This is my procedure for adjusting knife heights for a jointer. I should make a few remarks at the outset.
First, you should have an extra set of sharpened knives. And the sharpening is critical. You must locate a dependable service for this. I suggest calling a few local wood shops and find one that the majority uses. If you don't, you may find that the knives are not ground to the proper bevel, or they may not last very long. With normal hobby use you should get many months of service, maybe a year or two or longer. With heavy industrial-type use, you should have carbide knives and you can expect at least six months use. I'm in the middle. I have steel knives that last about 8 - 10 months when I'm working continuously. I don't make heavy cuts as a rule.
Second, Delta sells a magnetic jig for knife setting, called Magna-Set, their catalog #37-157. Costs about $60. I have the set but have never used it. Each time I get ready to change knives, I look at it, but I know it will take me a few hours to figure out how to use it, then it may take 5 minutes to set the knives. Problem is I need to get going on a job, and I know that my own procedure will take me 15 minutes. So I'm a candy-ass about it. Probably is a good purchase. Or it probably isn't. It consists of two carefully machined magnetic bars mounted on two chrome-steel rods. That's it. For $60. There are other magnetic jigs available. I'm not an expert on this type of adjustment.
Lastly, the procedure is written for my specific jointer, the 6- inch Rockwell/Delta which was manufactured in the USA about 30 years ago. It is certainly adaptable to other jointers. For instance I talk about three 5/16-inch gib screws and certain cutter-head orientations, and you will have to make allowances for that. Otherwise, the techniques will be the same. Now for the procedure:
- Disconnect The Power Source. (Yes, I'm yelling) Remove all the guards and move the fence to the rear so that the blades are completely exposed. Find a chair that will sit you at the proper working height. Locate the small wrench that was supplied with your jointer for this purpose. What, you lost it? Then find a very thin 5/16 open end wrench, Snap-on, or Williams, or SK. I don't think Craftsman will fit. Your jointer may use a different size. Find the tool that works.
- Raise the infeed table to the zero position. Move the cutter head so that no knives are exposed. Take a long 5-foot straight-edge, like the one Exact Level sells, and set it down along the infeed and outfeed tables. Adjust the outfeed table so that each table is exactly at the same height and the level rule lies perfectly straight with no gaps. Use a rear light to help. If you can't do this, then you may have a problem with the ways. That's a separate issue and must be dealt with before proceeding any further. Don't bother with the rest of this procedure until you straighten out that problem if it exists. Secure the outfeed/infeed tightener-knobs, usually in the back.
- Each knife is secured in the cutter head with three 5/16 nuts behind a plate which forces the knives against the head. Loosen the nuts and remove one knife. Clean out the slot with a small brush and vacuum. Place a new knife in its place with the bevel facing upwards. Those who have a jointer with a non-adjustable outfeed table should go to step 8.
- Now you will need a 12-inch straight-edge. I use the straight-edge ruler from my combination square. It has the right characteristics and is heavy enough for this step. Gently, very gently, tighten one end nut. Carefully lay the edge of the ruler on the knife at that nut position at about the 6-inch mark and cradle it between your thumb and index finger. Roll back the cutter head. With a pencil, mark the infeed table or the fence at the end of the ruler. Take a square and draw a line across the table or fence. Note Fig. 1 below. You can use any reference point, like the front of the throat (end of the infeed table), or a mark on the fence. I mark the infeed table.
- Roll the cutter head forward, the knife should contact the ruler at approximately the six inch mark. Continue to roll the head, the ruler is lifted by the knife as in Fig. 2 and then drops down. The ruler should move about one inch or so. Adjust the infeed/outfeed tables up or down to achieve this distance. Place a mark on the infeed table where the ruler drops, shown in Fig. 3. Use the square and mark that position across the table or fence. Again, if the knife is below or equal to the height of the outfeed table, it will not contact and move the ruler. Adjust the outfeed table down about a millimeter or less. You may also have to drop the infeed table by the same amount so that the knife is exposed above both tables. The knife should contact the ruler and move it freely one inch, or to your mark. Get this right before proceeding. This is the crux of the procedure. Look closely at the three illustrations to see this progression.
- Once you're satisfied, move to the middle nut and repeat the procedure. If you have to move the knife a good bit, then you will probably throw off the first adjustment. No fear, you will check it again. Physically adjusting the knife is not trivial. Practice this a bit until you get it. Maybe you can find a little tool to help you out, like a very small screwdriver. I use my fingers, but my jointer is old and things move smoothly by this time. You will repeat step 5 at all nut or gib screw positions and adjust the knife until at each position the ruler moves exactly the same amount. Now firm up the nuts. Again check all three positions. Voila. One knife is set. First try should take you 20 minutes, maybe 15 if you're good. Now repeat step 5 on the remaining two knives. It's tedious at first, but that's the practice you'll need. Done.
Note: An 8-inch jointer may have five gib screws and a spring assembly under each knife. Doug Murray suggests that you gently snug the middle nut at the proper height such that the blade will pivot at that point. Push the knife down at the rabbet side so that it is intentionally high at the fence side. Then use a small screwdriver to lever up the knife slowly from the bottom and set the height at the fence gib screw using the ruler technique in step 6 above. All you should need to do is set the knife height properly at two gib screw positions, the remainder had better be OK or the knife was not sharpened properly.
- All that remains is to set the outfeed table level with the knives. If you don't, then you will gouge board ends or taper the edges. Worse, you may get convex or concave profiles. Use the ruler for this. Adjust the outfeed table until it just barely contacts each knife across its length. If it doesn't do this just right, then re-adjust that knife. And don't be frustrated, you'll get it. This method of adjustment is what the pros do, and it will eventually take you 15 minutes. The glass-smooth cuts you will make will justify all the time you took to get it right.
- I have no experience with non-adjustable outfeed tables, like some Craftsman models. I can only suggest you try the following: The knives are set slightly above the outfeed table with these jointers, like 0.002 to 0.003 inch. Purchase two sets of feeler gauges from any autoparts store. Remove the appropriate gauges and lay them on the far ends of tables. Make sure each has the identical thickness measurement. Place an aluminum straight edge carefully along the feeler gauges across the cutter head (no knife exposed). Use wood blocks to keep it vertical. Roll the cutter head forward until the knife contacts the ruler. Move the knife up or down until you can move the cutterhead so that the knife just barely touches the ruler. Gently tighten the gib screw or nut at the ruler position. This is now the reference height. Return to step 4 and omit step 7.
I'm indebted to Loren Hutchinson for the Figures, and to those of you who took the time to respond after using the procedure to clear up the fuzziness and unclear descriptions. If anything remains unclear, or you would further diagrams, let me know. Be glad to assist.