While the table saw and surface planer are the tools of choice when a piece of stock needs to be cut to a certain thickness, neither of these tools can properly do their job until one edge of the stock is flat. The best power tool for that job is the jointer.
A jointer is little more than a rotating cutter head with two or three finely tuned blades in-between two small, flat tables. The stock is pushed across the first table, called an infeed, past the cutting head and onto the second table, appropriately deemed an outfeed table. The outfeed table is the same height as the top of the cutting head, whereas the infeed table's height is adjusted to determine the amount of material to be shaved off the stock. The jointer is also equipped with a fence that aids in placing a square edge on the board.
Using a Jointer Safely:
Although all jointers should have a spring-loaded blade guard which covers the exposed cutting head, care should always be taken to keep hands and clothing away from the blades. This is especially true when jointing thin pieces of stock. Using a push stick or wood paddles will help keep your hands clear of the cutter.
To straighten out a piece of stock along one edge, it is better to take a few passes removing a very small amount of material than to try and take a larger bite in one pass. This approach will cause less stress on the motor and cutting head and will produce a smoother, more predictable edge.
First of all, it always goes without saying, so that's why I always say it: be sure to read and follow all of the safety rules in the instruction manuals that accompany your new power tools. It is also a very good idea to use hearing protection along with your safety glasses.
Stand on the side of the jointer opposite the fence and place the edge to be jointed face down on the infeed, against the fence. Start up the jointer and wait for the motor to come to full speed - never attempt to begin cutting when the cutting head is at less than full speed.
Operating a Jointer:
Apply light downward pressure onto the stock as you begin to feed the board over the cutting head, enough pressure to comfortably control the stock. Once the front of the piece has safely passed the cutting head, shift your weight forward and place your left hand onto the stock on the outfeed as you continue moving the board ahead. As the rear of the stock approaches the cutting head, remove your right hand and move it onto the stock over the outfeed table, safely away from the cutting head. Keep moving the board until it clears the cutter. Safely lift the stock and return to the starting position for another pass.
How to Straighten a Bowed Board:
Straightening a severely bowed piece of stock requires a slight adjustment in technique. First of all, it is much easier to joint an edge where the bow is facing upward in the center of the board. Attempting to joint a piece of stock when it is rocking on the table is extremely difficult.
If you're forced to try to joint the edge with the bow in the center facing downward, you'll want to focus your pressure on the infeed table for as long as possible. This will keep you from simply repeating the rocking action as the stock passes the cutter. After a few passes, the board should begin to straighten out.
Squaring Up a Second Edge:
If the finished piece of stock is to have four square edges, the next step will be to square up one edge perpendicular to the edge that was just jointed. First, verify that the fence is square to the infeed and outfeed tables. Since the fence on most jointers can be beveled, it is a good idea to verify that the fence is at 90-degrees with a Layout Square.
Once the fence is 90-degrees to the table, squaring an edge is very similar to the jointing procedure described above. The main difference is that consistent pressure will be applied primarily to the fence in this case. Place the previously jointed edge against the fence and move the stock over the cutter, keeping your hands safely away from the blades. Once again, numerous passes will likely be necessary until the edge is against the cutter is perfectly flat and square to the edge against the fence.
Once you have two perpendicular flat edges, a table saw, or surface planer can trim the stock to the final dimensions.
Once you have a handle on the basics of using a jointer, there are some other useful functions you can experiment with. First, as mentioned earlier, the fence on most jointers can be beveled up to 45-degrees. This will allow you to joint mitered butt joints along a length of a board. You could also mark a start and stop point on the angled fence and create stopped chamfers on an edge by easing the stock down onto the blades, cutting between the marks and then raising the stock away from the cutter.
When appropriate, use of a featherboard to hold the stock firmly against the fence or the table would be a good idea.
A jointer can be used for cutting hardwoods and softwoods, but you should avoid cutting plywood, MDF or any other manufactured wood materials on your jointer, as this can chip the knives in the cutter.
It is a good idea to keep an additional set of cutting knives for your jointer on hand in the event you do chip a knife. A chipped knife will leave a raised line along the edge of the jointed board that would need to be removed with a sander, which reduces the effectiveness of the jointer. You should always inspect the knives in your cutting head before jointing an edge.
Always check your stock for any metal before beginning to joint a board. Any piece of metal in the stock can wreak havoc on the knives in a jointer. A woodworking metal detector, available at most fine tool suppliers, can help you find any hidden pieces of metal in the stock, particularly if you are working with recycled timbers.