26 



Hardwood Record — Veneer & Panel Section 



July 25, 1919 



iContiinird from page 24) 



The use of different species of veneer for the symmetri- 

 cal plies or veneer of different densities, may also con- 

 tribute to warping a panel. The differences in the magni- 

 tude of the shrinkage of different species for a given 

 change in moisture content, and the differences in the 

 stresses set up for the various species even for the same 

 magnitude of shrinkage may cause considerable Vifarping. 

 In a panel with maple veneer on one face and basswood 

 on the other the forces exerted by the maple in shrinking 

 were greater than those exerted by the basswood so that 

 this panel has cupped on the maple side. 



Effect of Changing Direction of Grain of Plies 



In the discussion of symmetrical construction it was 



SPECIALISTS IN 

 -DIFFICULT ITEMS- 



We Manufacture 



ROTARY CUT VENEERS 

 THIN LUMBER SPECIALTIES 



BIRCH DOOR STOCK 

 MAPLE PIANO PIN BLOCKS 



YEARS OF EXPERIENCE BEHIND OUR PRODUCTS 



.— MUNISING WOODENWARE CO. — 



MUNISING, MICH. 



North Wisconsin 



IRON-RANGE 

 HARDWOODS 



The finest and most beautiful 

 Birch, Oak, Basswood, Ash and 

 Ehn grown in this country. 



We use this quality stock exclusively for our: 

 ROTARY CUT LOG RUN 

 ROTARY CUT DOOR STOCK 

 ROTARY CUT FURNITURE 

 ROTARY CUT PIANO STOCK 



VENEERS 



ROTARY CUT HOOPS AND LINERS 

 THIN LUMBER SPECIALISTS 



REMEMBER we are specialists in 



LOG RUN VENEERS 



any thickness, any lengths up to 98 in. 



Let Us Figure 



Your Requirements 



Kiel Woodenware Co. 



KIEL 



WISCONSIN MELLEN 



understood that the successive pHes were always glued 

 with the grain either parallel or exactly at right angles to 

 the core. In careless construction this may not always be 

 the case. In Fig. 6, for example, the face veneer of panel 

 No. 4 is glued so that the grain of the face veneer makes 

 85 degrees wfith the grain of the core instead of the stand- 

 ard 90 degrees. Considerable twisting was introduced. 

 Panels No. 5 and 6 were glued with somewhat larger devi- 

 ations in the direction of the grain of the face veneer and 

 show more twisting. 



An extreme case of this kind is shown in Fig. I , in 

 which the plies were glued so that the grain of each face 

 of the panel w^as at 45 degrees with the grain of the core 

 and the two faces were at 90 degrees with respect to each 

 other. From the illustrations given and from many 

 observations, it appears that even quite small deviations 

 from the standard 90-degree construction may introduce 

 twisting. 



Relation of Density of Veneer to Warping 



Numerous tests have shown that the w^arping of ply- 

 wood panels when subjected to varying moisture contents 

 is least for the panels made of low density veneer, and 

 that in general, w^arping increases with the density. This 

 applies to panels in which all plies are of the same species. 



Effect of Increasing Ratio of Core to Toted Thickness 



Experience has shown that for 3-ply wood a high ratio 

 of core to total plywood thickness contributes to main- 

 taining a flat unwarped surface. In general, a ratio of 

 from 0.5 to 0.7 will give satisfactory results where flatness 

 is an important consideration. 



Of two 3-ply panels having cores of the same weight, 

 one of low density veneer and the other of high density 

 the panel having a low density core will in general shov^r 

 less warping than the panel having a high density core. 



Summary 



Wide deviations in the warping of panels seemingly 

 built in the same w^ay may occur because warping is often 

 the result of a combination of influences, some of which, 

 while not apparent, may be the more important factors. 

 The following points if borne in mind while designing or 

 manufacturing plywood should aid in minimizing the ten- 

 dency to warp. 



The moisture content is one of the most important fac- 

 tors that must be controlled. 



The moisture content of the panel when it leaves the 

 conditioning room should be about the same as it will 

 average when in use, and all plies should be about the 

 same moisture content before gluing. The limits of from 

 1 to 15 per cent moisture in the finished panel will usually 

 give satisfactory results when the panel is in service in 

 the open air. 



2. Plywood panels should be made of an odd number 

 of plies. For every ply of a given thickness and species 

 at a certain distance from the core, there should be a cor- 

 responding ply on the opposite side of the core of the same 



(CoHlimii-ii on page 28) 



