Setting Jointer Knives - Using the right tools

[Brent Ring]

Okay, after I explain what happened and what I did, I am sure someone will tell me to get a better straight-edge - but it worked for me. I have a bed project going right now and needed to plane some long boards for the headboard and rails. I decided it was time to change the jointer knives to sharpened ones. I have a 6" Jet jointer with around a 40" bed.

I took out the old knives and installed the new ones, and used a 12" straight edge from my combo square to adjust the knives to the correct height. I started edge jointing a project board and noticed I was getting rocking. I had correctly setup support for both infeed and outfeed, but was still getting rocking occurring. I got frustrated and quit for the day after maiming one board.

As usual, I came back to the creek to check out my technique and other ideas. Chip Lindley has "rassled" with some long boards on a 6" jointer.

Well the only thing that had changed was the blades, so this time I got out the 24" framing square, set it on the long edge, and held it in place with a wooden handscrew. I then loosened and re-adjusted the knife heights. Once tightened I took a few test passes with a practice board and sure enough, nice flat, perfect jointed edges.

I share this only to show that when the right type (Length in this case) of tool is used, things get adjusted and work just fine.

This makes me question the things like the jointer pal, which is set up to make jointer knife installation faster and more accurate. I probably have a bad 12" combo square, it not an expensive one, but I wonder how these short little devices can do such an accurate job?

Anyway, a little FYI and information from my recent shop time.

[george wilson]

Rafter squares are not a good choice because,if you look at the edges,they are chomped out,leaving a sheared edge. Some long time ago,the edges used to be ground,but not now. There MIGHT be a few expensive ones with ground edges. My memory is hazy on that.Still,I'd advise you to get a piece of precision ground 01 steel about 1/4" thick,and maybe 1" wide. The steel would be thick enough to stand on edge,and you won't have the tall leg trying to tip over.

DO NOT rely on any of the magnetic blade aids.They are useless,IMO. Soon as you start to tighten the gib screws,they let go. Magnets just aren't strong enough.

[Russ Kay]

I was taught to set jointer knives using a 12-inch Starrett combination square. The trick was to rest the square's blade, edge on, on the outfeed table, with an inch marker carefully aligned with the back edge of the table. Then slowly rotate the head by hand (jointer unplugged, please!) until one knife catches the edge of the square's blade and moves it a short distance toward the infeed table. You need to rotate the head until the jointer knife disengages from the square's blade. This is a surprisingly sensitive gauge of blade height, even though you're not measuring it indirectly. Instead, you're magnifying the effect of the blade height by converting the motion to linear.

-- Russ

[glenn bradley]

I wonder how these short little devices can do such an accurate job?

They assume your outfeed table is flat :).

[Brian Kerley]

I had tried to use the method of having my knives pull the square/straightedge forward, but it didn't work for me. Easiest thing I found was to go to harbor freight, get a magnetic base for a dial indicator, a dial indicator, and adjust the knives so each knife was parallel along it's length, and each was at the same height relative to the outfeed table. Beautiful cuts ever since

[Rod Sheridan]

I've used the dial indicator/mag base method and the having the knife drag a piece of hardwood method.

Both work well.

Now I have a machine that only needs an allen wrench, and no adjustment.

One step down from a Tersa head that only needs a wooden mallet.

Regards, Rod.

[Guy Germaine]

I'm with Brian on this one, only I don't use the dial indicator. I just use the magnetic base. Find top dead center of the knives, and scratch a mark on the fence for reference. Remove the old knives. Set a new knife in the slot and align it with your TDC mark. Take the magnetic base and set it on the outfeed table so it overhangs far enough to catch the knife, and turn it on. It will hold the knife in place while you tighten the bolts. Once I have the knife held by the mag base, I run the Gib screws up until they just touch the bottom of the blade, then tighten the bolts. 10 minutes and I have the blades changed.