T E C H N I C A L NOTE N U M B E R 223
UNITED STATES DEPARTMENT OF AGRICULTURE FOREST SERVICE
FOREST PRODUCTS LA130RATORY
MADISON 5. WISCONSIN REVISED December 1952
A FACTORY METHOD FOR WESTING HARDNESS OF GLUE JOINTS**
Woodworkers have found that glues are usu alf than wood
_fi! T17__j7oT-glued prod' ucts.
and that tools dull rather quickly in he mac n Some glues are harder than others, and, if the glue line of a, joint made with a very hard glue is allowed to pass its entire length several times over the same spot in a planer or jointer knife, a nick is very likely to develop that is expensive to remove. In many cases, therefore, it would pay a manufacturer to select a glue for his particular requirements that is the least abrasive of those having the qualities he desires.
The Forest Products Laboratory suggests the following test to determine the comparative hardness of joints made with different glues.
A number of pieces of wood (preferably softwoods) are glued together face to face, each individual joint being produced with one of the glues tobe tested. Since the effect on theknife depends, in part, on the thickness of the glue line as well as the type of glue, the gluing procedure should be the same as that most commonly used in the plant. After the laminated block has been allowed to condition for a week or more, one edge is cleaned of excess glue and squared up. The block is then run repeatedly over a jointer in such a way that each glue line passes over only one spot in the cutter. The depths of the respective nicks caused by the hard glue lines are then a rough measure of the abrasive effect of the glue s.
It is rather difficult. however, to detect differences in the nicks by a mere visual inspection. A better means of comparison is afforded if a smoothly planed softwood board is run across the dull knives. The accompanying illustration shows a soft pine board that has been run across the jointer knives in the manner described. In this case the nicks were the result of jointing a laminated block in which 17 different varieties of commercial glues were used. The block was run over the jointer repeatedly until about 60 linear feet of each glue line had passed over.
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