The preparation of joints as well as the adhesive itself plays a critical role in the quality of fingerjointed products. Most failures of fingerjointed lumber are caused by poorly machined and poorly fitted dry joints. The adhesive is a factor in heat and water-resistance, however, even the best adhesive available cannot make up for a poor-fitting joint.
Here’s some common problems, causes and solutions for fingerjointing.
1: Pressed fingerjoint blanks fall apart.
Probable causes: (1) Glue penetration into wood is not adequate. (2) Temperature is too cold to allow glue to form a reliable bond and chalking occurs.
Solutions: (1) Moisture content of wood is too high (above 15%). (2) Keep boards and surrounding work area above minimum use temperatures.
2: Joint fingers are too short or too long.
Probable causes: Fingers are too long: The fingers must be shortened by increasing the amount of wood the trim saw removes. Fingers are too short: They must be lengthened by reducing the amount of wood the trim saw removes.
Solutions: A good fingerjoint will have no gaps, allowing very little room for excess glue. This is accomplished when the fingers are equal in lamination.
3: Joint fingers have concave gap on side.
Probable cause: The cutters were not resharpened according to the hook gauge or the cutters have too sharp of a hook angle.
Solution: Grind the cutters to match the hook gauge.
4: Joint fingers have a convex gap on the side.
Probable causes: (1) The cutter stack moved during setup and rotated too far forward for the correct set-up angle. (2) The cutters were not resharpened according to the hook gauge or have too blunt of an angle. (3) The cutter stack was not pulled up to the alignment post.
Solutions: (1) Reset the head and check the alignment of the cutter stack. (2) Grind the cutters to match the hook gauge. (3) Set the cutter stacks against the alignment post of the set-up stand.
Source: Franklin Adhesives & Polymers. For more information call 800-877-4583 or visit FranklinAdhesivesAndPolymers.com.