How to Test a Jointer Table for Coplanarity
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A jointer’s infeed and outfeed tables must be coplanar: both surfaces need to lie in the same plane, without one corner sitting high or low. If they are not, the jointer can produce tapered boards, concave faces, or boards that rock as they leave the cutterhead. The problem is often blamed on knives or fence alignment when the tables are actually twisted.
You can test coplanarity with a straightedge and feeler gauges, or with a dial indicator. The straightedge method is quick and adequate for many home shops. A dial indicator costs more but shows exactly where the error occurs and makes adjustments easier to verify.
Contents
What Coplanarity Means
Coplanarity does not mean the tables are parallel to each other in every possible measurement. A jointer’s tables form one continuous reference plane, while the cutterhead sits below that plane by the amount of cut being taken. The outfeed table must be flush with the knives at their highest point, and the infeed table must be parallel to the outfeed table when set to the same height.
For practical woodworking, a small error is acceptable. A deviation of about 0.001 to 0.002 inch across the table length is generally very good. Around 0.003 inch may still be usable for rough stock, but it can become visible on wide boards or when making light finishing cuts. Errors over 0.005 inch deserve correction rather than workarounds.
Tools You Need
Use the longest straightedge that fits comfortably across the machine. A 24-inch machinist’s straightedge is useful on compact jointers; a 36- or 48-inch precision straightedge is better for longer beds. Do not substitute a framing square, level, or ordinary ruler. Those tools can be straight enough for construction but are not reliable for measuring a few thousandths of an inch.
You will also need a set of feeler gauges, a clean rag, and a bright light. A dial indicator mounted on a magnetic base or a jointer table gauge is optional. A 0.001-inch dial indicator is appropriate; a 0.0005-inch indicator provides more resolution but does not make a rough machine more accurate.
If you are buying equipment for occasional checks, a precision straightedge and feeler gauge set is the sensible low-cost choice. Buy the longest straightedge you can store without it being dropped or bent.
Prepare the Jointer
Disconnect the jointer from power. Remove the fence if it obstructs the measurement, and clean both tables thoroughly. Pitch, rust, and packed dust can easily create a false gap. Lower the infeed table below the outfeed table so the straightedge can bridge the cutterhead area.
Check that the machine is sitting firmly. A twisted stand or uneven floor can distort a cabinet and change the reading. Leveling the machine is not the same as making the tables coplanar, but a jointer that rocks on its base should be corrected before you diagnose table alignment.
Raise the cutterhead knives to their normal working height, or use the cutterhead body as the reference if the knives are removed. Keep your fingers clear of the cutterhead. Never rotate it by pulling on an exposed knife edge.
The Straightedge and Feeler-Gauge Test
Place the straightedge lengthwise over both tables, spanning the cutterhead opening. Put it near the front edge, then near the rear edge. Add a feeler gauge under any visible gap. Repeat with the straightedge along the left side, center, and right side of the tables.
Record both the gap size and its location. A consistent gap along one side suggests that one table is tilted relative to the other. A gap that changes from front to back indicates twist. If the straightedge rocks, the tables may have a high corner or a raised spot.
Next, place the straightedge diagonally from the front-left corner to the rear-right corner, then reverse the diagonal. Diagonal checks are important because a table can pass front-to-back and side-to-side checks yet still be twisted. Test across each individual table as well as across both tables together.
| Observed result | Likely problem | What to check |
|---|---|---|
| Gap remains the same along one edge | One table is tilted | Table mounting bolts, ways, and adjustment mechanism |
| Gap changes diagonally | Table or machine bed is twisted | Stand stability, mounting stress, and table casting |
| Straightedge rocks on a high corner | Local high spot or burr | Clean surfaces and inspect for damage |
| Tables align but board still tapers | Knife height, fence, or technique problem | Outfeed height and fence squareness |
Using a Dial Indicator
A dial indicator gives a more repeatable test. Mount it to the outfeed table or a rigid bridge so the plunger contacts the infeed table. Zero the indicator at one point, then move the mount or slide the table through the same front, center, rear, and diagonal positions used with the straightedge.
Do not trust a flexible magnetic base or a mount that shifts while you measure. The indicator should contact the table with light, consistent pressure. If the reading changes by 0.004 inch from one corner to another, that is table twist or tilt—not necessarily a defect in the indicator.
A 0.001-inch dial indicator with magnetic base is also useful for setting knives and checking planer tables. For occasional jointer maintenance, however, it is not mandatory if your straightedge and gauges are accurate.
Fix Common Problems
First, loosen and retighten the table mounting hardware according to the machine’s design. Tightening one bolt too aggressively can pull a table into a twist. Support the table while making adjustments, and change one variable at a time.
Inspect the table ways, dovetail slides, gib strips, and elevation screws. Chips under a gib or a damaged adjustment screw can prevent a table from settling consistently. Clean and lubricate only where the manufacturer permits; excess oil can attract abrasive dust.
Do not correct a warped cast-iron table by grinding it casually. Removing material may destroy the reference surface and can make the machine unsafe. A qualified machine-tool repair shop is the right option for a visibly bowed or cracked table. Replacement tables are often more economical on small benchtop jointers.
Verify the Result in Use
After adjustment, repeat every measurement, including the diagonals. Then set the outfeed table flush with the cutterhead knives. With the machine unplugged, place a short straight strip across the outfeed table and rotate the cutterhead by hand. The knives should move the strip slightly—typically about 1/8 inch—without lifting it or leaving it behind.
Make a test cut on straight, reasonably flat stock. If the board still tapers, check your feeding pressure: pressure belongs on the outfeed table after the cut begins. Also verify that the fence is square and that the knives are equal in height. Coplanar tables cannot compensate for poor knife setting or forcing a warped board against the machine.
For a maintenance kit, a jointer knife-setting gauge is a worthwhile addition. It addresses one of the most common problems that can look like a coplanarity error.
