One of the great things about woodworking is that there are often more than one way to accomplish things. This fact is true with setting jointer knives. There is really only one rule in setting jointer knives and that is NEVER let the knives fall below the height of the out feed table.
When this happens, when you joint wood, instead of your wood being nice and straight and flat, it will be flat but will come out "arced" and will look like your board has "sagged" along the edge that was just jointed. The lower the knives are from the out feed table the more exaggerated the arching or sagging will be. This of course makes it impossible to glue boards together, or in many case to even connect your wood together.
One of the tried and true methods I was taught many, many years ago was that when setting jointer knives the correct height is when you lay a steel ruler on the out feed table and slowly turn the jointer knives by hand, the knives will grab the ruler lift is slightly and move it ahead by about 1/8". This setting will joint or plane the board while leaving a very shallow amount of snipe at the back end of the board being jointed. (snipe is that small depression that jointers and planers can leave a the very end of boards).
BUT ... there are other ways of setting jointer knives, they can of course be set higher that what my suggestion is, but the can also be set even with the out feed table. The advantage of setting them even with the out feed table is that there if virtually no snipe when you joint boards. The disadvantage is that actually setting the knives exactly even with the out feed table can be challenging, especially when you are on your knees setting up the machinery, which can sometimes be in poor light, and your do risk setting the blades a bit low which creates the problem we want to avoid in the first place.
The other problem with setting jointer knives even with the outfeed table, is this ... when you run wood through the jointer you will, ever so slightly wear the blade down. The ques ion now is, how much wood (or what type Oak, Maple, Cedar etc) can I run, before the blade is now below height of the out feed table? The answer to that question is this, you need to keep checking your blades, and re-setting them as needed. Now if your jointer is easy to keep checking and re-setting jointer knives, this isn't a problem. If, on the other hand you can live with a bit of snipe (this often means cutting it off when you cut your boards to length) and you want to spend less time setting up machinery, then maybe setting jointer knives a bit higher is your best option.
I have never actually measured at what height above the out feed table, the jointer knives are when using the "lift and move 1/8" methodology, and so I set about to do that. It turns out that if you set your jointer knives with this setting, the blades actually approximately 1/64 of an inch above the out feed table. If you are not sure what a 64th of an inch is, it is roughly the thickness of 2 or 3 sheets of 20 bond paper (the same paper you probably you in the printer for your computer).
In the end, the choice is yours and it depends on many factors, what kind of wood you are using, what you are doing with the wood (are you making furniture of expensive wooden musical instruments), how much wood do you actually run through your jointer, and finally, how much time do you want to invest in setting up machinery to the accuracy you need. There are no right or wrong answers, only what works best for you, and that's the great thing about woodworking ... lots of different choices and we can vary them as needed. No wonder I love this medium so much ...
Copyright Colin Knecht