Part of the problem may be grain orientation, rather than the bit. To test it out, try the same thing on the short edges of the boards in the pictures.
I can't improve on the above member's advice.You are routing along the edge of the grain. Thats wrong 99 times out of a 100 (Maybe even more).
Box joints are for end grain, especially if you are using a router.
Jointing a long edge would usually be with glue or splines.
Tell us what you want to build for more detailed help.
HARRY, I sure would like to see your post about routing tips. Not being very right nor sound of mind I need a lot of help. Question, could you use a router and work against the grain, using a small bit Making more than one pass to get the width you want or need?I can't improve on the above member's advice.
I don't like using the router for box joints. Instead, I popped for the Ibox jig and the two blade set mentioned by CharleyL. If you want to do box joints on the long grain, you could probably do it with the Ibox, but I'd make sure you used a sacrificial piece behind the workpiece.
I love the Ibox jig (below) because it can work with any blade width you set up in a dado set. But for end grain boxes, the Freud double blade set will cut 1/4 to 3/8 th is a pleasure. The jig has a simple adjustment you can dial to any blade width.
I can see doing the pins and sockets on the long grain edge if you were making a tall box. If using ply, I'd go with Baltic Birch or Apple Ply. Its a nice way to make drawers
BTW, welcome to the party.
Pictures. Incra Ibox jig on table saw. Freud SBOX8 blade set. Diagram of box joint (pins and sockets)
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