A mill is a flow of material
A sawmill converts logs into lumber and byproducts. Looking at it as a flow is the easiest way to understand it. Logs enter at one end, pass through a series of machines that remove bark, cut the log into pieces, trim those pieces to size, and sort them. Everything that is not lumber, such as bark, sawdust and chips, leaves along its own paths. Because wood is a natural material that varies from log to log, the mill’s goal is to get as much useful lumber as possible from each log, a measure called recovery or yield.
How logs get to the mill is a separate subject, touched on under timber felling machines and log hauling equipment. This article starts at the mill gate.
The log yard
Logs arrive on trucks and are unloaded by loaders, grapples or cranes, then measured, sorted and stored. Sorting is typically by species, length and diameter, because each size is best sawn in a particular way. Some yards keep logs wet with sprinklers to reduce cracking and insect damage during storage.
Yard equipment is mostly heavy mobile machinery, which relates to the rough terrain trucks and loaders seen in other heavy industries.
Debarking
Before sawing, bark is removed. Bark contains dirt and grit that dull saws quickly, and bark itself is a byproduct that is sold or burned. Common debarkers include ring debarkers, which spin tools around the log as it passes through, and drum debarkers, which tumble logs in a large rotating drum. Debarking is a good example of mill processes that must be tuned to avoid damaging the wood beneath.
Breaking the log down
Primary breakdown is where the log is first cut. The main machine is often called the headrig or head saw, and the choice of saw type shapes the whole mill.
- Band saws. A long continuous blade runs around two large wheels. Band saws have a narrow blade, so they turn less wood into sawdust and are popular for larger logs and valuable timber.
- Circular saws. A round blade with teeth around the edge. They cut fast and are used in many high-volume mills. Thicker blades produce more sawdust per cut, though thin-kerf designs reduce this.
- Chipper-canters. Common in mills that process smaller logs, these machines use profiling heads to turn the outer log into chips while shaping the center into a squared piece, called a cant, which then goes to gang saws.
- Gang saws and edgers. Multiple blades mounted side by side cut a cant into several boards in one pass.
Logs are often scanned before they reach the saw. Lasers or other sensors measure the shape of each log, and computers choose how to turn and position it for the best yield. The ideas are those of inspection sensors applied to a raw material.
Kerf, the width of wood removed by a saw blade, is a recurring theme. Every bit of kerf turns into sawdust, so mills aim for thin, stable, well-tensioned blades. A blade that wanders cuts boards of uneven thickness, so saw maintenance is a serious function. Sharpening shops are part of many mills. The same logic applies to cutting tool wear in metal cutting, and saw blades are tensioned and leveled to cut straight.
Edging, trimming and sorting
Boards from the headrig have rough edges, often with bark remnants, called wane. An edger cuts them to produce clean, parallel edges and a defined width. A trimmer then cuts the board to length and removes defects such as damaged ends. Scanning systems often decide where to cut to give the best value from each board.
After trimming, boards go to a sorter that separates them by grade, thickness, width and length into bins or stacks. The grading step is either done by trained graders or by machine vision, following published grading rules for the species and market. The sorted lumber is stacked, often with small spacers called stickers between layers to allow airflow for drying.
Drying and planing
Freshly sawn lumber is wet. Drying is done either by air, stacked outdoors under cover, or in kilns, which are heated, ventilated chambers that remove moisture in a controlled schedule. Kilns are a specialized form of industrial dryers. Drying too fast can crack or warp the wood, so schedules are matched to species and thickness.
Dried boards may go to a planer mill, where machines surface the faces and edges so the lumber is smooth and uniform in thickness. The planers resemble those described in woodworking machinery.
Material handling
Conveyors dominate the layout. Chain, belt and roller conveyors move logs, cants and boards. Transfer decks, lugged chains and sorting gates direct them. Since a mill makes a continuous stream of pieces, a stoppage at one machine can back up the whole line. Understanding conveyor types and good upkeep of drives, chains and bearing wear patterns is central to uptime.
Byproducts
Sawdust, shavings, bark and chips are not waste to most mills. They may be sold for pulp, board manufacture or landscaping, or burned for energy to run the kilns. This means every mill needs systems to collect and move large volumes of fine material, which brings dust-related hazards, discussed under combustible dust.
Safety
Sawmills combine heavy loads, fast blades, moving conveyors and noise. Guarding, emergency stops, lockout and tagout in general terms, hearing protection and trained operators are standard. Work follows manufacturer manuals and local regulations. This article is a general overview only.