Jointer Tune-Up

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This article is from Issue 96 of Woodcraft Magazine.

Simple steps that yield smooth, square stock edges

By Paul Anthony

The jointer is one of those “heaven or headache” tools. When it’s working properly, it makes quick, sweet work of straightening and flattening boards. However, a poorly-tuned jointer can cause no end of frustration, yielding crooked edges, weirdly tapered pieces, and washboard surfaces.

Though a relatively simple tool, the jointer requires an exacting setup. Fortunately, it’s not that difficult to tweak the tool into proper working order. All you need is a good straightedge, a simple shop-made jig, a few common workshop tools, an automotive feeler gauge, and a bit of patience. The set-up sequence involves checking the tables for flatness and parallelism, correcting them if necessary, and then adjusting a set of sharp knives to the proper height. The entire procedure should only take an hour or so, paying big dividends in accuracy and time saved down the line.

Jointer at Work

In order to tune up a jointer, you need to understand the operating principle. As shown in the drawing below, a workpiece is fed across the jointer’s infeed table and over the knives, which are set at top dead center to the height of the outfeed table. The outfeed table supports the cut surface as the remainder of the board is jointed. If the tables aren’t parallel to each other or if the knives are too high or low, you won’t get a straight cut.

Feeling for flat. Check each table for flatness using a precision straightedge, gauging any gaps underneath using an automotive feeler gauge. You may be able to file or sand out a high spot, but there isn't much you can do for a dip.

Check the tables

Make sure your outfeed table is set to its proper height in relation to the cutterhead and knives. As shown in the drawing on the facing page, it should be level with the knives when they are at top dead center. Check this with a straight piece of wood (about 1/2 × 1-1/4 × 18"). With the machine unplugged, rest the piece on the outfeed table with about 2" extending over the cutterhead. Rock the cutterhead while adjusting the outfeed table until a knife just barely grabs the piece. Lock the table. 

Check each table for flatness as shown. Humps or dips can compromise operational accuracy. If a tune-up doesn’t correct problems, it may just be time to invest in a better machine.

Checking for parallelism. After adjusting the infeed table to be level with the outfeed table, extend an accurate 24-36" straightedge across both to check for parallelism. Press down firmly on one end of the straightedge while extending the majority over the opposite table as shown.

The tables on most jointers ride on dovetail ways. A flat metal bar called a gib rides between the mating dovetails on one side of the machine. The gib and its adjusting screws remove the slop from between the table and the base while still allowing the table to move for adjustment.

It’s best to shim the outfeed table to correct non-parallelism, but if your jointer doesn’t allow that, shim the infeed table instead. Loosen the gib screws enough to allow a bit of table lift, and insert matched metal shims between the ways to elevate the appropriate end. (Aluminum shims cut from a soda can work fine.) If the table needs raising near the cutterhead, insert the shims at the upper end of the ways. If the table sags at its outer end, place the shims at the lower end, as shown. When the tables are parallel, snug up the gib screws just enough to stabilize the table while still allowing it to move.

Adjusting for parallelism. To correct a misaligned table, insert shims in the ways, then snug up the gib screws and tighten their jam nuts.

Clean the Machine

Before tuning a jointer, you want everything working in your favor. That includes easy feeding of workpieces for testing purposes. So begin by wiping down your tables and fence with mineral spirits and scrubbing them with fine steel wool to remove any fine rust or gummy residue. Finish up with a well-buffed coat of paste wax to protect the metal and minimize feed friction. Also, clean up your work area to prevent losing springs, shims, and small tools among jointer shavings.

Replace and set the knives

Despite what you may think, changing straight jointer knives is surprisingly simple (and changing the cutters in a spiral head is even easier, but not discussed here). It involves determining top dead center (TDC) of the knife rotation, setting the knives in the neighborhood, and then tweaking them into a final, precise setting.

Many cutterheads employ either springs or jack screws to raise the knives in their slots, which makes setting the knives easier. Others have neither, which makes the job a little trickier.

Mark TDC. Hang a combination square on the fence. While rocking the cutterhead, lower the square’s blade until the jointer knife barely grazes it, indicating top dead center. Lock the square.

Determine top dead center

Lock your fence down and tape a piece of stiff paper to it. Then locate and mark TDC as shown. Carefully align the tip of the knife with the mark and wedge the cutterhead to prevent rotation. Quiet the shop so you can easily hear a knife grazing the straightedge.

Remove the knife and locking bar. Scrub the bar, the screw heads, and the cutterhead slot with fine steel wool and mineral spirits to minimize any friction that might impede easy knife adjustments.

Jig up. Lay the jig on the outfeed table with its sticks extended over the knife. For jack screw adjustment, press down on the jig while raising the knife until the sticks slightly lift off the table, and then lower the screws until the sticks just make contact with the table. Pressing down firmly on the jig, tighten only the outermost locking bar screws just enough to hold the knife in place.

Get in the neighborhood

The simple knife-setting jig shown here is a great helper. Make one from an MDF panel about 4" long by the width of your cutterhead. Glue two 1⁄2 × 1⁄2 × 41⁄2" hardwood sticks underneath the 4" piece at the ends.

Center the knife and locking bar side to side in the cutterhead slot with the inner locking screws free of the slot wall. While holding the jig firmly against the knife tip and outfeed table, snug up only the outermost locking bar screws as shown. With spring-loaded knives, press the jig down tightly until the sticks contact the table,then lightly tighten only the outermost screws. For jointers with neither springs or jack screws, tighten the outermost screws slightly with the knife too high, then push it down with the jig until the jig sits flat on the table.

Tweak the final adjustment

Your knife may still be a tad too high. Check by gauging for gaps under a straightedge laid across the knife, as shown.

With jack screws, make tiny adjustments downward and recheck with the straightedge and feeler gauge until the gap just disappears. Hold the knife down firmly as you snug up the two outer screws just enough to hold the knife in place. Don’t overdo the torque or you risk shifting the knife. Leave the inner screws alone.

With spring-loaded knives, press down on a straightedge lying across one end of the knife, and loosen the locking screw at that end. Still pressing on the straightedge, loosen the opposite screw just enough to allow the knife to rotate downward under pressure from the straightedge as shown. Tighten the locking screw at the straightedge end just enough to prevent the knife from lifting. (This maneuver requires a bit of trial-and-error to inform you how much torque to apply.) Now repeat the same procedure at the opposite end of the knife, as shown. For jointers without spring-loaded knives, simply tap the knife down with a screwdriver handle.

The knife should now be dead level with the outfeed table, but check for gaps with the straightedge and feeler gauge to make sure. If necessary, repeat the procedure to fuss the knife into place. Just do it. It’s worth the extra few minutes.

Remove the cutterhead wedges and rock the knife back and forth under the straightedge. You should hear the two kiss lightly without the straightedge sliding. Now carefully tighten all the inner locking screws to the same light torque you applied to the outer screws. Then tighten all the screws down firmly and move on to your remaining knives using the same technique.

Gauge for gaps. Extend your straightedge across the knives at each end, and try to slip a .015 feeler gauge blade underneath on the table. If there’s a gap, you’ll want to do a final tweak.
One end… Press the straightedge down on the freed end of the knife, and loosen the opposite locking screw enough to allow the knife to rotate without shifting upward.

…then the other. After tweaking the first end of the knife in place and lightly locking it down, repeat the procedure at the opposite end.

Test the tune-up

Square the fence to the table, grab two boards about 6 × 36" and joint one edge of each. Place the edges together and inspect for gaps. The joint should be perfect or nearly so. It’s not uncommon to find that the edges are very slightly convex, yielding tiny gaps at the ends of the joint. To fix this, lower the outfeed table by a hair, take another pair of cuts, and check them again. If the joint still isn’t perfect, lower the table a tiny bit further, repeating your test cuts if necessary until the joint mates perfectly along its length. If you go too far, a sniped (overcut) area will appear at the trailing end of the cut. In that case, raise the table in tiny increments until the snipe disappears. Your jointer should now be cutting perfectly straight.

Sharp and Straight

Freshly sharpened knives must be dead straight. Unfortunately not all sharpening shops are up to the job. Check your sharpener’s work against a good straightedge before installing the knives. If the job’s not up to snuff, have them redo it, or try another shop. I send my high speed steel knives to Ridge Carbide Tool: (800) 443-0992; ridgecarbidetool.com. It’s best to replace knives one at a time to prevent distorting the cutterhead. A spare set of knives allows this and prevents downtime while one set is at the sharpener’s. 

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