THE GLUING OF WOOD 17 
work joints are not entirely satisfactory where they are to be ex- 
posed to view. However, there has recently been a marked improve- 
ment in saws used for this purpose so that sawed joints are now being 
glued which are suitable for certain uses, such as edge joints in core 
stock. The saving of labor and material effected by gluing surfaces 
direct from the saw is important, and the practice is likely to 
increase. 
Machine marks caused, for example, by feeding the stock through 
a planer too fast for the speed of the knives, prevent complete con- 
tact of the joint faces when glued. Machine marks in cores of thinly 
veneered panels are likely to show through the finished surface. 
Unequal thickness and width which cause unequal distribution of 
the gluing pressure and usually result in weak joints may be due 
to the grinding, setting, or wearing of the machine knives. A small 
variation in the thickness of each piece of wood may cause a large dif- 
ference when a number of similar pieces are piled and glued in the 
same order as they come from the planer. 
On the theory that roughening and tearing up the surface fibers 
of wood gives the glue a better chance to adhere, it is customary 
in many gluing operations to scratch, tooth plane, or sand the sur- 
faces to be joined. This is done by machine or hand and results in 
tearing or cutting small grooves in the planed surfaces of the wood. 
However, the Forest Products Laboratory has made many compara- 
tive strength tests between smooth and rough surfaces which failed 
to show that better results are secured with the roughened surfaces, 
and, furthermore, studies in the penetration of glue into wood have 
shown the theoretical benefit of the roughened surfaces to be im- 
probable 16 {21). 
MACHINING SPECIAL TYPES OF JOINTS 
The plain, the tongue-and-groove, the circular tongue-and-groove, 
and the dovetail are four of the most common types of edge joints 
used in gluing boards into wider pieces. (Fig. 1.) The tongue-and- 
groove and the dovetail joints possess the theoretical advantage of 
having larger gluing surfaces than the plain joint. However, plain 
side-grain joints which are as strong as the wood itself may be glued 
easily on most woods, and therefore the extra gluing area of the 
tongue-and-groove and the dovetail joint is not needed. Moreover, 
the latter joints are more difficult to machine and to fit. Experience 
has shown that a lack of a perfect fit in tongue-and-groove or dove- 
tail construction often result in joints which are weaker than the 
plain joints. Furthermore, poorly fitting pieces require a heavier 
glue line, which result in slower drying and setting of the glue. For 
fast production, true, close-fitting surfaces are essential. The valu- 
able feature of the tongue-and-groove joint, hoAvever, is that the 
pieces of wood to be glued are held in alignment during the assem- 
bling and the subsequent setting of the glue. This makes possible 
faster clamping and less slipping of parts under pressure, which are 
advantages so important that some form of tongue-and-groove is 
16 The penetration of the glue into the cell cavities gives a much larger area of contact 
between glue and wood substance than can possibly result from a roughening of the 
surfaces. 
31595°— 29 2 
