Cut vs Rolled Thread Wood Screws

Discussion in 'Miscellaneous' started by Dylan Schoelzel, Mar 1, 2011.

  1. Dylan Schoelzel

    Dylan Schoelzel born in a canoe

    This topic originated from this thread http://forums.wcha.org/showthread.php?t=7167

    The discussion of rolled verse cut threads as applied to silicone bronze wood screws may in fact be a moot point as it is. Nearly 100% of the silicon bronze wood screws on today’s market are cut thread. From what I can decipher there really aren’t any suppliers or firms stocking, selling, or manufacturing rolled thread silicon bronze wood screws. They may be out there somewhere but nobody I talked to would even consider stocking them, selling them, or even special ordering them.

    If one were to read the literature on rolled threads it would naturally bring you to the conclusion that rolled threads are better than cut threads. A rolled thread has more thread strength, a stronger alloy can be used, and the alloy is strengthened through the manufacturing process. This sounds like good screw manufacturing to me. But is this really the case for a silicon bronze wood screw, the preferred choice of screws for most boat builders across the board? If the data suggests a rolled thread is superior, then why aren’t silicon bronze wood screws with rolled threads readily available or available at all? I don’t have the definitive answer but let’s take a closer look at some of the differences between cut and rolled threads as the two different manufacturing processes pertain to silicon bronze wood screws and boat building. Hopefully this will generate some conversation on the topic so we can learn more.

    For those that don’t know, the threads on a wood screw can be made one of two ways. The threads are either rolled or cut. When threads are rolled the screw blank is basically squeezed through a series of dies that roll up, or extrude, the threads from the stock. Threads on a cut thread screw are formed by cutting or removing material from the screw blank. Each method produces a screw with different characteristics.

    As far as I can tell the process of rolling threads was brought about as a cheaper and quicker way to manufacture threads and not because it produces a better screw. Less material is used to begin with and the screw can be made up to 10 times faster than a cut thread. Certain alloys cannot be used in the thread rolling process and some industry specifications call for the use of cut thread fasteners only. Not always is a rolled thread the preferred fastener for every application.

    A rolled thread has stronger threads than one that has been cut. This is due to the extruded way in which they are made. This process forms the threads so the grain of the alloy is oriented closer to (or with) the profile of the thread. But, have you ever seen a wood screw fail along its thread profile? I haven’t. When screws fail it is usually just below the head or at the shank/thread junction. Most failure can be attributed to faulty installation; from over tightening or an improperly bored pilot hole.

    Rolling threads allows for a stronger material to be used in the manufacturing process but just because a stronger alloy can be used doesn’t mean the manufacturer is using one to begin with.
    Bronze wood screws are largely targeted toward the specialized trade of wooden boat building. The industry standard alloy for a bronze wood screw is 651 or C65100. Why make a change from an alloy that has a proven track record within a trade for which the alloy as been deemed the industry standard?

    No one is really making a rolled thread from silicon bronze so we probably won’t know what alloy they are being or were made from. I would tend to think that if a rolled thread silicon bronze wood screw was being manufactured that it would follow the industry standard and be made from the same 651 as the cut threads are. In addition, bronze does rather well in withstanding metal fatigue. It can do so better than steel can.

    The strength of the alloy being used is increased due to the forces put on the material while rolling the threads. I’m no metallurgist but this is how I understand the mechanics of this strengthening in a nut shell.

    The strengthening of the material in thread rolling is achieved by work hardening. Work hardening strengthens the material by reducing its’ ability to stretch or more rather increases tensile strength. This in return (highly) increases the chance of adding brittleness making the bronze weaker in shear loads. When threads are rolled the material also becomes harder. If it becomes too hard little tiny cracks in the material can develop. This is not good.

    Even before thread rolling occurs, bronze wire has the potential to be somewhat torsionally brittle. This is due to the way the wire stock, from which the screw is manufactured, is made. This is especially true with screw sizes that are #12 and smaller which are commonly used in canoe and small boat construction.

    Furthermore, because of the extruded process used to roll the threads, rolled threaded screws can and are made from smaller diameter wire, or lesser material, than what the finished product will be sold as.
    As a result the shank diameter on a rolled thread screw is smaller than the outside diameter of the threads. In some cases if not all, this not only diminishes the shanks ability to completely fill the pilot hole but it diminishes the shanks ability to make an adequately water tight fit. This can also effect the mechanical way in which the fastener is suppose to achieve some of its’ strength because the shank will have a loose fit in the pilot hole.

    In summing up the manufacturing of a silicon bronze rolled thread wood screw you have a potentially brittle material to begin with that is compounded with an increase of brittleness. Add to this a smaller diameter shank of said brittle material with the potential for tiny cracks within the alloy. It’s not looking good for the rolled thread silicon bronze wood screw.

    A cut thread screw is a full bodied screw meaning the shank diameter is the same as the outside thread diameter. When appropriately bored, the shank will completely fill the pilot hole which creates a better water tight fit. A cut thread also eliminates the possibility of a situation in which the shank could have a lose fit. (This often arises when joining two pieces of wood together and the threads get caught between the mating surfaces of the two pieces. When this happens, the two pieces are driven apart instead of together. To solve this, a hole is bored on the top piece, or the first piece the screw enters, so the threads won’t catch it. No problem with a cut thread here as it will fill the hole, but this will leave too large a hole for the smaller shank of the rolled thread screw.)

    The process of cutting threads does not work harden the material and thus does not add brittleness or heighten the chance for cracks to form do to over hardening. But, cut thread stock is still subject to the initial brittleness that can occur when the wire is made.

    Cut thread screws tend to vary a bit from manufacture to manufacture. It is important to have a reliable source so you know there is consistency and quality.

    In summary I am not sure a definitive answer exists as to whether a cut or rolled thread is superior across the board or not. It seems to me that it depends on the application in which the fastener is being used and the alloy from which the fastener is made. The fact that no one sells or manufactures a silicon bronze wood screw ought to tell us something. Sometimes we just have to choose the materials that are available and which we feel are best for our needs. I can tell you from experience that cut thread silicon bronze wood screws are less prone to failure, have more than an ample amount of sheer, tensile, and torque strength as pertained to my uses in canoe and small boat building. I cannot say the same for a rolled thread.

    I try to keep up to date with products and materials we use in the shop as best I can. There are invaluable resources which can be draw upon in order to do so. Two of those resources I drew upon as reference for the material above are the following:

    Fraser, Aime Ontario. “A guide to modern wood screws” Fine WoodWorking March/April 2003: 46-49

    McClave, Ed. “A close look at Wood Screws part 1” WoodenBoat #54: 47-54

    McClave, Ed. “A close look at Wood Screws part 2” WoodenBoat #55: 43-52
     
  2. Dan Lindberg

    Dan Lindberg Rotten Wood Hoarder

    Dylan,

    Good work and good research.

    A couple comments,

    Agreed. With everything said, the bottom line is still to use a good quality Si/Bronze fastener.
    "The discussion of rolled verse cut threads as applied to silicone bronze wood screws may in fact be a moot point as it is. Nearly 100% of the silicon bronze wood screws on today’s market are cut thread."


    Agreed. Failures below the head, in my experience, are due to mainly 3 causes, an excessive torque load due to friction (improperly bored pilot hole) in the threads and body, a stress concentration underhead (poor quality) and hyrogen enbrittlement in high strength fasteners (poor quality) enhanced by a dose of salt water. (think Navy ships)
    "When screws fail it is usually just below the head or at the shank/thread junction. Most failure can be attributed to faulty installation; from over tightening or an improperly bored pilot hole."

    Threaded fasteners "in general" are designed for tensile loads, and when shear and/or torsional loads are applied, their load capacities decrease significantly. The usual or correct failure mode (on a correctly loaded fastener) is a lengthwise stretching with a reduced diameter at a minor diameter, until the load is greater then the material strength, and then the fastener breaks. And I agree, I have never seen this with a wood screw.

    Dan
     

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