Make a Marking Knife

Geoffrey Noden Posted on Feb 27, 2012 12:39 PM

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This article is from Issue 45 of Woodcraft Magazine.

Give new life to an old jointer blade.

When it comes to scribing tenon shoulders, dovetails, hinge mortises, and other joints and recesses, it’s hard to beat a traditional V-shaped marking knife. Beveled on only one face, the opposite one can register intimately against a square, straightedge, or other reference surface for a precise layout line. The V shape permits pulling the knife in either direction and using it right-handed or left-handed. Unlike a pencil line, a scribed knife line cleanly severs wood fibers in advance of a saw cut and provides accurate registration for a chisel when paring to the line afterward. Compared to handled marking knives, I find that a simpler flat-bladed version is easier to register against broad surfaces and better suited for reaching into tight spaces.

Commercial marking knives can cost upwards of $40, but you can make one yourself in an hour or so. All you need are some basic sharpening tools and a piece of tool steel about 1⁄16" thick. Good tool steel is available in the form of old high-speed steel planer or jointer knives that have become too narrow to fit the machine. If you don’t have any, you might be able to get one from a woodworking pal or friendly local sharpener.

Tooling up

The only tools you’ll need are a grinder and honing stones. A standard 3,450 rpm grinder and grey wheel will work, but you’ll greatly reduce your risk of overheating the metal by using a half-speed (1,725 rpm) grinder with a soft-bond aluminum oxide wheel. Also, it’s best to use a grinder tool rest that’s notched out to accommodate the wheel, providing good support for shaping the knife.

Remove the bevel

First, grind away the jointer knife bevel. Set your grinder rest to horizontal at the height of the wheel arbor. With the knife lying bevel-side up, begin grinding away the bevel, moving from one end to the other at a steady rate, as shown above. (If your fingers get uncomfortably hot, you’re applying too much pressure against the wheel and/or advancing the knife too slowly.) Take as many passes as necessary to completely remove the bevel. Then ease all of the sharp edges with 220-grit carborundum paper for safe handling.

Nose to the grindstone
Square off the beveled edge, holding the knife firmly down on the tool rest and applying moderate pressure against the wheel. Grind from one direction instead of back and forth, to let the steel cool between passes.

Form the point

Shape the tip next. I shorten the knife at the same time (to about 51⁄2" long) by grinding two opposing V notches. Once you’ve separated the two pieces, refine the point by taking short back-and-forth passes against the wheel. Make the angle slightly less than 90° so the point can completely access 90° corners. Use light pressure to avoid burning the metal at the tip, where it’s most susceptible to heat.

There’s no need to get too fussy about the exact angle of the two bevels. I grind mine somewhere between 25° and 30°, so set your grinder rest angle in that neighborhood. To help you grind parallel to the sides of the point, apply masking tape to the tool rest to serve as a reference. Hold the knife parallel to the tape, and grind in short side-to-side strokes, keeping the entire cutting edge against the wheel as much as possible while maintaining the knife angle.

Gradually work each bevel in turn, grinding a bit from one and then the other until they meet in the center. To prevent burning, reduce the pressure as you approach the finished edge, monitoring the heat by touch between grinding strokes. If it’s too hot to touch, you can quench the metal in water to cool it, but I find that using a light touch on a half-speed grinder outfitted with a soft-bond wheel keeps me out of the heat danger zone.

Hone the bevels

Using your chosen honing system (I prefer waterstones), remove any scratches from the back of the knife within an inch or so from the tip. I start with an 800-grit stone, rubbing away any grinding marks until I have established a consistent 800-grit scratch pattern. I follow up with a 1200-grit stone, and then finish up by polishing on an 8000-grit stone.

Next hone the bevels. On a freshly ground knife, you can begin with a 1200-grit stone, and finish up with 8000 grit. Grasp the knife in one hand, pressing down firmly against the back of the bevel, and then pull the knife toward you. Repeat several times; then check the bevel for a narrow polished band that extends completely across the edge. Avoid pushing the knife back and forth, which invites rocking that creates a rounded bevel face.

When you’re done, you should feel a tiny burr completely across the edge on the back of the knife. Rub the back of the knife on only the finish stone to remove the burr; then drag each bevel across the stone again, finishing up by rubbing the back one last time on the finish stone.

The Burning Blues

It’s not uncommon to get the blues at the grinder. That’s the color you don’t want to see on your metal because it means you’ve “burned” it, drawing its temper so it won’t properly hold an edge. If you burn the metal in a critical area (such as a cutting edge), your best bet is to grind away the affected area and start again.