My old fence extension table was just a piece of 3/4” MDF with plastic laminate on top. 8+ years with a PC7518 hanging from it put a pretty good sag in it. I reinforced it with some 1×6 poplar, but that was a temporary fix. Since it’s my main router table, I need something extremely flat, so I can do precision work with it. The plan is to build a torsion box table. I’ll be using the existing steel angle at the Unifence and outfeed table to support the front and back. This dictates that the top extends beyond the torsion box to rest on the steel. I’ll be building some legs to support the end. I’d like to build a cabinet for blade and bit storage, but I need to see how much room I have with a router hanging under the table.
I use an Incra Jig Ultra for my fence. It’ll be mounted to the top using 1/4-20 T-nuts inside the top, and studded plastic knobs. To give me more flexibility, I’m putting a series of T-Nuts, so that I can move the fence in 5” increments. It’s a 16” fence, but being able to move it will give me over 24” between the bit and fence, if needed. I can also mount the router fence at the end of the table, and get the full 50”+ use from my Unifence.
I’m also using an array of T-Nuts for featherboard and fixture mounting. It’s hard to allow for all possible situations, but hopefully the layout will be flexible enough.
I like my Unifence, but one issue is the fact that there’s nothing holding the back down, and that the back can flex a bit when pressure is applied at the back. To fix these issues, I extended the back row of T-Nuts for the router fence all the way across the table. I’ll be making some hold down fixtures for locking the ear of the fence in place when needed. I won’t use these often, but where high precision, or extra rigidity is needed, they’ll be available.
I started, by throwing the top on my CNC router, and routing the holes for the T-Nuts. I also counterbored them so the nuts are recessed, although I didn’t really need to do that. I also routed dado’s for the front to back torsion box ribs. This just made it a little easier to build. And last, while it was up there, I rough cut the hole for the router. I’ll CNC a jig that bolts to the T-Nuts for the finish plate opening, when I figure out what to do for plates. Btw, the top is 3/4” Medex (water resistant MDF). I got it for free, and wasn’t using it for anything else. The entire torsion box is built from scrap, and won’t cost anything.
Because of the motor cover door, the maximum thickness of the top can only be 3”. For extra rigidity, the two ribs on each side of the cutout are 1” appleply. The rest of the ribs are 1/2” MDF.
Before gluing in the ribs, I shimmed the top flat on my saw and sagging outfeed table. Once it was flat, I applied the bottom sheet (1/2” MDF) with glue and staples.
I have an idea for a durable top surface for this table, and a few others I want to build. I’m going to attempt an 1/8” endgrain veneer top. I’m hoping for the stablity of MDF, and the durability of butcher block. Here’s the plan. First, cut a million little endgrain “tiles”. Then, attach them to the top using CA glue. Just a little in the corners, as I don’t want to coat the entire bottom. When the entire top is covered, I’ll saturate it with epoxy resin. The intent is that the epoxy will flow all the way through, actually bonding to the MDF. This will basically create a plastic top, that should be very stable. Once the epoxy cures, I’ll surface the entire top with the CNC router to get it perfectly flat. I’ll need to thoroughly seal the entire box, to prevent any warping in the summer. If this doesn’t work, and cracks (or worse), I can always put it back on the CNC and route the “veneer” off. We’ll see how it goes.
Not sure how long this entire project will take. I start a new job tomorrow, and may need to invest a bit of my personal time in that for the next few months.
-- Gerry, http://g-forcecnc.com/jointcam.html