The Common Wood Screw
When it comes to assembly time, we usually reach for two basic products – glue and screws. Like modern glues, there are dozens of varieties of screws available today for just about every conceivable application. Nonetheless, the fundamental function of the wood screw has remained the same.
It All Starts at the Top
The typical wood screws with which we are most familiar have a flat head and generally a Robertson (a.k.a. square) drive. The major advantage of a Robertson drive is that it significantly reduces ‘cam out’ when applying full torque to a screw. Due to the design of the drive, Robertson screws are much less likely to fall off the end of the driver than most other types of screws, even when held at an angle, making one-handed driving much easier.
Many Heads Are Better Than One
There are some situations when a screw with a different head is a better choice. On very thin stock, where you don’t need to hide the screw head, use a screw with wide shoulders, such as a round head, pan head or washer head screw. Pan head screws, by the way, look very similar to round head screws, except they have a flatter head and chamfered sides. These three screws deliver a lot of holding power. Pocket hole screws, which rely on the holding power of the screw rather than glue, are essentially a washer-type head screw. In a pinch you can use an appropriately sized washer, or cup washers, with a flat or round head screw to simulate a washer head screw.
Some furniture designs call for screws to show. Oval and round heads are the common choices here. These come in a variety of sizes and finishes, with brass being a popular option. The major problem with brass screws is that they are weaker than steel screws and can easily break if too much torque is used when setting them. To avoid breakage, pre-drill the screw hole, then install an equivalent sized steel screw to thread the hole. Remove the steel screw and then insert the brass screw.
The Straight and Narrow
Traditionally, wood screws had tapered roots. Today, the standard wood screw typically has a straight root and a zinc coated body. If you hold a ruler along the body of a wood screw you’ll see that the threaded portion has a larger diameter than the shank. You can also see that it has a single, shallow thread. The thickness of the shank (the screw diameter) is expressed in a gauge number (designated by the pound ‘#’ sign), and range from 0 to 24. The most common sizes used by woodworkers are the #6 to #12 sizes. The smaller gauge screws come in one or two lengths. For example, a #1 screw is only available in a ¼” length. Larger gauge screws come in various lengths – the ever popular #8 comes in 14 lengths, from 3/8″ to 3 ½”.
Unlike wood screws, most brass screws continue to have tapered roots. If you look at a brass screw you’ll see that the shank is larger in diameter than the threaded portion. Brass is much weaker than steel, and the thicker shank on brass screws provides more structural strength, particularly in the small gauge sizes.
You’ll also find a wide range of specialty steel screws on the market, including flooring screws, deck screws, stainless steel screws and construction screws. These are typically manufactured from hardened and heat-treated steel. Like standard wood screws they have a straight root with a wide diameter thread, and usually have deeper, sharper threads, along with some kind of tip slot, which provides a self-tapping feature. Often these screws have nibs on the head, which facilitate countersinking, and are dry lubricated to facilitate wood penetration.
A Few Basic Guidelines
When fastening two pieces of wood together, you want to sink your screws as far into the second board as you can without having the screw protrude out the back side. A good rule to follow is sinking two thirds of the screw into the bottom board for maximum holding power.
In general, pre-drill screw holes, specifically for hard woods, and always when installing screws near the end of boards – it will prevent splitting. Ensure that you select the appropriate bit for drilling (see ‘Screw Clearance and Pilot Hole Reference Chart’). If you are using tapered screws it’s best to drill two holes: an appropriate size clearance hole for the shank, and a smaller sized pilot hole for the root. You can buy tapered drill bits from Lee Valley that enable you to drill the right size hole to accommodate both the shank and root in one fell swoop. Often, the exit hole on the top piece of wood will have some blow out; a countersink makes quick work of ‘de-burring’ the hole. Due to the weakness of brass screws, install the same gauge of steel screw in a pre-drilled hole before inserting a brass screw.
Most specialty screws are dry lubricated; however zinc screws are not. You’ll find it easier to sink the screw if you dip the tip into a bit of wax before installing. Don’t over torque when setting screws. Use a torque setting that sets the screw at the appropriate depth. You’ll have to do a bit of experimenting with your drill/driver to determine the correct setting. You’ll also get better results if you replace worn bit drives. Select high quality, hardened and heat-treated driver bits, as they are less likely to damage the screw head and will last longer.
Regardless of the application, there is a screw out there that will do the job. Next time you’re in your local hardware store, check out the selection – you may have more options than you thought.
Cam out occurs when a driver bit slips out of the drive as you are applying torque. The result is invariably a damaged screw head, premature wear on the driver bit, or worse, a nasty scratch across the surface of the piece you‘re screwing into. Robertson screws have a square drive with slightly tapered sides that virtually eliminate cam out – but only if you use a properly fitting driver bit. A worn or damaged driver bit can easily slip out of the drive head, so it‘s a good idea to buy high quality, hardened and heat-treated driver bits – they‘ll last a lot longer than bits made of mild steel. Other more exotic drivers, like Torx and Pozidriv, offer the same advantage as the Robertson.
By a wide margin, flat head wood screws are the most widely used on the planet. Whether you make furniture or cabinetry, build houses or craft products, or install decking or flooring, you‘re more than likely to reach for a flat head. They can be installed without countersinking, using the torque of your power drill/driver to set them flush with the work surface, or for a cleaner look, by countersinking a hole, and then driving the screws home. If you don‘t want the screw heads to show, you can counterbore the holes, install the screws, and then cover the holes with wooden plugs. There is even a slimmed down version of the flat head that finish carpenters use when installing trim work or moulding. Called, appropriately enough, a finish head screw, it gives more holding power than a finish nail, with a small head that is easily covered by filler or putty. Flat head screws with undercut heads (leevalley.com) are handy when installing piano hinges and drawer slides. These screws sit flush to the surface of the hinge or slide.