While a jointer can be used as a planer for small
Accurate jointing or chamfering necessitates that the cutter head knives be adjusted precisely with reference to the outfeed table. The edge of each cutter head knife must exactly at the level of the outfeed table: Not above or below it. In most cases, sharpening the cutter knives dictates that they be removed completely from the cutter head and then replaced and adjusted after sharpening. This is why I suggest the use of solid carbide as opposed to high speed steel knives: Carbide knives last a lot longer and that means less time and effort has to go into removing, replacing and tweaking knives. Buy two sets. That way, you can continue to operate your tool while the dull set is out for sharpening and you will always have a sharp set waiting.
Always unplug your jointer from the electrical outlet before beginning any knife adjustments. In my jointer, an 8' Rockwell/Delta classic, the knives are removed and put back by using a flat wrench that came with the jointer. This wrench is used to release the hex head machine screws that press against the knives and hold them in place in the cutter head. It is very easy to round over the hex heads, so I am very careful not to do so. I purchased a gadget that helps me align the knives with reference to the outfeed table. It magnetically affixes itself to the surface of the outfeed table and magnetically attracts the knives vertically and holds them in position, exactly level with the outfeed table, while I tighten the hex bolts. Each knife (there are 3 in my machine) must be in the extreme vertical head position before it can be perfectly adjusted and tightened. When all 3 knives have been set properly, they should just touch, but not lift, a flat piece of wood laid on the outfeed table, extending over the cutter head. They must do this across their entire length of each knife.
Jointer size is most commonly determined by the full width of the knives (knives). A 6' jointer makes a maximum 6'-wide cut. An 8' jointer makes a maximum 8' cut and so on. It would be rare to use the entire width of even a 6' knife set at once, so the real advantage of wide knives is that you can move the fence to use a sharper place on the knife when the knife becomes dull. The wider your knives, the more use you will get out of them before it is time to re-sharpen. I usually start with a sharp knife set and the fence all the way to the right end of the cutter head and move the fence, in increments, a bit wider than the maximum board thicknesses, to the left until the knives are all used up.
Sometimes, with curly or wavy grain structure, you will have to endure tear-out from the lumber edge even with sharp knives. Sometimes you can turn the plank around and run it through again in the other way with very shallow cuts until the edge is fully jointed and the tear-out is gone. At other times, you may have to settle for a sawn joint made on the table saw. Usually you can make fairly good glue joints this way, if you have to, but a jointed edge is always my first choice because there is a more consistent (smooth) gluing surface and, thus, a better bond.
The depth of cut is determined by the height of the infeed table with reference to the outfeed table. The lower the infeed table, the more wood is removed with each pass over the jointer. It is not a good idea to take off too much wood with a single pass. The chances of tear-out increase with the depth of cut and you may end up removing more precious wood than you really need to, to get your perfect joint. After all, the idea of jointing is always to remove just enough wood to create a straight, flat board edge. Removing any more than that is just a waste of lumber.