Alan Froome covers technological issues of bucking and debarking in a sawmill operation.
Debarking and Bucking
After trees are logged and delivered to the sawmill, the first job is to buck them to length and remove the bark. Some mills now prefer to buy logs already cut-to-length in the forest by hand or by ever more sophisticated logging machines. However, most do the job in the sawmill yard, so we will focus on that more common situation.
Tree lengths arriving at the mill must comply with trucking regulations for maximum allowable weight and length. Some areas in Northern Canada permit trees up to 100 feet long to be transported on the highway, but most local authorities restrict them to 60 feet or less.
There are regional differences on when to debark logs, but it is usually best to leave the bark on as long as possible because it protects the wood from damage in the log yard caused by cranes, loaders and other handling equipment. Any rocks or gravel picked up in the yard will also be eliminated when the bark is removed.
There are also regional preferences on how to store tree lengths. In the Southeast, many mills unload logs from trucks with a central crane and store them in a circular fashion around the crane. This is almost unheard of in the Northwest, where mobile loaders typically stack logs in long parallel lines.
These differences also apply to bucking and debarking. Many pine mills in the Southeast prefer to buck the trees with the bark on and debark the short logs afterward. In the Northwest, most mills, if they are not running cut-to-length logs that were bucked in the forest, debark tree length wood and then buck.
Bucking to the desired log length varies from mill to mill, depending of course on the product lengths produced. Many dimension lumber mills in the Southern pine belt buck to lengths from 8 to 16 feet while most in the Northwest cut to 8 to 20 feet. Hardwood mills cut other lengths, depending on their customers’ needs for flooring, furniture, pallets and other products. There are always exceptions to suit local or niche markets.
In the case of a stud mill, only one or two different lengths are required, so an inclined slasher deck with fixed saws at 8-foot nominal pitch across the deck will suffice as there is little to be gained from an optimum bucking system.
Some of the more advanced softwood dimension mills cutting 8 to 20 feet use elaborate log merchandisers or automatic bucking systems. These systems use electronic scanners to measure the tree length, diameter, taper and even sweep and crook, and a computer program calculates the optimum bucking decision. With an 8 to 20 foot mill, there often are hundreds of possible length solutions for each log; the operator with only manual controls does not stand much chance of matching a computer in terms of recovery. These merchandiser systems often run the same basic optimization
software as the primary breakdown machines downstream, so it is at the bucking station that a mill has the first opportunity to affect its overall recovery and profit from the log.
From an investment standpoint, payback is somewhat slower for an optimized bucking system over other machine centers in the mill, such as the head rig or a gang saw. So these systems are usually only found at very high volume softwood mills.
Whichever way the bucking decision is made, in the operator’s head or by a computer, most systems use an inserted tooth circular saw of 60 or 72 inches in diameter to make the cuts. Some use a long arm pivoting chainsaw to do the bucking, but kerf thickness is likely to be greater.
There are all kinds of clever ways to control the length of the cut. The most accurate and positive use a set of mechanical stop arms or gates against which the tree is positioned on a conveyor — usually butt first — while the saw makes the cut.
Often a ‘lily pad’ has to be cut first, and often butt damage has to be trimmed off at the same time, particularly if the tree was felled by a shear. This is an important operator decision although work is being done on camera systems that can ‘see’ defects or bias cuts in order to control cuts in the future.
Some mills cut a mix of metric and imperial (inch) length lumber, and there are many novel ways to move the length stops to accommodate the two. One example: stops mounted on a movable carriage with servo positioners or dual sets of stop arms in the cutting zone (one metric and one imperial increments) on opposite sides of the conveyor.
Length measurement systems can be quite simple, such as paint marks on a wall behind the bucking conveyor or chains suspended above it to match the preferred log lengths. Automatic measuring systems use a rotary encoder driven by the conveyor drive shaft, which counts electrical pulses as the tree moves down the conveyor — a given number of pulses corresponds to a known log length. This information can either be displayed to the operator on a TV monitor or sent to an optimizing computer in the case of a fully automatic merchandiser system.
Removing bark is basically done on one of three types of machine: drum debarking, the rosser head type, or the ring type.
Many pulp mills and chip plants use the huge drum debarkers. These ribbed drums rotate and tumble the logs, so that the bark is removed by abrasion as the logs rub against one another and the drum. Drum debarkers work well on large quantities of very small logs, but ring and rosser head debarkers are more widely used.
Most hardwood mills use the rosser head debarker. It rotates each log in a cradle while a spinning cutter head passes over the log and along its length.
Most softwood mills use the ring type debarker. The log is passed through a rotating ring of knives, sort of like a big pencil sharpener. This is a continuous operation, so it is ideally suited to high production mills.
The principle behind the operation of a ring debarker is to use rotating knives to peel off the bark at the cambium layer, the slippery bit between the wood and the bark. The first debarker using this principle was built in Sweden and in fact was called a Cambio for that reason.
There are many more makes available today using this principle. Tool pressure on the logs can be adjusted on some of the machines to suit the bark characteristics – like thickness, for example — of different species. Centering feed conveyors are used to feed the logs into the center of the ring irrespective of log diameter and are controlled by a weight sensing mechanism or sometimes a diameter scanner and a linear positioner.
The rosser head machine popular in the hardwood industry does not need a centering feed. It works something like a lathe in the way it cuts the bark off the logs. Logs in this case tend to be more crooked than most softwood logs, which is a factor to consider. Logs are kicked into the debarker from an infeed conveyor and kicked out in similar fashion after debarking to the outfeed conveyor. The log sits in a cradle with driven rollers that rotate the log while the cutter head — which is also rotating — passes over it. The operator decides when to move the cutter head along the log. This is by nature a slower method of debarking but nevertheless effective, and the machines cost less to buy and are simple to maintain. A dedicated operator is required to control a rosser head debarker.
Manufacturers of ring debarkers have done studies, and they contend that there is a payback even on hardwood with their machines. Ring debarkers often do not require a dedicated operator, and the
manufacturers report better recovery due to less tear-out and fiber loss. However, few hardwood mills have invested in ring type machines to really start a trend in this direction.
(Editor’s Note: This month we begin a new regular columnist for TimberLine. The column, SawLines, written by contributing author Alan Froome, will be devoted to concepts and developments in sawmill technology.
Alan is a mechanical engineer with extensive experience in sawmill engineering on an international scale. After graduating from technical college, he began working as a sawmill designer for a Canadian consulting engineering firm. He later moved into machine manufacturing and worked on the development of Letson & Burpee high strain bandmills. He worked as a designer for Kockums and then in technical sales of sawmill machinery and feed systems, traveling extensively in North America, South America, Europe, Africa and Australia. In the 1990s Porter Engineering named Alan marketing manager for scanning and computer controlled optimization systems.
Alan has written regularly for TimberLine and Pallet Enterprise in recent years, and we welcome him in this new capacity. SawLines will appear quarterly.)