Newsletter Article: All about Woodscrews
For fine woodworking glue, not screws, is generally the go to method of attaching two pieces of wood together. For fine woodworking, glue has several advantages over screws; however, all woodworkers, even fine woodworkers use screws so let’s talk about screws used for woodworking.
When to use screws. Screws are a quick, mechanical way to fasten two pieces of wood together. Screws will make a fine, strong joint, but they are generally deemed not appropriate for fine craftsmanship. I generally categorize the things I build with wood into three categories which also relate to when I am willing to use screws verses glue. Those categories are:
Fine Furniture Grade: Creating fine furniture grade pieces out of nice grain wood is my woodworking passion; it is why I am a woodworker. When I am building fine furniture, I use traditional wood joints and glue to fasten wood together. Yes, I do sometimes find a need for a screw in one of these pieces, but its not typical. If I do use a screw, I will always locate the screw, so it is hidden from view. Two exceptions to this rule are on attaching draw bottoms and when attaching hardware, such as a hinge, to a piece of fine furniture.
- Cabinet Grade: I classify projects as cabinet grade when they are not fine furniture using expensive, nice wood. A good example is the storage cabinets I am building for my workshop or the storage box my wife just had me build. I typically use sheet material (plywood or MDF) and softwoods rather than expensive hardwoods for most of these projects. When I am building these pieces, I have no issue using screws, but I do endeavor to hide the screws. Pocket screws are a great example of screws I use in Cabinet Grade projects.
- Shop Grade: These are projects that I use around the shop. Jigs are a great example. When building these I typically want them done quickly and I do not care if the screws show.
Now that you know when I use screws in my woodworking projects, let talk about woodworking screws in general and what screws I recommend. There are hundreds of types of screws on the market and even if we limit ourselves to just woodscrews, selecting the right screw for your application can be very confusing. To un-confuse things, let’s first talk screw basics. Below is the anatomy of a woodscrew:
Traditionally, wood screws had tapered minor diameter (also called the “root”). Today, the standard wood screw typically has a straight minor diameter. The major diameter, or threaded portion, has a slightly larger diameter than the shank. You can also see that it has a single, shallow thread.
Gauge Size. Wood screws are rated in gauges, which indicate their major or thread diameter. The larger the gauge, the thicker and stronger the screw. The thickness of the screw is expressed as a gauge number (identified by the “#” sign) and ranges from 0 to 24. The most commonly used woodscrew gauges range from #6 to #12. I typically keep gauge sizes #6 and #8, in various lengths. If I need a #10 or a #12 I will purchase it when the need arises at my local hardware store.
Screw Length. This is the most important feature for any woodscrew you use. When fastening two pieces of wood together, you want to sink your screw as far into the second board as you can without having the screw protrude out the back of the wood. A good rule of thumb is to use a length of screw that allows you to sink the screw two thirds of the way into the bottom board for maximum holding power.
Drive Types. There are too many drive types of woodscrews to go over them all, but there are only a handful of the commonly found drive types. These are illustrated below, and I suspect you have seen or even used all of them.
Drive type is my second most important consideration, after length, when choosing a woodscrew. For me drive type is all about preventing or minimizing cam out. 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.
Slotted, Philips, and combination drives are notorious for “cam-out” and I don’t like to use them. I prefer Torx and square (also called Robertson) drives because they almost eliminate cam-out. My go to drive head is Robertson. 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. Torx, is a German designed drive and offers the same advantage as a Robertson drive.
While Robertson drive screws are very common in Canada, they are harder to find in the U.S. I order mine online from Mcfeelys (https://www.mcfeelys.com/). When I buy screws at my local store, I will pick up the more U.S. common Trox drive screws, normally Spax (https://spax.us/) or GRK brand (https://www.grkfasteners.com/). Spax and GRK are a bit pricy but they also have toothed threads which really bite into the wood.
Head Shapes. Most wood screws usually have one of three head shapes: Flat, rounded, or oval.
Round head screws have a rounded top with a flat underside and are typically used for affixing thin objects to wood. Round heads come in several sub-types such as pan, domed, and truss.
Flat head screws fit into tapered recesses, such as the holes in hinges, and will be flush with the surface when properly applied. 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 screw home. If you do not want the screw head to show, you can counterbore the hole, install the screw, and then cover the hole with a wooden plug.
Finally, oval head screws are a combination of the previous two, in that they have a tapered bottom, like flat head screws, but also a slightly rounded top.
I almost exclusively use flat head screws in my woodworking. The one notable exception is when I use pocket screw joinery. This calls for a pan head screw. I keep flat head screws in my shop, along with an assortment of pan head pocket screws. If I need some another type of head I will purchase it as the need arises.
Screw Material. Wood screws are usually made of hardened steel but they are also available in brass, stainless steel, and I even have some aluminum screws (not sure why as they are not strong enough for any woodworking application, in my opinion). For most of my work I want a premium quality, strong hardened steel screw, however, if the screw is going to show on a piece of fine furniture (i.e. to fasten hardware) I like to use brass. It looks better, even though its not as strong as hardened steel.
Screw Coatings. Some screws have coatings to help prevent corrosion, but be aware that they may stain some types of wood. There are also screws that are coated with a dry lubricant to make it easier to drive the screw. If your screw does not have a lubricant coating you can also just dip the tip into a bit of wax before installing.
The Right Way to Install a Screw. In general, pre-drill screw holes, specifically for hard woods, and always when installing screws near the end of boards – it will prevent splitting. 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 that enable you to drill the right size hole to accommodate both the shank and root in one fell swoop. Instead, of countersinking you can also use finish washers.