How to Check Jointer Fence Flatness Along Its Height
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A jointer fence can look straight from end to end and still be unsuitable for accurate work. A shallow hollow, a high spot, or a slight twist changes how the board is supported as you move it across the cutterhead. The result may be tapered edges, inconsistent bevels, or a jointed face that is flat only over part of its length.
Checking the fence along its height means testing both its long face and the relationship between its upper and lower sections. This is different from checking whether the fence is square to the tables. Do the flatness test first; otherwise, you may spend time correcting a square-setting problem that is actually a bent or twisted fence.
Contents
Tools You Need
The best tool is a straightedge longer than the fence. For a typical 6-inch jointer, a 24- to 36-inch precision straightedge is practical. A known-straight steel rule, machinist’s straightedge, or quality precision machinist straightedge is preferable to a framing square or level. A level can be straight enough for rough work, but its accuracy is not guaranteed along the measuring edge.
You will also need a bright flashlight, clean paper or feeler gauges, a marker, and a stable way to hold the fence. A pair of small clamps can help, although do not clamp so hard that you bend a thin aluminum fence. A flat reference surface, such as a table saw table or a thick piece of float glass, is useful for checking the straightedge itself.
For a quick shop check, strips cut from ordinary printer paper work well. Standard paper is roughly 0.004 inch thick, so it can reveal a gap that is too small to see clearly. A proper feeler gauge set gives repeatable measurements when you need to decide whether a problem is acceptable.
Prepare the Jointer
Disconnect the jointer from power before putting your hands near the cutterhead. Lower or remove the cutter guard if necessary, but keep the knives covered whenever possible. Clean resin, dust, and rust from the fence face and from the straightedge. One chip trapped between them can look like a serious low spot.
Move the fence to a convenient position and lock it firmly. If the fence slides or pivots while you test it, the readings are meaningless. Put the infeed and outfeed tables at the same height if the machine design allows it, and make sure the fence mounting hardware is tight. Do not use the tables as your flatness reference unless you already know they are properly aligned.
Check the Fence Face
Hold the straightedge vertically against the fence, with its long edge touching the fence face. Start at the left end and look for light between the two surfaces. Repeat at the center and right end. Do not push the straightedge hard against the fence; light finger pressure is enough.
Try the straightedge at several heights: close to the table, halfway up, and near the top edge. This is the important part of the inspection. A fence may be straight at the bottom but bowed near the top, or it may have a raised corner caused by a damaged mounting bracket.
Slide a paper strip along the joint between the straightedge and fence. Note where it stops or becomes loose. Then reverse the straightedge end for end and repeat the test. If the gap moves with the straightedge, the straightedge may be the problem. If the gap stays in the same place on the fence, the fence is showing a defect.
Mark high spots and gaps with a pencil. A continuous gap in the middle indicates a concave fence. Light showing at both ends indicates a convex fence. A gap at only one corner, or a reading that changes from top to bottom, suggests twist rather than a simple bow.
Measure and Interpret Flatness
Use feeler gauges to estimate the largest gap. Place the thinnest blade that will enter the gap without force, then increase thickness until the blade will no longer fit. Measure at the center of a hollow and at any isolated high spot. Record the location and the fence height, not just the maximum number.
| Measured gap | Likely use | What to do |
|---|---|---|
| 0.001–0.003 inch | Most general jointer work | Usually acceptable if the gap is smooth and repeatable |
| 0.004–0.010 inch | Light work and short boards | Usable with care, but inspect jointed edges and consider correction |
| Over 0.010 inch | Precision edge jointing | Correct, shim, or replace the fence before relying on it |
These are practical limits, not universal machine specifications. A short board may bridge a hollow without trouble, while a long board can rock against the same defect. A high spot is often worse than a smooth hollow because it can tilt narrow stock and produce a repeatable angle error. Twist is particularly troublesome: the fence may appear acceptable at one height but shift the board as pressure moves upward.
Check for Twist Along the Height
Place the straightedge vertically at the left end and note where it contacts. Repeat at the right end. Now place it near the top and bottom edges, keeping the same vertical orientation. If the contact pattern changes from one end to the other, the fence may be twisted.
A more sensitive method uses two straightedges or two equal-height blocks. Hold one reference at the bottom of each fence end, then compare the distance from the fence face to a parallel line near the top. Differences between the ends indicate that the upper portion is not parallel to the lower portion. You can also use a digital caliper to record the distance from a reference edge, but only after confirming that the reference edge is straight.
Correct, Shim, or Replace?
Small errors may be corrected by adjusting the fence mounting hardware or adding thin shims behind the fence at specific mounting points. Shimming is inexpensive, reversible, and often adequate on a cast-iron fence with a slight alignment issue. It is not a good fix for a visibly bowed face, because the board will still contact only the high areas.
Aluminum fences can sometimes be carefully flattened, but aggressive filing or sanding can create a new hollow. A replacement jointer fence replacement makes more sense when the fence is bent, twisted, or damaged around its mounting slots. On an inexpensive jointer, compare the cost of the part with a longer straightedge and a replacement machine fence. If the error is below about 0.003 inch and your work is mostly short boards, the cheaper option is usually to keep using it and verify results with a test cut.
After any repair, repeat the straightedge test at three or more heights, then check the fence square to the tables. Finally, joint a straight test board and inspect the edge against a known-flat surface. Measurements find the defect; a controlled test cut confirms whether it matters in actual work.
