I would suggest making an adjustable mortise jig for the router. I made this one recently, to make the 40 or so mortises for a dresser. In my opinion it was a few hours well spent, to allow easy and consistent mortising. You can easily find plans on most of the major woodworking magazine sites.
It's basically an open box base, with a fence that holds the workpiece, and a small platform for the router that slides forward and back on two parallel wood strips. The platform is only attached by a t-bolt in the forward/back adjustment slot and a locking knob. The platform has a hole cut out along the path of the router so you can see the mortise.
You cut matching dadoes in the top of the base and the sliding platform, and put the wood strips on the base. There's a fixed stop on the right, and an adjustable on the left. You will find some plans that have you attach your plunge router base to the platform. I simply made two parallel fences that keep the router steady. I made a platform for both of my plunge routers.
Three helpful comments before you start.
1- Use a bit the size of the mortise. It's much easier than making multiple passes to get the proper width.
2- Mark all the mortises from the face of the board. Since this jig references off the edge of the bit, that front mark needs to be consistent.
3-Extend the layout lines for the length of the mortise to the edges of the workpiece. This allows you to line it up with a mark on the jig that shows where the bit starts cutting.
How to use it.
Set your plunge router for the finished depth of cut. Make sure you zero the bit while it's on the platform, otherwise you'll be short the thickness of the platform.
Using a piece of test stock, marked with the mortise width (don't worry about the length), put it face towards the jig (away from you).
Move the platform forward or back, and lock it down when the bit looks like it aligns with the face side (toward the jig) layout line. Make a short cut and adjust the platform forward or back until your cut is at the face side layout line.
Move the router to the right side fixed stop, and make a small plunge just to see where the right side of the bit starts to cut. Mark that on the base (or the platform). This is where you will line up the actual workpieces.
Make a test mortise in several passes of increasing depth (about 1/8" or so, you'll feel what the bit can handle). Check alignment, depth, how the router cuts as you plunge, etc.. If everything went well, move on to your workpieces.
With your workpiece, face towards the jig, align the right mortise layout line with that right side start mark. Next, slide the router to the left until the bit lines up with the left end of the mortise layout line, and set the left stop. Plunge in several passes until you get to the finished depth.
If you have a spiral bits, they make it easier than straight bits because they cut better as they plunge. Straight bits work fine, but since the bottom doesn't cut as well, you have to take smaller passes. It's not a huge issue. I cut 24 1-1/2" deep mortises using a 1/2" straight bit and it was just fine. It just took a little longer than when I used the spiral bit.
I hope this helps.
Last edited by sanchez; 10-02-2020 at 04:29 PM.