I was contemplating even attempting this. I knew I didn’t want to do Ng’ jig as I don’t have guide bushings and I figured I’d messup the mortise for it anyways.
Here is my version and it works incredibly well. I hope it can help some people and ease some tension on this critical step.
The first step is to secure some quality ply, the router and bit of choice and some patience.
The jig is constructed similarly to an “Exact width dado jig”. Make two of these guides.
Select the stock for the splines, and place it between the two guides. Clamp firmly but not so much as to compress the fibers. Next grab some scrap and screw it onto the sides to hold this position as in the photo.
This sets the width of the mortise.
Onto the “centering” portion to locate the jig and make sure the mortise will be centered on the edge. It took me a second to figure out how to do this but then I had an “aha!” moment.
Grab a scrap piece of hardwood and set the table saw height to cut about 1/2” depth. Set the fence so the blade takes off an 1/8”. Cut one side then flip. Move the fence in incrimentally and keep flipping until the tongue fits into the slot on the jig with as little play as possible.
Then grab two small scraps, one for each side, clamp them firmly to the block of hardwood and attach to the base. This acts as not only a clamp, but a stop. This, as well as the side stops, will be custom to your splines. Once you have a length, flush them to one side.
As a side not, when you make the “Exact width” portions, mark an arrow on the edge that you used to ride the rail. Use this as a guide when routing the actual mortise. I hope this isn’t too confusing and if anyone needs some clarification please don’t hesitate to ask. Sometime’s I am not the most thorough with explanations. Ask Red ;).
I did a quick Inlay on the top as well. I had a co-worker router a template for the recess. I thought I could have him route a template for the inlay, just 1/2” bigger to account for the bit but that didn’t work out so I had to trace it and use an OSS to finess it.
I actually went through about 8 of these inlays. The first 4 or so I tried doing ebony plugs but everytime the hollow chisel chipped out the maple. I finally scrapped that and will just do the inlay itself, or maybe just glue some pegs on after the fact.