When you need to flatten something wider than your jointer.
Any knowledgeable woodworker will tell you that a surface planer doesn’t flatten a board, it only planes the top face to be parallel with the bottom. A jointer is the only machine in the wood shop that will produce a truly flat face. This is true. But there is a simple way to machine a flat surface on a slab that’s wider than your jointer
Use a sled.
By using a sled for the slab to ride on as it goes through the planer and shims to take up the space between the uneven surface of the board and the sled, the planer can create a truly flat face.This works because the slab is held perfectly still on top of the sled as the feed rollers pull it through the machine. Because the shims are taking up the uneven space under the rough board, the feed rollers can’t push the board down against the sled. This allows the sled to act as the reference surface and the machine planes a face parallel to the bed, which should always be flat. This works. I do it all the time in my custom woodworking shop.
The sled should be made of a material that is consistent in thickness such as MDF. It will need a stop at the front, yes, the front, of the sled to prevent the board from being pulled off of the sled as the feed rollers pull it through the machine. (This is contrary to what is published on FineWoodworking.com but like I said, I do this all the time and the stop needs to be at the front.) The shims should be fairly short, so they don’t stick out the sides and get caught in the planer. The set up should be done on a flat surface like the top of a table saw.
Passing it through the planer.
I start by placing the slab down on the sled right against the stop. The board will move forward as the feed rollers grab it so it’s best to start with it at the front of the sled. Then I add shims as needed to prevent the slab from rocking. Once it’s sitting stable, I add more shims to take up any space at about 12” intervals down the length of the board. I tape them in place so they don’t move then I pick up the sled and move the whole unit to the planer.
I do light passes through the machine until the top face has been fully planed then I take it back to the table saw to check to see if it’s flat. I remove the slab from the sled and place the newly planed face down on the flat table saw top. If it’s flat, then I keep that face down and go back to the planer and thickness it as usual. Sometimes, if the slab was severely cupped, it will need further flattening on the sled. I do this in two stages. The first stage gets me close to a flat face. Then I reset the sled with the “mostly flat” face down. This second stage usually only requires a few small shims to hold it steady. After planing on the sled the second time, I check the new face for flat then mark it as the reference surface.
I use this technique for flattening boards and live edge slabs for all kinds of projects in my woodshop. I always take the time to make my live edge slab serving boards truly flat. I do the same for wide slab table tops in my custom furniture projects as well. My end grain butcher blocks are flattened in a similar way but I use the drum sander as the surfacing tool.