Introduction: Lawn Mower Blade to Custom Knife
Hi All, this is my first attempt at an Instructable so comments and advice are more than welcome.
Now onto the project itself: I'm going to outline the steps I took in order to turn an old lawn mower blade into custom, "one of a kind" full tang Knives. This was my version of a trash to treasure as the two main materials are simply old lawn mower blades and some firewood I rescued, and I end up with beautiful knives.
I am a chemical engineer and making these knives during exams was my method of stress relief. I do most of the work by hand so it's not perfect and is somewhat slow and tedious but I find the end product all the more satisfying. As some of the steps are quick to do but require some setup I normally make pairs of knives at a time, this saves time when doing heat treatment and grinding the blades.
My steps are all based off reading up on blacksmithing and playing around in my backyard so I'm sure I do some things in a very labor intensive way or not 100% correctly but the end product comes out looking amazing and has a properly hardened blade that hold an edge well. I also don't mind some scratches on the blade as knives are tools and should be used and each scratch or flaw gives the blade some more character.
As a general disclaimer for the project it involves: Fire, Power Tools, and of course knives (I.E. lots of fun things). All steps will typically require some form of safety equipment including gloves, safety glasses, ear protection and work-boots. In my experience even while wearing safety gear and being careful you will still accumulate cuts, bruises and burns so a first aid kit can come in handy. The main thing with working in a shop is to pay attention to what your doing, be comfortable with the tools you are using and have common sense.
The Steps I will cover include:
- Materials and Tools.
- Lawn Mower Blade preparation.
- Blank Blade
- Initial Grind and Sanding
- Heat Treatment
- Clean-up and more Sanding
- Final Clean-up and polish.
Step 1: Materials and Tools
- Old Lawn Mower Blades.
- Wood for handles.
- Firewood or charcoal, for heat treatment.
- Brass Pins (not 100% necessary but make the handles a bit better).
- Epoxy, for the handles.
- Your choice of wood finisher, I use beeswax or teak oil.
- A whole lot of free time.
Tools: (Most of these are general workshop tools nothing too fancy).
- Safety gear, use what you think you need for each step.
- Angle Grinder with cutting blades.
- Bench Grinder.
- Drill and drill bits.
- Some files
- Wire Brush.
- Sanding Paper of various grits (150 all the way up too 1200)
- Hair Dryer (be careful of who you borrow it from as it may get cooked)
- Sharpening stone/oil stone.
Step 2: Lawn Mower Blade Preparation
You can get old blades when you replace them yourself or if you go to most places that sell new ones. The local hardware store was more than happy to let me grab a couple that were lying around in their workshop.
Not all lawn mower blades are created equal. When looking at the blades there are a couple things to pay attention too:
- Where the blades are damaged and how thick the metal is, you want at least one good region to cut the knife blade from.
- Where the serial numbers are on the blades, I find these add a cool one of a kind look to each blade.
- Whether or not the metal can be hardened, IE is there enough carbon in the steel for it to hold an edge.
Most lawn mower blades will require a good cleaning so use a steel brush and get to looking at the blade. Look for where it's been damaged and why it needed replacing. Normally only the cutting edge is worn but there may be cracks or other damage to look out for. Also look at where serial numbers have been stamped into the metal as these are quite a cool feature to have in the end blade.
At this point you can get the metal nice and hot in the fire and use a hammer to get it as flat as possible, as a bonus any grass that was left on the blade will get burnt off. A normal wood fire with a hair dryer providing extra air gets hot enough to get metal red hot.
One of the key features you need to test for is whether or not the blade can be hardened. There are loads of articles and techniques for testing the metal and a quick google search can be a great help if you get stuck, I use a simple shatter test and skittering test. Basically all I do is light a fire get a corner of the metal red hot and dip it into either water or oil. First I do the water quench and once the metal is cooled down I hit it with a hammer and see if it shatters or just bends, this is a quick way to see if the metal can be hardened. If the metal shatters it's a good metal to use, if it stays soft you can try a different section but the blade may be a dud. After the water test I test it again with cooking oil (sunflower oil), I typically cut off a small piece get it red hot and dip half of it in the oil, I then check if a Hacksaw or file is capable of cutting into it or scratching it (known as the skitter test as the file will just skitter along the surface). I test the side which was quench against the normal side and its quite easy to see if it hardened well. I use cooking oil to check as my actual blade quench is done with cooking oil.
Step 3: Blank Blade
Once you have found a good lawn mower blade it is time to decide on the type of knife you want. Spend a bit of time on this as there is no point making a cool new knife for yourself if you wont actually use it. I'm not the most creative person so I usually just google designs and look at pictures until I find something I like. Once you have a design in mind you should draw it out a couple times or find a knife template online too print.
Cut out your paper template and trace it onto the lawn mower blade, I use typex for this as it's easy to see. I try to get the blades serial numbers onto the blade of the knife as a one of a kind sort of effect but this does limit me on the size of the knives I can make it also means I can only make one or two knives from each lawn mower blade.
Once your design is on the blade it is time to cut it out using an angle grinder. Try to cut as accurately as possible as this will be the shape of your knife, if you cut too much you will have an issue and if you cut it with a thick border you are just making more work for yourself when you grind it to shape. I like to have about 1-2 mm extra around the border at this makes sure that I will have a nice edge to grind smooth and clean off for the shaping of the blade.
Step 4: Initial Grind and Sanding
Now it's time to turn you blank piece of metal into an actual blade!
First stick your template onto your blank blade, you could also mark out where you want the edge and such to be with a pen or typex but I find it easier to just stick the paper to the metal.
Now follow the template and grind the blade first too shape and then put the bevel onto it. I like to do this quite slowly as you can't put metal back if you make a mistake! Also do this slowly and don't over heat the metal, the simplest way of preventing this is to work bare handed and if the metal is too hot to hold cool it down in some water. Be warned it's easy to over heat and damage the tip of the blade. You never want to see the blade get any blue spots or worse get red hot.
When grinding the edge don't make it thinner than 1mm otherwise you run the risk of it cracking during heat treatment!
After grinding it's time to begin sanding. This needs to be done now while the metal is still nice and soft, before the heat treatment. Use this time to clean up the edge of the blade, make sure the bevels are uniform and to do some general scratch removal from all exposed metal. Start with a course 150 grit and work your way up until you are happy with how the finish looks. I normally go up to a 600 or 800 grit as I get lazy and don't really need a perfect mirror finish, I like a slightly more rustic look.
Also drill the holes for the metal pins to go through for the handle. It is easier to do it now than after the heat treatment.
Step 5: Heat Treatment
Time to play with Fire!
I do a differential heat treatment on my blades; basically you pack clay around the knife to control how fast it cools when you quench it, this allows you to have a hard edge which will stay sharp and a soft spine which increases durability.
The steps for heat treatment are:
- Normalizing the metal, this relieves stress and prevents warping. You need to heat up the blade until its no longer magnetic and the allow it to cool slowly. I normally skip this step as I'm careful during the grinding step.
- Next you harden the metal by heating it up until its no longer magnetic and quench it in oil or water. This changes the metals molecular structure and makes it hard but brittle.
- Finally you temper the blade by heating it up to about 180 degrees Celsius and keep it at this temperature for an hour to soften the metal allowing it to be more durable.
Those are the broad steps to follow though they will vary based off which metal is being treated. I do a differential heat treatment so I put clay onto the blade before the hardening step this changes how the metal cools in the quench and gives a blade that can hold and edge while also being durable.Once again I will refer you to google if you want to read up more on heat treatment processes. Put simply a thick layer of clay goes around the spine and half of the handle so, while the edge of the blade is left exposed is quenched so it is hardened.
When I put clay onto the blade I go about half way up the tang in order to leave a section of metal to hold onto with the pliers when I do the quench. When I quench the blade I only dip the section with clay on it into the oil and allow the rest of the tang to air cool, thus the tangs of the blades aren't hardened and remain soft.
The steps I follow are:
- Cover the blade with clay and allow it to dry, as wet clay will crack off during heating.
- Get your fire nice and hot, heat your blade until its red hot and is no longer magnetic.
- Quench the blade in cooking oil and crack off the clay, you can see the effect of the differential quench on how the blade looks in the one photo.
- Cook your blades in the oven at 180 degrees Celsius for an hour, I cooked a couple sweet potatoes to eat while I was at it.
- Examine your now heat treated blade in all its glory.
Step 6: Clean-up and More Sanding
Now that your blade has been hardened it is time to clean it up and get it looking presentable. I do all this by hand so this step takes me the longest and it gets really tedious.
During the heat treatment a layer of rust will have formed on the blades surface you need to sand this off and sand the surface of the blade to the shine you want it at. So once again start with a grit and work your way up.
At this stage your blades edge is still about 1mm thick, so it is super blunt, and you will need to get it a whole lot sharper before you take it to an oil stone. The thing is you shouldn't use a grinder now cause if you over heat the metal you will undo/mess up part of the heat heat treatment you just finished! Thus what I do is put some sand paper onto a tile and use this as an initial sharpening stone to work the edge until it's thin enough for a real sharpening stone. Start at 150 grit and work your way up.
Depending on how you want your knife to look now is the time to get some polishing compound to buff out those last scratches and get your knife to a nice mirror finish. I don't mind some scratches as they give the knife character and allows me to use the knife without worrying about damaging the finish.
Step 7: Sharpening
Once your blade is reasonably sharp it is time to sharpen it for real.
I use an oil stone to sharpen my knives but it really is a case of do it how you want to. Also I'm no expert on the best way to sharpen knives on oil stones so I will once again refer you to the all knowing google as there are loads of video tutorials on the subject.
On thing I have found is it is easier to sharpen the blade before you put the handle on it. Also once you have sharpened your blade wrap it in some form of tape as this makes it easier to handle while putting the handle on.
Step 8: Handles
I like like full tang knives where you can see the metal all the way through the handle, thus this is the finish I go for. You could alternately grind the handles metal down smaller and inlay it into the wood but I find it quite a labor intensive process.
Here is how I make may handles:
First I wrap tape around the blade as this prevents any glue or scratches getting onto the blade and makes it far easier to handle without getting cuts.
Next I have a piece of lead wood (scavenged from some firewood I came across) which I use to make the slots for the handles as it has a nice deep red color. I cut a couple slots of lead wood which are about 8mm thick and are large enough to cover the handles. These slots need to be slightly bigger than the handles and flat enough that they fit flush with the metal. I trace out the rough shape of the handle and cut the wood to shape with a fret saw. Make sure to label which side each piece of wood fits on. If there are any cracks in the wood you can use a wood filler or saw dust (from the wood itself) and super glue to make them disappear, my lead wood has a nice grain with tiny crack along it so I always have to fill in a couple spots.
I then cut some brass pins, slightly longer than necessary, and drill holes through the wood and handle. Make sure everything lines up perfectly as you don't want to find that it doesn't quite fit when you are epoxying it together. These pins are not 100% necessary as the epoxy creates a very strong bond, but they do make it slightly stronger and give a nice finish to the handle. Some people just use brash pins and flare the ends in order to hold the handles together but when I tried doing this all I managed to do was crack one of my handles.
Wash the blade with a bit of solvent (paraffin, turpentine, ethanol, acetone, etc) or in soapy water (rinse it off well) to make sure there is no oil on the blade handles. Then it is time to stick it all together.
Mix up some quick set clear epoxy and as fast as lightning smear epoxy onto both slots and into the pin holes before clamping the handle together to set. I use quick set epoxy as I always have some lying around but you do have to work fast with it as you only have about 5 minutes of working time with it. I also find that clear epoxy leaves a better finish on the knife than grey/metal epoxy.
Once the epoxy has set solid (24hrs at least, to fully cure), it's time to make the rough handle shape into something nice to hold. As the lead wood I use is incredibly hard I simply take the whole thing to my grinder to get it to as close to comfy as possible, I also use the grinder to get the pins to sit flush with the wood. Once the handle is sort of to shape I use files and sanding paper to get a nice smooth finish that sits well in my hand.
Once you have a nice comfy handle check that there are no cracks and if there are fill them in and give it a quick sanding so it matches nicely.
Now that the handles are shaped finish the wood in whatever means you personally enjoy using. I like a raw wood feel so I just give the handles several coatings of a mixture of beeswax and teak oil. The handles come out with a deep red colour and fit comfortably in the hand, also after some use its easy to re-oil the handles.
Step 9: Final Clean-up and Polish
Its time to pull off the tape on the blades and look at your almost complete knives.
All that is left to do is a final polishing of the blade, a quick touch up with a sharpening stone, re-oil the handle and maybe make a sheath for your new knife.
I used a bit of leather to form a couple rough sheaths just to store my knives in and I'm still looking around to find some thicker leather to make a proper belt sheath.
You now have an awesome one of a kind knife which makes a perfect gift to any outdoors enthusiast or pocket knife which has a cool backstory to it.