I've had this idea for a long time of making a small knife that could be retained in a credit card sized sheath so that it could fit in my wallet. This one was made with recycled materials as it was a first test of the design while I am also working on the second version that would use a known steel and a 3D printed sheath for a slightly cleaner design.
NOTE: The steel I am using for this is from an old saw blade. It as a cut off piece from making my throwing hatchet. I do not go over the material hardness on this one because I covered it in that instructable. If you are interested in the steel properties you can find that instructable HERE.
WARNING: Please check your state or country laws to make sure that carrying this type of knife is not illegal. If you happen to know the legality of carrying this in your state or country then please add a tip or comment here so that it might help others know. Thank you.
Step 1: Create Your Design Template
To create this design I first needed to use a gift card/credit card as a template because I wanted the knife to fit inside my wallet. Now that I had the size it needed to be it was time to sketch the knife shape inside of the card template. It is then a good idea to cut out the design and make sure you can hold the knife shape comfortably. You don't want a design that will end up hurting you when you try to use it.
Step 2: Transfer the Template
Then use that cut out piece to transfer the design onto the metal you will be using. I am using a cut off piece from an old 7-1/4 in circular saw blade that I used for another project. This saw blade was 1/16 in thick and, as I showed with the throwing hatchet, can be hardened. This thin steel in nice because it will fit better in my wallet but you could probably go up to 1/8 in if you don't mind you wallet a little thick. My next version that I am working on is made out of 1/8 in steel.
Step 3: Cut Out the Design
Now it is time to cut out the design. I used my angle grinder for this. Just be careful not to cut too close to the line.
Step 4: Grind to Final Shape
With the knife cut to a rough shape it is now time to clean it up to the final shape. I did this with my belt sander then used my Dremel for the inside corners.
Step 5: Grind the Initial Bevel
I decided to make this a kiridashi which is a Japanese knife with a bevel on just one side. If you want to put a bevel on both side then you can but I just wanted it on one side. I used my belt grinder to grind the bevel.
Step 6: Heat Treat
I found out from my testing on this material that I would heat treat it to get it to 60HRC. This was not the most precise heat treat process but I heated it to red hot and non magnetic and then quenched it in some canola oil. Then I tempered it in the over at 400 degrees F for two sets of one hour.
Step 7: Cut Out the Sheath
I needed a way to hold this in my pocket without cutting myself and I also wanted it to be the same size of a credit card so I figured why not make a sheath out of old gift cards. Depending on how thick your material is it may determine how many cards you need to use. I used the knife as a template to mark out where to cut to make the slot where the knife will fit. I just traced the outline with a knife multiple times until it cut all the way through. There is probably a better way to do this but it worked. A scroll saw would have probably been nice. I found that if you heat the end of the knife up then you can melt through the card to save some time. When you have the initial slot cut out then you will want to test fit the knife and cut away an additional material needed to make it fit.
Now that I had one cut it was time to cut the second one. For the second one I intentionally kept some material where the blade is so that the material would prevent the blade from poking out if the sheath was bent. Don't want to hurt myself with this in my pocket. I only needed to cut two cards because this knife was so thin.
Step 8: Test Fit
With the cards cut it was time to stack the cards and test fit to make sure the knife can be inserted without any issues. Everything fit fine.
Step 9: Finish the Knife
With the knife heat treated I now cleaned the scale off of the knife, finished grinding the bevel, and then sanded it down until it was nice and shiny. I started with 120 grit sandpaper and then proceeded to 320 and finished with 600 grit.
Step 10: Sharpen the Knife
Now that the knife is cleaned up it is time to sharpen it. I used my Lansky sharpener to bring it to shaving sharp.
Step 11: Cut Out Slots for Rubber Band
I needed a way to retain the knife in the sheath and the easiest solution for me was to use a rubber band. To keep the rubber band contained I drilled four holes and then cut them into slots with a utility knife to hold the rubber band in place. I glued the two top pieces together before making the slots and then taped on the bottom piece to provide the hole locations for the bottom. I decided to cut the bottom piece differently because I wanted to make a slot for the rubber band so that it didn't add to the thickness of the bottom.
Step 12: Attach the Cards Together
Now with all the slots cut it is time to attach all of the card together. Since the top two cards were already glued together, I just had to glue the bottom card on. I was a little concerned about this holding together with just glue so I found two small rivets and used those to rivet the cards together in the corners with the most material. I couldn't add rivets to the other corners because there was not enough material there. I also decided to drill a 1/2 in hole in the bottom card so that I could push up the knife making it easier to remove.
Step 13: Paint Sheath (Optional)
If you want the sheath to have the gift card look then you can keep it the way it is but if you want it one uniform color then you can paint it like I did. I would normally use spray paint but because I live up north where it gets really cold in the winter I had to use brush on acrylic paint. I don't know when I last brushed on paint but I really likes how this turned out. Because it was a thicker paint it gave it a textured look which hid the fact that this was made out of gift cards.
Step 14: Stash It in Your Wallet Until You Need It
Now that everything is complete all that is left is to rubber band it together and put it in your wallet until you need a knife. This could be a good backup backpacking or camping knife so that if you really need a knife then you always have one on you as a backup. Just remember to remove it before flying.