This is an example of the improvising that happens when making tables. This example is a 9 foot by 50" wide slab of Claro-Walnut. It was over 3" thick and weighed 505 lbs as the Laden Weight listed.
So, first thing: Build a huge table made from 1/4" angle with supports. It has to be wide enough and Long enough and above all has to be seriously strong. Get people strong enough to lift onto the table.
The top of the rough slab humped up about a half inch, and dipped down almost 3/8" along one edge. It was shrink-warped.
So, whats the best way to level this ?
Step 1: Lets Make a Huge Mess....
So basically at this point the slab is on the table, thanks to some help.
You have to build a self-made template for the proper leveling of the table top. in this case, because the table was against a wall, i attached a 2x4 on the wall, at the same height along the side as the lowest point of the dip in the edge on the side not against the wall. Once i got it to the right height to take off the first layer/pass of the router, i used a level and went across to the other side of the table and made that the same height below the edge of the table top with a 2x4 that parallel and level to the one mounted to the wall.
This created a solid sliding surface for the router.
Next.... get (2) 1/"4 angle steel long enough to span the width of the table plus the 2x4's. Use a grinder flap wheel to remove slag off of the flange the router will be sliding on. And cover that with painters tape to aid the sliding. It'll chew up the plastic router base otherwise without grinding and taping it with something..
Lay them parallel to each other on the floor. The idea here is that 1/4" steel angle stock is thick enough that it won't sag over the span of 50", while you're pressing down on it etc. It should weigh about 60 to 70 pounds as a unit, with the ends welded on.
So now you should have 2 pieces of angle-steel stock and what you need to do is lay the router on the flanges of the angle steel, so that it slides on the flanges (by flange i mean the part of the angle steel piece thats laying on the floor), between the two pieces of steel. You don't want to make it tight however, or you won't be able to slide it. Don't leave too much room either. Give yourself an 1/8" on either side of the round router base.
We don't care too much about the exact width spacing of the two angle pieces, we're trying to level the surface of the slab. Now that determined spacing between the two angle steel pieces is set, weld pieces of metal on the ends to permanently hold the two rails in place, as a solid unit, looking like a thin rectangle in the end.
Step 2: As Long As You're Safe, It Is Kind of Fun Making a Huge Mess Temporarily....
SO, NOW THE MOST IMPORTANT PART --- PPE !!!!
Absolutely wear a Face shield, with ANSI certified Glasses underneath, a leather Apron and a dust mask- THAT WORKS !!!. Not some cheap paper filter mask. One that is either made by AO Safety or 3M. It has to conform to the contours of the face, with a self-administered pressure test (sealing it perfectly against the face, breathing in and out). Any air gaps will create serious problems, and according to OSHA, the dust becomes TOXIC at that fine-particulate dust level.
Pick an end, and start the first pass. you'll need a 1/2" router with a 1.5" dia. flat seat cut rabbeting bit. It looks ike a drum but with cutter blades on the sides and bottom where it flattens out the cut.
Have someone else hold a vacuum if possible. This will create the most unbelievable amount of mess. I cant say that enough..
Do the first pass at around 3/8" in depth. It'll take a while, no matter what. On my first pass with the router i hit an embedded nail that was inside it- completely grown over and hidden.
Walnut, Butternut, African Mahogany,Bubinga and countless other species create powdery dust that is so fine you have to wear a mask. African woods have silicate in them (dirt). Looks like smoke almost when your cutting it. A dull router bit will create tons of unnecessary fine dust also. The router shavings will fill the room with it if theres no-one there to hold a vacuum.
A large dust collector with a dust hood will be just about useless also. It will take at least 30 to 40 minutes to get to the other end. Thats just the first pass. I had to go over it again- you don't want to take off too much, it'll over-heat and ruin the router motor, or it'll rip the router right out of your hands. Huge bit, lots of torque.
This technique leveled the Slab so perfectly, all it took was about 5 minutes across the top to finish sand it. There is no planer big enough around here to plane this size slab at 50" wide. And it would be astronomical in cost due to the handling of something that is over 500 lbs. .
Works for any sized table that is roughly cut and not perfectly flat. Good luck and be safe !