60 BULLETIN" 150 0, U. S. DEPARTMENT OF AGRICULTURE 
suit in the ultimate failure of the joint. It is therefore necessary in 
the manufacture of such parts to use irregular shapes of joints, 
dowels, tenons, and other devices (fig. 2) in order to reinforce the 
joint or to secure a larger gluing area, but even then the stresses which 
recur with seasonal changes put a very severe test on the joints. It 
is very desirable to protect such glued joints against moisture 
changes. The strength and permanence of all t}^pes of end-to-side- 
grain joints depend upon the accuracy of the machining and upon the 
fit of the parts as well as upon their design. There are no adequate 
data, however, on the comparative strength of different designs. 
CORRECTING GLUING DEFECTS 
Every woodworking plant should have a competent personnel to 
detect the causes of gluing defects and to correct them, Serious glu- 
ing defects usually result from a combination of causes and are there- 
fore seldom due entirely to a single condition. Without a full knowl- 
edge of the manufacturing process it is not always possible to tell by 
inspecting a defective sample just what has caused the difficulty. 
Furthermore, the same general type of defect may be caused by 
entirely different conditions. It is not possible to present a complete 
key to the causes and remedies for all gluing defects. A number of 
the more common defects are listed below, however, together with 
some of their usual causes. 
KEY TO CAUSE OF GLUING DEFECTS 
1. Open glue joints. Some of the causes are the following: 
A. Weak joints, as a result of — 
(1) Incomplete contact, due to- 
la) Improper machining of pieces (p. 16) — 
Uneven thickness. 
Irregular surfaces. 
Poorly fit joints. 
Tooth planing. 
(b) Wrong condition of glue at time of pressing — 
Dried or jellied glue (p. 37). 
Foamy glue (pp. 10 and 24). 
Uneven spread of glue (p. 23). 
(c) Insufficient pressure (p. 24) — 
Warped stock. 
Jellied glue. 
Uneven application of pressure (j>. 26). 
Unevenness of press plates or cauls (pp. 26 
and 27). 
(2) Contact complete but joints weak, due to — 
(a) Character of glue — 
Low strength (p. 4). 
Mixed too thin (p. 10). 
Partially dissolved (p. 10). 
(b) Insufficient glue spread (p. 41). 
(c) Starved joints (pp. 37 to 40), caused by — 
Excessive heating of wood. 
Too thin glue mixture. 
Quick clamping. 
Excessive pressure. 
(d) Oil, wax, or other material on wood. 
(e) Pressing period too short (p. 27) . 
(/) Exposure of nonwater-resistant glues to moisture. 
