Routing a groove that matches the size of a router bit is a straightforward task. Just use the fence to line up the bit with a layout mark and make the cut, as you can see here. However, there are times when you need to cut a custom-sized groove that doesn't match any of your bits.
It would seem that all you need to do is adjust the fence and make another pass, right? Well, almost. There’s a catch. You want to make sure to adjust the fence in the right direction. A good rule of thumb is to always keep the material you’re routing to the outside of the bit and fence. To widen a groove, be sure to move the fence away from the bit. This way, you’ll feed the workpiece into the rotation of the bit. When the fence is moved closer to the bit, the waste is trapped between the bit and fence, which is a backrouting operation. And the bit will try to pull the workpiece out of your hands.
Another custom cut I often find myself making is a centered groove, I used to try and measure to get the bit perfectly centered on the workpiece. But the results were never quite accurate. Thankfully, there’s an easier way that doesn’t involve measuring at all. And that’s to do the job in multiple passes. Here, you start with a bit that’s narrower than the final width of the groove you need to make. Next, position the fence so the bit is slightly off center.
Then simply follow the advice earlier for widening a groove — with one difference. In stead of moving the fence, flip the workpiece end for end. I usually make slight adjustments so that I can sneak up on the final size.
