I made a box joint jig for my router table as shown on multiple Youtube videos but can't get the spline spacing right. After 8 tries I've concluded I'm missing something. My fence sled is smooth and tight. I have a new 1/4" Bosch upcutting bit in the router and I make a relief cut in the front board with it centered and clamped in place. I cut a strip of wood for the spline making sure it fits in the relief cut, not too loose or too tight (I think). I move the front board over using 2 layers of spline stock to set the "finger" size and make the cut for the spline, again with the front board clamped in place. I then drill and bolt the front board to the fence in it's centered location and insert the spline. When I make a trial cut I get close but the "finger" is always either too big or too small for the gap. It seems the error compounds the longer the joint so getting the finger size precise is essential. Anyone have any idea what I am doing wrong?

I am not understanding your procedure. On my box joint jig for my tablesaw I set the first piece against the index key, what I think you were calling the spline. I run it over the blade and cut the first slot. I then move the piece so it straddles the index key and cut the next slot and so on until that piece is done. To cut the pins on the second piece I butt it up against the bottom edge of the first piece then proceed with straddling the index key as before. That offsets the pins and slots so that the top and bottom edges of the panels line up.

It sounds like you’re index key is slightly off in width. It doesn’t take much at all and, as you mentioned, it will compound itself as you go down the line cutting slots and pins. After only a few inches or so you will really notice how off it has gotten.

My jig rides in the miter slots on the table and is adjustable. Does yours ride in a miter slot on your router table or against your fence?If it rides on your fence you could probably get some adjustment that way.

A small adjustment on the side allows me to tighten or loosen the pins and slots to get them perfect. The jig would be a real PITA without it.

The jig sled I made with multiple fences:





This is the adjustment I use.



The adjustment screw is accessed through the hole shown below. If the pins/slots are too loose I move the index key ever so slightly away from the blade. If they are too tight I make a slight adjustment towards the blade. I always use test pieces of the same thickness of the material I’ll be working with to dial it in before starting the actual project.



One of the first projects I made with the new jig; a box to hold my branding iron equipment. I made it out of an old skid, LOL.

Lugnut- nice shop, nice jigs and nice craftsmanship.
EdP- if you're sure on your index pin width you could also make a T-Sqaure fence that locates off the side of your router table, this would allow you to put (2) index pins between the fence and T-Sqaure during set-up. As Lugnut mentioned compounding is the most likely the contributing factor, if you're off .003 of an inch in a pin or slot, over the width of your box your problem should and will magnify. Some pictures would be helpful- good luck.

My method for cutting the fingers/slots is exactly as you describe, but I could not get the spacing right. Your adjustment screw is an important feature missing in my jig and is a feature not shown on any of the videos I watched. I was either too loose or too tight with my index key but finally did get the spacing right and cut a joint that fit together. However, my jig is not precise enough to give a quality result. There is apparently too much side to side slop in the sled that feeds the stock foward through the bit even though it appears tight to me. I will either have to start again and incorporate spacing adjustment and better precision or go with a store bought jig.

Thanks for the help guys.

Lugnut Nice looking work on your Branding Iron Storage box .Do you have any pics of anything you made that is more fancy ?