Japanese hammers with their handles are fitted together without the use of any adhesive or wedges, instead only relying on compression joint. The hammer head has an internal taper which forces the tenon to be compressed as it gets past it. When the tenon gets past the taper, it will become noticeably smaller. After that, the tenon would be allowed to swell back to its original state by adding a bit of water or preferably oil to it. Once it swells back, it forms a strong joint.
Started on this little side project on fitting the gumi handle to the hammer head. Even though I've did this several times before, but this time round I'm slightly more excited because this is my first time working with raw material like this. This gumi branch came all the way from Japan and I can't afford to mess it up!