Screwing
It
10th in a series by Barb Siddiqui
Driving
a woodscrew with a simple screwdriver seems so basic, it shouldn't need
instruction. But why does the thing always squeak like it's being killed,
and become a power struggle during the last half-inch, when it started
so easily?
There are
some tricks to this that are worth knowing. If you look at a standard
woodscrew, you'll see the shank beneath the cutting threads is smaller
than the unthreaded shank below the head. Drilling pilot holes a bit smaller
than the inner diameter of the threads will ease the screw into the wood
and prevent splitting, but without a wider shank hole at the top, the
screw has no clearance to do its work.
Ideally,
when joining two pieces of stock with screws, the woodscrew selected will
slide clear through the top piece, and all of the threads will seat in
the bottom piece, pulling the two together when the angled head settles
into its countersunk position. Yes, that is three pre-drilled holes: a
pilot hole, a shank hole and a countersink, and yes, it is a pain. Even
then, you may want a counter bored hole above all that, to set in a wood
plug and hide the screw head.
There are
ways around this, though old-fashioned wood screws are the strongest mechanical
application you might ever need. One way around it is to buy a set of
pilot bits for the drill that incorporate pilot hole, shank hole and countersink,
all in one operation. They are pricey, and you'd need one for each size
screw you commonly use.
Another
approach is to buy drywall or deck screws. They are somewhat weaker, and
have less holding power, but have a straight shank and are usually threaded
up to the head of the screw. On hardwoods, one size pilot hole, less than
the diameter of the shank, will do. On softwoods, a pilot hole is usually
not necessary at all. If you use these, clamp your wood pieces together
or the threads may push the two apart as they enter the lower workpiece.
There are
many types of screws available: pan-head screws with a straight shank
and an auger-cut tip are ideal for pocket-hole joinery, pulling the workpieces
together. There are special wood screws with serrated edges on the threads
to cut into the workpiece and avoid splitting. There are 'production'
screws with a straight shank meant for use in plywood and medium density
fiberboard (MDF.) You can choose a single thread or a double-thread screw,
flat heads, bugle heads, round heads or pan heads.
In spite
of the bewildering variety, there are some general rules to keep in mind
when selecting screws and their drivers. You should judge what length
screw to use by the thickness of wood you are using as a top piece. Generally,
each job requires a screw that is three times the thickness of the wood
being attached. Half-inch case sides? Use one-and-a-half inch screws.
Quarter-inch back board? Use three-quarter inch screws.
The screwdriver
heads or drill bit drivers matter also. Whether straight, Phillips or
square drive, be sure the tip of the driver fits and fills your screw
head. There are different tips for different size screws, and the way
your tip seats in the screw head will determine how much power you have
to drive the screw accurately.
By hand,
hold the screwdriver perpendicular to the workpiece and drive the screw
in a straight line (use short, stubby screwdrivers in tight spaces.) When
using a power drill, be sure it is a variable speed model, go slowly,
and push with some force after the screw has started itself, so the bit
won't slip out of the screw head.
It also
helps to roll the screw threads in a little beeswax for lubrication. Toilet-sealing
rings, found in any hardware store with the plumbing supplies, are an
inexpensive source of beeswax. Don't use soap; it will react with metals
and stain your wood.
One last
tip: brass screws are classy looking, but they are soft and the heads
strip out easily. If you want to use brass screws, first sink a steel
screw of the exact same size, remove it, and then insert your brass screw.
Brass is also best in oak, as steel screws react with its high tannin
content and lead to ugly stains on the workpiece.
If a project
is worth doing, then it is worth doing right, so take your time, learn
to use even the simplest of tools properly, and your woodworking results
will show it.
Click
on the links below to see the previous articles by Barb Siddiqui.
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