There are many different types of screws on the market today, each of which has been created to meet a specific need. Today’s woodworker is faced with many choices, both in the design of the screw itself and the design of the screw head. While traditional wood screws are still available, there are many woodworkers who don’t use those, having opted for other designs, especially drywall screws or “decking screws,” which are a modification on the design of drywall screws.
By and large, wood screws are flat-headed, allowing us to sink the screw all the way into the wood, through the use of a countersink, making the screw head less visible. These can also be used with wood plugs, where a counterbore (a round hole, large enough for the screw head to fit into) is drilled, rather than a countersink. Then, once the screw is installed, a wood plug is glued in place and trimmed off flush with the wood surface, hiding the location of the screw.
But there are also other options used with conventional woodworking, such as carriage bolts, which have hex heads and lag bolts, which have round heads, with a square boss under the head. This boss jams into the wood around a pre-drilled hole, keeping the screw from turning.
The point is, there are many options for the modern woodworker to choose from, making it possible to find screws which will work perfectly in just about any application. Whether or not those screws were originally designed to be wood screws or not is immaterial, as long as they will do the job.
Let’s Talk Threads
There are four basic types of threads we need to consider, when we’re talking about screws:
- Machine threads – designed to be used with a threaded nut or threaded part. Threads are short and thick, not designed for cutting. Doesn’t work well with wood, unless you are pushing the screw through the wood and using a nut and washer on the back.
- Wood threads – designed to push wood fibers aside, creating its own threads. Threads are thicker and not so deep, with a thicker shank, made of softer metal, to resist breaking. The thread normally doesn’t extend the full length of the shank.
- Sheet metal threads – designed to work from a pilot hole, pushing thin sheet metal aside so that the screw can thread into two pieces of sheet metal, holding them together. Threads are deeper and thinner than those for wood screws and extend the full length of the shank.
- Drywall screw threads – closer in design to sheet metal screws than wood screws, with thin, deep threads. Drywall screws are made of harder metal than wood screws are, which may cause longer drywall screws to break, when driven into hardwoods.
Even though not all of these screw threads are designed for use with wood, you can find all of them used at one time or another in woodworking. This is especially true when different materials are used together, such as attaching a sheet metal bracket or faceplate to wood, where a round head or pan head screw is needed.
Sheet metal and drywall screws can both be used with wood, even though their threads are not designed specifically for cutting into wood. When using longer screws, which are likely to stick and break, applying a small amount of soap to the point of the screw can be useful, as it will act as a lubricant for the screw. Once the screw is installed, it will dry, becoming a cohesive to help hold the screw in place.
In a sense, all of these screw threads can be considered to be self-tapping, with the exception of machine screws, when used in wood. While it is possible to run any of these into wood, without a pilot hole, especially the sheet metal and drywall screws, you are usually better off drilling a pilot hole, especially in dense hardwoods.
If a screw is threaded into wood, without a pilot hole, the screw must not only cut its own threads, but also push aside enough of the wood fiber for the screw’s shaft. While that is not much of an issue with softwoods, it becomes more and more of an issue, the denser the wood is. In some cases, that issue can cause screws to crack the wood, especially if there is an existing flaw in the wood or in the case of plywood. In others, the screw can jam in the wood and shear off.
Thread Cutting vs. Thread Forming Screws
What we’re referring to here is thread forming. Quite literally, the threads of the screws form their own mating threads in the material they are being inserted into. This is true whether we are talking about screws being inserted into wood or sheet metal screws being used to hold two pieces of sheet metal together. In either case, the screws are pushing aside the material they are being driven into, forming their female thread counterpart.
But there are limitations with what can be done with thread forming. As I mentioned a moment ago, dense hardwoods can resist those screw threads. Likewise, metal and dense plastic can do the same. In the case of sheet metal, it doesn’t require very thick metal at all, before it prevents thread forming. Plastics can’t usually stop thread forming, because the screws are harder than the plastic, but the plastics can crack from the pressure of the thread forming.
The solution in these cases, and others, is to use thread cutting screws.
Thread cutting screws can be used in a variety of materials and a variety of material thicknesses. While mostly designed for use in metal, they work well in plastics also. You can even use them in wood, allowing the screw threads to cut their mating threads, rather than forcing wood fibers aside to form a thread.
You can find thread cutting screws in both machine screw thread sizes and sheet metal thread sizes, although the machine screw is more common. In the case of machine screws, the screws are tapered slightly at the tip, not straight like they are normally manufactured. They come in a variety of sizes, with a variety of head styles, including flat, truss, round, and hex.
What makes the difference between a thread cutting screw and either a machine screw or sheet metal screw, is that there is a gash ground into the screw threads, working back from the point. If you’ve ever worked with a tap to cut threads into metal, you are familiar with this sort of gash; it’s the same sort that is found in the side of the tap, creating a number of sharp cutting points. It is these points that cut the thread, as the screw is inserted, just like a tap cuts threads into material.
Using machine thread cutting screws
In order to cut these threads, thread cutting screws have to be tapered, so that each successive thread is cutting deeper and deeper into the material. That’s why machine screws which are thread cutting are tapered. If they were straight, like machine screws normally are, they couldn’t cut the threads.
When using thread cutting screws of any type, it is important to start by drilling the right size hole. For machine screws, this would be the same size hole that is used for tapping the material, as shown in the chart below:
|SAE Screw Size||Drill Bit Size||Closest Fractional Bit||Decimal Inches|
Most of us don’t have number and letter drill bit sizes. For those cases, I’ve provided the closest fractional bit size. These will provide a hole which is usable with that screw size, but the threads won’t be cut as deep as they would, if the right sized bit was used. Therefore, the strength of the hole won’t be as great. Keep this in mind, when building projects where high strength is needed.
While not normally recommended, it is possible to cut threads into dense hardwoods with these screws. The screws will seat well into the wood and will grip fine, albeit not with the same strength that they would have in metal. The problem comes in if you try to remove and replace the screws repeatedly. The thread cutting action will actually cut wood fibers, which will come out as sawdust when the screws are removed. Each time they are removed and replaced, the screw to wood interface becomes weaker, until it won’t grip anymore after a few cycles.
Using thread cutting sheet metal screws
A pilot hole is also needed when using thread cutting screws with sheet metal type threads. Since these threads are of a different size, different size holes will be needed, per the chart below:
|Screw Size||Drill Bit Size||Closest Fractional Bit||Decimal Inches|
As with the machine screws, there is a difference between the drill bit size and the closest fractional bit in several cases. However, the difference will not make as much of a difference in the ultimate grip strength of the screw, as these threads are different, making them better able to accommodate the difference between the two sizes.
Drill Point Screws
There is another category of screws, which are often misunderstood to be thread cutting screws; these are known as drill point screws or self drilling screws. These do not have the distinctive notch that makes it possible for thread cutting screws to cut their thread, even though the drill flute may extend into the first screw thread. What makes these screws unique is that the point of the screws is the same as a normal twist drill.
Drill point screws are used extensively in cases where sheet metal or other materials are being attached to steel or aluminum tubing, whether square or round tubing. The drill point provides a hole through the sheet metal and into the tubing. While it can drill through fairly thick tubing, the threads of the screw are sheet metal screw threads, so they will not work in thick-wall tubing, where it would be better to drill and tap the material.
There is one manufacturer who provides a drill point screw which is also a thread cutter. These are referred to as Tek Screws. They provide the benefits of both thread cutting screws and drill point screws in one fastener.
There are also drill point screws which are made specifically for woodworking. The difference between these and other drill point screws is that they have a wider point, often referred to as “wings,” allowing the drill portion of the screw to create a clearance hole through the wood, before drilling through the metal tubing.
One Final Point
Years ago, I worked in a city transit bus manufacturing plant as a manufacturing engineer. We used both thread cutting and drill point screws extensively in the manufacture of those busses, because they were faster to use than any other type of screw. In that case, we were drilling and screwing into stainless steel structural elements of the bus. The screws were strong enough for that, because they were made of hardened steel or stainless steel.
The big advantage of using these screws is that they are much faster to work with, than any other type of screw you can find. The ability to drill and sink the screw in one continuous operation or attach something where the threads needed to be cut in one continuous operation was a time saver, more than compensating for the higher cost of the screws.
Since leaving engineering, I have made several enclosed trailers. Without exception, I have used truss head drill point screws for attaching the aluminum skin to those trailers. The time it saved me made it worth the cost of the screws.