Ziggy, I've written a few things in the past about the high-centerline. I believe it is a balance issue rather than a chipping issue. There are a few reasons for this:
- It is not historical documentation but theorising at an attempt to regain axe knowledge and skill which has given us the idea of the high-centerline.
- The only historical discussion I have found regarding the centerline is in "Woodcraft" by E.H. Kreps. It is clearly a matter of balance for the axe, placing the mass of the steel in the top third so that it may swing in a crescent shape properly. Only axes which require a full-circle of swing benefit from central balance.
- Many of the axes that tend to have a high centerline are American axes made in hardwood-dominant regions.
- Users may be misinterpreting the reason for the chip as most wood-reading knowledge has been lost. They may also be using different hardwood axes in softer or long-fibered trees.
- If precise measurements have not been taken on a large number of axes then we may be dealing with a mix of axes that were poorly ground from the factory, designed with splitting/dual-purpose in mind, designed with the high-centerline of balance in axes which were becoming wider (and so required a greater increase in apparent high-centerline), or designed for greater heel and toe penetration (a much more significant factor in hard woods).
- The high-centerline, understand as thicker cheeks, decreases efficiency as each depth of cut requires a much greater angle increase. As well, it increases the chance of glancing blows.
- In Nova Scotia and New Brunswick the preferred axes were flat and thin-cheeked axes like the Maine axes (Spiller is specifically mentioned in historical accounts as the best) - this is an area with a lot of red spruce, for the core of the shipping industry as well as general construction. Spruce is one of the hardest woods to chip because of the length and durability of the fibers.
- In the hardest and most dense woods it makes very little sense to rely on thick cheeks to chip as it is unlikely the depth of cut will reach those cheeks anyway. Most of the chipping is from the angle of cut, transition of primary bevel angle to secondary, and the depth of cut combined with plate diameter. It is when the edge cuts deep enough and at the right angle that the chip pops.
- Where the theory of the high centerline focuses it is over an inch behind the edge, essentially at a negative angle to the primary and secondary bevels. This means that it generates very little force compared to the primary and secondary angles. If anything besides balance it helps to relieve the partial plate from closing back in on the edge/bevel, creating an opportunity to pull the axe back out the way it went in.
- There is no physical difference in popping a chip with a high-cheek or flat cheek-axe given the same angles of primary, secondary, and hollow.
- To my knowledge there is no mention of the centerline in the Forestry Service, but their gauges specify the arch of the bit and a separate gauge for the heel and toe. Their focus is on a balance of cutting ability and durability from a general service axe, so it would be interesting to see if there is any documentation in regards to the development of the gauge. Perhaps that would confirm if the thicker cheeks are intended for chipping or edge release out of the corner (a corner is the connection between the uncut and cut sections within a notch or plate, where you are most likely to jam your axe if you don't leave the heel or toe out of the wood).
- Given that the toe is much harder to cut with and very little changes were made in the axe to make the toe cut better, then we have to imagine that axemakers created the centerline for swing and toe-release balance rather than popping a chip. It is good to keep in mind that your axe should come out of the wood the same way it goes in - meaning that if the heel goes in first and cuts inwards towards the centerline, then the exposed toe naturally creates a point of balance for the centerline to leverage the heel out of the corner.
- Over time axemen learned that cutting harder woods requires focus on heel cuts and so it would make sense to develop geometry which would improve that area.
- Here we can imagine a couple of things: first, that the lesser ability of the toe to cut requires less of a lever to pull the axe out, and secondly, that the different angle of leverage pulling out of the outside corner and the inside corner demands a high-centerline for cut exit. At both the outside and inside corners you are pulling the heel out first; the outside cut-exit requires that the heel comes out and away from the corner while the inside cut requires an exit whereby the toe leans against the corner as the heel comes out. Two points of leverage which makes the high-centerline appropriate in the upper third of the axe.
- The majority of plate and chip geometry stems from reading the wood, knowing wood types, and adjusting the angle of cut and notch size for that specific wood; axe geometry compliments this but is not the whole of it. Timbersports competitors have basically confirmed this knowledge in saying that axe geometry is preference and that one still must cut at different angles according to the wood type (Arden Cogar Jr. says this).
- My own thinking is that high cheeks are meant to keep pressure from closing in on the heel or toe of the axe when cutting the corners. This creates someting of a pop and automatic release of that edge from the corner.
None of this is necessarily correct, nor is it to say others are wrong (Axeconnected and others here have created and thought a lot about the ideas of the high-centerline), it is simply my own attempt to work through what is largely lost knowledge.
