Fingerjointing in woodworking and furniture plants.

Where it's been and where it's going.

Joinery is an ancient art developed to connect two or more pieces of wood together. Fingerjointing is the latest of the methods, preceded by the lap, pegs, the mortise and tenon, tongue and grooves, dowels and dovetails. Producing a strong, precise fit has always been the target of jointing, and fingerjoints are the strongest, and in many respects the most exacting, of all in their manufacture.

The fingerjointing process gained momentum in the 1920s when it was used to manufacture the wooden skeletons of automobiles, truck decks and furniture frames. For a true luxury automobile, the plain old cast metal steering wheel was demeaning. If a wheel made of beautiful high-grade hardwood could be devised, it would impress well-heeled buyers.

Unfortunately, nobody had yet dreamed up power steering, so the steering wheel had to withstand the torque necessary to control the car on primitive and often rutted roads. A sturdy hardwood wheel with its sections joined by strong fingerjoints was beautiful and got the job done. Tenoners and shapers did the machining.

Cutting the joints requires exceptional precision, and one of the tooling experts of that era was Frank Weaver. Weaver was cofounder of Wisconsin Knife Works in 1926, and his patent for fingerjointing heads was issued in 1929. Fingerjoint tooling remains an important element in the company's business today. [ILLUSTRATION FOR FIGURE 1 OMITTED].

By the 1940s, it was apparent that the supply of high-grade wood was not inexhaustible, and agile minds set about searching for ways to stretch the supply. A very visible loss of valuable wood was in the trim conveyors of planing mills and cut stock lines. A multitude of trim blocks up to 24 inches long (or even longer) were generated because of the practice of manufacturing lumber in standard rather than random lengths. [ILLUSTRATION FOR FIGURE 2 OMITTED].

Trim lengths (in.)                        Board lengths (ft)
                           8        12         16         20

24                     25.00%     16.70%     12.500%    10.0%
12                     12.50%      8.34%      6.250%     5.0
6                       6.00%      4.16%      3.125%     2.5%

The First High-Speed Fingerjoint System

Redwood and cedar were in demand for the housing boom following World War II, and a lot of 3-foot- to 6-foot-long material was generated in normal milling operations, plus shorts developed in cutup lines. Pacific Lumber Co. in Scotia, CA, was the largest in the redwood area. In 1948, the company formed a joint venture with Stetson-Ross Machine Company to develop an integrated system to make long lumber out of these shorts.

The modern end matchers that Stetson-Ross had recently developed became the basis for such a system, and glue applicators were added to the outfeed end of these matches.

The functions of cutting joints, application of glue, assembly of endless lumber, electronic drying of glue (high tech then) and cutting to length were all combined into a conveyorized line. [ILLUSTRATION FOR FIGURES 3 AND 3A OMITTED]. The immediate success of the system developed world-wide interest, and soon a half-dozen or more machinery companies were offering their creations. However, the basic Pacific Lumber Co. system for making horizontally-jointed lumber became the standard for high-production plants in America. The major improvement on the original system has been the use of a single-feed chain to move the pieces through both joint cutters, thus eliminating one operator.

The Vertical Joint System

Some have found that a vertical joint is more suitable for the products they manufacture. Basic joint types are shown in Figure #4. The vertical units cut joints a batch at a time using a reciprocating carriage to traverse the cutter-heads through the cut. This contrasts with the horizontal machines, which process individual pieces at a good rate of feed using a precision feed chain.

A number of vertical joint systems were prototyped during the 1960s and '70s, and the most successful were developed in Europe. A high production unit is shown in Figure #5.

The Timber Shortage Increases

In the decades since 1950, the price of timber has escalated. Revolutionary new log processing systems and successful kerf-reduction programs have increased yields. All of this has re-focused attention to fingerjointers as an additional way to get higher solid wood recovery. Earlier splicing lines concentrated on highest quality wood, but today 2 x 4s, construction lumber, roof trusses and similar items are also included.

The long scarf joints in laminated beams are a thing of the past, replaced by fingerjoints, which save material without loss of strength.

Some smaller operators are finding it economical to do splicing of high-value specialty products. Angled joints encourage manufacture of such items as arched window frames and other irregularly shaped millwork.

Micro-mini fingerjoints are used to make mitered pieces for picture frames and similar products. These splices make a very strong frame, and their small size eliminates visibility problems. [ILLUSTRATION FOR FIGURE 6 OMITTED].

Types of Joints

Nearly every conceivable joint shape has been tried: long fingers up to 1 3/4 inches, short fingers down to 5/32 inch, feather-thin tips, curved finger tips, male-female shapes and reversible patterns. To maximize strength, fingers with a slope of about one in 10 are common.

Regardless of type, the importance of a perfect fit of the fingers cannot be overstated, because this affects both the strength and appearance of the finished product. [ILLUSTRATION FOR FIGURE 7 OMITTED].

Machine Requirements

It is not possible in this short article to go into machine detail, because systems can vary in price from a few thousand dollars to a few hundred thousand dollars.

For those who want to do a smaller volume of splicing using a hauncher or single-end tenoner, there are some accessory items available such as an end-press/cutoff machine to assemble the product [ILLUSTRATION FOR FIGURE 8 OMITTED] and pneumatic or roller-glue applicators. First-class sharpening equipment is a must regardless of plant size.

Semi-automatic lines may process six to 25 blocks per minute, while fully automated lines handle well over 100 pieces per minute. Some plants are known to put out 30,000 to 40,000 lineal feet of fingerjointed material per shift.

Tooling: One Key to a Perfect Fingerjoint

Fingerjointing is one of the most precise of all wood-machining operations. In America, the circular bit cutterhead is the most frequently used, and bits for these heads are honed to a tolerance of [+ or -]0.0002 inch! [ILLUSTRATION FOR FIGURE 9 OMITTED].

So choice of cutters and heads and their maintenance is critical.

1. Cutterheads, bolts and knives must be accurately balanced.

2. Heads must center perfectly on a very accurate spindle, preferably with hydraulic locking.

3. When sharpening knives, a knife-angle template should be used so that the required cutting angle is preserved.

4. All knives must project precisely to the cutting circle. That way each knife takes an equal bite.

5. Trim-saws also need careful attention, because the cut they make may become the end of the fingers.

6. On chain-fed machines, feed rates must be in keeping with the number of knives in the head and its rpm. Too fast a feed causes tearing, while too slow causes heating.

For the Future: Two New Sources for Fingerjoint Stock?

1. Throughout the first half of the 20th century, millions of wood-framed homes and commercial structures were built using untold billions of board feet of lumber. Window frames, door frames and base boards were made of high-grade material 4 to 8 inches wide. Drop siding and V-Edge siding also were made of prime material. Sheathing and sub-floors were made of 64-inch- or 84-inch-wide T&G or Shiplap instead of plywood, particleboard and other panel products used today. Framing lumber, roof trusses and joists were, of course, solid wood.

A lot of these old structures are out-living their usefulness, so demolition crews have the opportunity to salvage prime material for fingerjoint lines. By more careful disassembly of old buildings, the pieces can be processed to remove nails and other defects, after which a lot of "old growth" can be spliced into valuable lumber.

The idea is not as far-fetched as it may seem. Ongoing and very detailed studies are stressing orderly demolition rather than destruction when old buildings are torn down, not just to salvage larger timbers, but also 1-inch and 24-inch lumber. As a bonus, it also reduces bulky waste in garbage dumps.

2. Green trim-backs and shorts in the sawmill are also fingerjoint candidates if current efforts pan out. It is not easy to glue green fingerjoints satisfactorily, but current efforts by adhesive experts and electronic wizards may do the job.

Efforts to prevent solid wood from going into sawdust or chips are never-ending, and fingerjointing is an important part of that effort.