From WoodworkersZone WoodWiki
When I first started flattening stock on a jointer, I found that many times the material was wider than the jointer I had. I went through the "cut the stock down and re-glue" phase, then a friend and mentor showed me how to accomplish this task without cutting up wide stock.
Basically the job is pretty straight forward. It involves the use of an auxiliary guard, which replaces the factory-supplied hinged guard, a set of winding sticks, and a good eye.
A fairly typical 8" jointer with the stock guard in place. For this process to work, it will have to be removed . For the sake of safety I fabricated a new auxiliary guard using two pieces of 4/4 stock. The guard looks like this installed:
It is pretty straight forward to build: I rounded over the edges on the guard (top and bottom, both in- and outfeed). Using a bit of glue and three screws, I joined the two pieces. It is plenty strong enough to take a hit if you bump into it, and covers the knives adequately to prevent injury. The screws are countersunk to prevent accidental contact with the knives.
The jointer is capable of flattening up to twice the width of the jointer (6" unit will flatten 12"; 8"=16" etc.)when done as described below. As a general rule, I try to keep the stock about 1" under the maximum, doubled width of the jointer. I am showing a 15" piece of glued-up stock below, which is the maximum width of my planer. The stock is skip planed to 15/16", edge-jointed, cut parallel, then glued up. I do not work overly hard at glueups at this stage, except to make sure the stock is as tight as possible to the clamps during the process. Introducing twist into a glueup makes the job much harder, and if it's bad enough, the stock will not dress out as thick as you planned.
As you can see in the above picture, it is very tough to discern that a slight twist does indeed exist (the right side is about 1/32" higher than the left at the back stick). I could see it with my eye, the camera had its own opinion. Also note a slight cup under the front winding stick. This is a pretty typical glueup for me. If you are flattening wide one-piece boards, the winding sticks are extremely helpful for determining twists and cupping. Their use shows you lots of valuable information before you start flattening the stock.
The Jointer Setup
First remove the stock guard from the jointer, then set the stock you intend to flatten over the cutterhead
Next move the stock toward the fence so that roughly a bit more than 1/2 the stock will be worked. Slide the fence to the stock, then set a piece of material about 1/4" thick on top of the piece to be flattened.
Now set your shop-made knife guard on top of the shim and clamp it to the fence.
Remove the shim and the stock and be sure the knives are covered by the fence.
Working the First Half
As you can see, I marked the high corner in this pic for clarity, and the opposite rear corner is marked as well. When you push the material through the jointer, make sure the high points stay in contact with the beds as you flatten, otherwise you are liable to create a larger twist (and more work for yourself) . Start the jointer and begin the feed. Once you have enough material on the outfeed table to safely continue the feed, move your left hand to that side. Continue the feed holding the stock firmly to the outfeed table. As you move forward, depending on the severity of the twist or cup the machine may or may not remove material. The goal is to get a flat, straight surface, and this can take more than a few passes to accomplish
This is the piece after I made two passes. The line shows a ridge in the stock, and you can see the left side (in the picture) is jointed more or less flat. There may be some places skipped (not cut), but the overall 1/2 is flat enough to go to the next step.
The Second Half
Now flip the stock end for end ( the first face should be down still) At this point I would like to state that you will likely be feeding the stock into the planer against the grain. This is not a major cause for concern with a well-tuned jointer (sharp knives and a slow steady feed). It may cause some tearout, but that should be minimal. If you do experience tearout you should still be OK--it can be dealt with at the planer.
Feed the stock the same way, pressure at the high point of the stock, continue to hold pressure on the outfeed table once it is clear enough to hold down safely, and feed.
This is the face-jointed side after one pass. As you can see the black ridge line is nearly gone, as well as the final letter "s" from the word "passes." The black line only shows in a couple of places, and after checking with the winding sticks, I found the twist and the cup to be gone.
The stock is now flat and ready for the planer.
As you can see, I will be using the full width of the planer here to make the top parallel with the flattened reference side. The first pass generally is a skip-plane pass (light pass), generally due to how much stock you may have removed on pieces with twist in them so the stock does not hang up in the planer.
This is after the first pass:
This is after the second pass:
I measured the stock thickness after the second pass as shown below:
Now I turn the stock over (the jointer-flattened side is now up) and run this side. It is OK to plane the reference side once both faces are parallel; this step takes out what little irregularity existed from flattening, and achieves removing stock at roughly the same amount as the opposite side.
This is the end result:
The stock is now flat, parallel, and ready for further machining (edge jointing, ripping parallel, crosscutting, creative joinery, sanding and finishing).