Hi @orob, you don’t want to use screws into the edge of an mdf board under most circumstances since it splits so easily. #1 here is what I used… just including #2 FYI
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The pattern I cut had tabs on one side and matching grooves halfway through the board on the other side… you put it together with some wood glue and clamp it (No screws). Wood glue is so good these days the joint will be stronger than the mdf.
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If you are in a situation where you want to join two mdf boards 90 degrees and you don’t have a groove you can cut a small board out of hardwood or pine and put it in the inside corner where the two meet, screwing through the face of the mdf into this board (also using glue). There is a pilot hole drill bit called a countersink that can make your screws flush with the wood then you can add wood glue and sawdust to cover the screw and sand flat.