A sawmill converts round logs into rectangular lumber through a sequence of debarking, primary breakdown, edging, trimming, sorting and drying. For pallet stock, the mill aims to cut many boards and cants of standard dimensions from small and low-grade logs while wasting as little as possible as sawdust, slabs and chips.
Overview of the flow
| Stage | What happens | Typical equipment |
|---|---|---|
| Log yard | Logs are received, sorted by diameter and length, stored or sprinkled | Loaders, log decks, scanners |
| Debarking | Bark is removed | Ring or rotor debarker; see debarkers |
| Primary breakdown | Log is cut into cants, boards and slabs | Headrig bandsaw, circular saws, chipper-canter |
| Resawing | Cants or thick pieces are cut into boards | Band resaw, multi-rip saw |
| Edging | Wane (bark edges) is removed and boards are cut to width | Edger, gang saws |
| Trimming | Board ends are squared and cut to length | Trimmer saws |
| Sorting and grading | Boards are graded by grade, size and moisture | Green chain, scanners |
| Stacking and drying | Boards are stickered, then air or kiln dried | Stackers, yards, kilns |
Log yard and debarking
Logs arrive by truck or rail, often in lengths of 2.5 to 6 m, and are scaled or scanned to measure diameter and volume. Sorting logs by diameter is important, because sawing patterns are set for a given diameter class. Mills may store logs under water sprinklers to prevent end checks and fungal stain in warm weather.
Debarking removes bark before sawing. Bark contains grit that dulls saws, and clean chips from the slabs are worth more to pulp mills. Ring debarkers rotate tools around a log fed through them, while rotor debarkers rotate the log against tools. Bark is typically burned for energy or sold as mulch. For wood packaging, debarking also helps satisfy phytosanitary rules in some importing countries; see the debarked wood requirement.
Primary breakdown
Primary breakdown, performed by the headrig, cuts the log into cants (squared beams), boards and slabs. Main methods are:
- Bandsaw headrig: a thin continuous steel blade running on two wheels. It has a narrow kerf (saw cut), which preserves wood, and it can cut large logs. It is flexible and suited to higher-value hardwood and mixed logs.
- Circular saw headrig: one or more circular blades, usually with a bit more kerf but high speed. Often used for small logs.
- Chipper-canter lines: common for small softwood logs. Rotating chipper heads cut the log's outer surfaces into chips and shape a cant, which then passes to gang or multi-rip saws. These lines are fast and produce standard-dimension lumber from small, straight logs.
- Portable and small mills: a single bandsaw or circular saw for local production; see small sawmill equipment.
Pallet-oriented mills often cut cants and then resaw them into boards of 15–25 mm thickness, matching the dimensions of deck boards, stringer boards and blocks; see resaws and band resaws and multi-rip saws.
Edging, trimming and grading
Boards from the headrig have rough wane edges. An edger removes wane and cuts the board to the required width. Modern edgers use scanners to optimize the cut: transverse scanners locate wane to produce the widest usable board, according to industry coverage of pallet sawlines. Then trimmers square the ends and cut to length, removing defects such as end splits.
Boards are graded for knots, wane and other features, either by graders on a green chain (the sorting conveyor for green lumber) or by scanners that detect knots, splits and rot by color and shape. The grade determines whether a piece becomes pallet stock, higher-value lumber, or is chipped; see lumber grading and timber defects.
Drying and finishing
Green lumber must be dried before use in most pallet programs. Boards are stacked with stickers and dried by air or kiln, as described in air drying and kiln drying. Some pallet mills sell green-cut boards to pallet plants nearby, which dry or heat-treat finished pallets. Export pallets are heat-treated and marked according to ISPM 15; see heat treatment.
Co-products and yield
Not all log volume becomes lumber. Typical co-products are:
- Slabs and edgings: chipped for pulp or panel mills, or cut into pallet parts and blocks.
- Sawdust and shavings: used for fuel, animal bedding or panels.
- Bark: used for energy or landscaping.
Lumber recovery depends on log size, taper, saw kerf, sawing accuracy and the product mix. A US Forest Service study of lumber recovery found that reducing kerf from 12/32 to 9/32 inch increased yield by about 7% on logs of 5.5–12 inch diameter, and that computer-optimized edging reached 97% of the theoretical optimum. Cutting patterns are explained in log yield and cutting patterns.
Mill scale and automation
Large softwood mills process hundreds of logs an hour with scanners, computers and optimizers controlling each cut. A pallet-industry article on sawline scanning notes that many high-recovery softwood canter lines are shut down if the scanner and optimization system stops working, because manual operation cannot match its recovery. Small mills use simpler, manually operated bandsaws or circular saws but follow the same stages.
Safety
Sawmills are among the more hazardous industrial environments, with moving blades, heavy logs, dust and noise. Guarding, lockout procedures, dust control and training are essential; see pallet plant safety.
Frequently asked questions
What is a headrig?
The headrig is the first saw in a sawmill that breaks a log down into cants, boards and slabs. It may be a bandsaw, circular saw or chipper-canter.
What is a cant?
A cant is a log that has been squared on two or four sides, ready to be cut into boards or used directly as a beam or pallet component.
Why are logs debarked before sawing?
Bark carries grit that dulls saw blades and contaminates chips, and wood packaging for ISPM 15 marking must be debarked. Removing it also allows clean chips to be sold.
How much of a log becomes lumber?
It varies widely with log size and mill technology. Optimization, narrower kerfs and accurate sawing improve recovery, and small logs recover less than large ones.
Sources
- Sawlines: Scanners, Computers Optimize Machines — Pallet Enterprise
- Lumber recovery (General Technical Report FPL-39) — USDA Forest Products Laboratory
- Wood Handbook: Wood as an Engineering Material — USDA Forest Products Laboratory
- Beware of Blue Stained Wood in Wood Packaging Materials — Northeastern Lumber Manufacturers Association