Fair Wind Fasteners only deals in cut threads when it comes to our Silicon Bronze wood screws, but often I'll tell that to a customer and be asked, "What does that mean?" which essentially translates to, "Why do I care?!"
There is a big difference between cut and rolled threads, especially when it applies to wooden boat building. Hopefully this article will shed a bit of light on the threading processes as well as the difference between them and what that means for the screw you're going to plank that beautiful boat with.
Machining Cut and Rolled Threads
The terms "cut threads" and "rolled threads" refer to the process used to form the threads on the wood screw (or any other machine screw, bolt, and a huge variety of other fasteners but we're talking about wood screws here). They are the two main processes that are used, and differ greatly in how they are done as well as how the end product performs.
We'll start with rolled threads. Rolling threads has a few advantages when it comes to the machining process, but for our needs as boat builders the end product is not the same. Rolling on threads is a much faster process than cutting, it allows the manufacturer to start with smaller (cheaper) wire stock, it's not as hard on tooling, and it can be done with much less complex (again, cheaper) machines. However when it comes to the end product, that is where the disadvantages begin to show.
To form the threads on a rolled thread wood screw raw wire stock is sandwiched in between two drum type dies that are turning at high speed while applying pressure to the stock. This pressure, along with the grooves on the dies "bends" the metal stock to form the threads. A good video showing the process can be found here
The diagram above shows a wood screw that has been made by rolling the threads. Notice that the shank of the screw is a smaller diameter than the threaded portion. The manufacturer was able to use that smaller and cheaper wire stock to form the screw by "bending" the thread metal outwards, which reduces the manufacturing cost. Add to that the less costly tooling, less complex machinery, and faster production times and rolling threads is what 99% of manufacturers do today.
Disadvantages of a Rolled Thread Wood Screw
However, when it comes to wooden boat building, a rolled thread screw only has advantages for the manufacturer, and several disadvantages for the end user.
To start, having that threaded portion of the screw wider than the shank is no good! The shank isn't completely filling the clearance hole made by the screw once driven in, and therefore isn't creating a completely water tight seal like a cut thread screw would do. Even if water doesn't get in, the air gap around the shank of the screw introduces oxygen which rapidly increases corrosion. Introduce a bit a sea water here and there and it's a recipe for disaster when used below the waterline.
Rolling threads also creates a weak point where the shank meets the threads. If you've been unlucky enough to be lured in by a low price and have bought rolled thread silicon bronze wood screws you'll already know that they tend to twist and break where the shank meets the threads if any significant amount of torque is put on them. That's a problem even if you don't realize it! What also happens is microscopic cracks can form when you're driving in the screw. All will seem well and it will feel "right", but that crack is there and as soon as any significant stresses are applied to the screw the shank will separate from the threads. If you're unlucky you'll be having a cocktail on the aft deck when your planks start popping off one by one! Yet another reason why the fasteners you choose for a boat build are so important - don't let your cocktails get interrupted by your vessel trying to join the seabed!
Then there is strength. Smaller wire stock for manufacturing = smaller strength. I don't need to present any scholarly articles on fastener research to drive that point home. Thicker body = a stronger fastener.
But what about "work hardening!?" I see you watched the video I linked above. It's true, a rolled thread "massages" the grains of metal rather than cuts through them resulting in the threads themselves being stronger. However, that doesn't much matter on a wood screw. LONG before the threads themselves fail, the wood will strip out. Thread strength matters in automotive applications, and industrial applications where the threads are actually holding large amounts of tension. If the threads were cut on an industrial bolt, the bolt itself might have it's threads stripped away, but we're talking about wood here - there is no amount of pressure that will strip the threads off of a cut thread wood screw before ripping out of the timber first.
Advantages of a Cut Thread Wood Screw
To the left here is an actual picture of one of the #14 Silicon Bronze Wood Screws that I pulled off my shelf here at Fair Wind.
Notice that the shank is the same size as the outer most portion of the threads. When you drive that screw home in your Chris Craft, Gar Wood, Century, Sparkman & Stephens yach
t, or even your massive Pilot Cutter
, the body of the screw will fill the voids created by the threads. You'll create a water tight seal on that beautiful boat and you'll have a strong fastener that will last a lifetime, resisting corrosion by being air and water tight.
Notice how the shank tapers in to the threaded portion of the screw as well? That quality completely eliminates the weak point mentioned above and experienced in rolled thread screws. These cut thread screws won't bend and twist when torqued in to place, and more importantly won't have those hidden cracks that only show themselves when it is too late.
Lastly is simply tradition. That Chris Craft you're restoring? It had cut thread wood screws. The S&S linked above? It did too. They all did. That's because that even at a production level when ordering tens and hundreds of thousands of fasteners at a time those big companies still splashed out on using the right fasteners. It would be a shame to put lots of time in to a vintage boat restoration only to sink inferior screws in to those old planks.
Disadvantage of Cut Thread Wood Screws
The heading is correct - disadvantage. Singular. One. That disadvantage is simply the cost to manufacture. We're not talking about bolts here that need the work hardened threads to do their job properly. Wood screws don't need to have their threads work hardened to suit their purpose, so the only disadvantage is the cost. The larger wire stock to create the full sized shank is more expensive, the machines required are larger and more difficult and time consuming to set up, the process is slower, and the dies used tend to dull quickly. All of that adds to the high cost of creating a cut thread screw.
All of that being said, the cost isn't very much higher! I've worked hard to try and keep my prices down in order to compete with the imposters out there selling rolled threads and unscrupulous alloys. I have to. Most customers don't know the difference (you do now because you read the whole article right?) and just buy whatever is the cheapest. When looking at the price difference in proper cut thread fasteners vs. rolled thread fasteners it really is a drop in the bucket and lots of people don't even consider the difference in quality. It's well known that pine, while cheap, would be a terrible timber to plank your boat with so most would use cedar, mahogany, or a multitude of other species of wood - but not many people understand the difference in fasteners is just as important!
Build it once and Build it Right
Trying to save a few pennies on fasteners can wind up costing much more further down the road (or down the river? out at sea?). All of the love and care that went in to each plank or frame is for naught if inferior products are used. You're not building a boat with a lack of love for it, nobody does. So why cut corners when it counts? Build it once, build it right, and build it with cut threads from Fair Wind Fasteners.