Q: I have two new custom-made table slabs and need to attach hairpin legs to them. Because of the fasteners that came with the legs, the legs require pre-drilling, I’m afraid I will drill all the way through the expensive wood. What’s the best way to control drill bit depth when I use an electric drill?
A: This is a really good question. Actually, this scenario comes up very often in carpentry and woodworking. Luckily, the solution is incredibly simple!
“Pre-drilling,” as you mentioned, is necessary when wood is too dense to drive screws through easily. (That’s the case with most table slabs). First, you bore a small hole with a drill bit. This allows clearance for the shaft of the fastener. This helps prevent wood from splitting as the fastener forces its way in.
Unfortunately, it’s difficult to tell how far into the wood a drill bit has passed. The bit spins in a blur. Also, your sight angle from behind the drill limits your view of what’s going on.
Pre-drilling thinner materials
This becomes an even bigger problem when you’re fastening into something that has two visible sides.
When drilling too deeply from one side, the bit can punch through the far side of your material. Naturally, this blemishes an attractive finished surface (like a nice table top). This hole is extremely difficult to hide afterwards. In fact, it may even be impossible to fix!
To prevent this minor fiasco, you need a reliable way to judge how deep you are pre-drilling.
Solution: one small piece of tape
Before you drill, wrap a bit of tape around you drill bit at the depth you’d like to stop at. I use painter’s tape because I always have it around. Also, helpfully, 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 will drive, to determine the depth to drill. Position them so they’re pointing in opposite directions. Then you butt the tip of the drill bit to the underside of the screw head. As you can see in the picture above, this accounts 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.
It helps to wrap your tape somewhat thickly. That way, you get a slight tactile clue, as well as a visual cue when you’re drilling.
An important note
Make sure that there is still enough of the drill bit shank left exposed for the drill jaws to grab onto, though. That’s because chucking the bit with tape wrapped around the end causes it to slip.
Next, mark your fastener location on your material. Then punch this location with a scratch awl or a fine-point nail set, and a hammer. In a pinch, use a trim nail. This little divot helps prevent the drill bit from wandering when you start boring 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.
Note on Pre-drilling Very Hard Wood
If the wood is very dense, back the drill out a couple times as you drill. This allows the waste wood–or “swarf”–to clear from the flutes of the drill bit before going deeper. Actually, this method helps when drilling deeply in soft wood, too.
Repeat this process for all your drill locations. You’re ready to drive your fasteners!
(As an extra tip, I always use titanium drill bits for my work. They’re vastly superior to black-oxide drill bits. Fortunately, they’re produced in such huge quantities these days that they’re almost as cheap. They last far longer, and won’t break as easily.
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