Glenn

Don Abbott Forging a Knife from an Old File

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BP0420 Forging a Knife from an Old File
by Don Abbott

 

We’ve had several questions lately on forging knives from old files, so I thought I would give a step-by-step procedure for a simple method that I use from time to time. Understand that these are simple instructions for a very simple blade. There are as many different ways of forging a blade as there are smiths who forge them. Everybody will have different ideas about how to get from file to finished knife, but this way works for me.


I did a live run-through by these instructions this past Saturday just to be sure I was not leaving anything out. I completed all of the forging steps in well under an hour using only a charcoal forge, hammer, anvil, and my wolf-jaw tongs. You will also need a cutter of some sort. I use a hot-cut hardy, but a handled hot-cut, or even a chop saw, will work fine.


There has been much said about which files to use, and general consensus is that you can’t beat the old “Black Diamond” brand. But like everything else, these are getting harder to find. Any file made of high carbon steel is going to work. Really old files are usually going to be good, but they say that many of the newer, cheaper brands of files, and especially rasps, can be casehardened mild steel. These are no good for blades. If the material is in question, you need to do a little research on spark testing. Perhaps you could even test a sample piece and see if it will harden when quenched. Also note that this same forging procedure would work with any rectangular piece of carbon steel.


NOTE: You will hear reference to what we call “critical temperature” and/or “non-magnetic”. To find this temperature, you will gradually heat the steel, continually checking it with a long-handled magnet of some sort. There will be a point where the steel will no longer attract the magnet. This is your critical temperature. Make a mental note of the color of the steel. In my shop, this will be a good uniform orange color. Do it enough and you won’t be nearly so dependant on the magnet.


NOTE: You will hear me refer several times to “a good forging heat” or “a good heat”. You must be aware that the higher your steel’s carbon content is, the tighter your forging temperature range will be. You might be accustomed to pushing your mild steel forgings to a bright yellow heat and then working them down below cherry red. You’re gonna have to tighten up for a thin, high-carbon piece like a file. Don’t heat your file much past critical, and go back in the fire as you lose heat down around cherry red. Work it too hot and you’ll get damaging grain growth. Work it too cold and you’ll get stress cracks. It ain’t hard to do, just keep a close eye on it.


NOTE: One more thing up front: You will hear bladesmiths speak of “normalizing” their steel. We will accomplish this by a three-step procedure as we forge our file blades. To normalize, gradually heat your blade to non-magnetic (critical) and leave it in the fire at that temperature for about a minute (sometimes refered to as “soaking”). Take it out of the fire and let it gradually air cool to black. Repeat this process two more times. Now your blade is normalized. The purpose of this is to relieve the stress that all of our forging operations will introduce to the steel. It allows the steel to relax and become accustomed to its new shape. Feel free to normalize at any point you think your blade might benefit.


NOTE: Before I forget… get yourself a good, stiff steel wire brush and keep it handy. Your steel will form scale and it will pit your blade and cause problems down the road. You cannot wire brush too much. Get in the habit of hitting it a quick lick with the brush every time you come out of the fire. A scale-free blade is a happy blade.


So that’s what we’ll start with… and an old file:


step1.jpg




Before forging, you can anneal the file and grind past the teeth for a clean start, but this is just a blacksmith’s quickie, so we’ll leave the teeth and deal with them later.


The first forging I do is to draw out the tang. This is going to be a “stick tang”, or “hidden tang” knife, by the way, so I need a little more tang than the file has to offer in its natural state (the intended size of your finished knife handle will dictate the length of the tang. I’d rather go long and cut it off than end up short).


Into the forge, and take a good heat at the tang end:


step2.jpg




Working over the edges of your anvil, simply draw that tang to your desired length:


step3.jpg




Now, unless you want to end up with a miniature sword, you’ll want to cut the blade end off a bit before you commence forging it.


Take a good heat on the blade end, and cut it on the diagonal. I usually line the blade of my hot-cut up with the teeth of the file, but it really is not critical. Cut as shown by the dashed line:


step4.jpg




Your file should now look something like this:


step5.jpg





Note that, in spite of the way the blade is shaped at present, the top will be the spine of the blade; the bottom will be the edge .


Take a good heat on the end of the blade to start forming your point:


step6.jpg




You want to begin forging the profile of the point end, causing the bottom edge to turn up (vary the angle of your blade on the face of the anvil to achieve the point shape you desire).

The dark arrows represent the approximate direction of your forging blows:


step6b.jpg




A WORD ON BLADE TAPER:
As you proceed in forging the profile (side view) of your blade, you will want to pay continual attention to the taper (top view) as well. When forging the blade profile, the thickness will naturally tend to bulge. You must continually correct this, ultimately aiming toward a good distal taper; kinda like a really slim diamond, or a really skinny fish. Something like this:


distal_taper.jpg




Here you should have established the final profile of your blade:


step7.jpg




At this point, you are ready to start hammering the bevel into your blade.

If you think about it, as you start to thin the steel on the edge of your heated blade, the material on the edge is naturally going to spread in all directions. This will leave your blade bending backward like a banana. You can try and see what happens if you want to, but I have learned to put a counter-bend in the blade first. Some folks correct it as they go, but after a time or two, this problem becomes predictable enough to fix up front.


Take a good, long heat:


step8.jpg




Then simply introduce a slight arch, making the edge become concave. You can bridge the blade between two pieces of pipe or some other specialty device; I simply bridge the spine of my blade over the step of the anvil and work it in there.

You should have something that looks similar to this:


step9.jpg




NOTE: every one of these procedures will potentially affect the natural flatness and straightness of your blade. ALWAYS restore your flatness and straightness before proceeding to the next step! A twist, warp, or bend will only get worse if not corrected.


Here we begin hammering the bevels into the blade. I tend to begin my forging passes at the tip and work back, but I suppose back-to-front will work as well. The key is to maintain a consistent pattern. Make as many passes as you need to establish the bevel you want.


hammer_sequence.jpg




Don’t hesitate to take more heat at any point in the process. If you lose forging heat in the middle of a pass, stop hammering and go back into the forge; just remember where you left off, then resume.

A lot of this stuff will not make much sense until you get your hands on it. This is really one of those things that you’re going to have to try. That’s why we’re using an old file instead of expensive steel!


As you forge your bevels, you must pay close attention to the angle of your blade to the face of the anvil. Remember, the anvil will “answer back” every blow of the hammer, so if you lay it too flat on one side, you’ll undo what you just did on the other.


You should be forging bevels at an angle similar to this:


hammer_bevels.jpg




As you forge the bevels into the blade, remember that we are going to do a good bit of stock removal to achieve our final shape. HERE is where we’ll get rid of most of those unsightly file teeth. So as you forge, DON’T WORRY ABOUT GETTING IT REAL THIN! Especially for beginners, you are going to get rid of a lot of surface imperfections; teeth, hammer marks, scale, etc., so forge it thick; we’ll grind it thin.


Think of your blades cross-section like this:


cross_section.jpg




So here is where we end up: tang drawn, profile forged, and bevels hammered in. Notice that the profile is a little “fatter” than your original profile before the bevels were forged. That’s good. We’ll clean it all up in the stock removal stage. MAKE SURE that everything is good and straight! At this point, you can put the hammer down. We are finished with the forging, but we will be going back to the fire, so keep it hot.


The blade should resemble this:


step10.jpg




Normalize here, if you want to. Might help; can’t hurt.


In order to do any filing, grinding, or drilling we might want to do, this steel needs to be as soft as possible. This process is called annealing. In our case, it’s not rocket science, but indeed, it can be. Since we are blacksmiths and not rocket scientist, we are going to keep it simple.


You will need a big, non-flammable can, bucket or box, preferably with a lid, full of some sort of insulating material. A lot of folks use powdered lime or vermiculite. I use one of those big 3 or 4 gallon popcorn cans full of sifted wood ash. What we are looking for is something to slow the cooling process way down.


The process is simple. Gently heat the blade to non-magnetic (critical temp), and bury in the ashes (or vermiculite, etc.). Put the lid on. Leave it until the next day. Don’t peek; just leave it.


After this slow cool-down, your steel should be soft enough to work with a file, a grinder, sand paper, drill bit, stones, etc., etc. The whole grinding and shaping process is a study within itself; one that I still need a lot of work on. Files are a good way to start. This gives you a lot of control and you can sort of make your decisions as you go. A good belt grinder will be quick, but you better know what you’re doing.


NOTE: Leave your edge roughly the thickness of a dime. Don’t take it to a sharp edge right now. If it’s too thin, you will increase your chances of cracking or warping during the final heat treating. We’ll worry about the edge in the final finishing stages. Just be sure it is good and smooth.


After filing and grinding everything, you can start cleaning up the scratches with descending grits of sand paper. Start around 100 or 150 grit and work down to 400. Use each new grit until you remove all the scratches from the previous grit. This is probably the most boring and least enjoyable process of all, but the results you get depend on your patience and the time you put in. Again, volumes have been written on blade finishing, but this is the 101 method.


Here’s what the almost finished blade might look like:


final.jpg




At this time, we are ready to finish up with the heat-treating.


Go ahead and normalize your blade, and then remove any scale or crud you might pick up in the process. A clean blade is important from here on out. We want it fully prepared for what is about to take place.


Now it’s time to harden the blade. You will need your forge, your tongs, and your blade (of course). Also some type of quenching medium and something to put it in.


I use an old brass flower planter that is about 6 inches square and 2 feet long. I’ve seen some folks use old valve covers from truck engines. Just be sure it’s not flammable, and doesn’t leak.


As for quenching mediums, I use vegetable oil. Any type of cooking oil is okay. I have seen several guys use automatic transmission fluid. It seems to work well, but it stinks. The cooking oils smell like, well, like your cooking. Just stay away from used motor oil and transmission fluid. If you can smell it, that means you’re breathing it.


I normally fill my tank with oil and then pre-heat it to roughly (very roughly) 150 Fahrenheit. I usually accomplish this by heating up a railroad spike and submerging it in the tank (I just leave it in there).


Then, you will take your cleaned and normalized blade and ease it back into the fire. Let it heat slow and gradual, and be very sure the heat is dispersed evenly, even if this means moving the blade back and forth a bit. This time, check it very carefully with the magnet. You can certainly take your time getting up to critical, but you definitely don’t want to exceed it. (Take your time, and don’t rush. At any time you feel like you need to scrub the mission, just re-normalize and start over.)


As soon as you reach non-magnetic (critical), you need to go into the quench. Different steels have different rates at which they must be quenched, but we’re not going to get fancy. Just be quick. Come out of the fire and into the quench edge first, just like you are cutting through the surface. Submerge the entire blade in the oil, holding it straight. You can move it front to back or up and down a bit, but do not wave it side to side… it could warp. Leave it there until it is completely cool. I know there are also many ways of quenching a blade, but this is the most basic; just quench the whole blade.


NOTE: Depending on the type of oil you use, you might get a little flash or surface flame when you quench. As in all things flammable, be prepared for the worst. Do this only in a protected area with a fire extinguisher handy. Gloves, glasses, etc… you know the drill. Safety first.


After your blade is cool enough to handle, it should be fully hardened. It will be blackened from the burnt oil, so you need to take the time to clean it back to bright. To test the blade for hardness, take a good file and try to cut the edge of your blade. The file should simply “skate” like a piece of glass. If the file cuts the steel of your blade, it is not as hard as it should be. But if you did it right, it will be fully hardened.


If the blade is fully hardened, it is too hard to be of any use as a knife. The steel is extremely brittle and too hard to sharpen. You will need to introduce enough heat back into the blade to pull back some of this hardness. This is what we mean by tempering. I normally use a very simple 3 step process here… well, actually 2 simple steps, and one that can be a bit challenging.


First, put your blade in a 400 degree oven for an hour. You can use a full size kitchen oven or a small toaster oven. Heat is heat; either will work. After an hour, let it cool.


Second, do it again; 400 for an hour. Let it cool. Now, take it back to a bright 400grit finish. It is important that you have a clean, bright finish for the third tempering step.


Third, you will need an exposed source of heat. I use the eye on the kitchen stove. Some folks use a block of iron or copper heated in the forge, some use a propane torch, some use a special set of heavy jawed tempering tongs, and some have the nerve and know-how to do it over the forge. I have found the front stove eye on the high setting is the easiest and most manageable heat source to use for these smaller blades.


You definitely need that bright finish on your blade, because you are going to be watching the oxidation colors “run” through the steel. I highly recommend that you polish a couple pieces of scrap steel (even mild steel will work) and practice with this before you go live. Make sure that your work area has enough light to see what’s going on.


I generally turn the stove eye to “high”. I grip the tang with tongs or pliers and lay the spine of the blade directly on the eye. The blade needs to remain edge up. Immediately begin to move the blade back and forth with the entire spine making passing contact with the heat source (kinda like you’re playing a slow tune on a fiddle). Soon, depending on the size and shape of your blade, you will see the first hints of a straw color appear closest to the heat source. Now you really need to pay attention. The colors are going to run in a manner similar to a sunrise. Straw will be followed by bronze, bronze will be followed by a range of purple-ish colors, and then comes the blue. Ideally, you want to let the heat flow from spine to edge, stopping the process as soon as the bronze band reaches the edge. If the blade is wide enough, you can end up with a full spectrum across your blade, similar to this:


temper.jpg






If this procedure goes as planned, you will have managed to reduce the hardness of the blade in a way the spine will be somewhat flexible, and the edge will remain hard enough to hold a good edge. If you find the edge is still too hard, polish it up and re-temper, this time letting the colors run a bit farther that the last time.


That will about do it. All that’s left is to do the final polish and mount the handle. We have taken it to 400grit at this point; how fine you want to go is up to you. Same with the handle… bone, antler, horn, wood; what ever you have in mind.




Here’s a couple of my “early works” made according to these instructions:


K2b.jpg




kt6.jpg




Again, our experienced bladesmiths will automatically see all sorts of things that can be done differently (and probably better), but this is simple, and it works for me.


Have at it!


Don

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Thanks! I can tell that took alot of your time and I appreciate it. I'm gonna give it a try!

 

Jamie

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Don, you sure put a lot of time and care into your presentation - THANK YOU!!!!  It's folks like you and the rest of this fine group that makes the internet so GREAT!!!!  

 

Ken H>

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This was a awesome job!! And I can't wait to get some time off work to try this out. Thanks again

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Any time I have started a project with file steel I have put the file in the oven as high as possible heat, 450' and let it heat for a couple hours. I don't know if it is of value but it does soften it up some before start working it.

This is a great article, thanks

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Most excellent Article well written and easy to follow Thank you Don & to Glenn for posting this .

 

Sam

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That was a Great read!

Don, thank you very much for taking the time and effort to write this up. 101 is where I am at, so this is HUGE for me!

 

Thanks again.

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Great stuff I have a 5 gallon bucket of files laying around waiting for a worthwhile project.

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