Make a Hybrid Marking Knife

If you spend as much time watching my Instagram feed as I do, you’ll see TONS of cool wood/acrylic hybrid blanks. After seeing my first marking knife with hybrid scales, it was on my to-do list. After a couple years of slowly gathering pieces, making vacuum chambers and pressures pots, I’ve finally gotten to a point to actually make it happen. I replaced my jointer blades a while back stashing away the old ones  and this summer I collected my last piece of material when I traded a slab of mesquite for a pile of burls.

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Hybrid Knife Scales

hybrid castings before/after trimming
Hybrid blanks before and after being trimmed to size.

If you’re not sure what a hybrid knife scale or blank is, it is typically a piece of wood that has been stabilized and then cast in some type of resin. Typically you’ll see burls cast in resin but cactus, pinecones, lathe shavings and many other things can make interesting blanks as well. For my hybrid blanks, I’ll be using a boxelder burl cast into a florescent green resin.

I won’t be going into great detail on stabilizing and casting because I plan on making detailed videos and articles on the topics in the coming months. I will give you a high-level description as well as some additional resources where you can find out more.

Stabilizing Wood

What You Need

To stabilize wood you’ll need a vacuum chamber w/pump, stabilizing resin, woodish stuff and a toaster oven. If you want to read more about making a vacuum chamber, you can check out my previous article, “How to Make a Vacuum Chamber“. The stabilizing resin I use is Cactus Juice by turntex.com and the toaster oven was something I picked up on Craigslist for about $20. In my case, the woodish stuff I was stabilizing was some boxelder burls but you can pretty much stabilize any kind of wood.

What is it and Why do it?

The general idea of stabilizing wood is that all of the air is removed from the wood and replaced with a resin that is then cured. To do this, the wood is placed into a vacuum chamber and submerged in a stabilizing resin like Cactus Juice. The vacuum pump is turned on generally for a few hours, the air is sucked out and then the seal on the vacuum chamber is broken and the vacuum pump is turned off. Since the wood is submerged, the stabilizing resin starts to soak into the wood replacing all of the voids that used to have air. (This process is usually done over night.) After you are satisfied that the wood has soaked long enough, it is then placed in a toaster oven and cured until the internal temperature has reached 190-200°F. (Generally a few hours.) At this point, the wood that was previously soft, punky, rotten or just crappy in general is now 2-4 times heavier and much harder.

Casting Wood in Acrylic

After the burls are stabilized and cured, they are cut down fit into a non-stick form. There are many different shapes and sizes of forms as well as material they can be made from. In a pinch, plywood or MDF would work but you’ll have a lot more work removing the castings. My forms are all made from HDPE plastic because the acrylic resin doesn’t stick making it very easy to release the casting. Before pouring the resin into the forms, the two part resin is weighed, mixed together in equal parts and optional dyes & powders are added to give the clear resin some character. Before the resin begins to cure, it is placed into a pressurized container (pressure pot) to leave it curing between 40 and 60psi. This pressure reduces the size of air bubbles inside of the resin making them virtually  impossible to see.

A few hours later when the casting has cured, it can generally be removed by taking one side off of the form and giving it a good smack on the table. If you used another type of non-stick material as a form, you’ll have to work a bit harder to get a release. The hybrid castings are cut down to the required sized blanks exposing the irregular shape of the burls inside.

Additional Resources

Ok, so that was a lot more detailed than I expected… Maybe I’ll shorten it up after I get a dedicated stabilizing article out. If you want additional information on the topic of stabilizing and casting, I have found that https://www.turntex.com is a great resource for both stabilizing and casting info.

Jointer Knife as a Blade

Jointer knives/blades are generally made out of high speed steel which is a very hard material. There are a few other consumables in the shop that are also a hardened steel and can be repurposed so keep that in mind before tossing out those old blades!

Grinding the Tip & Keeping its temper

In order to use the jointer knife, it’s sharp edge needs to be removed which will require a great deal of grinding or sanding which will generate quite a bit of heat. The last thing you want to do during this step is to get the metal so hot that it loses its temper (hardness). The best way to  maintain the high level of hardness is to prevent it from getting too hot in the first place.  To do this, you’ll need to dip it in water every few seconds keeping it cool and preventing it from getting too hot during the sanding/grinding process.

To help when forming the tip, you can draw some angled lines for reference on your sanders table or use a miter gauge if you’re sander is equipped with one. (I drew and set mine at about 35°.) To bevel your blade, you can either go as far as your table will let you or do it by hand. My table only went to 45° so I had to finish it by hand to get it to a 25° bevel. It was a bit tricky getting a perfectly flat 25° bevel while constantly going from the sander to water so next time I’ll definitely be making a jig to help keep everything straight.

To sharpen the marking knife, it’s really a personal preference. I’ve been through my fair share of sharpening methods and I’ve been sticking to a diamond plate and leather strop.

Assembling the Marking Knife

At this point, you should have the hybrid knife scales, the marking knife blade ground to a point and sharpened and some brass pins ready to go. I haven’t talked about the pins yet but you can generally pick up a small rod at your local big box or metal supply store. The ones I got were 5/32-inch diameter and about 12-inches long. If you don’t want to drive around looking, you can also order them from here: http://amzn.to/2bDy3wT. To use the brass pins, you’ll need to cut them to length and drill through both the hardened blade and the knife scales.Drilling through high speed steel with carbide bit

Drill Through High Speed Steel

To drill through HSS, you’ll need a carbide bit which isn’t always the easiest to locate locally. The specialty store sold one bit to me proclaiming it was made to drill through HSS but it didn’t even scratch the surface. Neither did my center punch. After being unable to even put  a mark on the blade, I was in a hurry to get a new drill bit and started calling industrial supply stores. I found one at Grainger but paid a little extra for my impatience but also had no issue drilling through the hard metal. If you have a  little more patience than I do, you can order this one from Amazon for about $9 less.

Using brass pins to align knife scales while drillingAligning Knife Scales with Blade

After drilling the holes through the hardened metal, the brass pins will come in handy to align the knife scales to the blade.  After transferring one hole from the blade to the scale, you can insert a brass pin in that hole and then align the scale to the blade for the rest. When doing the opposite side, just push the pins through and play a little musical chairs with them until everything has been drilled out.

Gluing, Clamping & Finishing

If you want to do any shaping on the front of the handle where it will meet the front of the blade, now is the time to do it because the next step is to get everything epoxied and clamped together. I just use the every day 5-minute epoxy and try my best at covering the blade with painters tape to help with squeeze out. The rest of the squeeze out  will be cleaned up on the sander when doing final finishing. After the epoxy has had a good chance to cure, you can cut or sand the brass pins flush and then give the rest of the handle a good sanding to remove any epoxy squeeze out and to shape it how you want.

After getting the shape you want, move on to hand sanding through the grits and using polishing wheels on the blade if that’s your thing. Through this whole process, make sure you’re careful with the blade because if you remember, we sharpened it a while back before sticking the scales on. Since the blank is partially acrylic and the rest is stabilized wood, finishes that need to soak in won’t work so well so you’ll want to pick one like polyurethane that creates a thick film on top.

I hope you enjoyed this article because I had a good time figuring out this entire process. If you end up making something similar, send me some pictures so I can share them on my social media!

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