My magnetic chuck for grinding bevels works well and I am still using it but it is unsuitable for establishing the bevels on a huge blade like this. I have actually been thinking about this for some time, and the kukri commission was in the end just a suitable excuse to play for two days with magnets and exercise my grey matter a bit.
The thing I came up with was a combination of a magnetic jig and the sharpenatrix. That alone could not work because it does not allow me to get as close to the belt as I need. And also it has a fixed length, so in certain positions, the blade like the kukri would actually be partly above the tallest point on the belt. Thus I established that I need:
- a telescopic arm
- a switchable permanent magnet
Both of those things can be bought, sometimes even in conjunction. But they are really expensive and for my purposes, even the cheapest and smallest ones are needlessly bulky and heavy. Yes, at long last, finally a chance for me to just dick around with various scraps and it is really economic use of my time!
After some trial and error, I have gotten the best results with just two magnetic arrays from two broken speaker magnets and four flat pieces of mild steel from a broken clamp.
The magnets are oriented in both arrays with the north in the same direction on both sides of the pipe in the middle. And since they were broken into irregular pieces, I have glued them in with a mixture of steel dust and epoxy to better facilitate the transfer of the magnetic field into the steel. With one exception – the side that is going to hold the workpiece has a bit of brass between the steel bars, so the magnetic field does not extend there all the way to the surface between them. The piece of stainless steel non-magnetic pipe in the middle allows me to connect the two magnets with an axis around which they can swivel freely. When the poles of both arrays are aligned, they repulse each other but the whole assembly sticks to steel on the sides very strongly. When they are misaligned, the whole thing is nearly non-magnetic all around.
Here you can see me testing it. A threaded copper rod is fixed to one of the magnetic arrays and will connect it to the telescopic arm later on. A stainless, non-magnetic steel rod is also fixed (riveted) into that magnetic array. The second array can rotate freely on the top. At this stage, I got my first bonus – both extreme configurations are stable without the need for any mechanical locking mechanism and the outward magnetic force builds up/disappears quickly, not gradually.
Now you can see it nearly finished. The telescopic arm consists of several parts:
- the 8 mm copper rod with stainless steel nut fixed into the array
- thin 12 mm steel tube lined with 10 mm brass tube in the upper half to ensure a tight fit for the copper rod.
- 10 mm steel rod with thread at the end on which the ball from sharpenatrix can be screwed
- 2 screws go through threads in two pieces of thicker tube and through all the tubing to lock both the steel and the copper rods in fixed positions. There are brass inserts under each screw to ensure they do not scratch the surface of the rods. Hopefully.
The knob was only added so I do not poke myself with the sticking screw during work and it turned out to be a second bonus – it allows me to hold onto the blade with one hand and comfortably hold and switch on/off the magnet with the other.
With that, the arm was not finished yet, but it was functional, so I went on and tested it. And it worked really well. Not ideally, but it did help a lot, especially with a complicated grind like this. Kukri changes the blade width over the lenght of the blade, so to reduce the weight, keep it strong, and optimize the cutting capability towards the end of the blade the primary bevel has to be steeper on the wider portion of the blade than on the narrow part. So I had to grind it in two steps. The first step was to establish the less- steep bevel on the whole blade (approx 5°).
The second step was to establish the steeper bevel on the wide portion of the blade whilst carefully feathering it into the narrow portion. The grind on the intermediate portions is a bit funny-shaped, which I will have to correct with a file. Later during polishing (this will only go to 100 or perhaps 120 grit), it will smoothen out, I did make blades like this already, although not of this size and not with a belt grinder.
I made this grind in about an hour, which is speedy, especially considering that I was working with a new jig. I slipped up on two parts on the other side before I figured out how to best use it, but nothing that would not be corrected in polishing.
As a final touch, I have encased both arrays in alluminium housing so they do not gather steel dust. And I painted ON/OFF markings with a sharpie to have visual clues during work.
If it were a bit stronger, I would not have those two slip-ups that I had, but it is strong enough to be useful – it has over 2,5 kg lifting force, which is in my opinion impressive given that the initial magnets on their own have barely lifted anything.
Overall I am very pleased with the result. I now know how to make small switchable permanent magnets. I still have some ordinary black magnets to play with, but I will probably also buy some small neodymium magnets and build myself a variety of magnetic holders with high force. Even a small flat magnetic plate can cost several hundred €. With some care and planning, I think I could make a useful one myself for a fraction of the cost in just a few days.
nothing particularly interesting to ad.
I just find this project interesting, and wanted to comment, as I find the topic and this series of posts interesting, and would like to read more about it.
I’m kinda fond of the general design of the kukri -- in my experience, it seems to be excellent for “chopping” type tasks, and if you are looking for a knife large enough to fell small trees, seems to me to be an apt choice.
Granted, it’s not suitable for a lot of tasks, but I’m not sure it’s any less suitable for smaller-blade tasks than any other large blade knives/machetes, and I think it may weigh less or be more compact than other options. I’ve no idea how this stacks up in comparison to a medium fixed blade and hatchet combo, but that’s really getting into the weeds…
Anyways, the bevels/grindlines/whatever look amazing, and the overall outline looks promising. Looking forward to further updates, and thanks for sharing with us.
@lochaber, I do not have much personal experience with kukri but one of my friends absolutely swears by it. He is using it to remove branches from tree logs when making firewood. And from what I have been told, that is also the purpose this one will be put to.
I have cleaned the bevels with a file today and made them as symmetrical and smooth as I can at this stage. I would need some rain now so I can start a charcoal fire outdoors without risking putting my whole neighborhood on fire.
Due to drought and probably some nitwit, Czechia now has large forest fires in a national park, the firefighters have been trying to stop it for three days now and already there are villages reduced to ashes -- a thing that did not happen in my lifetime.
They’re great at chopping, pretty much a decent compromise twixt a large knife and a hatchet. So, probably pretty well suited for what your friend uses it for.
I just don’t get how so many older folk are in denial of climate change, I feel like anyone who has been alive for just twenty years or so, should be able to notice that weather patterns are quite different now than they used to be.
I sometimes almost wish I were religious, so that I could believe the people responsible for promoting these falsehoods and disinformation would face some sort of retribution for their crimes against humanity and nature. Unfortunately, I believe they will live a very comfortable and easy life, and pass on a considerable sum of wealth to their offspring…
None are so blind as those who refuse to see. My parents both believe in climate change ever since the 90s because ever since the 90s the changes in weather patterns were visible around here -- generally shorter winters and warmer and drier summers. In the last decade, it became more extreme, but the trend was visible, even subjectively, for several decades now.
As if summoned by magic, today morning it rained a bit. Not much, but enough to wet all surfaces, tufts of old grass, and piles of leaves. So I felt safe to start a charcoal fire and harden the kukri, three knives, and one platen for the belt grinder. It did not go very well, although I managed to harden everything in the end. But it took me much longer than it should have…