Turn defects into details
Not every face frame and cabinet case go together like a match made in heaven. The frame may slightly overlap the case sides, or hairline gaps may emerge after the glue-up. That's when you need the perfect solution for an imperfect world: a flush-trim V-groove router bit.
These bits look and work much like straight flush-trim bits with one exception: A V-shape groove-cutter extends about 1/16" past the bottom-mounted pilot bearing and cutters, as shown below. This V forms a groove on the joint line between the face frame and the case; the rest of the bit trims the frame edge flush, as shown right.
Grooves hid cracks
On cabinets with a clear finish, the groove helps mask hairline gaps. On painted cabinets, it works like an expansion joint in concrete, concealing cracks that inevitably form between the face frame and the case, as shown right. Such a shallow groove won't interfere with pocket-hole screws or other fasteners.
Anatomy of the bits
Bits from most manufacturers use cutters running straight up and down. If you're working with figured wood or species prone to tear-out with straight cutters, choose a bit with an angled (down-shear) edge, right, that slices as it cuts for smoother results.
Find your groove
Size the face frame or rough-cut it so you trim off less than 1/16" of waste to avoid tear-out. Place the router base on the face frame, as shown above, and adjust the bit depth until you center the groove over the joint. Check that the bearing rides against the case.
Then start the router and move it counterclockwise, trimming first along the end grain on the frame stiles. Routing the sides last removes tear-out. To smooth any rough spots, use 220-grit sandpaper wrapped around a hardwood scrap block.