Troubleshooting Finger Joint Gluing

Franklin Adhesives & Polymers has a complete line of non-structural finger jointing adhesives, each designed for a particular end-use requirement. To determine an appropriate product option for your application, please browse our products or contact one of our experts.

The preparation of joints as well as the adhesive itself plays a critical role in the quality of finger jointed products. Most failures of finger jointed 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. The fit of the dry finger joint should be checked before gluing begins.

The following tips may help you achieve a properly fitting finger joint or trouble shoot problems in your operation guidelines. Additional information on  can be found in the full version of our Finger Joint Gluing Guide, or by contacting our technical support team.

Cutterheads

• Knife-set - are all knives measured? New knives and shims should be balanced and dialed in.
• Knife stack - be sure to check overall knife stack for accuracy. Keep cutterheads in pairs and properly cleaned.
• Cutterheads should be sharpened as a set.  Knife set should cut only .010” to .030” of wood.

Knife grinding

• Knives should be sharpened after running approximately 30,000 board feet (wood species  
  could cause this to vary).
• 0.003" to 0.004" should be ground off upon each sharpening. No more than 0.00005" of metal should be taken off on the final cut.
• The hook angle is important in the finger profile and knives should be fitted to templates after three to four grindings.
• The set-up post should be used for realignment.
• The head should be balanced every three to six grindings.

Cutting machine

• Make sure cutterhead spindle is set vertically with no wear or play in the bearings.
• Chain carrier lugs should be squared with the trim saws and cutterheads. These should be checked periodically.
• Make sure trim saws are set true.
• Check bed rails for wear on a regular basis.
• Check hold down pressure to provide sufficient pressure to prevent movement of stock while cutting the joint.

Joint assembly

• Pressure should be held constant until joint is cured.
• End pressure should be set to provide 150-200 psi pressure for non-structural joints and 350-400 psi for structural joints.  
• Crowder wheels should be aligned to match fingers accurately.

Adhesive application

• Sufficient adhesive spread will provide a uniform coverage that should cover 1/2-2/3 the length of the finger on both sides in a thin continuous film. Make sure fingers aren't skipped and that the adhesive is applied to the whole joint, not just the tips of the fingers.
• Were adhesive ingredients properly mixed?
• Excess adhesive squeeze-out can cause arcing in a radio frequency tunnel. It also causes adhesive build-up and poor adhesive efficiency. Too much adhesive can cause a hydraulic effect in finger joint.

Additional information on assembly gluing can be found in the full version of our Finger Joint Gluing Guide, or by contacting our technical support team.