This thing fits onto the 5mm square valve stem on my 10ft^3 acetylene tank. My tank had no knob which seems rather common on the smaller tanks and I grew tired of fiddling with the adjustable wrench every time I needed to use the torch. I finally had enough and designed this knob. This is the second iteration as the first had no filleted edges and was uncomfortable to grasp. I wanted this to be rather sturdy so I printed it with 5 shells and a 75% infill. It still only took about a half-hour or less to print.