Timber Joint Strength

As a former mechanical engineer, one of the areas that interests me is the strength of the various types of joints used for furniture construction.

Joint strength is of critical importance when designing and building furniture. Failure of joints can occur either from excessive deflection or cracking / breaking of the joint. Glue strength is not typically an issue with joints, as the glue is stronger than the timber provided you are not gluing end grain. Gluing end grain is difficult and results in a very weak joint, which is why joints like a mortise and tenon are so good (the tenon gives a large long grain surface for the glue to adhere to).

Matthais Wandel, a fellow engineer and woodworker, has done extensive testing on the commonly used woodworking joints. He used a test rig that applied a bending moment to the joint and measured the force required to brake the joint.

The joints tested are those commonly found in furniture construction, namely;-

1. Mortise & tenon

Mortise & tenon

2. Dowel joint

 Dowel joint

3. Pocket hole screws

 Pocket screw

4. End grain screws

 Screwed joint

The test results clearly showed that mortise & tenon joints are the strongest joints for furniture construction followed by dowel joints.

Joint failure table (https://woodgears.ca/joint_strength/)

Joint failure strength table

* All screw joints deflected to such a large extent before failure that the ultimate failure strength is useless as far as furniture making is concerned.

Low end and flat packed furniture is typically constructed using screwed joints. The results above show why screwed furniture is not built to last.

Full Joint Strength Article by Matthias Wandel




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