Q: I have two new custom-made table slabs and need to attach hairpin legs to them. The fasteners that came with the legs require pre-drilling, though, and I’m afraid I might drill all the way through the expensive wood. What’s the best way to control drill bit depth while using an electric drill?
A: This is a really good question, because this scenario actually comes up very often in carpentry and woodworking. And, the solution is incredibly simple!
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“Pre-drilling,” as you mentioned, is necessary when wood is too dense to drive screws through easily (as it is in most table slabs). Boring a small hole with a drill bit first allows clearance for the shaft of the fastener, and also helps prevent wood from splitting as the fastener forces its way in.
It can be very difficult to tell, though, how far into the wood a drill bit has passed. The bit spins in a blur, and your angle from behind the drill limits your view of what’s going on.
This becomes an even bigger problem when you’re fastening into something that potentially has two visible sides.
If you drill too deeply from one side, the bit can punch through the far side of your material, blemishing an attractive finished surface (like a nice table top). This tiny hole can be extremely difficult to hide afterwards–and it may even be impossible to fix!
To prevent this minor fiasco, you need a reliable way to judge how far into the surface of the material your drill bit is passing.
Solution: one small piece of tape.
Just wrap a bit of tape around you drill bit at the depth you’d like to stop at. I like to use painter’s tape because I always have it around, and the blue color provides a sharp visual contrast to the black or gold of most drill bits.
Lay the drill bit down beside the fastener that you intend to drive to determine the fastener’s length. Position them so they’re pointing in opposite directions, though. This way, you can butt the tip of the drill bit to the underside of the screw head, accounting for the full depth of the “shank.”
Now, take a short piece of tape and lay it across the drill bit at the point of the screw. This is your screw depth. Just wrap the tape around the bit evenly here, and press it tightly with a rolling motion.
I think it’s okay to really wrap your tape somewhat thick because, again, it provides more of a visual cue when you’re drilling.
Make sure that there is still enough of the drill bit shank left exposed for the drill jaws to grab onto, though. Chucking the bit in with tape fully wrapped around the attached end could cause it to slip.
Mark your fastener location on whatever your drilling into, then punch this location with an awl, or a fine-point nail set, or even a trim nail and a hammer. This little divot will help prevent the drill bit from wandering when you’re starting to bore into the wood.
Drill at this location with high speed and minimal pressure, making sure that the drill is perfectly perpendicular to the material. Bore until the tape is just about to touch the surface, then stop. You don’t want to drive until the tape is touching the wood, because this can force the tape back, ruining your sight line.
If the wood is very dense, you may want to back the drill out a couple times and allow the waste wood–or “swarf”–to clear from the flutes of the drill bit before going deeper.
Just repeat this process for all your drill locations, and you’ll be ready to drive your fasteners!
(As an extra tip, I always like to use titanium drill bits for my work. They’re vastly superior to black-oxide drill bits, and they’re produced in such huge quantities these days that they’re almost as cheap. They’ll last far longer, and won’t break as easily. Here’s a good rundown on the variety of twist bits being manufactured these days.)
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