Here is an effective method for making mortises with a plunge router and a simple, intuitive jig. I find it particularly useful for mortising table legs.
Why it works
In mortise and tenon joinery, with few exceptions the mortise is made first, then the tenon is cut and adjusted as necessary to fit it. Thus, it is advantageous to machine mortises with repeatable accuracy. The strong point of the setup I will describe here is that it uses two fences, each snugly aligned to opposite sides of the work piece as shown below, to very reliably control the path of the router parallel to the length of the work piece.
I use an older model, clear plastic jig, shown here, made by Woodhaven that still works very well. Their currently available version (#145H, $34.95) is the same basic idea but made of aluminum track. It must be attached to an adequately sized base plate according to the manufacturer's instructions.
A few words are in order regarding safety because woodworking is potentially dangerous and can result in grave injury, even death, especially when power tools are used. I show a method that I use in my shop. Ultimately you must decide what is safe for you to do. Please learn proper safety procedures from the many sources available, read and follow the manufacturer's instructions for using your tools, and never proceed if you have any doubts about your safety.
Layout and setup
For mortising a set of legs, I lay out the first mortise on one leg and set the router (unplugged) and jig directly from that. These settings then obviate the need to layout the other legs. Make the mortises while the leg stock is still rectangular, before any shaping of the legs.
First, the mortise depth is set. Various routers have different mechanisms for this but the one shown here is typical. The upcut spiral bit is chucked in the router. The height stop is set so the tip of the bit is about 1/4" above the wood surface. Plunge to bring the bit just in contact with the wood surface, then lock the plunge lever and bring the plunge stop to rest on the lowest turret step, as shown in the first photo below. Zero the depth adjustment scale. Then use the scale to elevate the plunge stop to the desired mortise depth, here 1 1/16", as shown in the photo below.
Now set the right side fence so the outer edges of the bit align with the mortise layout as in the first photo below. Then bring the left fence to snugly meet the wood as in the next photo. Note that accurately dressed stock is a must for this method.
Of course, end stops are necessary to define the length of the mortise. Register the position of the leg blank on the bench top with a stop block, then secure it with the tail vise and a bench dog. Then position the bit at one end of the mortise and clamp a block to the bench to limit the base of the jig at that point. Do the same for the other end of the mortise. These end stops remain in place so when each successive leg blank is positioned against the leg stop block, each mortise will be located in exactly the same place.
Below, the right side stop block meets the base plate to define that end of the mortise.
Caution: The photos show the bit lowered to contact the surface of the wood only to make the settings. Before plugging in and starting the router, the motor is raised to meet the height stop so the bit clears the wood by about 1/4", as noted above. Never start the router with the bit in contact with the wood!
Make the mortise
The mortise is cut in shallow passes. For this 5/16" wide x 1 1/16" deep mortise in a light hardwood, I would typically use about eight passes with the motor speed at 18,000 - 20,000 RPM. However, it is important to listen to the router while working (with ear protection) to sense if it is being overtaxed. The RPM, depth of cut, and lateral advancement speed are altered as needed.
I plunge the bit, lock the lever and push the router. At the far end, I release the lock, plunge again, relock and pull the router back, and so forth. The intermediate steps on the turret are helpful to avoid accidentally plunging too deep but they are spaced too far apart so I simply estimate smaller plunge depths within the built-in steps. The end point is when the depth stop meets the lowest turret step, which was used in the depth adjustment process.
The result is a very clean, accurate mortise, precisely placed.
Narrower stock such as members of a 3/4"-thick frame to be mortised with this method should be clamped to a stout auxiliary piece to increase the width of support for the router and jig.
Like all mortises produced with a router, the ends are rounded. The ends can be squared with a chisel, but it is easier to simply round or chamfer the edges of the tenon to fit.