How to Diagnose a Jointer That Produces Concave Edges
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A jointer that leaves a concave edge has removed too much wood from the middle of the board, or too little from its ends. The defect is often called a hollow or scalloped edge. Before changing the table settings, check how you are feeding the stock: a small change in hand pressure can create the same result as a machine fault.
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Confirm the defect before adjusting the jointer
Mark the edge with a pencil and take a light pass, about 1/32 inch or less. Check the result with a reliable straightedge, or put the board against a known-flat reference surface. A short straightedge can miss a gradual hollow, so use one nearly as long as the board. If the edge is straight after a light pass, the previous cut may simply have been too aggressive.
Check the board as well as the machine. A twisted or bowed board can rock on the infeed table, and a cupped face can make it difficult to hold the edge upright. Joint one face flat first, then use that face against the fence. Keep your hands positioned to support the stock without pressing it sideways into the fence or forcing it down at the ends.
Rule out feed pressure and cutting technique
During the cut, apply downward pressure over the infeed table ahead of the cutterhead. Once most of the board has passed the cutterhead, shift that pressure to the outfeed table. The outfeed table supports the finished portion; it should not be used to lever the board down onto the infeed table. Keep pressure close to the cutterhead, but never put a hand over it.
Too much downward pressure near the infeed end can bend a long, thin board into the table. The cutter then follows that deflection, producing a hollow after the board relaxes. Pressing down hard on the outfeed end before it is fully supported can also tip the stock and worsen the cut. Make two or three light, even passes rather than trying to remove a large amount at once.
If only one operator’s cuts are hollow, technique is a strong suspect. If the defect repeats with a short, stiff test board and careful pressure, inspect the machine.
Match the symptom to the likely cause
| What you see | Likely cause | First check |
|---|---|---|
| Hollow appears mainly on long or thin boards | Pressure flexes the stock during the cut | Use lighter pressure and support the board across both tables |
| Hollow repeats on short, rigid test stock | Table alignment or cutterhead setup | Check the outfeed table against the knives or inserts |
| Edge is straight near one end but hollow near the other | Uneven pressure, board twist, or fence alignment | Check the board for twist and verify fence position |
| Cut has pronounced ridges or scallops | Dull, nicked, or uneven knives; feed too fast | Inspect cutting edges and slow the feed |
Check table alignment and straightness
Unplug the machine before inspecting or adjusting it. The outfeed table should be level with the knives or cutterhead inserts at their highest point of rotation. If it sits too low, the board may dip as it leaves the cutterhead; if it is too high, the board can catch or fail to cut cleanly. Follow the jointer manual’s adjustment method and use a straightedge or a setting gauge suited to that cutterhead.
Also check that the infeed and outfeed tables are coplanar. Place a long, accurate straightedge across them in several directions, including along both sides and down the center. A gap that changes noticeably across the table suggests misalignment or a warped table. Do not assume the fence is the cause of a hollow edge: fence squareness affects the angle of the edge, while table or pressure problems are more likely to create a lengthwise hollow.
For a detailed setup, a jointer knife-setting gauge can make knife height checks easier. A straightedge is cheaper and often adequate for initial diagnosis, but it must be genuinely straight and long enough to reveal the error.
Inspect the knives or inserts
Unplug the jointer and follow the manual before opening the cutterhead area. Look for resin buildup, a chipped edge, loose fasteners, or one knife sitting higher than the others. A damaged knife can leave ridges, while uneven knife heights can make the cut inconsistent. Replace or reset knives as a matched set where the manufacturer requires it. On a spiral or helical head, check that inserts are clean, seated flat, and not chipped.
Do not try to correct a dull or uneven cutter by lowering the outfeed table. That changes the support geometry and can introduce a new defect. If you are not comfortable setting knives, paying a service technician may cost less than losing time to repeated adjustments—or damaging the cutterhead.
Choose the least costly useful fix
Start with technique and a light test cut; they cost nothing and solve many cases. If the setup checks out but the cut remains rough, replace worn knives or damaged inserts with parts specified for your machine. A compatible set of replacement jointer knives is usually a sensible purchase when the existing edges are dull or nicked, but buying knives will not fix misaligned tables.
If the tables are not coplanar, consult the manual before deciding whether to adjust them yourself. Some jointers have straightforward adjustment points; others are difficult to align and may require service. A precision straightedge is useful if you expect to maintain the machine, but an occasional user can borrow one or ask a repair shop to verify the alignment. Spend on a measuring tool only if it solves a recurring setup need.
After any adjustment, take a shallow pass on a short, straight test board. Check the edge with a long straightedge, then repeat with the longest stock you commonly joint. If the short board is straight but long boards still come out hollow, revisit pressure and support before changing the machine again.
