(Editor’s Note: In his regular column, Alan Froome continues his discussion of sawmill ideas and methods and how they evolved, along with a review of some recent developments in technology. This series of columns is intended to be a generic overview of different methods used in the sawmill industry in response to readers’ requests.)
Previous articles discussed the processes used to produce rough lumber boards, cants, and so on, and then finish them by drying and planing. Now we examine the art of saw filing. This includes the selection of saw types, tensioning and straightening, and sharpening the teeth.
Saw Filing, Selection
There has always been a lot of secrecy attached to the work done by the saw filer or ‘saw doctor’ as some like to call him.
The LRF (Lumber Recovery Factor) and production at each mill depends to a great deal on the work of the filing department. These specialists usually are reluctant to show anyone their methods and guard their secrets carefully.
However, manufacturers of saw filing and benching equipment have developed automatic machines in recent years, and this equipment has removed at least some of the mystery.
Early man found that pieces of flint with chipped edges could cut meat very easily, and even wood and bone. This was in effect man’s first saw.
Later in ancient Egypt, bronze saws were made, and some even had inserted teeth of precious stones, giving them longer life. That idea is not so different from today’s inserted tooth cut-off saws! These early bronze saws were used by pulling — not pushing — the blade through the cut, similar to the thin pull saws used in Japan today. The action is similar to the two-man log saw that was used to fell trees before the advent of the chain saw – it was nicknamed the ‘misery whip’ for good reason.
Circular Saws
As we discussed in an earlier column, the first saws used in the sawmill industry to cut raw logs (scragg saws) were circular. Many sawmills still use circular saws for primary breakdown, but the saw kerf (thickness of cut) is larger than a bandsaw doing the same job.
However, the potential higher recovery (LRF) of bandsaws is somewhat offset by the lower maintenance and simplicity of the circular saw blades.
Generally, mills select the smallest saw blade diameter they can. Larger diameter blades (with more teeth) are more difficult to maintain and kerf thickness also increases. In practice, if there is only a small number of very large logs, most mills will by-pass them completely rather than use a bigger saw. Some use inserted tooth saws but the kerf is larger, so solid swaged tooth blades are best for a head saw. Inserted tooth blades are rugged and often are used for log bucking, where kerf is less important.
Later in the sawmilling process, typical edger and trim saws also use circular blades. Edgers are much smaller in diameter, and with modern splined arbor design and lubricated guides, they can have very thin kerf. Guided edger circular saws in some high-tech mills, cutting 4-inch softwood, are down to 0.100-inch kerf. For edgers without guides, the Strob saw (named after Keene Strobel, a filer who invented it) with special slots to remove the sawdust and prevent over-heating, has many advantages.
Besides saw kerf, correct saw selection considers several other variables:
• bite per tooth (the distance the wood advances between successive saw teeth)
• tooth profile (design of the tooth point and gullet shape)
• depth of cut (maximum thickness of wood passing through the saw)
• saw speed (outer rim rotation speed)
• feed speed (speed the wood moves through the saw)
• wood species
(For final saw selection and ideal tooth profile, it is best to refer to established manufacturers of saw filing machines, such as Armstrong Manufacturing of Portland, Ore. Armstrong, now part of Simonds Industries, has been in the business since 1902 and has contributed a great deal to saw technology.)
Optimum saw selection is different for softwoods and hardwoods. As a rule of thumb, many consider 0.125-inch as a good bite per tooth for softwood and 0.100-inch for most North American hardwoods.
The use of carbide-tipped saw teeth has become widely accepted. Carbide tips stay sharp longer. In fact, carbide tipping can reduce tip wear by a factor of nine with many wood species.
Some have found that stellite gives longer tooth life with cedar and exotic wood species because it has better corrosion resistance than carbide. Cobalt chromium alloy is a new hard tip material now available that may combine the best characteristics of carbide and stellite. All these materials require special grinders for sharpening.
In the ideal depth of cut, the saw tooth gullet is just filled at the specified bite per tooth. This is the amount of wood that is carried away as each tooth passes through the wood. Saw problems, like snaking in the cut, can be caused by over-feeding the saw (by over-heating the blade).
Optimum saw speed varies by wood species. Many consider that 10,000 fpm (saw rim speed) is best for softwood while 8,000 fpm is suitable for most hardwoods. In frozen wood, these speeds normally are reduced by 25%, and a different saw tooth profile may be used (a ‘frost tooth’).
The minimum practical saw kerf is vital as this directly affects the sawmill recovery factor (LRF). Mills seek the minimum kerf they can run without sacrificing saw stiffness or strength. Most saw filers will make a compromise in order that the blade will last between saw changes and not over-heat and crack, but low kerf and high LRF are always the target.
Bandsaws
Most of the above comments about circular saws apply equally to bandsaws with respect to bite per tooth, saw speed, depth of cut, and feed speed.
Bandsaws can be solid swaged tooth type but these days most of them are tipped with carbide or some other wear-resistant material.
Bandsawing developed over 100 years ago as a way to make deeper cuts. The sash gang was one of the first designs and still is popular in Europe. Later research and development gave us high-strain setting bandmills that have the ability to make thinner cuts in large logs than are possible with a circular saw.
One of the most popular sawmill upgrades is to change the primary breakdown machine center from a circular scragg saw to a bandsaw. Typically saw kerf is reduced from 0.375-inch on the circular scragg (on a 60-inch diameter blade, for example) to 0.140-inch on a bandmill (with 6-foot diameter wheels). Some mills use an over-and-under circular saw (two blades) as a way to reduce kerf, but most upgrade to a bandmill at the head rig. As a rule of thumb, many accept that 55% of the kerf savings from this kind of upgrade translates into extra lumber. This represents significantly more lumber in the course of a year.
Filing Rooms
The filing room in a large, modern sawmill is best located close to the machines it services. Saw tips now usually are sharpened by a grinding machine rather than by filing.
It is common in large West Coast softwood mills to locate the filing department upstairs, above the sawmill operating floor. Trapdoors in the filing room floor are positioned directly above the bandmills so that new saws can be lowered by electric hoist to the machines below and the worn blades can be lifted up for servicing.
The head filer and his crew usually change saws every four hours, sometimes longer. The filers have a wide variety of machines available to service both circular and bandsaw blades. The equipment includes semi-automatic levelers and sharpeners, side grinders, automatic swagers and shapers, CNC wheel dressers, as well as the many hand tools used for the job.
Bandsaws are benched differently from circular saws. The ‘top’ wheel on a vertical bandmill is ground with a small crown (about 0.005-inch). In addition, the bandsaw has to be tensioned to be slightly concave on the back so that it tracks straight over the wheels.
Even with all the latest automated filing equipment on the market, a lot of skill and experience is still needed in the filing room in order to keep a sawmill cutting and running efficiently.
As a final note, many medium size mills find it more efficient to send their saws out to a shop for servicing on an exchange basis instead of employing their own filer.
Also, the excellent, small, horizontal resaw bandmills, which are popular in the hardwood and pallet industries, typically use 1-inch wide, 0.035-inch kerf blades. These blades often are used only one time and discarded after one shift.
