Flatten wide boards on a narrow jointer
The best way to make a really wide board flat is to rip it into narrow boards first, face-joint them, and then glue them back together. Not only does this solve the jointer-capacity concern, but it also results in a more stable panel because the rip-joint-reglue process relaxes tension in the wood. And, if you're careful, the joint line blends almost seamlessly into the grain.
But, what if you want to flatten the face of a highly figured board without a ripline interrupting the figure? Or what if the board is just a little too wide, and not worth the hassle of rip-joint-reglue? Here's a quick way to joint it that requires only a strip of scrap plywood and your planer.
Note: This technique will work on boards up to about twice the jointer's capacity, but we suggest no more than one-third of the board's width be unsupported or overhanging the jointer table —any more and the board may tip as you pass it over the cutterhead.
Changing of the (cutterhead) guard
After jointing one edge of the board, replace your jointer's cutterhead guard, which will interfere with a wide board, with a shop-made guard of 3⁄4 " plywood like the one on the first slide. It should cover the length and width of the cutterhead entirely. Clamp the guard securely to the jointer's fence.
Next, set the jointer to take a 1⁄16 "-deep or less cut, depending on how much extra thickness you have to work with. Keep the jointed edge against the fence as you make a pass. This will produce a wide rabbet in the face of the workpiece.
Because the rabbet is the full width of the jointer's cutterhead and infeed table, an attempt at another pass will leave the rabbet resting flat on the infeed table while the unrabbeted edge overhangs. Depending on the design of your jointer, that edge may catch on the rabbeting ledge. The good news: You only need two flat points on the rabbet for this technique to work. One pass should be adequate.
Cut a 6"-wide spacer from 1⁄4 " or thicker plywood an inch or two longer than your board. Secure the spacer to the workpiece using double-faced tape, one edge abutting the rabbet shoulder, photo below. Then run the stack through the planer.
With one side now flat, remove the spacer; then run the board through the planer with the just-planed face down to remove the rabbet.