A Mortise Jig for a Router for Flush Rails

[TomInNC]

I am finally at the joinery stage of a grill cart project I have asked about on here a few times. The cart legs are 4x4 posts, and the rails are 3 inches wide and 1.75 inches thick. The design I went with requires that the rails are flush with the legs. There will be a bunch of mortises to cut (28, to be precise), all of which I am making with a plunge router and a guide bushing. To make things as  repeatable as possible, I want to build a jig that is the full length of the leg that allows me to make the mortises for the top and bottom rails without repositioning the jig. I aim to construct the jig in such a way that it runs the full length of the legs and slips over them with some shoulders (e.g, the second video linked).

The width of the mortise is wider than the width of the bit, so I will need to take multiple passes. The rails need to be flush with the leg, and the rails are a different thickness than the legs. So the left side of the top of the jig (e.g., 5:39 in the Cremona link below) will need to be sized in such a way that the rail is flush. 

I have used jigs similar to what you see here in the videos, but in all previous projects where I did this, the rails were actually inset from the edge of the legs. Have any of you seen articles or videos about building jigs for building a mortising jig for a router with a specific emphasis on getting the rails as close to flush as possible?

My plan at this point is to just take very careful measurements when figuring out the width of the dimensions that I need for the spacer in the jig. Given the sheer volume of mortises, however, I would feel more confident of what I am about to do if I knew my general approach was sensible. I am envisioning a nightmare scenario where I make 24 mortises, cut 24 tenons, and realize I'm an 1/8 out of flush and spend the next 2 months sanding. 

 

 

 

 

 

 

 

 

 

[Tpt life]

32 minutes ago, TomInNC said:

This is the value of test pieces. Almost every machine setup requires a verification trial for me, before I mill work pieces. 

[Dave H]

I make my jigs very similar to the video with gluing up pieces of wood to make the jig but I only use one fence on the face edge I have a center line on the inside of the jig my jig is sitting on top of the coffee table

with this jig I made the joinery for a bread board edge. If you register the edge off of the face edge of your leg and the face of your rails and you locate your center lines of your mortise every thing should align perfectly my method you only need one jig, I only clamp the fence of the jig to the faces of the work piece and usually put it in the my bench vise to cut the mortises. I couldn't do that with the live edge coffee table I had to stand the table on it's end and cut the mortises standing on my 2' ladder.