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I built this Rappel Hammer by tuning two pieces of 1018 carbon steel. The spool is 3 inches (75 mm.) in diameter and 3-1/4 inches (82 mm.) long. The full diameter sections at each end are each 3/8 inch (9.5 mm.) long. The central section is 1-1/2 inches (37.5 mm.) in diameter and 1.6 inches (40 mm.) long. The central section is drilled and tapped for a 3/4"-UNC thread, and counter-bored to accept a short unthreaded length of the shaft.
The shaft is 12 inches (300 mm.) long. The lower 3 inches (75 mm.) are 1 -1/4 inches (37 mm.) in diameter, while the remainder is 3/4 inch (19 mm.). The attachment hole is 5/8 inches (16 mm.) in diameter, and the shaft is thinned to 0.35 inch (9 mm.) in this area.
My original Rappel Hammer was (intentionally) a poorly constructed device assembled without access to major metal working machinery. Once I acquired a large metal lathe, I decided to make a better Rappel Hammer. This is the result. I did not try to make a truly safe version (if I had, I would have added rope guides); instead, I insisted that this Rappel Hammer could effectively drive tent stakes. That is why the end faces are flat and not hollowed out to save weight. The result is a very heavy rappel device that would make a more effective effective speleomedieval weapon than practical descender.
The Rappel Hammer design dates from the days of natural fiber ropes. The Rappel Hammer is rigged by looping the rope once around the spool on one side of the shaft, and once or twice around the other. A superior rigging is to rig it like the rope path on a Figure 8: bring the rope down one side, pass it under the horizontal spool (90° bend), 180° around the vertical shaft, and then back up and over the top (270° around the spool). Passing the rope this way around the shaft helps keep the rope on the spool piece.
There is no guarantee that the rope will stay on the Hammer.
Considering its size and weight, I can't imagine anyone making and using a Rappel Hammer except for academic interest.