Thursday, June 1, 2017

Suitable Woods for Axe Handles

The properties desirable in a handle for a striking tool are:

1. Hardness. (Don’t want the handle deforming inside the eye or getting too many dents where we grip it.)

2. Modulus of rupture - how much of a load the wood can withstand perpendicular to the grain.

3. Modulus of Elasticity. This is the stiffness - how much the wood will bend perpendicular to the grain. In other words, how much shock it will absorb rather than transmit to your hands.

Of these, modulus of rupture is probably the most important consideration for an axe handle. This is usually measured in pound-force/square inch (lbf/in2) or a metric pressure unit called megapaschals (MPa).

Bear in mind that individual pieces of wood can vary greatly, but:

The gold-standard wood for axe handles has long been shagbark hickory, at 20,200 lbf/in2 (139.3 MPa).

White oak is 14,830 lbf/in2 (102.3 MPa). It was commonly used during the golden age of lumbering in the northeast, because it was the best of what was locally available in commercial quantities. Obviously it worked.

White birch averages 12,300 lbf/in2 (84.8 MPa). It was almost exclusively used in far northern lands simply because that’s all there was. While not optimum, it was clearly adequate.

Other North American woods, in descending order, that would seem to be very suitable:

Black locust: 19,400 lbf/in2 (133.8 MPa)

Osage orange: 18,650 lbf/in2 (128.6 MPa)

American Persimmon: 17,700 lbf/in2 (122.1 MPa)

Yellow birch: 16,600 lbf/in2 (114.5 MPa)

Sugar (hard) maple: 15,800 lbf/in2 (109.0 MPa)

White ash: 15,000 lbf/in2 (103.5 MPa)

American Beech: 14,900 lbf/in2 (102.8 MPa)

A few more interesting tidbits:

Grain orientation is important with ring-porous species (which have pores in the spring growth rings between the summer growth rings, such as ash). This is why ash baseball bats are wielded with the trademark up, so that the edge grain is what makes contact with the ball.

Grain orientation is much less important with diffuse-porous woods (where the density is even across both spring and summer growth rings, like sugar maple). Such woods actually resist impacts on the flat grain better than on the edge grain. This is why the best orientation for maple baseball bats is the opposite of that for ash baseball bats.

With ring-porous species, faster-growing wood (second growth, fairly young trees, which are characterized by a low number of rings-per-inch), is stronger than relatively slow-growing wood (old growth, large trees, with many rings-per-inch), because they have fewer pores.

With diffuse-porous species, rings per inch make no difference in strength.

With any type or species of wood, the overwhelmingly important consideration for use as a handle is slope-of-grain, or run-out. The closer to parallel the grain of the wood is with the longitudinal axis of the handle, the stronger it will be. This is why split billets make stronger handles than sawn billets.

Bottom line: while many woods make suitable axe handles, shagbark hickory is king, at least among what’s readily available. It’s a ring-porous species, so young, second-growth trees are strongest. The grain should follow the handle as closely as possible, and the edge-grain should take the impacts rather than the face-grain.