“Precompression” technology allows Bethel Furniture Stock to make wood twist and curve like never before.
Looking for a neat party trick? Try this one:
Take a wood dowel, several feet long and about a half-inch in diameter. Then, with your bare hands, tie a knot in it. It’s very possible to do with wood — just not ordinary wood.
Bethel Furniture Stock of Bethel, ME, has the technology to produce that wood. It is far more than a party trick, though. Bethel uses that technology to make bent solid wood components with curves so sharp they could previously only be made by laminating pieces of veneer together.
The process is called “precompression.” Bethel bought a press about a year and a half ago from Danish manufacturer Compwood Machines Ltd. Bethel president Leon Favreau says it is the first press of its kind in the United States.
The machine compresses steamed wood to about three-fourths of its original length. The process changes the wood at the cellular level: compression causes the normally rigid cell walls to fold over on themselves. Compwood describes the precompressed wood as being “like a bendable straw.”
While the wood is wet, it can be bent much more easily than wood that has just been steamed. But once the wood dries, it regains nearly all of its original strength, making it suitable for use in the furniture components that Bethel produces.
From Theory to Reality
Favreau says the company was intrigued by the precompression process since learning about it in 1992. The high cost, however, as well as difficulties with the process, kept the company from leaping at it. Bethel finally did buy a press about a year and a half ago and received it just under a year ago, after the price had dropped and the equipment had improved.
In addition, Favreau says, “The timing was right for us to take the gamble to try to develop it. If I have a strength as a businessman, it’s being able to sense trends, and I just felt that for the strength of our business, we were going to just need an added touch.”
Making the precompressed wood is a tricky proposition with a number of technical details to work out. For example, a compression rate of 85mm per minute might be ideal for one species of hardwood. In another species, though, that might create compression that is not uniform throughout the piece, which will make the wood unsuitable for use.
Getting the details worked out has been a process of experimentation. Favreau says that Compwood has had only a small involvement with troubleshooting the compression process. “They’re not benders, they’re equipment manufacturers, so each company [that buys a press] has to develop the bending processes on its own,” Favreau says.
Bethel has been aided by the Maine Technical Institute, which gave it a grant to help with research and development of the precompression project. Favreau used part of that money to bring in a professor from the University of Maine to serve as a consultant. He also hired John Lucas, who had recently finished a master’s degree in wood technology from the University of Maine, to work on the project.
“I wanted him to take a researcher’s approach to developing the process,” Favreau says.
Bethel recently hired a craftsman who is making working with the precompressed wood his specialty. “Because the precompressed wood is different than regular bending wood, we wanted somebody who is creative and who can help our customers develop products,” Favreau says.
‘A Designer’s Dream’
“Some products right now are done only with veneers because of the sharpness of the bend,” Favreau says. “Some of those, maybe not all, but some of those could be done with precompressed wood,” which would eliminate lifting and visible laminations. Favreau says he has also noticed that some of the bends that have a tendency to wrinkle on one edge bend more smoothly using precompressed wood.
While the sharpest curves need to be bent in a machine press, gentler curves can be hand-bent around a jig. That allows Bethel to add variety to its slate of products; components with bends in more than one dimension or that twist as they curve, for example.
“This should be a designer’s dream,” Favreau says. “I had always heard that furniture companies need to keep developing new products to keep the customers interested. This certainly can do that.”
But he does not claim that precompressed wood will replace traditional steam bending. The extra precompression step adds cost, both from the extra processing step and the fact that some precompressed wood will be rejected if the press’ settings are not set perfectly. Compwood says the wood also loses about 10% of its strength in the precompression process, and there are some limits in machining the wood while it is wet.
“It’s basically to augment our operation, but it’s certainly not going to replace our regular bending operation,” Favreau says, noting that he recently bought some generators and presses with an eye towards expanding capacity for traditional bending. “I’d be surprised if it gets to be more than a third [of our bending operations].”
The Press in Action
The machine holds the wood for three to six minutes, before its hydraulic rams pull back. The hold time allows the wood to cool in its compressed state, which helps it to retain its flexibility.
As the pressure decreases, the wood regains most of its length — all but about 3 to 6 percent, depending on how much it had originally been compressed and how long it was held there. The entire precompression process takes about 15 minutes.
The wood can be bent immediately, but it will also stay flexible as long as it remains moist. If the wood is wrapped immediately in plastic to retain moisture, Compwood says it can be stored in a cool place and remain flexible for up to 12 months. In fact, Bethel does sell wrapped precompressed lumber.
“There is a market for the small company that doesn’t have the facilities to do its own bendings or to do just a few,” Favreau says. “They can buy the precompressed wood and do their own bending by hand.”
Bethel hand-bends some of its precompressed wood around a form, particularly for parts that curve in multiple directions. The wood can also be machine-bent in the same manner as its steam bendings. Bethel has 10 bending presses, most of which it built itself, to wrap steamed lumber around a form. The presses use RF generators from Radio Frequency Services to dry the parts quickly.
In addition to hardwood bendings, Bethel produces edgeglued panels. In fact, bendings are the more recent addition to Bethel’s offerings — it has been making panels almost since its founding in 1958, while it entered the bending arena in 1986.
Bethel produces parts in oak, ash, cherry, soft maple, hard maple and yellow birch as standard offerings. “We have been known to bend some other species when the customer supplied the wood,” Favreau says.
One of the company’s niches is supplying panels to companies that do not have their own rough mill. Another is machined components, both from panels and bent wood. Favreau says that about 40% of the company’s sales are in fully machined parts.
Bethel uses a 5-axis CNC router from CMS North America to machine many of its parts. It bought the router in 1995. “When I made the decision to buy the five-axis machine, we were being encouraged to do more value-added work,” Favreau says. He adds that while the company looked into less expensive, non-CNC equipment, “I felt that if we were going to get into machining, we had to be able to do it as well or better than our customers,” and chose the repeatability a CNC machine offers.
The company writes its own programs for the machine. “We’re doing some pretty fancy work on some chair seats that are ‘sculptured’ and ‘scooped’ out,” Favreau says. The scooping program defines an outer edge to the scoop, and then bases all of its cuts from there.
Bethel also has three smaller CNC machines from Maxym Technologies to augment its machining, as well as a Weinig Hydromat 23C moulder and a Maxym tenoner.
Yield the New Driving Force
“We used to feel that it wasn’t worthwhile to reduce some of our offal from our dimension area where we prepare wood for bending,” Favreau says. “Now we’re taking the extra time to save this wood that we would ordinarily put into the chipper.”
Favreau says the idea to focus on yield came from employees on the shop floor and that any kind of transition like it requires their input. “If it’s their idea, it’s so much easier to implement. We’ve basically empowered them to make decisions,” Favreau says.
The company is currently installing an ERP (enterprise resource planning) system in the plant to help with scheduling, setting product standards and accounting. “Our goal is to have computers throughout the plant and to have operators have access to the information,” Favreau says.
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