I've been doing so many handles lately that I can't even begin to write a boring blog post about each one. But, I've been busy doing a bunch of cross wedged hammers. Most of them have gone on to their owners so I should be able to get durability reports over time and I look forward to that. Just the same, all indications are that it's a solid technique and not difficult at all. What I have found is that hammers have more apparent taper in the eye, at least ball-peens, blacksmiths hammers, sledges and the like do. Some are extreme in my experience, where the size of the eye when viewed from the top or bottom is actually significantly larger than it is inside and the handle must be whittled down much smaller just to pass through. The result is an eighth inch gap all around the handle, give or take, before it is wedged. I doubt you will find a single hammer like this on the shelf past or present that doesn't have a metal wedge and often larger hammers will have two, or the cone shaped wedge, as well as a wood wedge. In order to fill the eye, it's pretty much mandatory to jam all these objects into the handle. Of course a hammer is small, or more compact than an axe of similar weight and there is less iron to wood contact keeping them together to withstand the sort of abuse hammers are put through. So, it's also mandatory that the taper exist to really lock the stick into place. I just don't like metal wedges. I would venture that they were more common in hammers, even in centuries past, because of the need to really secure a hammer head, and less common in axes. By the same token, how hard would it have been to design an eye punch that only create taper on the sides of the eye and not the front or rear? This would create a solid lock, but require only a single wedge. What's more, every video I watch of blacksmiths working, they are tapping their hammer handle on the anvil to drive the loose head down. Presumably, the amount of heat that a blacksmith hammer is subjected to has the tendency to loosen handles by pushing most of the moisture from the wood and it may be an exercise in futility to think I can overcome this. I can live with that.
As for the technique itself, it's very simple. I fit my handle as usual and when I get it just about to where I like it, I pull it for the last time and check the depth of my kerf. At this point I cut it deeper if necessary. I believe that you want a good third, possibly more, of the head below the kerf. I believe this because I have driven a wooden wedge completely through a handle before and I think, when you consider the taper (which probably begins at the halfway point or so), you want some solid wood inside the eye. Sometimes you see people say you have to have the handle crazy tight as you are fitting it. I don't believe so. For one thing, it isn't possible with a hammer. If you can get the handle through a hammer eye, it will be loose at the top, there's nothing you can do to avoid it. But, you know you can fit the lower portion of the head tight and that will help to support the wood and prevent splitting or checking. There is really no point in cutting the kerf into that good tight wood. The illustration might suggest that you could stop the kerf at the halfway mark, and you probably could, but my theory is that if you get it just a bit lower, the head is supporting solid wood, and also the bottom of the kerf so that you aren't able to split it. I realize this is pretty academic and I want to emphasize that it isn't worth going crazy over - get it close and call it good. There are too many other variables to consider anyway, like how steep your wedge will have to be to make it all work.
Next step is to cut the second kerf. I typically cut it shallower than the first, again, just for a little more support. I then hang the head, drive it down good and tight. I then drive the cross wedge and the important point here is that the wedge must be the same width as the handle, and no wider. Remember that it has to spread later as the main wedge is driven and if it is wider than the handle, it will prevent you from getting a good wedging effect. Drive the cross wedge until the eye is filled front to back.
I use a chisel to split the cross wedge enough that I can drive the main wedge. Turn the tool upside down, rest the wedge on something solid, and strike the bottom of the handle to drive the wedge. Use a wooden mallet and be very gentle to avoid cracking your handle. Driving wedges this way will drive them in straight, and prevent splitting them.