May 2005 - The Logging and Sawmilling Journal
High-strain band sawing
Although it is now considered an industry standard, the development of high-strain band sawmilling is an interesting story in itself, with a significant breakthrough discovered by accident.
By Alan Froome
High-strain band sawing is now considered an industry standard, but that wasn’t always the case, and how the method came about is an interesting story in itself.
High-strain band sawing 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 then a small group of talented people came together to change things in a dramatic way. And one of their most significant breakthroughs was, in fact, discovered by accident.
Letson & Burpee (L & B) was established in 1893 when J M K Letson and F W Burpee formed a partnership to design and build canning machines and pressure cookers. The company was incorporated in 1903, and Burpee moved to Bellingham, Washington, to better serve the many fish canning companies in Alaska. The company later built winches and after World War II, they started building woodroom and sawmill equipment.
L & B subsequently manufactured bandmills under licence with the Monarch brand name. These were what might be called today low-strain machines, strain meaning the load applied to a band saw blade to tension it. It’s important to note that the type of strain mechanism used is critical to the performance of any bandmill.
It had been well established that you have to apply sufficient tension to keep a blade straight as it cuts wood, to prevent it from snaking in the cut. Like all the other machines on the market in the early days, a heavy weight hanging on the end of a lever arm was the standard method of tensioning the band saw blade on Monarch equipment. To reduce friction, the arm itself pivoted on hardened steel knife edges with this design. Incidentally, many of these low-strain machines are still in use today, which is a testament to their heavy construction and durability. The drawback to the low-strain equipment, however, is that they have to run thicker kerf blades, with resulting lower lumber recovery.
An Accidental Breakthrough
In 1965, L & B management decided to build its own bandmill to improve on the Monarch design. At the time, only single bandmills were in use and the first machine built at L & B was a seven-foot (wheel diameter) headrig machine for a customer in Quebec.
Design engineer Noel Jenkins worked with Ed Allen to draw up the new bandmill, which carried the name Letson & Burpee for the first time.
The concept behind the design of this machine was to retain the strain knives as part of the mechanical strain system, still using a weight arm to apply the strain. As part of the new design, the Monarch-style gibbed slideways for the top wheel lift were replaced by cylindrical plunger tubes. “We knew clearly that friction was the enemy,” says Jenkins.
The new seven-foot bandmill was packed and shipped to the customer in Quebec. Shortly after the machine was installed and put into service by mill personnel, a follow-up phone call from the mill revealed that the customer was not only pleased with his new bandmill, he was raving about its outstanding snake-free cutting accuracy on frozen logs. Then, after running for a time, the bottom arbor suddenly broke. L & B sent a new arbor, which also soon broke. This puzzled works manager Dick Crawshay, who flew east with another replacement arbor, and to find out what was going on. What Crawshay found was a shock.
Despite warning notices, the customer had inadvertently been running the new machine with the shipping blocks still in place. It was normal practice at that time (as it is today), to fit shipping blocks and wedges into the upper wheel lift assembly and other moving parts to prevent movement and potential damage while in transit. It seems the mill personnel had been raising the top wheel until the motor stalled out, ignoring instructions. After removing the shipping blocks and fitting the new axle, Crawshay supervised start-up of the machine with the correct (low) strain setting. Immediately the sawyer exclaimed: “What have you done with my lovely machine? It worked so much better before.”
Back at L & B in Vancouver, Chief Engineer Ed Allen figured out the strain at which the mill had accidentally been running, which was way more than recommended. In effect, this incident later turned out to be the first true high-strain bandmill installation. Allen realized that if they could design a bandmill to take the strain accidentally experienced in Quebec, they could achieve much greater accuracy and recovery.
Allen was concerned, however, about wear on the strain knives at the higher loading. The company decided to design a new machine with an improved strain mechanism, both to find the optimum shape for the pivot knives and hopefully get much faster response and damping times. The broken arbors also caused a complete machine re-design which resulted in dead arbors (fixed axles) then becoming a standard feature of L & B bandmills.
The new machine was designed to withstand a maximum strain of 25,000 lbs in the case of a five-foot model. It is interesting to note that at that time, a typical five-foot bandmill was running with about 3,000 pounds strain and using a 15-gauge (0.070-inch) saw.
In 1967, others were thinking about higher strain and thinner kerf. Jim McMillan of McMillan Contractors, who had a mill just south of Williams Lake, BC, was getting an LRF of 8.5, which was fantastic at a time when most were happy with a factor of 6.0. One difference at this mill was its saw filing and benching. Fred Clark had a saw shop in Williams Lake and serviced McMillan’s saws. Clark was an outstanding filer and an innovator in his own right, and developed his own methods of rolling tension into the blades. McMillan and Clark wanted to use thinner saws and run 17-gauge or even 18-gauge blades at the mill. Clark visited L & B in Vancouver to discuss his ideas and suggested they modify the bandmill to suit thinner blades and higher strain.
Another notable innovation introduced by L & B in 1967 was the use of twin bandmills with a Sharp Chain log feed. The first of these was installed at Jacobson Brothers in Williams Lake, BC and attracted a lot of industry interest.
We’ll take a look at the further research that went into high-strain bandsawing in the next issue of Logging & Sawmilling Journal.
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