When I was fixing my Monotype caster to do some composition casting a few months ago, I broke the small pin wrench that I use for turning some of the adjustments on the caster. The wrench seemed to be too brittle because I was applying very little force to it when it broke, and the broken surface had the appearance of a brittle failure.
Rather than tracking down a replacement, which might have the same brittleness problem, I made my own.
I started with some ⅜″ round low carbon steel rod mounted in my lathe between a three-jaw chuck and a live center, cut off a skim layer to reveal clean metal, and used my knurling tool to form a grip area on the center of the blank. This process applies a lot of lateral pressure to the part, which is why I had a center supporting its right end.
Once the knurling was complete, I turned the end area of the work down just enough to remove the rough end of the knurling, then cut a freehand taper down to the desired tip diameter (0.100″). I smoothed off the taper and transition to the cylindrical sections with a file and used fine sandpaper to polish it up a bit. I stopped the lathe, cut off the end with a hacksaw leaving the right amount of tip length behind, and used the file and sandpaper to finish the tip.
I mounted the work in the lathe the opposite way, and did similar shaping on the other end, except that I did not cut off the tip.
This end should be angled a bit so the wrench can reach places where the pin hole is facing a tight place. I use an oxyacetylene torch to heat the metal partway up the taper so I could bend it without the tip itself bending. Once that was done I again used a hacksaw to cut off the end and a file and sandpaper to finish it. I soaked the wrench in Evap-O-Rust for a day or two to remove the oxidation from the torch work, cleaned it up with a brass brush and oiled it a bit to prevent rust.
This shows the finished wrench above the broken one. Dotted lines mark the missing pieces of the broken wrench. When I was using the torch to heat the metal for bending, I applied it a bit too concentrated and actually melted the metal a bit, leaving some small craters (arrows).
If I wanted to copy the wrench more closely, I should start with 5/16″ round rod, and get a pair of diamond knurling wheels for the knurling tool. The replacement is a bit bulkier, but for its main purpose when running the caster, adjusting the quad size (width of the type body) it works just fine! Because it is low-carbon steel I expect that if I abuse it by trying to apply too much torque with it, the tip will bend rather than snapping off.