The Crank Disc
I decided to make the crank disc today. This is a bit bigger than 1.5 inches in diameter and 0.25 inch thick. It has a 0.25 hole to fit the crankshaft (press fit or locktite), and similarly a 0.25 hole offset 0.562 from the center for the crank pin.
I had some brass for this disc, but decided instead to go with 1018 cold rolled steel which I had in 2 inch diameter bar. So in the chuck it went and I turned it down to the final diameter.
Next I drilled and reamed for the shaft so it stays concentric. I don't want this disc to look all wobbly
I'm getting better at parting off on the lathe, but didn't want to press my luck so I went with the bandsaw this time. This cheapo bandsaw has been an arm-saver! Also, I had made a plate for mounting holding fixtures for the saw a few months ago and I find I use it for nearly every cut. I have a drill vise that mounts on it also (vertically) which is great for holding small parts.
Next I had to clean up the bandsaw side. This facing head is seeing a lot of work, too.
Next, I setup my RT. I happened to have a morse taper plug I had made for flywheel use that has a 1/4 inch pin and this was wonderful for holding the disc onto the center of the table. I drilled and reamed for the 1/4 inch crank pin hole.
Now the book recommends to carve a 1/16 inch deep recess on one side of the crank to greatly add to the appearance. I could see from the print what was suggested, but there were no dimensions. Obviously I was supposed to be "artistic" on this
. I happen to be learning how to use Fusion 360 (Lars Christensen's youtube channel is great for this!), so after doodling on paper for a bit I decided to hit my computer and make a sketch. Here it is...
Ok, I know that sketch is busy, but it told me what I needed to know. In particular (and not shown in the photo I posted) I figured out offsets and angles. In the next photo I'm starting with a 1/8 inch end mill to form the center hub. I had to go with a tiny 1/8 inch because that was the space I had. As you can see I'm right smack in the middle between the two holes in the part.
Next I switched to a more "beefy" 3/16 inch end mill and am sweeping out the outer part of the cut. I had to start and end 32 degrees from the centerline between the holes. This is the angle I got from my fusion 360 sketch. It was 74 cranks around. I know because I made a few passes to get to depth!
Once I got to full depth, I was able to move the cutter in X toward the center to take out the rest of the material. This took a few passes in X.
Now the middle was mostly carved out.
Next I unclamped the disc, positioned my RT center pin on the crank pin location, and here I've dialed over in X and using a 1/4 inch rod in the mill head to get the part aligned along the X axis for clamping. I really didn't need to do this as I eventually decided to just "wing it" for the angles for the cut around the crank pin.
Perhaps this next photo is more clear about what I'm doing. The disc is rotating now around the crank pin hole and I'm cleaning up that edge. This operation ended up going from -56 to +56 degrees.
And finally, here's the crank disc. I need to deburr it and clean it up a bit, but I'm pretty happy with how it came out.
Next I'll probably make the crank pin, eccentric, and connecting rod. Thanks for checking in!
Todd