Jointer Tune-Up

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This article is from Issue 28 of Woodcraft Magazine.

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.

How a Jointer Works

In order to tune up a jointer, you need to understand the operating principle. As shown in the figure 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, a straight cut won’t result.

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 coat of paste wax to protect the metal and minimize feed friction.

Check each table for flatness using a precision straightedge, gauging any gaps underneath using an automotive feeler gauge.

Adjust the tables to the same height and then check their parallelism using a straightedge.

Check the tables

Make sure your outfeed table is set to its proper height in relation to the cutterhead and knives, as shown in your owner’s manual. If that’s missing, adjust a knife so its shoulder behind the bevel projects no more than about 1/16", as shown in Figure 1. Then lay a straightedge across the outfeed table and knife. Rock the cutterhead while adjusting the table height until the knife barely grazes the straightedge. (This preliminary setting doesn’t need to be perfect along the length of the knife.) Lock the table.

Use a precision straightedge and feeler gauge to check each table for flatness (Photo A). Humps or dips can compromise operational accuracy. Unfortunately, there’s not much you can do about them, but at least you’ll be aware of the problem. If a tune-up doesn’t correct problems, it may just be time to invest in a better machine.

Check the parallelism of the two tables. Adjust the infeed table so that it’s level with the outfeed table, then extend a straightedge across both to check for parallelism. If your individual tables are flat, a 24" straightedge will do the job. Press down firmly on one end of the straightedge while extending the majority over the opposite table as shown in Photo B. If the tables are out of parallel, you’ll need to adjust one of them. If the tables are parallel, you can focus your attention on the knives. Adjusting the tables for parallelism 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 allow removing slop from the table attachment while still letting it slide on the ways for height 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 in the ways track 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 top of each track. If the table sags at its outer end, place the shims at the lower end of the ways, as shown in Photo C. When the tables are parallel, snug up the gib screws just enough to stabilize the table while allowing it to slide on the ways.

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

Locate top dead center of the knife’s cutting arc by slowly rocking it against the end of a combination square’s blade.
With the knife at top dead center, align the square’s blade to the knife tip, and mark the location on the jointer fence.

Replace and set the knives

For one reason or another, most woodworkers do everything they can to avoid knife-changing. As a result, they put it off until their knives are too dull and nicked to produce good work. Despite what you may think, the process is surprisingly simple. Whether your jointer’s cutterhead employs spring or jack screws (see Figure 2, below), replacing the knives involves the same basic steps: determining top dead center (TDC) of the knife rotation, setting the knives in the neighborhood, and then tweaking them into a final, precise setting.

Mark top dead center

Find and mark top dead center of your knives’ cutting arc. Lock your fence down and tape a piece of stiff paper to it. Hang a combination square on the fence, as shown in Photo D. While rocking the cutterhead, lower the square’s blade until the jointer knife barely grazes it, indicating top dead center. Lock the square. Then mark TDC as shown in Photo E. Carefully align the tip of the knife with the mark and wedge the cutterhead to prevent rotation.

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.

Get in the neighborhood

Regardless of whether you’re dealing with jack screws or springs, the simple knife-setting jig shown in Photo F 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. 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. With spring-loaded knives, press the jig down tightly until the sticks contact the table. Again, lightly tighten only the outermost screws.

Tweak the final adjustment

Your knife may still be a few thousandths of an inch too high. To check, extend your straightedge across it at each end, and try to slip a .015 feeler gauge blade underneath on the table (Photo G). If you can’t, the adjustment is fine. If there’s a gap, you’ll want to do a final tweak.

While holding the jig firmly against the knife tip and outfeed table, snug up only the outermost locking bar screws.

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, it’s just a bit trickier. Pressing down on a straightedge lying across one end of the knife, 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 (Photo H). Tighten the locking screw at the straightedge end just enough to prevent the knife from lifting. (This maneuver requires some nuance, but a bit of trial-and-error will quickly inform you how much torque to apply.) Now repeat the same procedure at the opposite end of the knife (Photo I).

Check preliminary knife adjustment by gauging for gaps under a straightedge laid across the knife.

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.

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

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.

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.

Check a newly sharpened knife against a straightedge to ensure the cutting edge is dead straight.

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. 

About Our Author

Paul Anthony is a woodworking writer, photographer, and teacher living in Riegelsville, Pennsylvania. His latest book is Taunton’s Complete Illustrated Guide to Tablesaws, Taunton Press (Woodcraft #149696).

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