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Product Design #2: hewing .. axe? (the Hui)

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Blog entry by RobynHoodridge posted 320 days ago 889 reads 0 times favorited 2 comments Add to Favorites Watch
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We have a project coming up that will need rough shaping of branches into square-ish forms. So we’ll need a broad axe to so some hewing. However, instead of going to the effort of sourcing a hen’s tooth here in South Africa, and paying out my adz for it, we set about making one for ourselves.
Now we don’t have forging facilities, so this was never going to be a re-hash of the master smith’s methods. We had an old blade from something like a guillotine. Which we aimed to turn into a hatchet by “reductive machining”.
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The Finnish product (because the blade has “made in Finland” printed on it)
Here’s what we eventually settled on whittling from the guillotine blade. We call it the Hui.
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The idea behind this form is that you ‘need’ both an axe with the flat face on the left and one with the flat face on the right. This is especially true if you like to let your left hand have a little of the fun sometimes. There are ways to get around this. Even : http://youtu.be/_3J5wkJFJzE?t=12m55s swapping the direction of the axe head on the fly. But why not include both blade shapes in one tool that can just be used however required. In our case that worked out as a central handle with a ‘head’ on each end of it.
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By holding such a tool at it’s centre (of gravity slash momentum) you’re able to impart a spin to it, rotating around your wrist, as the axe is swung. This way, when the blade hits the work piece the momentum of the opposing head is creating a force (torque) through the handle and around your wrist, which adds to the cutting force. Even though the weight of the second head isn’t directly behind the cutting edge.
The advantage of this is twofold. First, you don’t have the large mass extended away from the gripping position that the wrist then has to deal with supporting against the multiplying effect that the handle (lever) creates, every time the axe is lifted. You’re pulling almost exactly at the centre of gravity to lift the tool for the next swing.
Second, there’s very little rebounding shock from the handle against the wrist as there can be in a long handled axe when it impacts. Because a whack of the force is coming from the rotation of the blade which just .. stops rotating when the cut is made.
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I’ve only made this an entry in the “Product Design” series of blog entries to discuss the fundamentals of this hewing tool compared to the typical broad axe. In the modern world we’d probably use a chain saw for shaping tasks. And even in a world where broad axes were going to be developed as products, i’d never submit that they should be ‘whittled’ out of a guillotine blade. But the general principle of having two heads that add mass yet balance each other out seems incredibly functional. So we can learn something about finding alternate means to achieve an outcome in a product.
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Ax-citental discovery
I shan’t pretend that all this reasoning behind why the two headed hewing axe works was in our thoughts from the start.
In fact, the cutting and shaping of the ‘stock’ to end up with two separate axe heads was fully under way before we thought of leaving them attached through the handle.
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The handle was going to be split in two down its length so that each head could have a long section as a handle or tang of a wooden extension handle. You can imagine each of the sketched out axe heads in the pic below becoming their own axe. With the full length of the continuous piece between them as the beginnings of a handle.
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But when we held it in the intermediate joined state we found that we could do such a range of chopping actions with it, and discovered the balanced rotational effect.

Hatcheting a specific plan
The centralized handle is quite long so that you can vary how centrally exactly you want to hold it. More or less of a swing-arm. More counter-weight or less. And also so that you can choke right up on the head, as is traditionally possible with hewing hatchets.
We did have to cut away more of the cutting edge at the center of the Hui than originally planned, because some knuckles tend to move out from their close grip on the handle as it rotates in the spinning action if it’s employed.
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Our use for this tool isn’t hewing massive tree trunks. So it doesn’t matter that the handle isn’t set away from the plane of the flat face which is done to avoid banging one’s knuckles on the workpiece. Nor does it matter that the greatest range from the hand is only about a foot. However, our particular case of using a guillotine blade has left us with two countersunk holes through the Hui. These we plan to use to attach handles to the Hui. The option we’re actually going to do is to have a detatchable handle (countersunk bolt in place which you hold where it protrudes perpendicularly from the non-flat side of the head) which gets installed at the ‘front’ and allows two handed operation of the tool. A bit like a sythe’s handles, but used in the chopping motion. The point of mentioning this though is to get to the fact that we could bolt on a long handle using both holes. And the Hui would be freed from close combat.
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Ax-cident waiting to happen?
There’s a size limit to this design that there isn’t in a traditional broad axe. There’s only so much space between your torso and your wrist for the second head to swing up into as the tool rotates into the cut.
When the Hui is wielded, the action of the second head is similar to that of the handle when a traditional hewing hatchet is used the way it most usually is: http://youtu.be/qyT87p16m1g?t=1m27s tending toward (but not) slapping the inside of the forearm. And because this is the back side of the Hui and not the blade, any missed swing that couldn’t be fully controlled by the wrist alone would at worst bang against the forarm.
From the little bit of flinging this tool around that i’ve done i’ve found that it’s not all that dangerous having an additional large blade swinging around. It’s route is predictable, out of the way, facing away from you. But i think that the safety in this Hui’s use is again in the fact that much of the energy involved is locked up in somewhat balanced rotational movement. It simply doesn’t ‘want’ to spring outward as much as a long handled axe.
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Not as gay as your axe
the Hui’s cutting edge is very straight. And the flat face is very flat. Broad axes usually have a curved cutting edge to various degrees, and actually have a slightly convex flat face too. So i hear a choir of LumberJocks saying “it won’t work cause it’s not just like the traditional tool that evolved out of generations of use”. There certainly is a difference. But as any user of a straight chisel will tell you, it’s still possible, just different. And what you end up with is a very flat piece of wood.
It’s also really nice to have a swing-able 8 inch wide flat chisel laying around. For all sorts of things it turns out.

-- Never is longer than forever.



2 comments so far

View Bluepine38's profile

Bluepine38

2876 posts in 1671 days


#1 posted 319 days ago

I just happen to have an old heavy planer blade that was given to me to make a drawknife , but it was too
heavy for my design, it is similar in shape you your original blade, and my broadaxe and similar Keen Kutter
hatchets have grown legs, seem to have grown legs, so I think I will try to copy your design, if you do not
mind. Thank you for sharing, and please show some of the end results of your Hui’s work.

-- As ever, Gus-the 75 yr young apprentice carpenter

View RobynHoodridge's profile

RobynHoodridge

126 posts in 915 days


#2 posted 319 days ago

Aah, if there’s someone going to do similar i have a few things to say. I’m not entirely sure you need to remove the central part of the continuous ‘guillotine’ blade. We did so before we changed our minds about what it was going to become. Your fingers need more room in front of them than their width. Maybe 1 and 1/2 to 2 times their width. But if your stock is deep enough to have such a gap behind the cutting edge then you might be able to retain a longer edge (if you wish to work hard sharpening so much blade. There are tonnes of other ways to get hatchets out of such a blade. I’ll post pics of some i drew out soon. Just plan carefully before you cut. The biggest frustration with the process was cutting without overheating the blade, and edge particularly. Very. slow. going. You could ignore this and have to re-harden and re-temper the steel when you were done cutting. These types of blades CAN have tricks up their sleeves. They’re not all high carbon steel. Some are HSS, some are laminated, etc. http://jeffpeachey.wordpress.com/2012/06/19/has-anyone-else-ever-wondered-what-the-inside-of-a-board-shear-or-guillotine-blade-looks-like-if-you-saw-through-the-soft-parts-grind-the-hard-cutting-edge-and-break-the-rest-of-it/ Spark tests could help you determine this. My guillotine blade had obviously had a hard life guillotining. Cause it has a few cracks at the cutting edge. The following pic highly exaggerates them because the filings from shaping and sharpening are attracted to the cracks (the blade is magnetized). But you should know this is possible.

I’ve also considered cutting a drawknife from my blade. Better still maybe to dedicate the steel to that, and split it up into many pieces that get forged into drawknives?

-- Never is longer than forever.

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