its a simple modification of a tongue and groove joint Rob. figure 1/4" for your tongue and 9/32 for your groove depth so your tongue is 1/4x1/4 and your groove is 1/4x9/32.
This little added depth is going to allow a tight fit at the edge where you want it. Also, space the tongue and groove accordingly.
Although this is a good system, and I have used it on several occasions one has to be a bit more particular in the faceframe and box construction as there is less margin for error. Also, you cannot used this system if you are going to go with a prefinished cabinet interior which imo is far more labor intensive than the time savings on this joint. Typically with an 1-1/2" stile I dimension the box to be 1/32-1/16 wider than the opening dimension of the ff. Given that plywood runs about a 32nd undersize this allows for me to skin a piece of finish grade plywood that is species specific to the project and if its a little fat then I can belt sand the back. then edge glue to the ff and run some 1-1/4 screws from inside the box. also this is only needed on your finished ends all the rest, having that extra space between the outside of the box and the edge of the ff gives you more than enough fudging room for wall variances and allows for a tight joint when attaching cabs together. Another plus to dimensioning the box to the opening is in the installation of hardware such as euro hinges and drawer slides. There is no labor to having to pad out the sides. And when installing partitions run the face to one side so that only the other has to be padded.
When you are doing several cabinets all this time savings adds up pretty substantially. And gives you more margin for error.
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