This would be the basic set up with all parts collapsed. There are basically 3 pieces plus the locking devices. With all parts collapsed the offset between the top 2 parts and the bottom part determines the maximum blade width.
You would set the Kerfmaker to your blade or cutter width by sliding the middle part out until the offset between it and the bottom part matched your blade and then lock it down. I used a 1/8" saw blade for this example.
You would set the device to your material thickness by sliding the top two parts (which are now locked together) until the gap between them and the bottom part matches your material and then lock them down. My example shows a 3/8" material thickness.
This shows how the device is butted against the stop for the first cut. Notice that the stop has to be narrow enough that the stop will slide into the notch created by the offset of the device. In my example I show a cut at the very end of the Kerfmaker for illustration purposes. You can set it up to make a cut anywhere along your board.
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