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What tool makes pilot holes?

What tool makes pilot holes?

February 2012 Wood News Online

by Steven D. Johnson
Racine, Wisconsin

This month:

• Gimlets Please (Hold the Turkey)

• Go «Green» In Your New Workshop

• No Bad Tools

• Frozen Wood

Gimlets Please (Hold the Turkey)

Heading out to do a little emergency trim repair, my neighbor was helping me carry some tools. I said, «Grab those gimlets over there, too.»

«Gimlets? I don’t understand… you want turkey guts?»

«Where I come from we say ‘giblets’ with a ‘j’ my friend… I said gimlets,» with some emphasis on the first syllable.

«Ah, gin and lemon juice… sounds good, but can we still fix my trim?»

«No, no, gimlet, from the French word ‘guimbelet,’ meaning to bore or twist.»

My neighbor was unamused. He, like most of us, gets pretty interested when I show him big burly noisy power equipment and nods appreciatively at finely crafted heirloom hand tools, and even feigns interest when he sees me doing something as mundane as sharpening. But my little set of gimlets did not excite him at all. They should have, because in the Down To Earth Woodworking Shop these are must-have, constantly used tools.

A set of seven of these indispensible little augers costs just $17.00, but are the go-to tools for making a quick pilot hole or through-hole for a screw, a hole for a nail (to prevent splitting), or a starter hole for a larger power-driven drill bit. In a pinch I’ve even used the sharp point of a gimlet as a scratch awl. Gimlets will get into places that drills sometimes cannot, they are precise, they look pretty cool, and they feel great in your hand.

Figure 1 — The sharp point on a gimlet makes
placement for precise hole-starting a breeze

And what a timesaver! With one or two pilot holes to make, finding the right size drill bit, chucking it up, and drilling the holes can take several minutes. Grab a gimlet, place the sharp point precisely on the mark, and with a couple of quick turns, a perfect pilot hole is made. There is no need to worry about over-drilling or using depth stops, either. With a gimlet, work can be very precise.

Installing nine sets of drawer slides in the new shop desk / work bench / glue-up table / clutter catcher (click here to see the video series), I used a gimlet to quickly and precisely make pilot holes dead center in the elongated holes in the slides. I could not have been as precise with a drill bit or by marking with a pencil and drilling. Nor could I have used a self-centering bit in the elongated holes. If a power drill had gotten away from me I could have easily drilled right through the side of the cabinet.

Gimlets are formed with a sharp screw thread on the first quarter- to half-inch. That screw thread then changes to a familiar auger-cutting pattern. That configuration is why you will sometimes hear these tools called «auger gimlets» or «gimlet augers.» The screw tip pulls the point into the work effortlessly, and if you continue to turn, the augur grooves enter the hole and with each turn cut and pull the debris from the hole.

Figure 3 — Click image to enlarge, or
CLICK HERE to download a full-size PDF of the chart

Gimlets are sized (nominally) in metric measurements, and for a while I had a difficult time matching gimlet to screw, especially for pilot holes. You can do what I did for a long time, and simply hold the screw up to each size gimlet in turn until you find the gimlet that best matches the screw you want to use. To simplify, though, we have created a chart you can use.

Try them, and in time, the gimlet may become one of your most-used tools as well.

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Pilot hole

In construction, a pilot hole is a small hole drilled into a piece of construction material. Its purpose may be:

  1. to guide a larger drill to the appropriate location and ease the job of the larger drill
  2. allow for the insertion of another hole-making tool, such as a knockout punch, that will produce the final-sized hole, or
  3. locate, guide, and provide clearance for a self-threading screw in wood or plastic to prevent damaging the material or breaking the screw.

Pilot for large holes [ edit ]

A pilot hole may be drilled the full extent of the final hole, or may only be a portion of the final depth. The pilot drill may be a standard twist drill, another type of drill bit appropriate for the material, or, when the primary purpose is precisely locating a hole, may be made with a short, stiff center drill.

The pilot hole also reduces the power needed to turn a large drill bit, and reduces the large bit breakage risk. For twist drill bits, the pilot size is usually selected so that the chisel point of the larger drill does not need to remove any material, which reduces the chance of splitting the web of the bit. A pilot thus sized also prevents a larger drill bit from slipping on the material and guides the larger bit effectively. Very large holes may be stepped by drilling successively larger pilot holes before the final size drill is used.

On harder materials, such as most metals and many plastics, and sometimes on softer materials like wood, a center punch is used before drilling the pilot hole to ensure that the smaller pilot drill bit does not slip and that it starts at the correct location.

Pilot holes for screws [ edit ]

Pilot holes may be used when driving a screw, typically in wood, concrete, or plastic where the screw cuts its own threads.

When a screw is driven into a material without a pilot hole, it can act as a wedge, generating outward pressure which can cause many materials to split. By drilling a small pilot hole into the material, into which a screw is then driven, less wedging takes place, thereby reducing the likelihood of the material being split.

When a screw is driven without a pilot hole, or with too small a pilot hole, the core of the screw may bind and lead to the screw being broken. The appropriate pilot hole will prevent binding while providing sufficient friction to keep the screw from loosening. For common wood screws, the pilot providing clearance for the core of the screw may be followed by a larger bit to shallower depth to provide clearance for the larger, unthreaded shank of the screw. For standard wood screws, special pilot drill bits are manufactured to produce the correct hole profile in a single operation, rather than needing several different drill bit sizes and depths.

Screws driven into concrete must have the appropriate size pilot hole, or they will either break on insertion, strip the hole, or not provide the rated holding force. [1]

A hole drilled for tapping machine screws or bolt threads in metal or plastic may also be referred to as a pilot hole.

References [ edit ]

  1. ^TAPCON reference material retrieved 2013AUG9
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