Sharpening a concave blades like Kukri or similar Answered
Every now and then you have someone approaching with the odd job.
This time it was in the form of an old Kukri / Gurkha knife.
Wasn't expecting this when I was asked a few days ago if I could sharpen some old knife so it can be used for camping.
The knife had a few marks from hitting hard stuff or maybe the occasional nail.
But the worst was that for as long as guy had that knife it was only "sharpened" using a belt sander.
You know, these tiny machines advertised to give your (kitchen-) knife the perfect edge.
We could now argue about the pros and cons of having a knife edge that is literally rounded.
But once it was done so many times that the edge really looks rounded it becomes obvious why this method only works for thinner blades.
Adding to the problem was the fact that the belt used was just over 1.5cm wide.
Appereantly so it is easier to do the concave part of the blade.
Lets just say lengthwise it looked like someone created a wave pattern LOL
There was nothing "straight" on this nice blade anymore.
Now, if you look up how to properly sharpen thise Kukri knifes then you can find all sorts of really useful tips.
One I really likes was to use some eraser and toglue sandpaper on it.
Small and flexible enough to cause minimal damages to the curve towards the handle.
Another nice one is to use half round diamond file, preferable of a finer grit in the 600 region.
Should work fine - if you plan to invest an awful lot of money on such a file.
I however like things quite often done the old fashioned way.
The oldest trick in the book....
Whether you are using chisels and work on wood, just love to keep your knifes sharp or go on long camping or hiking trips - sharp knifes and tools just become your thing.
The main thing everyone tells you is a super flat surface for whatever is supposed to sharpen your blade.
For the normal stuff that is fine and good and you only need to flatten out your stones every now and then.
But what about these odd jobs?
Imagine you would need to sharpen a long paper cutting blade on some machine.
Might be over a meter long and it has to stay with a perfectly straight edge.
Back in the day this task was not done with some very expensive stone of large size....
Instead sandpaper of various grit was used on a flat steel surface.
I actually prefer a small pane of glass and tape my sandpaper on it.
Hard to find anything finer than 1000 or 2000 grit but you might be surprised how well this stuff polishes onces clogged up a bit.
Its all about the right level of wetness...
Anyways, for our Kukri in question I decided it is time to do the same but in a way that does not harm the blade, constantly cut into the eraser and still is solid and "flat".
If you still work with a sickle then you already know where I am going here ;)
I used a small diameter spray can as my surface to hold the sandpaper.
Of course a piece of PVC pipe, round wood or similar would warok the same way....
Sticky tape does not work well with sandpaper unless you use double sided stuff.
But it is enough to wrap one round on the top and one on the bottom of the sandpaper on the can to hold in place.
So much for the basics....
If you know how to sharpen a knife then you also know that there is a prefered way of doing it.
Depending on the blade and stone in question you literally try to cut a thin slice out of the stone with every stroke.
Either stright or with a cutting motion.
This works fine with sandpaper on a flat surface, not so much however on a round surface.
Try it and you see how you cut off the sand from the paper and constantly ruin your edge.
The only way to do it is to move with the edge.
You start from the heel and stroke to the tip.
The can is used likea sharpening rod and shall always stay at a 90° to the curve of the blade.
Takes a bit of practice to find the right grip to hold the blade while moving and twisting the can but well worth it.
The rounded surface only allows for a very thin area of the sandpaper to work on the edge.
I started with 120 grit!!!
It left a trail of destruction on the edge, at least in the rounded up section....
Once I only had a very thin bit left on the edge from the old sharpening I switched to 240 grit until a flat edge formed.
As the Kukri was a disaster this process still tok over 4 hours to complete.
That blade was properly hardened too...
The start of the finnishing was done by jump right to 600 grit paper.
The first can was just slightly smaller in diameter than the concave bend in the blade - perfect to smooth out those nasty bumps.
But with a burr forming now on the edge and minor mishap with angle of the can towards the curve of the blade would mean cutting into the can while sharpening the concave bit.
Meant I used my emergency insect repellant can as I did not like the idea of hoping my pepper shaker would start leaking while sharpening ;)
If you blade is not too damaged you can of course start right away with a smaller diameter.
The process is the same as before.
Move along the blade and keep the can at the 90° angle towards the curve.
Once you feel a burr forming on the side turn over until you have a bur on the previous side again.
Repeat until all the marks from the coarser grit are gone and the edge has a uniform shine.
Switch to a finer grit and go as high as you can here.
I had to stop at 1000 grit as my supply of 2000 and 4000 grit is out.
Hints and tricks along the way....
It really helps to do this sandpaper sharpening under running water.
The paper won't clog up, you won't risk a losse grain making really deep marks...
But on a bad blade this can take several hours and would do it with a small aquarium pump or so and some gloves.
A fine but stiff brush and soapy water however do wonders to clean up used sandpaper!
I prefer to use these re-used pieces before switching to a finer grit.
In most cases they are already finer than the next grit and create a nice polish that makes the visual confirmation of your right angle and angle of attack easy.
A kukri is a working blade!
It is mot meant to make fish filet or shave you legs.
It is somewhere between axe, big bowie knife and hatchet.
That mean if you would dare to give a 8° angle either side of the edge you would have a pretty damn wide edge...
Stick to the original in width but keep it nice and flat.
It is good compromise between cutting sharpness and durability when for example chopping wood for your camp fire.
DO NOT USE A BELT SANDER!!
I said it before but have to repeat it again as there is people using a big belt sander with enough free space to add a set of wheel that creat the curve I got from my spray can.
The guys in India that make these knife do this blind folded....
It takes years of practise to get the steady hand required not to cut through the belt.
The beginners start in reverse, meaning the belt runs towards the edge.
These guys only to the basic forming of the edge with really coarse grit.
Basically to remove the marks from the forging.
After that the pro takes over the blades and he has the belt running towards the edge!
If you are silly enough to try it at home be prepared to have the belt flying in your face very violently!!
The reverse sanding can't be used to finnish a blade as you never get a proper sharpness and flatness right on the edge.
So just stick to manual and take an hour or so longer but then be able to enjoy a cold drink when done.
You need surprisingly little sandpaper in terms of clogging up and getting useless until you get to the finer grits.
If you use a wooden dowel or similar then make it a bit longer and add strips about 6cm wide of sandpaper.
This way you have all the grits you need in one place and can take them with you to keep your blade sharp ;)
If you glue it onto the stick it is also quite easy to give it quick brush clean when done.
The really tricky part starts from about 800 grit onwards.
Every mishap on the concave part can mean damage to your paper or to your edge.
When using stone most beginner think that using a lot of pressure is a good way to remove the material quickly.
In reality however it is just a sure way to wobble the blade over the stone, especially if the blade is not fully straight.
Sandpaper can be more aggressive than your stones as in our case you only work with a little area and every time you turn the can only a little bit you have a fresh piece of paper working instead of a slurry building up.
This mean you really do't need much pressure at all.
It is the repetition, not the pressure that gives you the edge if you don't mind the pun here. ;)
For a real working knife stopping at 1000 grit once you do single strokes either side of the blade is sufficient.
The tiny burr left will disappear quickly during use and the Chakmak can be used for a quick refurbishing after every longer use.
Should mean you only need to get the sandpaper out once you edge actually started to get blunt again.
The final stroke....
There are those people that don't have a kukri to go camping...
Some people like to collect them.
Restoring an old kukri can be done like with any other knife.
That is until you want a razo sharp edge that is also highly polished.
This is quite possible with the original edge width on the kukri.
But of course you can only go so far with sandpaper....
Modern technolgy provides us with the solution in several options.
Firstly we have the ceramic sharpening rods.
Unless you can do with kitchen variety thickness you need to pay a lot of money.
A short 8cm diameter rod can set you back over 100 bucks with ease.
Especially if you want something that provides a mirror like finnish.
And alternative that is often available relatively cheap is a ruby rod.
They can often be found with slight damages that make them useless for laser applications.
Even burnt out rods are still fine as long as they are not cracked.
It is quite hard (literally) to give them a satin finnish but I found that good quality sandpaper is sometimes capable of doing it.
I like one side smooth and the other half of the rod with a satin finnish to prepare the edge.
On the budget there is quality wet and dry sandpaper as commonly used in paintshops.
If used dry the finer grits tend to clog up on such a wide edge.
Once you have a piece of 1000 or finer grit that is fully clogged up you can use to give the edge a final polish.
With this you won't even need a leather strop anymore but as said it takes a lot of practise so you won't cut the paper in the concave area.
The steel rod....
If you happen to have a hardened steel rod, like from a motion rail, small drive shaft or a big drill then give it a try.
When using a drill:
Of course use the end of the drill, not the working part ;)
Also make sure it really is motth as any burr from the chuck or such will cause deep scratches on your blade.
If it starts to feel sticky after a few good stroke you know the drill method is working.
If it continues to feel very smooth and you don't see any polishing effect at all if tried on a small area only then you blade is of really good quality.
But then again you would have confirmed that already by the ongoing swearing during the endless hours trying to remove some material from the edge...
A word of advise for the first time user of a kukri:
Although a good kukri is hard to damage without hitting a stone or metal, you can make blunt very quickly.
It is top heavy blade and requires a steady hand when working on other things than meat.
Chopping into some wood and letting the blade slip can deform your edge.
A little mishap can be fixed with chakmak but not if hit hardwood badly a couple of times.
And tempting as might be to use it as a small hatchet or axe to split your kindling:
Never hold a piece of wood and then hack into it from the top with your kukri!
Not only can you miss the wood and hit your hand, the wood can also split far easier or in unexpected directions!
If the kurki is sharp you then have a good chance to loose a finger or two!