Routing With A Template

Each of the doors has a curved groove that accepts the outside panel . It would be impossible to route this dado accurately by hand, so you will need to make a template that will guide the router's cutting bit along this line. These grooves are 3/8" wide, and approximately 1/2" deep .
There are two ways of using a template to cut an irregular dado like this. One way is to use a bearing guided bit like the ones shown earlier in this blog, except the bearing would be above the bit as opposed to below it. But a better way for this application would be to use a guide bushing that will allow your cutter to fit through it. As the guide bushing rides along the template the cutter makes a groove just off set from the template. To determine the amount of off set, you take the size of the bushing's outside diameter (5/8"), and subtract the dia. of the cutter (3/8"). Now you have determined the bushing is 1/4" larger than the cutter, you divide this in half . This will give you the off set of one side (1/8"). Knowing this, you will need to make the template Off set from the groove you want to make by 1/8".
Given the depth of the dado you are making, it will take a few passes to reach the full depth of 1/2". Guide bushing sets are available with about six or seven different size bushings to accomidate a variety of cutter sizes.
Most bushings will require your template to be at least 1/2" thick. Also another good way to determine the off set of your template is simply to set up the cutter in a bushing that fits closely but does not touch, attach a fence to a sacrificial piece of wood, make a shallow pass. Ensuring the bushing is staying in contact with the fence, as it will sometimes want to wander depending on the direction of your cut, measure the distance from the edge of the fence to the edge of the dado. This is your off set.
This technique is useful whenever you must route a dado that does not run parallel to any side of the piece being worked.