Fitting a gennou head

[Devon Curtis]

I bought a couple of gennou heads from Tomohita Iida, and it's time to fit some handles to them. Having always bought my gennou with handles attached, I'm not sure how much of a taper the socket should have. Just a slight amount close to the rim, or a more pronounced taper towards the centre? The one, a copper head by Kondo, has nicely finished, parallel walls. The other, a funate by Susa, is a bit more crude, and actually slightly concave on one wall. How much do I need to file them to give them the right shape?

[Jim Koepke]

Howdy Devon,

Here is an old thread that might help:

https://sawmillcreek.org/showthread.php?243999-Part-Deux-The-Gennou-BTS

Here is a link from Derek Cohen's site:

http://www.inthewoodshop.com/ShopMadeTools/AHandleforaGennou.html

Between the two you will likely learn enough to keep you busy for a while.

jtk

[Rhys Hurcombe]

Derek's blog is one of my favorites to read and his attention to detail, picture quality and descriptive writing style are easy to follow. I'd file the funate square then just make sure your tenon is a nice tight compressed fit. Should be fine. The taper should be gradual and even on both sides.

[Devon Curtis]

Thanks guys. I’ve read Stanley’s threads, as well as both Derek and So’s articles. I’ve just never seen described anywhere just how pronounced the hourglass shape of the socket should be. I’ll square up the funate and give it a gradual tightening towards the middle. I kind of figured that should be the case, so that was my approach with the copper gennou. We’ll see how it goes once I get a couple of handles shaped.

[Robert Hazelwood]

A tip I'll pass along: just before you do the final fitting (with the handle still a hair oversize) dry the handle out where it will insert into the gennou. Use a gentle heat source- I like to stick it between the fins of a radiant space heater, but you can also stick it in the oven on the lowest setting for maybe 30 minutes or so. Then immediately do the final fit and install the head. The point is to dry the wood to its lowest MC, and therefore it's most shrunken state, so that it can only expand after being installed.

It may just be an issue in humid places like GA, but I have had axe heads and the like become loose due to wood shrinkage. But every tool that I have pre-dried the handle for has remained rock solid. This is especially important if you are using air dried wood for the handle.

[Devon Curtis]

Good tip. I will try that. I've got some very dry hickory set aside for the handles, but it is particularly humid here in southwestern Ontario right now, so better safe than sorry.

[Dom Campbell]

Very hard to put a measure on how hour glass shaped it should be, but it should definitely be there. I would prefer more rather than less. Toshio Odates Japanese Tool Book has a section on genno heads, shapes, etc.

Also make sure the makers mark goes toward the handle. That will put the taper in the right direction.

[Matt Lau]

Man, I'm glad they didn't delete Stan Covington's threads...

[Devon Curtis]

No kidding. Miss him.