June 2005 - The Logging and Sawmilling Journal
Taking high-strain band sawing further
Once work had started on high strain band sawing, the innovators behind it took it further, developing the famous Letson & Burpee air strain system, featuring a custom-built air cylinder strain mechanism that protects thin kerf blades by its quick response and short damping time.
By Alan Froome
In the first article on high strain band sawing—published in the May issue of Logging & Sawmilling Journal—we looked at how it was developed by a company called Letson & Burpee in Vancouver , BC , in the late 1960s and early 1970s. For 50 or so years before this, bandmill technology had remained static. But a small group of talented people came together to change things in a dramatic way.
In 1970, research scientist Andy Porter started working at L & B as a consultant. Porter had done some earlier work on the stability and stress in band saws while working at the Forest Products Laboratory in Vancouver (Canadian Forestry Service). This research indicated that it was possible to use thinner blades if somewhat higher strain was used. Like Clark , Porter knew that to prevent saw cracks in thinner blades (due to the bending stress as the blades go around the band wheels), you actually have to increase the strain. In order to quantify actual performance numbers, Porter was given the task of testing the L & B weight strain machine and measuring it in comparison to another new L & B design of strain system. This was the first full air system, without weights or knives, and was drawn up by L & B engineer Daniel McGuire. It used Bellofram airbags, one under each upper wheel bearing.
The first air strain tests took place in October 1970, on a seven-foot headrig bandmill at the Fort Hill Lumber Co in Grande Ronde , Oregon . The new air strain components were retrofitted to the seven-foot machine just for the duration of the tests. “The filers at the mill liked the new system and didn’t want us to take it away, as sawing accuracy was improved and the saws needed less work,” McGuire recalls. “However they didn’t really trust the strain shown on the gauge; they were used to seeing actual weights.” A patent was granted for the air strain system, based on this design, in the names of Dan McGuire and Ed Allen.
The general manager of L & B was Robert (Bob) Baldrey, who later became company president. He recognized the sales potential and that much of the high-strain, thin-kerf technology Allen and his team were developing needed testing, so Baldrey initiated a series of weekend sawing tests in the Vancouver workshop.
These tests were carried out during December 1970 and January 1971 on a five-foot bandmill. According to Andy Porter’s test reports from that time, sawing accuracy was tested using strain levels of 13,600 lbs and higher with a 19 gauge saw (0.043 inches). It is worth noting thatat the timethis was five times the conventional strain level for a five-foot machine.
The testing was followed by further development of the L & B bandmill design, to fit it with a more refined air strain system that could react even faster. Dan McGuire and Ralph Wijesinghe, who joined the company as a designer in 1969, worked with Ed Allen on these drawings, starting in 1970. This improved final design later became famous as the Letson & Burpee Air Strain system. Basically the system uses a custom-built air cylinder strain mechanism. The mechanism protects the thin kerf blades by its quick response and short damping time, momentarily dropping the strain loading if the blade hits a knot, for example.
In 1972 and ‘73, industry demonstrations were carried out using the same five-foot machine, under a tent in the yard at L & B’s foundry in Surrey , BC . This involved sawing cants up to 12 inches thick against a linebar to show its exceptional accuracy using a thin kerf blade. Fred Clark, who had by then joined the L & B staff, personally benched the saws and was a large contributor to the success of the demonstrations. Fred went on later to train many of L & B’s customers’ own filers in the benching techniques he had developed, necessary to maintain high-strain, thin-kerf saws.
The L & B machines became known as the Rolls Royce of bandmills and it was some years before other machine makers devised their own high strain system using hydraulics and other methods. As a result, L & B air strain bandmills were shipped all over the world and sold as single headrigs, twins, quads and horizontal resaws. A variety of different feed systems was also built to complement them.
The development of high strain bandsawing at L & B was the result of a team effort and was strongly supported by the L & B board of directors headed by managing director John L Letson and his father, Col Gordon Letson. But Ed Allen was very much the main push behind its design and development.
Allen was a natural born engineer. He was largely self-taught and didn’t have the benefit of a college education. Allen started at L & B as a machinist and rose to become chief engineer. He ran the drawing office at L & B’s plant in Vancouver for many years and was usually seen around the office with one of Texas Instruments’ huge early calculators hanging from his belt.
The L&B designs were eventually purchased by mill equipment
Allen developed what became many of the standard industry formulae used to calculate maximum feed speeds, band saw tension, band saw guide pressure, and saw blade fatigue.
Widely recognized as a pioneer in high-strain band-sawing, Allen died in 2004 after a long and distinguished career. He was given the Award for Excellence at the Portland Wood Clinic in 2000 and became an honorary member of the Canadian Society of Mechanical Engineers, the New Zealand Institute of Engineers and the National Society of Professional Engineers of Oregon.
In later years, Kockums Industries purchased L & B and later still, merged with long time rival Canadian Car to become Kockums CanCar. The company did not survive one of the sawmill industry’s many cyclical downturns, however, and the L & B designs were eventually purchased by US Natural Resources (USNR) of Woodland, Washington, who still build the L & B air strain bandmills today. One of the original design engineers, Ralph Wijesinghe, is occasionally called in as a consultant on the machines. “In my opinion, over thirty years later, it is still the best bandmill available,” says Wijesinghe.
In researching the high-strain band saw story, thanks go to Andy Porter, Noel Jenkins, Dick Crawshay, Ralph Wijesinghe, Dan McGuire, Ed Komori, Bruce Allen and others for test reports, photos and personal memories from the period.
Where is High Strain Today?
Although the general concept has become widely accepted, most filers and mill owners have struck a compromise between cutting accuracy, kerf size and saw maintenance.
One of the most technically advanced sawmills, the Seneca Sawmill Company of Eugene, Oregon, believes strongly in the advantages of high strain and recently revealed that they run their bandmills at the following remarkable strain levels:
L & B six-foot Twin headrig - 40,000 pounds
L & B five-foot resaws - 27,000 pounds
Seneca five-foot horizontal headrig - 24,000 pounds
Seneca five-foot resaws - 24,000 pounds
“We have tried even higher strains but you have to balance the cutting accuracy with maintenance issues,” says Ed Komori, chief engineer at Seneca. “Our belief is to run strain levels only high enough to achieve our standard deviation targets for lumber accuracy.”
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