A commercial version of this tool is available for sale, but I wanted to try my hand at 3D printing a version. My version is more compact than most - this limits somewhat the width of dados that can be cut, but I expect that it will work in almost all practical cases. My version is also 100% 3D printed, and uses cams instead of screws  to lock the sliders in place.

See here for a wooden version, and an explanation of how it works :

https://www.christofix.com/diy-kerfmaker-perfect-dados-grooves-free-plans/#how-to-use-this-kerfmaker

Assembly

Note that one end of the axle pieces is slightly enlarged (to make a tight fit for final assembly). Refer to the photos to see how the pieces go together.

Start by inserting the Kerf Block into the matching dovetail groove (not the proper direction). With the Kerf Block in place, insert one of the Cams with the lever pointing away from the cam stops, and the thin side down. Insert the thin end of one axle half fully to capture the Cam, and then insert the second axle half to lock the axle in place. Test the Cam to ensure that it will lock the Kerf Block in place when rotated against the stops.

Insert the Base Block and Cam in a similar manner.

Note in the photos that I marked “matching” faces of the unit with magic marker to show which faces are to be used to align a workpiece.

Use Instructions

See the link above for more detailed instructions. Briefly [See also photos]:

1.  Set the blade kerf width by loosening the appropriate Cam and setting the width against the cutting blade before locking the Cam (a more accentuate method is to make a test cut, but the method show works for most purposes).
2. Set the Base Block by “measuring” the width of the piece for which you are cutting a dado, and then lock the Cam in place.
3. You will need to make a stop for use with your saw fence; I just used a scrap of wood clamped to the fence. Note by “fence” I am referring to the rear “wall” of my table saw sled.
4. Align the two outer cuts of your dado as shown in the photos, and then remove the waste between the cuts.

Photos

1. The completed kerfmaker
2. Measuring and setting the blade kerf width
3. Measuring and setting the dado width
4. Setting up for first cut
5. Setting up for second cut
6. Completed test dado
7. Faces marked with felt tip pen for first cut alignment
8. Faces marked with felt tip pen for second cut alignment

Print Instructions

The print consists of the following pieces :

• Dado block
• Kerf block
• Base Block
• Axle half (4 needed)
• Cam (2 needed)

Print in PETG using the 3mf files provided; in general :

• 3 perimeters
• 40% triangle infill
• fill angle 90 or 0 (see below)

The Dado Block is the only piece that requires supports. Supports are needed under the cam stops in the bottom half. In addition, I added a long thing support running the length of the “arch” in the bottom half, as otherwise the top of arch tends to sag a little, and this impacts the fit of the Base Block which slides along it.

Note : The reason for setting the fill angle to 0 or 90 is make sure that the mating faces of the sliding blocks have their top fill angle at right angles to the direction of sliding, so as to maximize friction when the sliding pieces are locked in place. 

CAD

The OnShape 3D CAD files for this are here :

https://cad.onshape.com/documents/fed710b9013371a2d2ebe4d5/w/e34268c7d84eb6fd80f96f43/e/8231086df62902475337ac8f