for inexact work - you can tell because the drawings are in simple fractions, with no tolerances stated. But I wanted to make it as close as possible, and worked to get it exact. As part of that, I wanted to compare my two height gauges. Conventional wisdom would say the digital one would be the one to use, but it's easy to get 0.001 or 0.002 of error in the digital one just because it's cheap. It won't always return to zero. Sure enough, the photo below shows the dial indicator on 1.276 (which I believe to be the correct measurement) and the digital on 1.2775. More resolution, but no use if it's wrong anyway!

I put the digital away - I'll just use it for woodworking, where tolerances are much looser. I got it exact according to the mechanical dial:

Once I had it to correct height on both legs, next was the exact width. I decided to put the workpiece in the vise diagonally, just because it left a cool diagonal pattern on the edges:

The spec called for a round-over on both legs. The "official" milling cutter looks just like a round-over router bit, and since I had one in the correct size and since carbide cutters will easily cut aluminum, I used it. Careful measurement plus a bit of trial and error on the test piece gave some reasonable alignment. I took several pictures while feeding the work, to get an in-focus picture with an aluminum chip flying through the air.

In the home stretch! The plan calls for a 1/2" radius "dip" for the finger. I simply used a 1/2" milling cutter. Of course, it wasn't until I typed this up that I realized that only makes a 1/4" radius. Maybe I'll recut it with 3/4" - the largest cutter I own.

If you look at the plan above, you'll see that dip is incompletely specified. The radius is shown, but the placement and depth is "whatever." I just eyeballed it, and cut 0.010" at a time until I liked it. The oak piece firmly held the aluminum angle, but could safely be cut away by the end mill.

After that, just some careful measuring and drilling with a brand-new bit, and the piece is done for now:

I started with this piece because, at first, I couldn't see exactly how to make it. I probably spent 5 hours messing around with it. A real machinist could do it much faster, but he/she would probably also say, "I remember when I made one of those in school. It took forever." I'm just thrilled to have a finished piece without using up all the raw material!

Continue on to Part 2