Hack a Swiss Army Knife to Have a Carbon Steel Blade

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UK freelance General Hacktitioner, health IT specialist and wannabe coder, lifelong hacker of any...

Intro: Hack a Swiss Army Knife to Have a Carbon Steel Blade

OH how I love my Victorinox Forester SAK. It has everything a resourceful and civilised gentleman should need in an Every Day Carry knife, such as a corkscrew. One thing that isn't quite great though is the quality of the steel used for the blade. Stainless steel looks good, but - for most types of SS - that's about it. Its edge is lost as soon as it gets good use. Carbon steel, however, holds a great edge for much longer. The other thing about the Victorinox blade is that the shape of the blade ("the grind") has a long taper with a secondary bevel - really I prefer short single bevels ("scandinavian grind" or "scandi" for short) which is personal taste but it is very easy to sharpen in the field.

I looked at other knives to see if there was anything out there with similar features to the Forester but with a carbon steel blade. Nope.

I looked out there to see if any clever cutlers are making retro-fit, aftermarket, Carbon-steel Scandi-grind Victorinox replacement blades. No such luck.

So, armed with some YouTube-learned metallurgy, and my Dad's garage (which is set up for metalworking, as opposed to mine which is set up for a band) - I hacked it.

This Instructable will work fine with most types of SAK although the method of mounting/hinging the blade and removing the scales might vary. If your SAK has a non-locking blade then the hack is MUCH easier since you don't need to worry as much about getting the "tang" exactly the same as the original - for the (locking) Victorinox Forester the tang has a "liner-lock" [wikipedia] which means the tang has to be very accurate for the locking mechanism to work properly, if at all.

Step 1: PLANNING! (I Usually Regard This As an Optional Step in My Projects)

MATERIALS:
  • STEEL - I measured the thickness of the existing blade: 2.4mm at the thickest bit. I couldn't get tool steel in this thickness so I settled for 2mm and figured I would have to use a washer/spacer. I got "O1 Ground Flat Stock" - about £10 from eBay, and I still have LOADS left to do other tools with!
  • PIN - during dismantling you will remove the hinge pin which is a 3mm diameter brass pin which is caulked over (hammered) on both ends and has a little round ferrule around it (difficult to describe, and so small I couldn't really get a good picture, you'll just have to see for yourself) - when you remove this it will be useless so you will need some other 3mm diameter piece of metal - I used a 3mm diameter nail and it worked perfectly. It will rust but is probably stronger than brass.
  • WASHER - as mentioned above I needed a small washer (see final assembly stages) which I had to make by cutting it out of a bit of 0.5mm brass sheet (cannibalised from the inner shell of an old carbon brush from a motor)

TOOLS:
  • general metal working tools - files, vice, hammer etc
  • gas torch for heating metal to harden it (but note that this can actually be done using a charcoal fire - see the GreenPete Knife Making Video)
  • domestic oven
  • optional extras that make life easier - bench grinder

KNOWLEDGE:
http://vimeo.com/album/1577465
I found this great video a few years ago which shows how you can make a bushcraft knife out of an old file in the woods. This guy is great and I cannot thank him enough for putting this video together, it is quite incredible what he manages to do with virtually no equipment - I felt mildly embarrassed about the fact that I used something so unmanly as a gas torch for the hardening process!

Step 2: DISMANTLING

  • Prise off the scales (plastic handle pieces) with a sharp-pointed screwdriver or similar tool.
  • There is a brass pin on which the blade hinges. File off enough of the brass pin on one side so that you can pull it out the other side. The knife blade should fall off. On the Victorinox Forester SAK, the wood saw blade and the tin opener will also fall off. Other SAK's may be different. The removed pin probably isn't any use now, you will need something else to make into a hinge when rebuilding the knife.

Step 3: COPY THE BLADE SHAPE

In an attempt to be all 'geek' about it, I tried to scan the blade into JPEG format and then edit the shape using GIMP. Unfortunately, due to my lack of skill in this area it failed miserably, I could not satisfy myself that the digital tempate was EXACTLY the same size when printed as the original. I couldn't really be bothered to spend any more time on it as I was eager to get my new blade done.

So, I did it the Old Navy Way, copying round the original blade with a scribe, direct onto the tool steel blank. I added a thumb hole to enable one handed opening (see later) - giving the blade a definite Spyderco look - in the end for various reasons I ended up changing the shape back to the normal victorinox blade shape after experimenting with a thumb stud (see pics)

I then cut this shape out with a hand saw and did the final work with a grinder, getting it pretty close to the original.

Then I located my pivot hole on the new blade and drilled it - since this point is the most important bit of the whole blade.

Once that was done I essentially riveted the old blade and new blade together through that hole, once fixed in this way I spent a bit of time filing so the two were IDENTICAL. The most important bit to get right is the EXACT shape of the tang, as this is crucial in getting the linerlock system to work right.

Step 4: BLADE BEVEL

After a bit of internet research I found that for a single bevel "scandi" grind you need to file an angle of about 12 degrees into each face of the blade, together this makes a blade with a bevel of 24 degrees.

Clamp the blade horizontally in a G-clamp, which is itself held in a vice (see picture)

I used a protractor to get a rough angle for the bevel, and then more or less did it by eye, making sure the bevel was similar both sides. I used a file although I think the "pro" custom knifemakers use a belt sander.

It's important to get all scratches and gouges out of the metal now before it's hardened, after hardening it will be a lot more difficult to remove any imperfections. I used a heavy/coarse file, then a fine file, then several gauges of sandpaper (finest was 240 grit). Then I buffed it on a buffing mop wheel.

Step 5: TRYING TO OUT-CLEVER THE SWISS

I thought, since I was making myself a nice new blade and all, I could add a feature I really like on my CRKT knives - one-handed opening.

Initially I thought that I could do this by enlarging the blade and putting a thumb-hole in it (see Victorinox's own one-hand openers for what I mean) - you can see the intention in the initial blade-making pictures. However, when I actually tried to use it the hole was not really in the right place and it was harder to open than it was before!

My next attempt was to remove the metal bearing the hole and put a small stud in the blade for thumb-opening (like on the CRKT M16 knife) - all went well until I suddenly realised why this won't work on a SAK - the thumb stud obstructs the opening of the adjacent blades!!! DOH.

So I ground it all off and so now my blade is the same basic shape as the original. I left the back of the blade quite rough and with sharp 90 degree un-smoothed edges (where I had ground off the thumb hole and thumb stud experiments) as this will provide a striker surface for firesteels.

Serves me right for trying to out-clever the Swiss.

Step 6: HARDENING AND TEMPERING

Once the blade was shaped right I did some sanding & buffing to make it nice. Then comes the fun part - hardening.

Basically the process is to heat the metal up to Very Hot, when it stops being magnetic due to a change in the crystal structure. Then you quench it cold very quickly by plunging it into some oil. It is now VERY hard, BUT very brittle. So you have to soften it a litte, a controlled softening, using an oven (there are other ways to do this but I thought the oven would be most consistent). This creates a blade that is hard enough to stay sharp, while being much less brittle (so it won't shatter of you drop it like a file would). Depending on the temperature you use to temper it, you can make it varying degrees of hardness, I chose 250C as it is supposed to result in a Rockwell Hardness of about 58 (ish) which is meant to be about right for a bushcraft knife.

There are people on the Internet who actually understand this and there is loads of information out there so I will let you get any further information you require from the Green Pete Knifemaking Video and from other Internet sources, I'll just stick to describing what I did.

1) Preheat your domestic oven to 250 Celsius. In fahrenheit that is 482 F.

2) Holding the blade in locking pliers, heat up with the gas torch until cherry red. Then start checking with a small magnet if it is still magnetic. Once it loses it's magnetism the structure has changed, hold it at this temperature (judged by colour I suppose) for a few minutes, then plunge into a bucket of oil AND COVER IT (in case it flashes into fire!!!)

3) Remove and wipe down. Sometimes the metal has distorted a little from this process, mine acquired a small twist to the left less than 1 degree and I was happy to ignore this. I suppose if it was worse you could try re-heating it to above the critical temperature again and re-quenching but I'm not sure if this causes weakening.

4) Stick it in the oven at 250 C (482F) for 2 hours, then switch off and allow to cool slowly.

5) Sanding and buffing again, to remove all the black stuff!

Step 7: REBUILDING THE KNIFE

As I mentioned earlier you will need to make or find a small washer to make up the difference between the 2mm tool steel blank and the original blade which is about 2.4mm thick. I cannibalised a piece of brass plate that was about right and drilled a 3mm hole, then cut a circular washer using tin snips around this hole, so that the final was about 8-9mm diameter.

Then it's just a case of putting everything back in the same order it came off and using a new hinge/"axle" (I used a plain steel nail that was exactly the right diameter - 3mm)

Cut the nail (or whatever you've used as the axle/hinge off) so that about 2mm sticks out each side of the knife.

Caulk this over using a hammer, I would advise repeatedly checking the friction on the blade while doing this - you want it tight enough that there is very little lateral wobble on the blade, however if you make it too tight it will be hard to open the blade. Experimentation is the only way. If you go too tight you can always saw one end of the axle off, pull it out and do it again.

I was very happy with my result and can report that the lateral "play" or wobble is no worse than with the original blade.

Clip the scales back on, and youre done

Step 8: FINISHED

Clean blade, sharpen blade & oil the hinges and you are done.

possible future instructable:
It would be very easy to make a set of nice decorative wooden scales by copying the shape of the plastic ones and then gluing them on with some good glue like evostik contact adhesive.

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    42 Discussions

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    JHG332

    1 year ago

    Is it just me, or does this PDF not download? Mine is showing 0 kb in size.

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    wolfy_9005

    5 years ago on Step 3

    Why not get a small opinel, open the hole up, and insert it into your SAK?

    1 reply
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    TimmyC1wolfy_9005

    Reply 3 years ago

    Opinels have a larger hole, at least for the 91 mm sak

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    r_harris2

    5 years ago on Introduction

    This would be a good hack for a multi-tool as well. Besides the explanations in the instructable (and many comments below), there is another great thing about having at least one carbon steel blade: firestarting with natural flint. Carbon or stainless both work fine for sparking a synthetic flint, but for natural flint, I have never managed to get a spark with stainless. If it is not impossible, I think it must be much more difficult. When using a synthetic flint, it is the ferrocerium material that is scraping away and creating the hot spark. When using natural flint and steel, it is the steel being scraped away (which is why you should use the BACK of your knife if possible). Perhaps stainless does not scrape away and make good sparks as easily. I know you can make stainless sparks with a grinder, but I have not succeeded by hand with a flint.

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    dichotomous

    6 years ago on Step 6

    SAK knives are at 56 on the rockwell scale. as hardness is the primary indicator of edge retention, this would only marginally improve edge retention. the harder steel, and now thinner, is more likely to snap, which springier sak blades are already known for, and rust. also, the sak is likely more accurately built. Wouldnt retempering (or rehardening then retempering) of the factory blade be better at getting to 58rhc?

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    Wulf187

    6 years ago on Step 3

    Do not cut knife blades with a grinder or cutting torch. Especially stainless steel. (which you can not cut with a torch). The blue and black discolouration is a sign that the steel got overheated and thus wil not be as good as it was intended. Always cut a blade with a hacksaw and use a file to file it down to the final shape. Your blade will not have overheated and would be at optimum performance.

    14 replies
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    markybWulf187

    Reply 6 years ago on Step 3

    After it was 'overheated' by cutting, the steel was heated to about 1300F with a gas torch during the hardening and tempering process. I can't see how a bit of overheating BEFORE hardening makes any difference.

    Sure, excessive heating of a blade AFTER hardening and tempering will ruin it.

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    Wulf187markyb

    Reply 6 years ago on Step 3

    Prevention is better than cure. If you burn off some carbon in the overheating process, you detroy the one thing that give the blade its strength. If it is damaged it is damaged. You can minimize the effects by heat treatment but it will never be as good as a blade that was never overheated.

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    djimdyWulf187

    Reply 6 years ago on Introduction

    See above. There are 2 things giving carbon steel its strength: Carbon-influenced crystal structure and Work-hardening. You can't "burn off" carbon, but you can remove the carefully planned work-hardening that the original forging created.

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    flamesamidjimdy

    Reply 6 years ago on Introduction

    aactually....it is possible to loose/gain carbon from steel...you see, carbon has a diffusion rate through very hot steel/iron (can't remember the temperatures and rate right now)
    this means that if you put iron in a carbon-rich enviroment, with as little oxigen as possible (sealed box full of charcoal), then heat to (very hot-don't know for sure)..the carbon migrate into the iron veery slowly, and after a while, creates "blister steel", which can then be processed into "shear steel". the reverse process is also possible (and happens, to a degree, in hardening)

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    djimdyflamesami

    Reply 6 years ago on Introduction

    Haha, an *excellent* point (sans the misspelling of lose ;-)

    But considering the relatively low carbon differential, even if done in a vacuum, this effect would be glacially slow, providing a thoroughly insignificant reduction in carbon, especially considering that one is removing significant layers of metal just by sharpening alone, not to mention using a gas torch is actually introducing extra carbon to drive the differential back the other way!

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    Wulf187djimdy

    Reply 6 years ago on Introduction

    I thought you could NEVER remove carbon from steel ? A cutting torch can NEVER put carbon INTO steel. A torch uses the principle of oxidising the metal where it needs to be cut. In other words you are "rusting" the plate into 2 pieces when you cut it. (By introducing oxygen) Now you know why you can not cut stainless steel with a torch.

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    flamesamiWulf187

    Reply 6 years ago on Introduction

    if you couldn't remove carbon from steel, you also couldn't put carbon into iron to make steel, could you? Especially not in the way shear steel is made. try googling "traditional steel making", or something like that.

    Also, a heat source can add or remove carbon from steel (in very small amounts), it depends on the amount of oxygen mixed with the fuel before it burns.
    Lots of oxygen-->carbon is lost and the metal oxidises, forming "forge scale".
    "not enough" oxygen-->less carbon is lost/carbon is gained and the metal oxidises far less.. this is why some people place an iron pipe in their forge, then place a bit of fuel(coal/charcoal) in it, before heating the finished blade inside the pipe to heat-treat it. the coal burns and removes the oxygen from the pipe, reducing scale formation

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    djimdyflamesami

    Reply 6 years ago on Introduction

    So it is a high-temperature process of rust, and because rust is (as you indicated) oxidized metal and that rust falls off, it also takes whatever impurities such as Carbon that was in the layer with it. If the alloy is consistent throughout, this should not be an issue, but if it is just in the top layer, the proportion of lost metal relative to the diffusion layer of Carbon in the metal could be too great and you'd be exposing lower-Carbon doped Iron beneath.

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    flamesamidjimdy

    Reply 6 years ago on Introduction

    hmm... I don-t think I explained it very well...if you heat metal in a vacuum, then you would not loose the outer layer(s) to oxidization, because there is no oxygen... but you would loose some carbon (not a lot, but some). If you heated an identical pice of metal in a carbon and oxygen-rich environment (impossible), you would loose iron to oxidization, but not carbon. an environment rich in oxygen, therefore give loss of both carbon and iron, but the carbon loss is far overriden by the iron loss, so its not that important...

    clear as mud, aren't, I...

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    Wulf187flamesami

    Reply 6 years ago on Introduction

    I think the best option is never to overheat steel in the first place. Keep steel as cool as possible while working with it and don't be lazy and cut steel with a torch and you will not have to get too technical into explaining to people why your method of overheating-underheating-impossible-carbon-adding-carbon-removing-oxidizing-back-summersault-heat-treatment is AAAACTUALLY the best way to work with steel and all the other people are so much dumber than you.

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    flamesamiWulf187

    Reply 6 years ago on Introduction

    overheating isn't really going to do anything though, otherwise forged blades would be cr*p. I was just trying to clear up a couple of points about carbon transfer and scale formation. And how scale formation can be reduced in an carbon rich environment.

    Also, I'm pretty sure you can cut stainless steel with a cutting torch, because it works by melting a hole in the steel, same way as a laser cutter does.

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    Wulf187flamesami

    Reply 6 years ago on Introduction

    Sorry. I did not mean "you" when I said other people are dumber than "you" I think I am on your side. English is not my first language so I use the wrong word order sometimes. I make knives using the stock removal method. I use various types of steel. Forging steel is different to production steel. One type (Bohler N690) I once overheated. They told me that once the steel changes coulour you can stop right there and throw it in the dustbin and start from scratch. Alternatively you can remove ALL discouloured steel (not just the outside layer because a knife is not an onion) Heat sink and all that. Anyway forging blades is a different beast alltogether. You keep on replacing the lost burnt off carbon everytime you reheat the blade in the coal fire. Coal=carbon.
    As for cutting stainless steel with a torch I suggest you try it first before you comment on it again. Stainless can not oxidize (rust) therefore it can not be cut by process of oxidation. The oxygen's sole purpose is to oxidize the steel. The process is accelerated immensely by first heating the steel til red hot to excellerate the reaction. If steel could be cut by mere high temperature you would only need one type of gas to do it. Lasercutting can well cut stainless by means of high temparature because it uses temps much higher than that of a torch. I hope I explained OK. (my first reply got lost in cyberspace)

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    thoraxeWulf187

    Reply 6 years ago on Introduction

    It was never overheated though. Are you implying that a large enough portion of the edge got heated to beyond ~1800F for a long enough time to lose some amount of carbon? Steel doesn't lose carbon that easily.

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    markybthoraxe

    Reply 6 years ago on Step 3

    Thanks thoraxe, I agree - I don't think the 'overheating' at this stage is significant. I think Wulf187 might be forgetting it's a 'hack', not a component for the international space station. There is meant to be an element of homebrew rootsy imperfection..... ;-)