bos joint jig - need help

Gang-
Made a very nice box joint jig with micro-adjustment feature. Very nice. Using this on a router table with a PC895 router and 1/2" straight cut bit. Taking about 1/8" - 1/4" off with each pass. Stock is 1/2" oak plywood for drawer sides and backs.
Problem - Even with a backer board, I am getting a lot of tearout on almost every cut. Is it the stock, the bit, the router speed, what? The bit is almost brand new, speed is roughly16K rpm.
These drawers are for the router table itself, so they do not have to be perfect. But shop equipment projects are my "live and learn" pieces before I try something real for the house.
Thanks for all input!
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bit.
almost
Cutting straight into the side of a board with a straight bit can cause tear out on both the front and back side of the cut. Is all the tear out on one side of the board? If you find it on both sides you may want to try a backer on both sides of the board. Also, it sounds like your almost brand new bit may not be sharp any longer or your feed rate may not be slow enough.
With than in mind, I use a spiral bit for cutting box joints with my router and have no tear out to speak of.
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