Adjusting jointer knives

There are two designs of cutter heads in terms of how the knives are adjusted. The type pictured at left is with jack screws (A) behind the blades, which are used to lift the blades to the right height.

Another set of screws, seen from the side (B) are used to clamp the blades in place. The heads of these screws push against the head, pressing the bar against the blades. The blades are held in just by friction.

To remove the blades, start by loosening the four screws holding the bar in. This is done with a small wrench. Pushing the wrench away from you loosens the screws. Turn them until they are loose, and perhaps a half a turn further. Not loosening them too far makes reinstalling them easier.

With all four screws loosened, the knife can be pulled out. It's easier to remove the knife before removing the bar, as the slot is wider towards the bottom, and the screws would have to be loosened more for the bar to come out first.

After removing the knife, the bar lifts out easily. The next step is to clean out the slot, so that any dust in there won't affect the alignment when you reinstall the knives.

Once you have the knives out, presumably, you want the knives to get them sharpened.

Once you have your knives back from sharpening, it's time to put them back in. Put the bar in first, then the knife. Next, tighten the four screws in the bar, and then back them off a bit so that they make contact but aren't tight.

Use a straightedge resting on the outfeed table to check the height. With an alan key in the jack screw, and the straight edge right next to it, set the height of the knifes so that they just touch the ruler at the highest point.

Animate image

Set the height of the knives so that if you rest the ruler on the outfeed table, and turn the planer head by hand, it will pick up the ruler, and move it forward by 5 mm, or about 0.2". Measure it just above the jack screws. It's a bit of an iterative process, and I usually just leave the alan key in between checking.

For a typical 2.5" diameter cutter head on a 6" jointer, having the ruler be moved by 5 mm works out to having the knives protrude 0.004" (0.1mm) above the outfeed table. However, once you tighten the four screws to lock the blade in place, the knife will actually come up a little bit, just from flexing in the steel. On my jointer, after tightening, the ruler gets carried about 6 mm, which works out to closer to about .0048" or 0.12 mm. When you adjust the blades, ignore this change in height after tightening. After installing, adjusting and tightening all three blades, check that they all come to the same height. If they all came up by the same height after tightening, you are good to go.

The amount of protrusion from how much the ruler gets carried can be calculated as follows If r is the radius (not diameter) of the cutter head, and d is the distance the ruler is carried, then the protrusion (p) of the knives from the outfeed table can be calculated using the Pythagorean theorem. For a right angled triangle, the pythagorean theorem states that the sum of the squares of the length of the sides forming a right angle is equal to the square of the length of the diagonal.

Thus, for the geometry at right, a² + (d/₂)² = r²

and a can be calculated as a = sqrt(r²-(d/2)²)

Where sqrt denotes square root.

And finally, p = r - a

Thus:

Protrusion p = r-sqrt(r²-(d/2)²)

For a 2.5" diameter cutter head, using inches, this works out as follows:

Distance
ruler pulled Knife
Protrusion
0.10" 0.0010"
0.12" 0.0014"
0.14" 0.0020"
0.16" 0.0026"
0.18" 0.0032"
0.20" 0.0040"
0.22" 0.0048"
0.24" 0.0058"

Setting knives on a springs-backed knife style cutter head

The other type of planer head has no jack screws, but some springs behind the knives. This type of head is also used in most thickness planers.

Rather than a pair of jack screws, the jointer blades are pushed up two little springs.

On this type of cutter head, the blades are set in place using a jig. Thickness planers generally also use this approach, as there is no outfeed table to measure the blades from.

When you take the blade and the bar out, you might as well loosen the screws by several turns, as getting the blade and bar back in against the spring tension is very fiddly. Also, ifthe springs are loose, take them out and put them aside, so they won't fall out at the bottom when you turn the cutter head.

Your jointer should have come with a jig something like the one at left. To use the jig, crank the infeed and outfeed tables down a bit to make room for it.

To set the knives, tighten and then back off the four screws holding the bar in place like you would with the head that uses jack screws. It should be possible to push the knife down, and have it come back up from the force of the springs.

While pressing the jig firmly down on the knife, tighten the screws. Really, just one screw tightened towards either end will do it. The screws are a bit hard to reach while the jig is in place.

Once you have all the blades set, wind the outfeed table back up, then adjust it so that the tips of the blades will drag a straightedge by about 5 mm, as described above. Hopefully, all the blades will be at a consistent height. If not, it's possible that you slipped with the jig while tightening the blade, so use the jig to check if any of your blades are set too high.