Sharpening Screwdrivers

by Michael Podmkaniczky

In 1964, I was blessed with a Latin teacher who was as happy to avoid the drudgery of classical studies as his charges were. His nonacademic interests were wide and varied, and he was easily sidetracked by his resourceful students. On one such ram bling day, while discoursing the development of the internal combustion engine by BMW, he observed that in order to make some vital adjustment, a properly sharpened screwdriver was necessary. This brought a back-row dozer to sudden, albeit sleepy, attention: “Sharpen a screwdriver, sir?”

“Indeed, scholar Westcott. . . Sharpen a screwdriver.”

I don’t remember just what tangent we managed to steer the screwdriver tale toward, but the vignette came back to me the other day when I was asked about the same thing.

The first requirement for a screwdriver is that its blade positively engage the slot of a (wood) screw well enough to re­ main in place while you turn and tighten the fastener. The second is that this must be accomplished without mangling the surrounding wood, or, if the screw is to be countersunk and plugged (as is usually the case in boat building), without de­ forming the bunghole. Screwdrivers straight from the hardware store don’t perform either task very well, but with a little “sharpening” they will.

Since the screw manufacturer kindly provides a slot across the whole width of the screw head, you might as well take advantage of it. You therefore want a screwdriver tip that’s exactly as wide as the screw head and that fits tightly in the slot, so as to bear along its entire width. Thus, you really need a set of drivers, individually matched to each and every screw size you use.

A screwdriver tip that’s too wide will overhang the ends of the slot. When driving a counter­ sunk screw, it will ream out the bunghole, resulting in a poorly fitting and unsightly bung.

If you’re trying to tighten down a screw flush with the surface, that last turn will score the wood around the head, or raise nasty burrs on brass hardware and fittings.

Most manufacturers make screwdrivers with spade-shaped tips, which means that the blade will make the hole even bigger as it goes deeper into the wood. You can prevent this by grinding the tip to a constant width.

A screwdriver tip that’s too thin will bear only at its corners, defacing the screw slot and increasing the likelihood that the tip will jump out of the channel and gouge the woodwork. This problem, bad enough with flat-head screws, is even worse with round-heads because the slot is so shallow at the extremes. Ask yourself why you push so hard when tightening a fastening with a stock tool. The answer is that you’re trying to keep the tip from parting company with the slot.

Because the threads of a screw do all the work, pulling it-

tightly into the wood, you should have only to apply torque; forward pressure should be unnecessary. But the faces of a stock screwdriver taper slightly, preventing the blade from squarely contacting the slot’s sides, and the tip therefore tends to ride up and out when torque is applied. The harder you twist, the greater the tendency of the tip to pop out, and the greater the force required to keep it jammed in place. If the tip does jump out, all the force you’re exerting will be directed at the surrounding wood, too bad ! Yankee-style screwdrivers can apply only as much “push” as the spring is strong, and they invariably pop out if not dressed properly. The result is a less than-decorative “Yankee Doodle” across your pride and joy.

The solution is to dress the tip of the screwdriver so that its faces are parallel to the sides of the slot. Bits designed to be power-driven with an electric drill are invariably ground this way by the manufacturer-they would be lethal otherwise. You can grind a screwdriver to the correct shape as easily as you would hollow-grind the bevel on a chisel. The tip will wear in use, and now and then you’ll have to go to the grinder to square up rounded edges. Such touch-ups will gradually shorten
the blade, but you should be able to drive a few thousand screws before you have to hollow-grind the blade again.

For major league screw installation, such as in boat planking, maximum torque is supplied by a brace and screwdriver bit. Once in a great while, this may even break a screw, but a properly sharpened screwdriver bit will engage the slot so well that even a screw that’s been broken above the threads can be coaxed Out of the bunghole by turning it counterclockwise with the brace and gently pulling it. Try that with a stock bit. The ultimate touch, the pièce de résistance of the craftsman’s ego, is to ever so slightly grind away the corners of the sharpened tool to make the tip conform perfectly to the bevelled edges of the screw slot.

Michael Podmaniczky is a boat builder and Windsor chair maker. He lives in Thomaston, Maine.