Woodworker's Journal 2004-28-4, страница 22

Drive Designs and Drivers

Preparations for Inserting a Screw

For hardwoods, such as maple, oak, and cherry, you need three bits to make a countersink, a clearance hole and a pilot hole. The best results are achieved when the clearance hole in :he captive piece allows the screw to freely pass through, and the pilot hole :n the anchor piece is the diameter of the core of the screw. Use your vernier gauge to determine the core diameter. Finally, the screw head sits better if a countersink is prepared for it using a social tool. A rose countersink bit, like the one shown ii the photo below, is typical.

For scfter wosds, such as poplar or pine, the three bits may be substituted by a single tapered bit with attached countersink. Adjust the length of the tapered bit using the stop collar.

Yet another alternative in softwood is a bradawl, which avoids the need for a drilled pilot hole. A bradawl has a sharpened end that resembles a fine screwdriver. 'Hie cut is started by pressing hard across the grain to prevent splitting, then turn the bradawl left and right several times to make a small hole. Screws hold well because the bradawl removes no wood

during the cut.

I he three basic drive designs are shown above. The slot was the first method of turning tiie screw Once screws became standardized, slois became standardized, and screwdrivers followed suit.

The first departure was the square drive, developed by Peter Robertson in 1907. His correspondingly shaped screwdriver fitted with firm engagement — within 1,000th of an inch.

Henry Phillips was granted a patent for his cruciform recess design in 1937. It was used extensively for wood and metal screws in World War II and was a commonplace sight in mass-produced products. The Phillips driver also fits with a firm engagement. However, the perimeter of the cruciform is slightly radiused, which makes tae screw well-suited to automatic driving, because once the screw is fully set the tip of the driver pops out of the recess.

Countersinking Without Thinking

cess.

As Ian indicates above, to prepare a countersunk screw hole in hardwood, you need bits to create two distinct borings with a cone-shaped finish (photo, top right). If you are working with c. softwood, a tapered bit and countersink combo bit works well (photo, lower right). The latest and greatest option is a system that combines multiple drives,

taper-countersinks or even self-centering bits. Very handy.

— Roo Johnstone

Woodworker's Journal A ugust 2004 22

With any driver, (hand, battery-powered «r corded], avoid cam-out by slowing drive speed and increasing pressure between driver and screw head.

Squa^re

Cruciform

Tiree operations to sink a woidscrew in hardwood are minumum. Four, if you count driving a steel screw into the lole to prep for a brass screw — also a good technique.

Vie new Insty-Drive System frcm Rockier (800-279-4441) lets you move fast /vithout re-chucking.