TECHNICAL NOTE NUMBER 227
UNITED STATES DEPARTMENT OF AGRICULTURE FOREST SERVICE
FOREST PRODUCTS LABORATORY
MADISON 5. WISCONSIN REVISED December 1952
TOOTH-PLANING OR SANDING OT-T
EFFECT STRONG GLUED W OD JOINTS
Thi opinion, frequently expressed, that ri ug ended u cas are produced by tooth-planing, sanding, and similar a re-ceSsary to make strong glued joints is not bQ .-otby experiments conducted by the Forest Products Laboratory. An extensive series of tests with Sitka spruce white oak, and sugar maple demonstrated that, if good gluing conditions are employed, it is possible to make, with smooth flat surfaces, joints equal in strength to those produced with surfaces that have been scratched or otherwise roughened.
Animal glue mixed in the proportion of 1 pound of glue to 2-1/4 pounds ofwater, and casein glue prepared according to a Forest Products Laboratory formula were used in the experiments, and the strength of joints produced with tooth-planed and sanded surfaces was compared with the strength of joints produced with smooth-planed surfaces.
The woodusedwas allowed to condition in a room at 30 percent relative humidity to a constant moisture content of about 7 percent. The boards were then cut to about 3/4 by 5 by 12 inches and matched as to density. Approximately 30 depressions per inch, 1/32 inch deep, were made in a part of the boards with a tooth plane. For sanding, comparatively coarse sandpaper was used. Both tooth-planing and sanding were done by hand.
In the gluing operation, pairs of blocks surfaced by smooth-planing were alternated with those surfaced by tooth-planing and sanding so that all conditions of gluing might be exactly comparable for each type. Under this plan, any consistent difference in strength of joint must be ascribed to the method of surfacing the blocks. The blocks were glued together face to face.
IU. S. Dept. of A agriculture Library
C/o Agricultural Experiment Stations
Att, Mrs. 1. K. Cresap, Librarian Library 209 Horticulture Bldge
The regular Laboratory block shear test, by which the joint is subjected to a compressive shearing force in a specially designed ma..hine, was used to test joint strength. The strength of joints in pounds per square inch and the nature and percentage of wood failure! were recorded.
The experiments were conducted under three sets of gluing conditions
- good gluing conditions, starved-joint conditions,. and chilled-joint conditions. A starved joint is one in which the film of glue between the wood surfaces is not continuous; it results when thin glue is used with high pressure. A chilled joint is one made with chilled glue.
Under good gluing conditions, smooth-planed surfaces gave slightly stronger joints than tooth-planed surfaces in two out of three cases. The differences, however, are not great enough to conclude that toothplaning weakens the joint to any marked extent.
Under conditions that normally produced starved joints, tooth-planed surfaces on two of the three -species gave higher joint strength than smooth surfaces. The joints produced in this way were not so strong, however, as the smooth-planed joints under good gluing conditions.
Chilled joints were stronger with smooth-pl aned than with tooth-planed surfaces. This may be accounted for by the fact that it is difficult to apply the extra pressure needed to force the chilled glue uniformly into the depressions produced in the wood surface by tooth-planing.
-Wood failure is the shearing apart of the wood fibers near the glue joint
which indicates that the bond between the glue and the wood is stronger
than the wood itself.
The follow-"ing table shows the results of the strength tests with animal glue under the three different gluing conditions:
Species :Normal :Comparative strength of joints
of : condition --------------------------------------------wood :of glued : Tooth-planed Smooth
:joints ------------------ :.-----------------Strength : Wood : Strength :Wood : failure : :failure
------------------------------- ------------ -------:Lbs. petr sq. :Percent: Lbs. per sq.: Percent.
in. :. in.
Sugar :Good : 3132 : 62 3148 : 69
maple :Starved : 2261 : 4 : 1993 : 6
:Chilled : 2718 : 36 : 3014 : 27
White :Good : 2401 : 51 : 2317 : 66
oak :Starved : 2019 : 28 : 1786 26
:Chilled : 2501 : 64 : 2508 : 70
Sitka :Good 1792 : 78 : 1853 : 88
spruce :Starved : 1932 94 : 1941 : 96
Chilled : 1649 : 33 : 1803 : 61
The experiments show that while occasionally the average strength of glued joints obtained with tooth-planed surfaces is higher than that obtained with smooth-planed surfaces, such results are apparently corefined to starved-joint conditions. Where starved-joint conditions exist, the remedy is to improve the gluing conditions rather than to roughen the surfaces of the wood.
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