Hidden Tang Clip Point Knife

70% Handle!

While cleaning out some of my old stuff from the shed, I found this hunk of steel. Specifically, it was a 1/8″ thick precision ground hunk of O1 tool steel. This used to be a nice 2″x 10″ bar, but as you can see (or not since there’s no size reference in the picture) there’s only about a 0.75″x 6″ piece left. What makes this steel special is its chemical composition. Tool steel, also known commonly as High Speed Steel, is a high-carbon steel. This doesn’t mean it’s carbon content is huge (its actually less than 1%!) but it is much greater than, for example, mild steel, which is the construction steel you typically find at the hardware store. What this extra carbon does is give the steel the ability to be hardened. This makes it an especially handy material for making tools,because it can we easily shaped in its soft (or annealed) state and then hardened to make it resistant to abrasion or deformation.

This particular steel is part of the O, or Oil hardening, series. O series tool steels contain blend of elements like Manganese, Chromium, Tungsten, Silicon, and Vanadium added to the steel to allow the allow the alloy to be hardened by oil quenching. The process for hardening this type of steel involves heating it to an elevated temperature and quenching it in a bath of oil to quickly drop the temperature. Whats nice about O1 is that this whole process process is fairly robust, and doesn’t require expensive heating ovens, carefully controlled temperature cycles, or long heating times like some other steels. Moreover, the temperature it needs to be raised to before being quenched is relatively low- about 1500°F. These qualities makes it ideal for making tools at home.

The tool I was particularly interested in making at home was a knife. I had made a couple of knives out of this stuff in the past (which is where the rest of this steel bar had gone) so I figured I’d break out the files and make another one and not let this piece of steel go to waste.

Designing The Knife

The first thing I did was trace out the steel blank on paper several times and I began to sketch out different designs for a knife. Since there just wasn’t very much stock material, the knife would have to be on the smaller side. Once I settled on a design I liked, I traced and cut it out of a piece of cardboard (in this case a cinnamon toast crunch box).

Cutting the design out of cardboard is actually a step I take when designing a lot of things because it’s a really good way to get a better sense of how the final, three dimensional object will look and feel. Doing this also allows me to make adjustments like changing the curvature of the handle, lengthening or shortening certain sections, and making changes to the locations of all the contours by physically holding the design in my hand and seeing what feels better. This really is one of those ‘ounce of prevention is worth a pound of cure’ situations because after all, its a lot easier (and cheaper!) to make these changes on the cardboard than it is to make them on the steel.

When I was happy with the design, I traced it onto the steel. This particular design is for a hidden tang knife. This just means that the tang (the skinny part in my drawing), the non-blade part of the knife that secures it to the handle, is hidden completely inside the handle, which is usually a single, solid piece of material. This is in contrast to a full-tang knife, where the tang is visible as the middle layer of the handle and flanked on either side by separate handle scales.

Hidden tang knives tend to be a little more difficult, because of the care that needs to be taken towards the fit and finish of the mating parts. But, since my past knives had all been full tang designs, I decided this would be a fun challenge to try.

The clip point descriptor comes from the fact that it looks like the top third-ish of the blade leading up to its point has been ‘clipped’ off. This is a fairly popular style especially among hunting/survival type knives.

Cutting out the Blank

With the design all drawn out, I headed outside to cut it out. The first thing I did was use a cut-off wheel on an angle grinder to cut off the unused steel and create a number of slits that follow the profile of the design. This is a quick way to remove the bulk of the material so that I can use a slower grinding wheel and carefully bring everything closer to required shape.

In the above image, you can see that I took care to leave around 50 thou clearance away from the lines I dew. This was so that I could then use hand files to bring the knife to its final shape. This gives me more control and allows me to carefully shape all the small details and features, including making sure the curves are smooth and natural, putting points where they are supposed to be, and squaring up the transition between the blade half and the tang half.

At this point I noticed a bunch of small pits all over the knife blank. These are actually from the jaws of the vice, which were pressing into the still-soft steel. Unfortunately, I don’t have any kind of softjaws for my vice so I’ll just have to be careful with how I clamp from now on. Luckily most of these indents were just on the tang, which would be hidden anyway.

Putting in the Bevels

The next step (and since I don’t have a belt-sander, the most labor intensive step) was to put bevels on the knide. The bevel is the sloped part of the knife that leads from the flat body to the sharp blade. In this design, there are four bevels that I need to put in-one on each side of the main blade, and one on each side of the ‘clipped’ section.

To begin, I colored in the side of the knife with a sharpie, and with a pair of calipers sett to half the thickness of the material, I marked a centerline around the blank. This line will be a visual guide that will allow me to see when the bevel has reached the middle of the blade and make sure the bevel is constant all along the blade.

To actually create the bevels I would have to file them into the blank by hand. To do so I set up this jig using some scrap wood. This beveling jig is based on the design by Gough Custom (check out that video here!) Gough Custom actually has a lot of really great videos for beginner knifemakers and its where I learned a lot of the techniques when making my first few knives.

Basically, an eyebolt is used to maintain the angle of the file and adjusted as needed to bring the bevel to the knife to the desired specification. The brass colored screw sticking out above the knife acts as a solid stop for the file. This ensures a nice and clean transition from the flat steel to the bevel, called the plunge line.

First a shallow bevel is filed all the way down to the center-line, then the file angle is reduced to push the further up the blank. With each adjustment, I color the bevel in with a sharpie and file until all the marks are removed. Doing so gives me a visual indicator to know when the entire beveled surface has reached the same angle.

I filed the main bevels to about three quarters of the way up the steel and filed the top bevels much shallower. I tried to get everything as symmetric as visually possible, but it wasn’t perfect. Its especially noticeable when looking at the plunge lines from the bottom of the knife.

Pre-Heat Treat Sanding

With the bevels filed, the knife steel is about halfway done. Before hardening it however, I needed to do a rough sanding of the blade up to 220 grit to clean up the surface remove the scratches left by the file. These large surface imperfections will naturally be much harder to remove after hardening.

To help with the sanding, I 3D printed a little jewelers sanding block I found on Thingiverse. I used this sanding tool to first sand the blade with 120 grit sandpaper and finish it off with 220 grit. The knife was now ready to be hardened.

Heat Treating

This was my knife heating setup. It is admittedly a little sketchy but it did the trick. Charcoal briquettes were placed in a pile inside an old cast iron skillet. Pointing into this pile is a metal pipe, on the other end of which is a hair dryer. The hair drier is used to push air (and with it oxygen) into the burning coals at a controllable flow rate to drastically increase the temperature they burn at. This coal pile can reach temperatures up to 1900°C and creates an easy way to bring the knife up to the quenching temperature.

To quench the steel I used some old peanut oil that I poured into a metal container. Oil is used to quench the steel rather than water for several reasons. First of all, oil actually does a better job of wetting the surface of the hot steel while it’s being dipped in, since the leidenfrost effect keeps water from being directly in contact with the blade. Secondly, water actually does extract heat much quicker than oil, and as a result, leaves the steel harder. However, because of this faster cooling, quenching in water greatly increases the stresses within the material and can lead to the steel cracking or breaking. Therefore, by quenching in oil, we sacrifice some of the hardness for a little extra toughness.

The bar you see sitting in the coals in the picture below is just a piece of scrap steel. By heating this up and quenching it in the oil before the knife, I can preheat the oil. I’m not entirely sure why this needs to be done, perhaps it decreases the viscosity of the oil or creates less of a temperature shock when the knife goes in, but it seems to be convention.

After the oil was preheated, I replenished the coals and pushed the knife steel into the pile. This is always my favorite part, because the steel undergoes some really beautiful color changes as it heats up. The reason for this color change is actually due to thin film interference. As the surface of the steel reacts with the air to form a thin oxide layer, constructive and destructive interference of visible light waves produces these really vibrant, fascinating colors as they interact with the oxide layer and the surface underneath. This also means that the colors are temperature dependent, because as the oxide layer grows with added heat, the interference changes. These color changes have been used in metallurgy for a very long time to as a visual indicator of the steel’s temperature, and an experienced metalworker could probably tell the temperature of the steel very accurately just by looking at its color. Here you can see it change from a bright golden hue to deep purple and blue.

The knife is heated until it gets to a bright cherry red color. The color change now is not due to interference, but black body radiation. The steel accumulates more energy as its temperature rises and some of this energy is radiated out as light closer to the infrared part of the spectrum. The color of the steel here can also be used to tell its temperature, but for someone like me who doesn’t know the first thing about metallurgy, there’s an easier way to know when the steel has reached the necessary temperature.

The curie point for steel is around 1420°F. This is the temperature at which steel looses its ferromagnetic properties and becomes completely non-magnetic. Although that’s interesting on its own, we can actually use this property of steel to determine when the knife is ready to be quenched. To do so, I used a small magnet to periodically test the steel. Once the magnet would no longer stick to the steel, I knew the temperature of the steel and that I would only need to heat for a few more seconds, until the blade reached 1500°F, before quenching it in the oil.

You’ll notice in the picture below that only the blade part is being heated in the coals while the tang portion is outside. This is OK because since the tang isn’t really ever going to be doing any hard work, it doesn’t really need to be hardened

After a dip in the quench oil, the knife is covered in black oxide and hard as glass. A file is no longer able to cut into the steel and if it were dropped, it would probably crack or shatter. While quenching the steel has introduces stresses inside the material to increase the hardness, it has also increased its brittleness. Since extreme brittleness is not an ideal quality for a tool, we need to temper the knife and exchange some of this hardness/brittleness for toughness/flexibility.

Tempering is done by bringing the blade to an elevated temperature and keeping it there for a period of time. This relaxes some of the internal stresses and makes the steel more tough at the cost of hardness. I tempered this knife in my oven at 400°F for two hours. In the chart below for heat treat response for O1 steel, you can see that as tempering temperature increases, hardness drops and toughness increases. By tempering at 400°F, I am bringing the final hardness of the knife to around 62 on the Rockwell Hardness C scale.

Coming out of the oven, you can see spots of straw-colored steel. This coloring is once again a result of thin film interference and this particular brownish yellow color is characteristic of steel that has been raised to around 400°F. Cool!

Finish Sanding

Now I could start finish-sanding the blade. The goal of this round of sanding is to remove all the oxides from the heat treating process and bring the surface of the metal to its final consistency and polish. This was also a pretty time-consuming process, but it needed to be done because the surfce finish achieved here would be the finish on the final knife.

The whole sanding process took a couple of hours as I worked up to finer and finer grits of sandpaper. Fortunately this little Western Fence Lizard hung around the whole time presumably just to keep me company. These lizards are identifiable by those little blue spots that run down their back and near their belly. It also has pretty crazy looking feet.

As I got to the finer grits, starting from 500, I switched to wet sanding. This just involves keeping the surface wet while sanding to keep the steel from clogging up the sand paper and prevent scratches as the metal approaches a mirror finish. As I was working on one side of the knife blade, I had to be careful to keep the opposite side dry otherwise little rust spots would quickly begin to form and ruin the surface.

The following picture shows the amount of sandpaper it took to get the metal to its final form. Each of the piles is a different grit of sandpaper starting from 120 grit, and moving up to 220, 500,1000, and finally 1200 grit.

Here is the blade after sanding with 1200 grit sandpaper. The surface is smooth and shiny, but unfortunately not exactly a mirror finish. For this I would have had to use higher grits of sandpaper or buff the surface with a polishing compound, neither of which I had on hand.

Forced Patina

So, instead of a mirror finish, I decided to try another metal surface finish known as a forced patina. This involves using an acid to etch the surface of the metal and force the formation of a fairly durable layer of Fe3O4 on the surface, also known as black iron oxide or magnetite if it’s in its mineral form. The main benefit this type of finish, other than looking really cool, is that its a great way to keep the blade from rusting. An unfortunate pitfall of high carbon steels is that they rust extremely easily, so they typically need to be kept with a thin layer of oil on the steel to seal it from oxygen and moisture in the air. By forcing a patina to form on the blade however, we can create a protective layer of black iron oxide (the good kind of iron oxide) that protects the underlying metal from the environment in the same way.

Also, cutting acidic things with the knife is going to create a patina anyway (and usually a pretty ugly one). So it’s better to be one step ahead and create the patina myself, where I have control over its consistency and color.

For my acid solution, I used a 50/50 mixture of regular white vinegar and water. Normally people use boiling, full-strength vinegar to carry out this reaction, but I found that with a dilute solution, the reaction progresses much slower and therefore gives me a lot more control over the process. So, I poured the picture into a narrow bottle, placed the knife inside and let it sit for a couple of hours, checking back periodically.

After about an hour in the vinegar, this is what the steel looked like. I wasn’t too happy with the direction this was going in- the resulting oxides were forming pretty unevenly and were having a hard time actually adhering to the base metal. This was likely because I sanded the metal to too high of a grit before etching it, which had created too smooth of a surface and as a result, nothing for the oxide layer to grip onto as it formed. To try and fix this, I sanded the blade back down to 500 grit and reattempted the patina.

The following images are the blade after 1 one hour, 3 hours, 6 hours, and 10 hours inside the acid bath. Every few hours, I would take the blade out and scrub it with a sponge and dish soap to leave only the oxide layer that had adhered firmly to the base metal. As you can see, this method resulted in a dark and very even grey-colored patina across the whole blade. At the end of the 10 hours, I cleaned off the surface of the knife and rubbed in some oil. Because the the surface was now microscopically porous, the oil was absorbed and left the steel dry to the touch. The knife now had a durable, non-toxic, and most importantly low-maintenance coating.

Cutting Out the Handle

With the blade complete it was time to work on the handle. I decided to use this nice slab of 1″ thick dark walnut wood. I chose the corner with the grain pattern I liked the most and sketched out a handle based on the cardboard prototype from earlier.

I cut the rough shape out of the slab using this little coping saw (a fantastic tool btw) and brought in the shape using a flap disk on the angle grinder. The final shaping, as usual, was done with hand filing. In addition to matching it to the sketched out profile, I also beveled the corners visually to about a 45 degree angle to make it more comfortable to hold. I didn’t round the handle completely and instead left it in this faceted form as a combination somewhere between a western knife handle and the hexagonally faceted handles in traditional Japanese knives.

After I was happy with the shape and feel of the handle, I moved on to making the guard. The guard is the bit of metal that goes between the handle and the knifeblade to pretect your hand from the knife or the thing you are cutting. Originally, if you remember the cardboard mockup from earlier, I had not planned on including one of these, but I figured if I was trying out so many things for the first time, I might as well add this to the list.

Cutting Out the Guard

I chose to make the guard out of this plate of 0.29″ thick hot-rolled mild steel. Mild steel is ok to use here since this component is mainly aesthetic and doesn’t need to be hardened. Ideally, I would have preferred to use brass or some other interestingly colored metal, but this was the only reasonable thick piece of metal I had around.

I cut a small piece off the plate and began creating the hole in the center through which the tang of the knife would fit through. I started by drilling a coule of 1/8″ holes (the thickness of the knife) and used a small file to shape the hole to a size that allowed the tang to fit through.

Then, I drew a profile that I thought would look good and shaped the steel until I was reasonably satisfied with it. The orange coloring in the second picture is just the remnants of a failed copper plating attempt using a copper acetate solution. Unfortunately, I had a really hard time getting the copper to adhere to the steel, so I decided to abandon that and just use the plain steel as it was. It also looks a little asymmetric in that photo, but I fixed the profile later on.

Shaping and Sanding the Handle

Before putting more work into the handle and guard, I had to make the hole in the handle where the tang knife would go through. I did this at this step because if I messed up making this hole, I would have to start the handle all over again and i didn’t want to have invested too much time into it before that.

To make the hole, I used a tiny bit of superglue to temporarily attach the guard to the handle where it could act as a guide when making the hole. I first used the knife itself to trace out roughly where the hole would need to go, and then began clearing out the wood using a 1/8″ drill bit.

Unfortunately, I wasn’t paying enough attention to where the drill bit was going and I accidentally broke through the surface of the wood. Now I know what I said earlier, but I didn’t really want to start over, so I decided to try and fill in the hole. A common way to fill in gaps and cracks in wood is to use sawdust from that same wood and mix it with wood glue. This creates a filler that correctly matches the color of the base wood, at least in theory. I will note that this method is usually used to fill very thin gaps, and is not really ideal for a hole of this size.

In my first attempt, I used Titebond II wood glue mixed with some wood shavings left over from when I was filing the handle earlier and worked the mixture into the hole. Unfortunately, once this was cured and sanded back, it ended up being quite dark, and didn’t really match the wood around it.

Unhappy with the result, I dug it out and reattempted it. This time I used cleaner and finer sawdust that I made using some sandpaper and I mixed it using some Elmers glue, which doesn’t add any of its own color unlike the wood glue. Unfortunately, this ended up giving me the exact same result. I decided this was good enough and I continued with the handle.

I started first by removing the guard with some heat from a lighter, and sanding it up to 1200 grit. I also used some polishing wax from my knife strop to try and give it floser to a mirror finish. It doesn’t reallly look that way in the following picture, but this actually worked quite well.

After this, I reattached the guard to the handle, this time with more superglue, and used a combination of small files to bevel its corners to better match the profile of the wooden handle. I sanded everything down with 220 grit sandpaper to create a seamless transition between the handle and the guard and finished the sides of the guard with 1500 grit sandpaper to give it a brushed look.

Final Assembly

With the handle done, it was time to mate the two parts of the knife. Before anything, I used the angle grinder with a cutoff wheel create a bunch of notches on the tang. This creates surfaces that the allows the glue to better hold on to the steel. Without these the glue would have trouble holding onto the smooth steel.

This is the equipment I used for attaching the two parts. For the glue, I used regular 5-minute epoxy, which I would mix using the popsicle stick and bottle cap. The syringe could then be used to squirt the epoxy into the hole in the handle before the knife steel is inserted.

What I failed to consider however was how warm it was outside when I was mixing the epoxy. The elevated temperature of the epoxy accelerated its curing time to under 2 minutes and most of the mixture ended up curing in the syringe before I was able to get it into the handle. I was able to get in a little however, so I inserted the steel anyway and hoped it would be enough to hold.

As a final step, I wiped on a coat of Danish Oil to finish and seal the wooden handle. This finish is not a stain, but rather a drying polymerized oil, which soakes into the wood and seals it from contaminants while simultaneously accentuating the color and grain of the wood. Danish Oil is pretty similar to Boiled Linseed Oil (BLO) but in my opinion, gives a slightly better, shinier finish. In the pictures below, you can see how much the finish adds to the look of the wood.

As a bonus, with the oil applied, the filled hole on top of the handle was blended in quite well. Of course its still noticeable if you’re looking for it but it fits in jest enough that you might think it was just a knot or other natural imperfection in the wood.

I used this Lansky kit to grind all the edge down and used a leather strop to hone the burrs to until everything was razor sharp. You can see how thin the black oxide coating actually is because it’s ground away wherever I sharpened the edge and over all I think that contrast is a nice look. Overall, I’m pretty happy with how this knife turned out.