I have parts for a 1000x1500mm OX that I’ll be putting together.
I’d be interested in reading different experiences folks have had making nice flat tables as good references.
I’m used to leveling my lathe with a master precision level to the limits of its precision, and I do expect similarly to level the router table as well as make it flat. That part I’m not worried about. But I’d like to hear options and experiences, good and bad, for a sufficiently flat table for a router of this size.
This shows what I am using:
Planned on leveling this surface but to date have not had to do so for the kind of work that I do.
I have a surfacing bit but to date not used it. That said I can imagine this is not accurate enough to engrave PCB’s but for most of my woodworking its fine and simple…
I’ll use an MDF spoiler board. But I’m thinking of the table the router sits on. 
I use a plywood torsion box on my OX. I actually have the rails attached to the sides of the box so I did not need any cross pieces.
The top is not exactly a torsion box but it is reinforced and have had no problems.
@donkjr regarding specifically making the top surface flat, the summary is 3/4" MDF, on dimensional lumber planed flat, right?
Yes, I used 3/4 mdf on top of rails that were planed. Then made sure the assembled upper frame was flat and square.
Just curious, what do you plan on cutting with it? If only wood, then you’re probably fine. If I had to do it again, I’d probably go with a screw driven system instead of belts.
@SirGeekALot Primarily wood, some light aluminum work (like, cutting it forward). Heavier metal work goes on a mill.
I’m building the table top from 3/4" MDF.
I don’t have a planer to plane dimensional lumber, so instead I used 4" wide MDF strips from the same shset for sides and stringers. I had them glued together into a frame before I realized that the 4" cuts weren’t exactly the same as each other, so I’m going to have to repair that a little bit. 
I haven’t yet fastened the top down (because I have to buy more screws). Just resting the top on the frame, though, I used a 40x80x1500 c-channel section and a feeler to see how close to flat I’m starting with. It’s almost entirely within .01" and mostly with .005" which isn’t machining tolerances but probably good enough for a 1500x1000 OX — the deflection of 20x80x1500 carrying a 1000mm gantry may be more than that.
I’m trying to decide between 3/4" and 1/2" MDF for the bottom of the box. I’m not eager to make it even heavier if it won’t benefit me. Interested to hear thoughts.
You can use the 1/2" it will stiffen the box just as well as the 3/4" I’m with you on the weight reduction.
Oooh, or maybe hardboard. Thoughts?
It would probably work out ok to use 1/8" hardboard, providing it is glued and screwed. But if it were me, I would use the 1/2" . My first OX was a torsionbox design with 3/4" cabinet grade plywood on top and 1/2" cabinet grade ply on the bottom with all the ribs made of the 3/4" material. I actually bolted the Y axis rails to the side of the torsion box so I did not need any of the cross bracing and made the rails stiff as can be.
I came to the same conclusion and just finished loading the 1/2" MDF into the car, along with pieces to make the legs for the table.
Turns out what I loaded into the car is what will become (part of?) a spoil board, because I got the minor dimension too small by slightly less than an inch, and it won’t quite reach to both edge pieces. 
The plywood saw at Home Depot or Lowes cuts straighter than I can with the saws I have, at least easily, and getting as many cuts as possible there reduces the amount of MDF dust in my workshop. (Once this OX project is done, I want to hang a dust curtain between the wood and metal sections of the shop to contain the MDF dust.)
I got that board wrong because I remembered that I had rounded up 1 meter to 40 inches to have a little extra room (the stores have never heard of metric, anyway) and 1.5 meters to 60 inches. However, I forgot that I took 4" cuts and made the table the remainder of the 49 inch wide sheet of MDF, so the real size of the table top I got was 41 inches wide minus two saw kerfs. It turns out that one saw kerf was 5/32, so my tabletop was actually 40 11/16" wide.
So far, the box I’ve made isn’t a true torsion box, just most of a torsion box. I have a box of 4" wide 3/4" MDF with a 3/4" MDF top and a 1/2" MDF bottom, and two 4" wide 3/4" MDF stringers across the width. I can’t figure out that I actually need anything more to support an OX…
For the legs, I’m using fir 4x4 pieces 36" long, with pieces of MDF screwed to the sides to stabilize and stiffen them.
To level the table, I bought 2" long 1/2" stainless steel bolts and 1/2" nuts, and 1/2" coupling nuts, one of each for each leg. I drilled 13/16 holes as deep as the coupling nuts are long into the base of each leg, and seated the coupling nuts in the holes. I threaded the nuts onto the bolts, and then threaded them into the coupling nuts on the bottoms of the legs. I will level the table with them essentially the same way as leveling a lathe.
Since you are just resting the OX on the table, and not using it as part of the machine structure like I did, I think you will be fine. Good idea for the leveling feet. You may want to rethink the leg height, with the addition of the machine on top the spoiler board height may feel a little high for working on, depending how tall you are. I set my Sphinx on top of a 33" table and it feels just perfect to 6’1" of me.
Much easier the cut the legs shorter than to lengthen them. I tend to prefer substantially higher work tables than normal people.
I didn’t take pictures as I went along, too much in the moment. I’m not sure it’s a design anyone else would want to copy anyway, I kind of made it up as I went along.
If it isn’t rigid enough, I can gusset it. But I think I’ll move on to building the OX before I worry about the table any more. 16 more screws and it’s ready to level and build on!
Have fun with the build. That is my favorite part, the actual building of the machine. I tend to sell them off and build another one after a few months time.
