Drilling Deep Pilot Holes
Recently, I was working on
a simple bench for my mudroom. Since it was held together with just woodscrews,
the construction was pretty easy.
In most cases, I drill the holes for
the screws in one step with a drill bit and countersink set. But these screws
were extra long, and I was a little worried about splitting the wood. So
instead of one step, I did it in three, see photo. Here's how I went about it.
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 Counterbore. Since I planned to cover
the screws with wood plugs, the first thing I did was drill a counterbore. It's
simply sized to accomodate the screwhead (#8 in this case) and the wood plug.
Shank Hole. Next, I drilled a shank hole. Usually, this hole
needs to go through the outside workpiece. But if you look closely at the photo
above and drawing at right, you can see that with longer screws the shank hole
should be drilled into the mating piece. So to keep everything aligned, I
clamped the two pieces together and drilled the shank hole to the depth of the
unthreaded portion of the screw. (Remember to account for the counterbore, see
the detail in the drawing at right.)
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 Pilot Hole. After
drilling the counterbore and shank holes, the last step is to drill the pilot
hole. When doing this, you could leave the pieces clamped together, but unless
you have an extra long bit, the pilot hole won't be as deep as the screw.
I decided to
play it safe and unclamp the bench pieces first, as shown in the drawing above.
(The last thing you want at this point is to break the head off of a screw.)
With the pieces separated, a standard bit will drill the pilot holes the full
depth of the screw, see the detail in the drawing.
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