Likewise the pin shape here is usually square,which probably reflects the fact that yesteryear's builders had no access to powerdrills
Hello,
Also because in Japanese timber building a round(ish) peg is thought of, (as it is), as inferior in both function and appearance.
Well not only is the 8 sided peg easier to make properly but it will always fit as there is a degree of cutting action. With a properly fitting round peg as you can imagine the fit is 100% compression and that requires more force to achieve the same surface contact. It's the same with round or wire nails and lets say cut nails - being also tapered - but because they cut through the wood fibers instead of pushing their way through they create the maximal surface contact with intact wood fibers and so grip better along the entire length.
Also a rounded off opening exposes a maximal amount of end grain, undesirable in any case but on the verge of offensive in Japanese woodworking.
An allowance for wood movement is also well accounted for in Japanese timber construction. Tenons never bottom out in a mortice. In Japanese joinery books I think a 5mm space is allowed for.
Greetings,
Don Wagstaff
