26 



Hardwood Record — Veneer & Panel Section 



August 25, 



block would be too wasteful, hence the various pieces are 

 shaped so that when fitted together the blank roughly 

 conforms to the shape of the finished propeller. Since 

 the pieces are of different shapes and lengths it is pos- 

 sible to adjust the patterns to the lumber so as to make 

 the most of the material, avoiding knots, worm holes, 

 checks and other defects and using for the outer lamina- 

 t'ons some short lengths which would otherwise have to 

 be scrapped. 



The lumber has to be carefully inspected for defects 

 which will affect the propelller in any way. Large knots 

 are, of course, not admissible, but small pin knots may 

 do no harm, especially if located in the hub. The aim 



a carload of this wood which was decidedly moldy. This 

 is requiring a change in the type of kilns which can be 

 used since the problem of drying green lumber is quite 

 different from merely completing the process after nat- 

 ural seasoning. Mahogany offers less difficulty to the 

 kiln operator than white oak and black walnut, but it 

 will case-harden and check if abused. Accordingly hot- 

 air kilns are being modified or replaced so that the 

 humidity, the temperature and the circulation can be at 

 all times controlled. Drying in the kiln is continued 

 until there is not more than 8 per cent moisture in the 

 wood, sometimes only 6 or 7 per cent, as determined 

 by samples taken from different locations in the heating 

 chamber. If the 

 circulation i s 

 bad the wood 



may dry very 



unevenly, s o 



that some sam- 

 ples will show 



the requisite 



dryness and 



others not. If 



the wood de- 



v e 1 o p s case- 

 hardening it has 



to be steamed 



and redried. 

 The properly 



dried lumbei 



goes to the 



planers, where 



it is surfaced 



smoothlj . Then 



the lamination 



patterns are 



applied and 



marked out. 



To build up a 



r e c t a n g ular CUUTIS SCOUT Pitoi 



.\IT()M.\Tir I'HdI'KI.I.KK SII.VI'ER. 



is to get as near perfect material as possible, and while 

 some of the defects are recognizable at a glance others 

 jTiay be difficult to detect. Among the latter class are 

 checks or cracks which have closed but which remain a 



source of weak- 

 ness. Fiber 

 breaks in the 

 tree due to 

 windstorms or 

 to felling may 

 occur in any 

 timber, but are 

 very common 

 in mahogany. 

 Fort unately 

 they usually 

 show rather 

 plainly in this 

 wood as con- 

 t r a s t e d with 

 those in spruce. 

 M a h ogany is 

 aluo subject to 

 red streaks 

 composed of 

 short, brittle 

 cells with little 

 strength. An- 

 other very seri- 

 ous defect not 

 always easy bo 



