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In soft woods a pilot hole of about 50% core diameter will give maximum holding power. For woods of imtermediate density a pilot hole of 70% of core diameter will give maximum holding power. For hard woods such as oak a pilot hole of 90% of the core diameter will give best holding power. Then pilot holes of from 70 to 90% of core diameter will give maximum holding effciency depending on the hardness of the wood. The seasoning of the wood should also be considered. Soap, or parafin may be used as a lubricant. In choosing between two adequate screws use the smaller diameter and the longer length when practicable. |