Finger Jointing

Finger jointing lumber can reduce waste and utilize marginal material - and in depth article -1999

By Kent Pitcher

Finger jointing short pieces of lumber has become an increasingly popular method of reducing wood waste and utilizing shorts to realize maximum profit from the steadily rising cost of raw materials.

In some instances, finger jointed lumber is actually preferred to solid, unjointed lumber. With rise in the popularity of finger jointing lumber for a variety of reasons, almost everyone in the adhesive industry eventually becomes involved with it to some degree.

Finger jointing lumber for both structural and non-structural uses has proven to be very successful over the years, and is now fully sanctioned by the building and lumber industry and trade organizations associated with these industries.

When a finger jointing operation is performing correctly and a good product is being produced, many of the important things that make it all possible are lightly regarded and taken for granted.

However, when a weak or faulty product occurs, the reason for this becomes of immediate concern and the search begins. Often, this search for the reason for faulty finger joints is fairly superficial, with the idea that it can be easily explained away.

Since finger joints usually involve the use of an adhesive, a common and easy explanation for failure is that the adhesive is no good. This may be true, but is seldom the case. More often than not, the fault lies in a poorly machined and poorly fitting dry joint. A good, properly fitting, dry joint accounts for about 87% of the success of a good finger joint. The best adhesive in the world will not cause the formation of a good finger joint if the dry fit is poor.

In order to defend his product and to assist the customer, it is beneficial to the adhesive supplier to recognize the importance of a properly fitting, dry finger joint, and what is needed to produce such a joint. When the call comes complaining that "the glue is not good", some knowledge of finger jointing can be very valuable.

While adhesive suppliers are not expected to be expert machinery set-up men, they should be aware of the details involved in making a good finger joint. To this end, the following items should be considered:

  • Cutterheads

  • Knife Grinding

  • Cutting Machine

  • Adhesive Applications

  • Joint Assembly

  • Joint Cure Out
  • A brief discussion of each of the above items follows:

    Knife set - Have all the knives been measured? Have new knives and shims been balanced and dialed in with a micrometer?

    Knife stack - Is overall knife stack height okay? Are cutterheads kept in pairs and properly cleaned? Are knives set so that only .010 to .030" wood cut is made?

    Cutterhead - Has cutterhead been properly balanced?

    Knife Grinding
    How often are knives sharpened? Knives should be sharpened after approximately 30,000 board feet of lumber has been run through them. This may vary due the species being cut. Each grinding, about .003 to .004" metal should be removed from the knife. Final cut should take off no more than .0005" metal. At this rate, knives should last at least 1 (one) year.

    Are knives ground to conform to hook angle template? Knives should be fitted to templates every 3-4 grindings. Hook angle is extremely important in determining the finger profile.

    Are knives realigned using a set-up post? After grinding, heads should be balanced every 3-6 months.

    Cutting Machine
    Is cutterhead spindle set vertically with no wear or play in bearings? Are trim saws set true? Trim saws should be at least 3/16" thick for good rigidity. Are chain carrier lugs squared with the trim saws and cutterheads and checked periodically? Is hold down pressure sufficient to prevent movement of stock while cutting the joint? Are bed rails worn? How often are they checked for wear?

    Adhesive Application
    Has adhesive been prepared properly? Were ingredients weighed out in recommended proportions and thoroughly mixed? Is adhesive spread sufficient and uniform on every finger?

    Adhesive should cover 1/2 - 2/3 the length of the finger on both sides of the finger in a thin, continuous film. Too often, there are skipped fingers, and adhesive applied only to the very tips of the fingers.

    Is adhesive application too heavy? Too much adhesive may cause arcing in an R/F tunnel due to excess squeeze-out. Excess squeeze-out also is messy, may cause build-up on electrodes, and gives poor economy of adhesive usage.

    Joint Assembly
    End pressure - Infeed machinery should be set to force adjacent fingers together with about 150-200 pressure for non-structural joints and 350-400 psi for structural joints.

    Continuity - Once the joints have been forced together, the pressure should be held constant until the joint is cured. Frequent starts and stops lead to disturbance of partially cured joints. This is especially serious in R/F cured joints.

    Line speed - Infeed speed should be set to allow time for complete cure of adhesive and to accommodate assembly of pieces of the length being run.

    Alignment infeed - Infeed should be aligned so that crowder wheels match fingers accurately.

    Joint Cure Out
    For melamine-urea adhesives, joint temperature should reach a minimum of 180F with 200F preferred.

    Wet wood - Wet wood is probably the most common reason for undercure of joints. Water is a good conductor of electricity, and will rob available R/F power away from joints. The recommended maximum MC of joints to be cured by R/F energy is 14%. At 12-14% MC, about 30% of available R/F power is absorbed by the wood, with the remaining 70% available to cure the glue line.

    As wood MC increases, R/F power to the joints decreases. It may be possible, by addition of chemicals, to make the adhesive more conductive, and thus attract more R/F power away from wet wood, but the best method is to start with wood that has 6-10% MC.

    Cold wood - Cold stock will cause a slower cure because more energy is needed to reach the desired curing temperatures. With cold stock, the machine speed may have to be lowered to reach the desired curing temperature, thereby reducing machine productive out put.

    Arcing - Arcing may be through the joint or to the machine frame. In either case, it involves a type of short circuiting that robs R/F energy from joints in the R/F tunnel and may cause undercured, soft joints.

    Arcing is most frequently caused by sloppy adhesive application, bits of metal or carbon in or on the wood, or by air gaps in poorly fitting joints. Arcing can also be caused by using an adhesive that is too conductive for the R/F machine.

    R/F machines vary widely in their ability to operate with a single given level of conductivity.

    By giving close attention to teach of the six above items, it should be possible to make a good strong finger joint. It is hoped that this data sheet will serve as a trouble-shooting guideline if difficulties are encountered.

    This article was provided by Kent Pitcher of Custom-Pak Adhesives, Inc.