Kerf, sawing variation and target size

What kerf is

Kerf is the saw plate thickness plus the set of the teeth, and every saw line turns that strip of wood into sawdust. The more cuts a pattern needs, the more kerf is lost, which is why kerf matters most for patterns with many cuts and for small logs, where boards are thin relative to the log.

Evidence on kerf

• Steele (1984, USDA Forest Products Laboratory, North America) cites a 1962 study in which cutting kerf from 12/32 to 9/32 inch raised board-foot yield by an average of 7 percent for logs 5.5 to 12 inches in diameter.
• In his worked example an 8.3-inch, 16-foot log gained 8.3 percent when kerf fell from 0.240 to 0.180 inch. The gain did not come from an extra board; a pattern change allowed wider lumber. A 10.1-inch log gained 4 percent through longer boards. Kerf savings are discontinuous: they appear when they tip the log into a better pattern.
• A Polish study of Scots pine (Wieruszewski et al. 2023; Europe) compared a horizontal band saw with a vertical frame saw on six large logs (375 to 435 mm mid diameter). Mean yields were about 72.8 and 71.8 percent in the tables, but 72.6 and 69.7 in the text and 72 and 69 in the abstract, so the paper is internally inconsistent. The frame saw had a 2.2 mm plate with 0.9 mm set per side; the band saw a 1.0 mm plate with 0.75 mm set per side. Counting set on both sides gives cut widths of roughly 4.0 and 2.5 mm (portal calculation, dependent on the authors' definition). The yield difference was not statistically significant, and with six logs the test had little power.

Sawing variation and target size

Sawing variation is the scatter in thickness of boards cut at the same setting, caused by saw wander, vibration, feed problems and worn guides. Because planing needs a minimum green size, mills add an oversizing allowance proportional to variation. Reduce the variation and the allowance can shrink, so each board is thinner and more boards fit in the log.

• Stern and colleagues (1979), cited by Steele, estimated that improving sawing accuracy from 0.2 to 0.1 inch on a machine with 0.125 inch kerf would raise board-foot yield by 6.0 percent (North America, estimate). Steele adds that planing allowances are often excessive and can usually be reduced.
• Kilborn (2002, Alaska; North America): 23 studies at 22 mills, 1,666 straight sound logs. Thickness variation exceeded the industry standard of 0.030 inch in 17 studies and oversizing was a problem in 18. The three most efficient mills were at 0.020 inch or below. Kilborn estimated roughly 2.5 percent more recovery for each 0.030 inch (about 0.76 mm) removed from product thickness. Recommendations: reduce target thickness, align and maintain equipment, run a quality control programme, even weekly or monthly.

The effect size on a modern high-speed European line may differ; no open European measurement is available in the articles.

Worked examples

Cut width. Band saw: plate 1.0 mm + 2 x 0.75 mm set = 2.5 mm. Frame saw: plate 2.2 mm + 2 x 0.9 mm set = 4.0 mm. A cant resawn into 12 boards needs 11 saw lines. Kerf lost: band 11 x 2.5 = 27.5 mm; frame 11 x 4.0 = 44.0 mm; difference 16.5 mm, which is about half of a 33 mm board. This illustrates how kerf accumulates (our arithmetic).

Oversizing. A mill removes 0.090 inch of oversizing. In steps of 0.030 inch that is 3 steps, so the estimated gain is 3 x 2.5 = 7.5 percent. Treat this as an order of magnitude, since the source states the rate per 0.030 inch and linear scaling is our simplification.

Checklist: a first kerf and variation audit

• Record saw plate thickness and set for each saw and compute the cut width.
• Measure board thickness at several points along and across boards (calipers) and quantify scatter within and between boards.
• Compare target thickness against the final planed dimension and question the allowance.
• Check saw guides, alignment and feed before buying thinner saws; a thin saw with poor guiding raises variation and gives back the gain.

Key points

• Kerf = plate thickness + set; cut width is plate plus set on both sides if both sides are set.
• Kerf savings are discontinuous: they pay when they enable a better pattern (Steele, North America).
• The six-log Polish test cannot prove or disprove a kerf effect.
• Variation forces oversizing; Kilborn estimated about 2.5 percent more recovery per 0.030 inch (0.76 mm) removed (Alaska).
• First tighten variation, then consider thinner kerf.