Tromner style neurological reflex hammer Created for fun to experiment with different filament properties (density, flexibility, etc.) but fully functional (and despite the light weight, still works better than the crappy tomahawk shaped ones).
Requisite disclaimer that this should probably not be used as an actual medical device (i.e. use at your own risk). Though a reflex hammer should generally not get significantly soiled (otherwise you are using it wrong?), it is likely inherently impossible to properly sanitize/disinfect a FDM 3D print with all of its gaps/holes between surface layers.

Creality

Ender 3

No

Yes

0.10-0.20

80%+ head, 15% other parts

Rotate parts in slicer to orientation shown in the picture. All parts are essentially structural so I increased extrusion width to 0.5 mm (for 0.4 mm nozzle) for strength and layer bonding. Threads should print fine with any layer height 0.2 mm or below, use whatever is strongest on your printer.

Head: Use PLA with a high density infill (i.e. gyroid, 80-90%) and 3-4 perimeters. On the actual hammer, this part is machined metal and rubber, so you want it to be as heavy as possible as preserve the same weight balance. No support needed if printed in the recommended orientation (on the front face).

Retaining Ring: Use same settings as head, it's a small enough part that it will likely be completely solid anyway

Front bumper: This is made of a firm rubber on a real hammer (i.e. does not collapse, feels similar to a new pencil eraser or an O-ring); use TPU/flex filament and experiment with infills/perimeters. Otherwise, be gentle if using a hard filament ;)

Handle: Recommend PETG or ABS, 4 perimeters, 15% infill. These filaments have more give than PLA and the springiness helps especially if you don't have a flexible bumper material. They are also less dense than PLA, which preserves the weight distribution (this whole creation was just an exercise in material properties). Use support only for the 2 ends (the floating round peg, and the end wedge); can use support enforcers, or just adjust overhang angle until only those areas are supported. Shaft has no overhangs >45 degrees.

The head screws together and is pretty self explanatory; there is a keyed notch for the handle. It should friction fit without the need for glue; due to the printing orientation, there may be some distortion of the round sections, so they may need some slight cleanup with a small file. Otherwise If the handle is forced in, it may split the head along the layer lines.