A rare opportunity to compare the inside and outside geometry of a finished handle core. Historically an old tsuka would be split open for repair or adjustment or even re-purposing for a new blade, however a newly crafted tsuka is always glued together before the outside is shaped.
In this unusual case the core was opened and adjusted at a late stage in shaping, providing an example to study and observe. The proportions in the example below are generally based on study of a range of antique sword tsuka, with the main difference being the kataki (hardwood) style lacking samegawa and silk cord wrapping.
First recall how the tang sits slightly off center in the block, the omote side is carved slightly deeper than the ura along the edge side to support the steel and keep stress off of the glue joint.
The edge of the tang is cradled by the omote (left) but fades away on the ura (right).
The handle flows along with the spine of the tang.
The spine of the tang is supported half by the omote (shown top) and half by the ura (shown bottom).
The mekugi peg comes through from the omote (top) towards the ura (bottom).
The off center split is visible, omote sitting on the bottom, ura on the top.
A view of the joint fitment, omote on the bottom, ura on the top.
Ideally the grain lines up and runs across the handle slightly diagonally.
The tang sitting in the omote, ferrule removed, because this is a kataki (hardwood style) handle the ferrule sits lower, ending up flush with the surface.
The tang sitting in the ura, note the spine of the tang is half supported on each side.
Tang in the omote, note the mekugi-ana in the tsuka is smaller than the mekugi-ana in the tang.
Tang in the ura, note how the mekugi pulls the tang back pressing against the mekugi-ana to create tension on the tang between the ana and the machi and compression on the tsuka between the ana and the seppa/tsuba.
If the ferrule were a proper fuchi there would be a small space between the tsuka and the tsuba/guard.
The edge of the tang sits fully in the omote and is flush with the surface.
More about the interior of a traditionally crafted tsuka and saya: