Forty years ago, I built a 13′ sail boat fastened with galvanized wood screws. The screws were closely spaced around each plank and deck panel. There were many hundreds of fastenings. I didn’t know anything about special bits for drilling pilots holes for screws, and I didn’t know about Yankee screwdrivers and had never tried driving screws with a bit brace. For each screw I drilled a pilot for the threads, a larger hole for the shank, and then switched bits once again to countersink for the head. Luckily I didn’t need to counterbore for a plug as well. Since then, I have been introduced to the various bits designed to do the above operations all in one shot.
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Of the many brands on the market, I, like many others, have settled on Fuller’s tapered drill bits and countersinks. They are of high quality, make a clean hole, and are adjustable. That means you have to buy only one drill countersink unit for each diameter of screw. Thus, the builder of small boats will find that four units (#6, #8, #10, and #12) will take care of nearly all his needs.
Several years ago my partner, Dave Thompson and I took a look at our methods of work to see if we could identify inefficient practices. One thing that seemed to take an inordinate amount of time was the adjusting of the tapered drill bit. We were always changing the length of the drill bit, sometimes to match different screw lengths and sometimes to allow for different densities of wood. The set-screws that lock the position of the countersink on the drill bit eventually wear and begin to slip. Sometimes, this is discovered only after several too-deep holes have been drilled.
It occurred to us that ifwe had a set of drill bits and countersinks that was fixed for the most-often-used sizes, the time we would save would justify their costs. A further saving in time would be realized if these bits were stored on the wall in a well-labeled rack.
Tapered drills are considerably more expensive than straight ones. The nine sizes we felt were needed for the screw sizes we most often used represented a formidable outlay of cash.
One way of saving money was to shape our own tapered bits from straight bits. (We would still buy Fuller’s countersinks to fit these bits.) This had further appeal as a replacement bit, suitable for tapering, and would be available from the local hardware store. We began to look closely at tapered bits in order to understand what it was that we were trying to reproduce.
The ideal drill bit would start out as a large as the shank (unthreaded part) of the screw. It would quickly step down in size where the threads begin, and then be parallel sided and slightly smaller in diameter than the root of the screw.
The threaded portion of screws is not tapered, although it appears so in the shorter sizes because of the last thread of two near the tip—which do diminish in diameter (see “A Look at Wood Screws” by Ed McClave in WoodenBoat magazine No. 54). A gradually tapering drill bit is, therefore, not the ideal shape but is a compromise necessary to achieve an adjustable tool. Only one place on the tapered drill is the proper diameter for a particular job. Very short screws will have undersized holes drilled for them and may split the delicate pieces they are fastening together. Long screws will have only part of their threads engaging the wood and will suffer a measurable loss of holding power. The tapered bit for #10 screws reaches the full diameter of the shank at a distance of 7/8″ from the tip. Any threads beyond that length are not fully contributing to holding power.
We had decided to give up adjustability and so were free to grind a drill bit to produce steps that matched each length of screw. We tested the holding power of the stepped shape against the tapered shape by driving two screws to equal depths in spruce. One pilot hole was made with a tapered bit, one with a stepped bit. We then engaged the head of the screw with a goosenecked wrecking bar. This bar had a stout stick of wood clamped to its shank to increase the length of the bar to 4′. We attached a spring scale to the end of the bar and pulled on the scale until the screw was torn from the wood. The force required to remove the screw was recorded.
All tests indicated an increase in holding power for the stepped bit. Using #8 x 1V4″ screws, the increase was 10%. For #10 x \lA” screws it was 5%, and for the #10 x IV2″ it was 15%.
Another disadvantage of the tapered bit is the amount of power required to bore a hole with it. When the bit is new and the edges of the flutes are sharp the power needed is not excessive, but it will still absorb more power than a straight bit because the entire length of the taper is cutting the wood. When the flutes become worn the bit becomes a dull wedge which burns its way into the wood. This is not a significant problem if you are using a 110- or 220- volt drill with plenty of power. It is a big problem with cordless drills, especially those in the lower voltage range. In contrast, the stepped bit needs only to have its point sharp. (The original sharpened point is not dulled during the shaping process.) The parallel sides of the bit slide easily into the hole made by the tip. In theory, the step itself (where the diameter increases from root to shank) does not receive a proper cutting shape from our grinding method. But, in practice, this makes no appreciable difference.
We have been using these home-grounded stepped bits for several years now, and I have no doubt that they have saved enough labor to pay for themselves many times over. They increase the holding power of screws while at the same time the screws are easier to drive. Often they need no lubricant. When we break a bit, we can grind a new one in five minutes at one-third the cost of a tapered bit.
We still use tapered bits for the occasional / screw that falls outside the range of our stepped bits, but we are quite addicted to the new system. The thought of all that adjusting now seems slow and inefficient.
Hold the drill bit and screw side by side with their points aligned. Using a felt-tipped marker, make a line around the drill corresponding to the point where the threads of the screw end and the unthreaded shank begin.
Dress the stone on your bench grinder, if necessary, until it has a flat face and sharp corners. A good wheel dresser is indispensable if you hope to do more than the crudest class of work with the grinder. Now position the tool rest as close to the stone as you can without touching it. Chuck the drill bit in a power drill and hold it across the rest parallel with the grinder’s motor shaft. Have a can of water close by.
With the drill turning in the same direction as the stone (meeting faces going in opposite directions), begin grinding down the diameter of the drill bit between the marked line and its point. Go slowly, being careful not to produce a taper or to grind past the marked line. Dip the bit in water every few seconds to keep it cool.
To check your progress, hold the drill between you and a light source. Pull the trigger to make the bit spin, and you will get a good sense of its shape. Make the step, or transition, from root diameter to shank diameter as abrupt as you can. Some taper here, as shown in the drawing, is inevitable and contributes to the strength of the drill bit. Keep grinding until the diameter matches the root of the screw. If you hold a screw directly behind the spinning drill bit you should be able to see the threads sticking out on either side. (As you make this observation, look through one eye only.) When the shaping is to your satisfaction, position a Fuller countersink on the drill bit and lock it firmly in place. (Stock numbers for the three sizes of countersinks we use are: C6, C8, and CI02.) We have the bit protrude from the countersink the same distance as the screw’s length measured from the underside of its head to its tip.