Saw mills have a number of individual wood sawing machines that are interconnected by a conveyor system. For the most part, all facets of a saw mill's operation are handled by machine. The saw mill operators operate the machine controls and occasionally free machine jam conditions.
Just as all industries have standard phrases for their process, the saw mill also has standard phrases. A cant is a log that has had one or more of its rounded edges cut off. When only one edge has been cut off and three round edges remain, it is called a one-sided cant. One-, two-, three-, and four-sided cants are all possibilities. Four-sided cants are sometimes sold, as is, for landscaping, railroad ties, and rough construction. A four-sided cant that is sold, as is, is called a timber. When a slab is cut off a log, the edges are still rounded. The rounded slab has a thinner side and a thicker side. The thinner side is called the "wane" The wane side must be up before a slab is fed into an edger or other type of saw.
Saw mill equipment falls into two categories: Wood handling equipment and wood processing equipment. Wood handling equipment holds, sorts, and moves the wood through the mill. Wood processing equipment cuts, smoothes, and treats the wood.
Logs are delivered to the saw mill by railcar, by over-the-road trucks, or by a waterway. The logs are unloaded and placed in the dry-storage yard. The dry-storage yard serves as an inactive storage area for logs that are waiting to be transferred to the wet-storage area or milllog infeed area. Logs are transferred from the dry-storage area to the wet-storage area as needed to maintain the inventory. Logs that are waiting to be transferred to the mill from the wet-storage area are subjected to a continuous water deluge spray. The water spray serves two purposes: it softens the bark to aid in the debarking process, and it wets the wood to reduce splintering during the sawing process. Some older mills still store logs in a pond, which keeps the logs continuously wet. However, this method is more labor intensive and there is less control over the amount of moisture that is absorbed by the wood. When logs are required by the mill, the crane operator positions the grapple to the log pile, picks up a number of logs, and transports the logs to the bucking saw infeed chain conveyor.
Because wood is too expensive a commodity to waste, modern mills use every part of the log. Electronic optimizers scan each log, cant, and board, and with the aid of the computerized programmable logic controller (PLC), position the wood and processing equipment for the most efficient cuts needed to maximize the final products produced from each log.
Conveyor systems are used for a wide variety of purposes in a sawmill. Chains, belts, blower systems, and rolls all play a part in the movement of material in the modern mill. Sawdust is an ever-present by-product that is frequently removed by a continuous chain conveyor that travels in a U-shaped trough. The conveyor is fitted with lugs. Sawdust and chips may also be handled by blower systems or belts. Boards and logs are usually moved by chain conveyors. Boards are sometimes handled by a conveyor that is made up of a series of rolls. Various types of gear and/or chain drives are used to power the conveyors.
One type of conveyor is used to move logs lengthwise. This conveyor, called a "log haul-up" or "jack-ladder," consists of an endless chain that is fitted with log dogs. Some sawmills use parallel chain haul-ups to elevate the logs side-ways instead of lengthwise. The speed and nature of the chains depend upon the daily capacity of the sawmill and the size of the logs. Usually, the main drive gear and head sprockets and shaft are found at the upper end of the conveyor. The tail sprockets are connected to the shaft at the lower end of the conveyor. Haul-up conveyors are subjected to severe strains and shock load. The logs may be washed by a stream of water during the haul-up to remove stream of water during the haul-up to remove unacceptable material. Logs that are received by train or truck are sometimes dumped into a flume where a current of water carries them to the foot of the log haul-up.
Small diameter logs can be carried on a type of conveyor called a "sharp chain." A sharp chain is a conveyor made of lengths of chain with sharp points mounted on the links. Pressure rollers are lowered to the surface of the log to force the log onto the teeth of the chain. The chain holds the log steady while it is being processed.
Log kickers transfer the logs off a conveyor to change the log's path flow. One log kicker is used to remove the small diameter logs while a second log kicker is used to remove the larger diameter logs. As the log passes through a log diameter detector, the log kicker computer receives a signal from the diameter detector to operate the log kickers. If the incoming log is smaller than the predetermined value programmed into the computer, the small diameter log kicker "kicks" the log onto the infeed chains of the sharp chain system. If the incoming log is larger than the predetermined value, the log kicker computer allows the log to pass by the small diameter log kicker to the large diameter log kicker. Larger diameter logs are kicked onto head rig infeed chains.
If both the sharp chain conveyor and the headrig conveyor are full, the log kickers can be reversed to kick the incoming log onto storage chain conveyors that are located behind each log kicker. Log carriages are hydraulically or pneumatically actuated pistons that feed the logs into saws. The log carriage has dogs, or stops, that hold the log through the use of a hydraulic cylinder or an air (pneumatic) cylinder. The log carriage is mounted on four or more wheels, depending on the weight and size of the loads. The wheels that are closest to the saw have a flat tread and run on a flat rail. The wheels that are farthest from the saw have a V-grooved tread that fits into a specially made V-planed guide rail. The V-grooved tread and guide rail eliminate any lateral movement of the carriage. The logs are supported on horizontal ways or head blocks and rest against vertical knees.
The carriage is moved past the saw by a feedworks. The feed works drive the carriage back and forth and control the rate at which the log is fed into the saw. Feed works make use of electric, pneumatic, or hydraulic drives. Hydraulic drives are the most common. The drives may be connected to the carriage directly, or they may be connected through a drum and cable arrangement. Hydraulic feed drives use a hydraulic motor to activate a drum and cable arrangement to pull the carriage back and forth. In the single piston hydraulic feed, a cylinder with about half the travel of the carriage is attached by a wire rope and pulley system so that the carriage moves twice as far as the piston.
At the end of each cut, the carriage returns to the starting point and, if required, a remotely controlled log turner turns the log over in the carriage. The carriage controls, called the "setworks," are usually remotely operated. The operator can control the carriage to vary the thickness of the slabs that are cut from the log.
Older mills may still use steam piston drives. The steam feed, called a "shot gun," consists of a piston head in a single long cylinder that is directly connected to the carriage and extends the full length of the carriage travel. A variation of the steam piston feed consists of piston heads in short twin cylinders that deliver power through connecting rods to turn a crankshaft that is directly connected or geared to a drum. Steam pressure can be applied to either side of the piston head to advance or reverse the carriage.
Various viscosity grades of anti-wear hydraulic oils are used in hydraulic systems. Recommend the proper viscosity MONOLEC® Hydraulic Oil or MULTILEC®. Steam cylinders should use a compounded cylinder or valve oil. The exposed screws that control movement in the head blocks may be lubricated with an industrial gear lubricant, typically the 1600 series VPGL, or with the same oil used for steam cylinders. A lighter lubricant may be applied on the ways of the headblock, but with product consolidation of importance, a heavier gear oil is often used.
Logs and lumber must be aligned prior to being fed into any sawing device. As the wood is moved along conveyors, it may become skewed or pieces may overlap. The unscrambler correctly arranges and aligns the wood. The unscrambler consists of multiple chains that are connected in pairs. Each set of chains is equipped with bar lugs. Spiked teeth in each bar lug extract boards from a V-notch holding area. As the bar lugs travel up the initial face of the unscrambler, singulating cams lift the leading edge of each board, which causes any "piggy-back" boards to fall back into the V-notch. The individualized boards continue onto a set of even-ending rollers and onto the turn/pass unit.The boards stop at the turn/pass unit. The operator must review the stopped boards for two conditions. First, the operator must analyze the board to see if it is good enough to make a board. If the board is a piece of slab or is just plain trash, the trash tipple is actuated, which allows the trash board to fall by gravity into the "under-floor trash recovery system." At the completion of the trash dump, the trash tipple is closed. If the board is good, the operator must next determine the position of the wane. If the wane is upward, the operator allows the board to proceed. However, it the wane is down, a turn mechanism is actuated to flip (turn) the board 180 degrees.
Sorters separate the finished boards according to length, using the same concept that change sorters use to separate pennies, nickels, dimes, and quarters by size. The finished boards that enter the sorter are electronically scanned, coded, and placed in a sequential order that is established by the scanning information received from an optimizer and an electronic encoder. The sequential order position information is transmitted to the sorter's computer. The computer also receives board width information from another scanner that is installed just before the sorter. Each board is discharged onto the sorter skid plates. The boards are pulled along by continuous running drag chains. As the electronically monitored boards travel down the skid plates, tipple arms raise and allow the board to fall into the nylon slings of the soft sling pockets. The computer program determines which tipple arm and soft sling pocket receives a specific board. When a predetermined number of boards are in a sling pocket, a latch arm automatically releases to allow the boards to discharge gently onto haul-out chains. The nylon slings retract, latch hooks actuate to capture the sling Clevis, and the sling pocket is ready to receive sorted boards. The haul-out chains transfer the boards to the stacker.
The boards that are received from the sorter are directed into a V-notch of the stacker incline conveyor. The incline conveyor transports the boards into the V-notch of the unscrambler. The incline conveyor begins the separation of the pile of boards.
The boards are removed from the V-notch of the unscrambler by bar lugs that are attached to parallel chains. As the bar lugs remove the boards from the V-notch, singulating cams knock any "piggy back" boards back into the V-notch. When the single boards crest the top of the unscrambler, the boards feed onto even-ending rollers. The even-ending rollers move the boards toward the near side of the machine while chain conveyors move the boards onto sequencing chains.
The sequencing chains feed the boards onto the stacker arms in a nonstop, side-by-side manner. As the boards feed to the stacker arms, the boards pass a second even-ending device. When the lead board comes into contact with the stacker arm hooks, the boards begin to back-up and block photocells. When the photocells are blocked, the sequencing chains are stopped. An operator counts the number of boards on the stacker arms that sit between the stacker arm books and the pinch point. The arm hook position is adjusted to obtain the correct number of boards. When the correct number of boards is established, the stacker is placed in Auto. In automatic operation, the boards feed forward until the correct number of boards are on the stacker arms. The stacker arms lift and move forward to deposit the stack layer on the board elevator. The board elevator declines as each stack is placed on the elevator. The cycle continues until a predetermined height of boards has been reached on the declining elevator. When the stack is complete, the elevator drops below chain-driven discharge rollers that remove the stack. The stack is transported to the exterior shipping area where forklift trucks transport the stack to drying kilns.
As previously discussed, every piece of the log (the bark, the wood, and the sawdust) is used. To extract the most from each log while minimizing the amount of waste, several different pieces of equipment are needed. This equipment includes barkers, saws, and kilns, which are discussed in this section.
Barkers remove the bark from logs while minimizing the amount of useful wood that is removed. Several different styles of barkers exist. The barker uses abrasive wheels to remove the majority of the bark. The abrasive wheels are set at an angle and rotate about the circumference of the log. The debarked log is moved to either the infeed of a bucking saw or to the infeed of a sharp chain. The operator chooses the path of the log.
In the past, bark and sawdust were nuisance waste products. Today, bark and sawdust are valuable assets that are not wasted. The bark that is removed from the lag is transported by a conveyor to either be burned on site in a boiler plant or sent to another converting facility that uses the bark to create other wood or agricultural products.
The basic variations of saws that are used in saw mills are circular saws, band saws, and reciprocating saws. Most rough cuts and sizing are accomplished with circular saws. Circular saws are also used to rip where small or low-grade logs are handled and investment must be kept to a minimum. Band saws consist of a continuous band of steel from nine to sixteen inches wide that is stretched around upper and lower wheels. The band may have teeth on one or both edges. The upper and lower wheels are forced apart to maintain a tension, called a "strain," on the band saw blade of approximately 35,000 pounds. The tension is monitored and automatically adjusted to maintain the strain at the predetermined level. The proper strain is maintained to ensure the maximum production of accurately sawed lumber. Reciprocating saws consist of straight-tooled, steel blades that are held between two arms with the proper amount of tension. A rotary cam imparts the reciprocating cutting motion to the blade.
The saws that are used in a mill are named according to the saw's use. These saws include the bucking saw, the head saw/slab saw, the resaw saw, and edgers. A light paraffin oil is used to lubricate oil-lubricated bearings. A lithium EP, 3751 or 3752 ALMAGARD® Vari-Purpose Gear Lubricant grease should be used for grease-lubricated bearings.
The bucking saw, is a large circular saw that is used to cut excessively long logs into usable lengths. A mill may have more than one bucking saw. One bucking saw may be set up prior to the barker. Another bucking saw may be used to cut standard log lengths of smaller diameter logs for direct feed to other saws.
To minimize waste, the first step in the saw mill process is to electronically scan the logs to measure the length and determine how many standard-length logs can be evenly cut from the log. When the number of standard-lengths is determined, the operator stops the feed conveyor at the first standard length position and sets the log to the bucking saw. When the log is in position, the operator actuates the saw to cut the log. The cut log continues along on the conveyor belt to be farther processed.
The head saw, or slab saw, cuts the rounded edges off a log to create a cant. The head saw may consist of a single band saw, two band saws, or two circular saws. One design uses two, hydraulically loaded circular saws that can cut either one or two slabs from a log. If one slab is to be cut, only one blade is moved into position. If two slabs are to be cut, both saws are moved into position. After the slabs are cut, they fall away from the cant to a trash collection system.A resaw is used to cut the cants into boards. A resaw may have several circular saws mounted on a single shaft, which is called a "gang saw." The distance between the groups of blades is varied to cut different thickness boards. A light oil, such as a pure mineral oil, is applied to the shaft to ensure free movement of gang saw blade sets. The mineral oil will tend to dissolve gums from the tree sap that would cause the sliding parts to stick.
Another type of resaw is a double horizontal band saw. The double horizontal band saw makes a double or singe cut that forms' one, two, or three boards. The operator decides how many boards will be cut and what the thickness of the boards will be as the boards enter the infeed conveyors of the resaw. The operator also notes if a slab cut will be required. When the determination of how many boards of what thickness is made, the controls are set and the automatic board feed drive rollers are actuated. The board feed drive rollers feed the board width-wise through the saws. Lower guide rolls and retractable pressure rolls hold the board in position during the sawing process.
The sawed boards exit the resaw onto a tilt table. If a slab is cut, the tilt table automatically tilts up to dump the slab and then lowers to permit the good boards to be conveyed to the edger. The cant is pressed into the "line bar" of the resaw by rolls. The line bar is an accurately machined bar that holds the cants while the cants are being fed to the resaw. The bar guides the cut so that an accurate thickness is maintained in the board. To prevent a build-up of errors that are caused by the saw "leading" when it starts the cut, the cant is rolled by a special conveyor while the cant is being returned for the next cut. Because several cants are sawed in rotation, the resaw is always kept busy.
A saw called the "sash gang saw" is also used to resaw cants. The sash gang saw uses parallel straight saw blades that are stretched vertically in a frame. The ram is reciprocated vertically by a crank and connecting rod. As a cant passes through the reciprocating blades, a series of boards is cut. The sash gang saw can also be used for sawing whole logs into boards.
Boards that are cut from a log or a cant that has not been completely squared have rounded edges or wane. The round-edged boards are passed lengthwise through an "edger." An edger has two or more saws on a single shaft. The saws are spaced to trim off the wane and to leave a standard-width board or boards. A "bull edger" has a large number of saws. A bull edger is used to process a large cant into several boards in a single pass.
A planer is used to smooth rough lumber. A planer consists of multiple knives mounted on "cutter heads." Rotation of the cutter heads at high speed provides a smooth cut. Specially shaped knives can be mounted on the cutter heads to cut tongues and grooves, or other shapes on the boards. The chips are transported to a collection site by blowers and fans.
Because of the high speed of operation, the arbor bearings of planers and matchers require a light paraffinic oil. Consult the OEM requirement and then we will recommend the proper Lubrication Engineers oil. A lithium EP grease may be used for bearings that require grease. A lithium EP grease may also be brushed on open gears of hand-operated mechanisms, but where gears are power operated, the heavier film of a high viscosity EP gear oil may be preferred.
Where gear boxes are oil-tight, an industrial gear lubricant (1600 series Vari-Purpose Gear Lubricant such as 1601, 1602, 1603, 1604, 1605, 1606, 1607, 1608 and 1609) would be satisfactory . However, if the gear cases tend to leak, a semi-fluid EP grease such as 4701 MONOLEC® Industrial Lubricant may reduce leakage while still affording adequate lubrication.
Green lumber is normally dried by air or in a kiln. Air drying in preparation for kiln drying consists of stacking the boards so air can circulate freely around them to reduce the moisture content. The stacking and unstacking of lumber are mechanized. The boards are fed by a chain conveyor to a pallet on an automatic stacker. The automatic stacker may be a platform elevator that descends as the board pile builds. Narrow pieces of wood, called "stickers" are inserted between the boards to allow the air to flow through the stack. The stickers are the same type of wood as the lumber pile, and they are automatically fed in between the boards from a magazine as the wood is being loaded into the kiln.
One layer of boards on the pallet can be pushed off by an L-shaped automatic arm, which is called an unloader or unstacker. Unloaders are tilted so that one layer of boards on the pallet can be pushed onto a conveyor chain. Electric motors power the stackers and unstackers through gear reduction units and chain drives.
Kilns eliminate surplus moisture by ventilation. Kilns are heated with either steam coils or electric radiators. There are three basic types of kilns. Ventilated kilns use chimneys to circulate air and to eliminate moist air. Condensing kilns use cold water sprays to condense the excess moisture in the air. The condensing of the moisture creates air circulation. Forced-draft kilns use high-capacity blowers to create circulation and to eliminate the moist air.Some kilns are loaded with wood at one end and an equal amount is removed daily at the opposite end. Other types of kilns are fully charged and fully discharged at one time. The temperature inside the kiln will vary from 235°F to 350°F. The drying period depends on the system, type of lumber, quality of lumber desired, and intended end use of the lumber.
Bearings that are located in and around kilns may operate at high temperatures and should be lubricated accordingly with synthetic greases. High temperature grease is required in rolling bearings above 300°F . Above 3000°F, it is particularly important to lubricate frequently and to completely flush out the residues of old grease from the bearings.