Another bike another coaster brake arm needed to fit around a rear sprocket on an adaptor. I started with 1" x 1/8" cold roll box tubing. Why such a stout choice of material for a coaster brake arm? Because I had a 10 ft. length literally laying around on the floor. I lopped off 20", just eyeballed it. Then I split it length wise about 1/4" thick, again I just eyeballed it. At this point it is hard not to notice that my work piece warped as I split it on the saw. This is to be expected when ever you apply friction and/or heat to thin metal and will be easily dealt with later in the process.
So now I have turned a piece of 1" box steel into a piece of 1" x 1/8" x 3/16" x 20" steel u-channel. I eyeballed the u-channel and sawed it in two. I chucked each 10" length one at a time onto parallels in a power lock vise and milled them 3" at a time down to 3/16" thick. Than using a belt sander I de burred the u-channel's and ground a 1/16 x 45 degree chamfer along the length of the outside edges. Again I just eyeballed it.