An Easier Kerfmaker

Someone showed a group of us a simple kerfmaker he built from scrap. I was reminded of a nice aluminum kerfmaker Instructable posted by Steliart and I thought about building it. This Instructable describes an easier kerfmaker I built from some hardwood. It is not a disposable kerfmaker fitted for only one tool. The precision sliding parts are so easy to make well that it is almost foolproof.

A kerfmaker is initially set by you for the thickness of your cutting tool (saw blade, router bit, bandsaw blade). From that point, it allows you to set the kerfmaker for the thickness of a piece of work so you can make a precise dado for that piece without further measuring.

Materials

• 3/4 inch oak
• #8 screws, nuts, and a Tee-nut
• Tempered fiberboard
• Glue
• Paraffin

Tools

• Table saw
• Plane or sanding drum and small table for smoothing and truing
• Drill and bits, including countersink bits
• Router and bits, router table
• Clamps
• Hammer
• File
• Pliers

I ripped some scrap oak to strips 1/2 x 3/4 inch. (I would use 1/2 x 1 or 1 1/4 inch next time, if possible.)

I smoothed and trued the pieces. (second photo) I have a 2 1/4 x 3 inch sanding drum on my radial arm saw. I often use it and a small table to push pieces under a sanding drum. The table acts like the bed of a plane. The pieces are smoothed and straightened at the same time. It is important to keep the work moving so it does not burn or get cupping. That means several light cuts and flipping the workpiece often.

The length of the pieces is determined by the width of the largest dados you will ever cut. One pair of pieces are around seven inches long, each. The other pair are about an inch shorter. I could make a dado almost as wide as a 2 x 4.

The two longer pieces will be glued face to face, but also need a screw slot down the center. I used a router in a table to remove half of the #8 screw's thickness from each face. Remove about 3/32 inch from each inner face, but leave some of the face untouched near both ends as shown in the photo. (My router bit is old and leaves burn marks.)

I removed about 3/16 inch of material from what will be the top surface of the two longer pieces. I could have used the router, but decided to use my homebuilt table saw. I moved the miter gauge during the process and worked from both sides of the blade to keep my fingers farther away from the spinning blade.

I used a bevel or chamfer router bit to make an angled surface on one edge of each of the four pieces. When the pieces are glued together in their proper order, one set will make an external "V" and the other will make an internal "V." (See the second graphic.) This makes two parts of the kerfmaker that slide over one another in a very precise and smooth manner without the multiple operations involved in making and fitting a raised surface and a corresponding recessed surface and then making it all fit closely. I carefully marked the end of each piece on one end with a diagonal pencil line so I would not become confused and make a cut in the wrong direction. (I have another fence I made that encloses the bit while it is spinning, but my wife cleaned my workshop and I have not found it again.)

You will need to trim a tiny amount from each glued up assembly to make certain the ends are square and smooth..

I cut a piece of tempered Masonite 1 x 1 1/4 inches for the end of the longer slotted piece. I glued and clamped it to dry.

See the second photo. The two main pieces are resting one on the other. I added a screw to add extra security to the glued joint. The head is countersunk, but I still needed to file and grind a little to make sure the end is smooth.

See the text box in the second photo. While the nascent kerfmaker was in this position, I drilled a hole for a #8 screw vertically near the closer end of the screw slot from step 2. I drilled a countersink for the head on the underside of the bottom piece.(My photo of this was very blurry. See one of the text boxes in the photo from the next step for a photo that includes the screw. When viewing that photo and text box remember that the screw appears to be at the far end because the piece has been turned end for end.)

For smooth operation, I rubbed a little paraffin on the mating surfaces.

The soft jaws on the vise are from another Instructable I posted.

I cut the bottom piece so it is about an inch shorter than the upper piece. (See the very last step for dimensions.) I am using a Tee-nut for an 8-32 screw. I drilled a hole straight into the front end of the bottom piece for an 8-32 machine screw 1 1/2 inches long. I expanded the hole near the surface to 1/4 inch in diameter to accommodate the threaded portion of the Tee-nut. In order to make certain the screw aligned with the longer hole inside the bottom piece, I threaded a substantial amount of the screw into the Tee-nut. I tapped the screw enough to start the prongs on the Tee-nut. (Be careful, tapping too hard makes it is easy to damage threads on either the screw or the Tee-nut.)

I removed the screw and drove the Tee-nut home. This aspect of the kerfmaker allows very precise adjustment for the thickness of the cutter. The screw acts as an adjustable spacer. It can be extended or retracted a few thousandths of an inch and locked with a hex nut against the Tee-nut. Making it is much lower tech. than what usually goes into a homebuilt kerfmaker, but it is also very easy to do.

I used a file to grind the Tee-nut flush with the bottom surface of the kerfmaker.

I am setting the kerfmaker for the saw blade I normally use on my table saw. Kerfmakers can be used with dado blades, router bits, and saw blades.

Place a straight piece of wood against the saw blade. Use a square to align the kerfmaker with the straight piece of wood. Be sure the lower piece is slid tightly against the Masonite. Turn the adjusting screw so it just touches the near edge of a sawtooth. Tighten the locking nut without allowing the screw to turn out of adjustment. Check again to be sure the setting is accurate. (If the screw is adjusted too tightly against the sawtooth, it will be as if you erroneously made the setting for a thinner blade and your dado will fit more loosely. If the screw is adjusted too loosely against the sawtooth, it will be as if you erroneously made the setting for a thicker blade and your dado will fit too tightly.)

If you use only one cutting tool to make dados, you can leave this adjustment undisturbed forever. Otherwise, you will need to make this adjustment each time you use a different cutting tool.

You are making a dado to fit closely around a shelf or a cleat. Fit the kerfmaker to the wood for the shelf or the cleat. Tighten the adjusting nut with your fingers. Without allowing anything to move out of adjustment, tighten the adjusting nut a little more with a pliers. (There really is not enough space for a wingnut.)

I have made a pencil line where I want the left side (as viewed) of the dado to be, and I have aligned it with the saw blade so the blade cut will be inside the dado.

The dado I am cutting will open to the right of the pencil line in the photo from the previous step. Regardless the actual size of the two parts in the kerfmaker, look at it now as adjusted for the thickness of the wood that will fit into the dado. The half with the adjusting screw is longer than the side without the screw. I am setting a stop clamped to the saw table. The short side of the kerfmaker sets the space between the 2 x 4 into which the kerf is being cut and the stop. Make a saw cut.

Turn the kerfmaker end for end. The long side of the kerfmaker now sets the distance between the end of the 2 x 4 and the fixed stop.

See the second photo. The outside edges of the two saw cuts define the dado. Now all that is left is to cut out the waste between these two saw cuts.

Pictured is the result of sizing a dado using a kerfmaker. It fits acceptably well, but a little more loosely than I expected. I did discover the very tip of the sawtooth was able to dip into the recess for the screwdriver in the screwhead and that distorted the accuracy of the set up in step 7. I could have filled the screwhead with a bead from a welder, but not everyone who reads this has a welder.

See the second photo. I cut the head from the screw with a hacksaw. I laid the screw end on a piece of steel and flattened the threads a little at the end of the screw with a hammer. I turned on a nut from the other end and jammed it onto the slightly crushed threads. I ground the face of the nut and screw flat to be flush with one another. I went back to step 7 and set the screw for the thickness of the sawblade.

See the third photo. I set the kerfmaker for this piece. My dados were still a little looser than I expected, although acceptable, so I turned the screw a third of a turn into the kerfmaker (made the screw shorter) and locked the retaining nut. The dado in the third photo is nicely snug like I was expecting it to be. A third of a turn is a tiny amount of difference. I am finding it is easy to be a tiny bit inconsistent in how much pressure is applied when fitting the kerfmaker to the piece that will fit into the dado in step 8, or when adjusting for the cutter thickness in step 7, or even when pushing the kerfmaker against the stop or the work against the kerfmaker when making the cuts. Each of these changes the snugness of the fit in the dado. (If the dado fits too snugly, there is less room for glue on all sides of the dado. Much depends on your needs. I will probably make a test dado or two if I need a special fit.)

The photo shows superimposed dimensions taken from my kerfmaker for anyone who wants a starting point in building his or her own.

Here is a very complete video on using a commercial kerfmaker.