We’re going to talk about screw point forms for making threads today. These are in a category known commonly as thread-cutting screws, thread-forming screws, and thread-rolling screws. We are not discussing self-drilling [Tek®-style] in today’s article as they are not thread-cutting screws, but I promise that blog post is coming soon. Please remember that there are many more types in addition to those listed here. This is a discussion of just some of them, specifically the more common ones you’re likely to run into. If you have more detailed questions about the differences and benefits of each kind, just ask us! We at Hardware Everywhere are more than happy to educate you.
Thread-cutting screws and thread-forming screws are always hardened when made out of either steel or 400 series stainless. Because of the properties of 300 series stainless, parts made out of that alloy can’t be hardened, and as a result, the screws simply don’t work as well in most materials.
The purpose of a thread-cutter is to go into a pre-drilled pilot hole that is unthreaded and then cut its own threads into the hole so that they mate with the screw perfectly. They are not designed for use with already threaded holes. Thread-forming (thread-rolling) screws do the same thing only using a different mechanism.
Like all screws, different types are designed for different purposes. We at Hardware Everywhere have a large range of these screws available for immediate purchase and can have anything you’d like made to your specifications.
TYPE F (MACHINE SCREW)
This thread-cutting screw is the most common, by far, of all thread-cutting screws. It’s versatile and readily available in multiple configurations. It may be used for fastenings into castings (both ferrous and non-ferrous), forgings in both brass and bronze, heavy gauge sheet metal, structural steel, plastics, and even resin impregnated plywood. Type F screws provide tight, vibrationless fit with high resistance to torque, shear, and tension. There are four equidistant cutting notches around the tip.
TYPE 25 (SHEET METAL SCREW) sometimes called TYPE BT
This hardened steel screw for plastics is characterized by a wider thread-cutting slot with an acute cutting edge and wide spaced threads. Particles won’t clog the threads and less driving torque is usually required. This design reduces internal stress of plastics as the screw is driven and therefore reduces the possibility of fracturing brittle plastic substances.
TYPE 17 (Primarily on PARTICLE BOARD SCREWS)
This is typically used as a reamer/auger for wood applications. However, it’s often seen on roofing screws (sheet metal-type) as well. It greatly helps with ease of installation and is extraordinarily popular as a standard feature on particle boards screws, though it is seen in sheet metal screws used in certain applications such as roofing. Every now and then you’ll find a T-17 on a drywall screw resulting in it being sold as a deck screw. Drywall screws are never deck screws – see our previous blog posts.
TYPE 1 (MACHINE SCREW) – also known as TYPE D
Type 1 thread-cutting screws are made for use in metals. You can also use them as general-purpose fasteners for almost any fastening purpose in harder metals such as steel, special alloy steels, and cast iron. They perform their thread-cutting function in metals of any thickness and may be reused. When driven into untapped holes (drilled or punched) they remain firm in their self-cut, perfectly mated threads. (A TYPE G is when the cut goes through and across the entire shank and not just halfway through as in the photo. The type G is extraordinarily rare.)
TYPE 23 (MACHINE SCREWS) – sometimes called TYPE T
These specially designed self-threading screws are for use in zinc, aluminum, and other soft metal die castings. Employing a wider thread-cutting slot, they allow ample clearance for particles. The off-center slot presents an acute angle, with sharp cutting edges for best cutting performance. Type 23 screws are often used in plastics where a standard machine screw thread is desired. They can also be used in harder metals where the advantages of a wider cutting slot are beneficial. They work well in soft electrical components of copper, brass, or bronze.
TYPE 7513 (MACHINE SCREWS)
This a thread-cutting screw in which the multiple grooves spiral all the way up the threaded part of the shank. As this is a DIN specification, these are only available off-the-shelf in Metric, but can be made in any configuration. They will cut its own thread as it is driven into an unthreaded pilot hole. They create chips from the material that they are cutting into. They generally require less torque to install than thread forming screws and can be used in applications when the torque required for other types of screws is undesirable. When installed correctly, they create a tight fit with the mating part and normally do not require the use of a washer to prevent loosening. These are especially good in heavy gauge metals.
TYPE Y (SHEET METAL SCREW)
This form is widely spaced with a slightly tapered blunt point. To the eyes it can appear to be checkered threads. It is used specifically for thermoset plastics (Bakelite, Epoxides, and Polyimides, for instance). It has no practical use in other applications.
TYPE CA (MACHINE SCREWS)
This a thread-cutting screw specifically designed for heavy-gauge sheet metal. It’s a machine screw, only it has a point like a tapping screw and the threads continue all the way down the point. Often, they are later removed and replaced with a standard machine screw once the object (such as a refrigerator where these are commonly used) is in place. Higher driving torque is required. The type CA is commonly used when there are two slightly mis-aligned holes. They are not typically available off the shelf except in a few specialty sizes designed for a particular application. (The type C doesn’t have a point).
Thread Forming (Rolling) Types
TYPE TT (TAPTITE®) (MACHINE and SHEET METAL TYPES)
This a thread-forming screw. Some materials are referred to as “notch sensitive.” This means that notches, such as cut threads, almost completely destroy the strength characteristics of the material. In these materials, formed threads (such as generated by Taptite® style thread formers) can alleviate this problem. Taptites only deform material as opposed to removing material by actual cutting. The Tri-lobular shape of a Taptite is the key to its ability for form threads. The high areas (referred to as the lobes) do the actual material deforming or thread forming and the flats between the lobes provide the needed stress relief for the material being moved between the flanks of the thread. The trilobular configuration extends the entire length of the Taptite in contrast to most other thread forming screws which have a forming configuration of only a few threads at the tip (see the F, 23, 25, and 1 types above). This is the reason Taptites generally having a lower driving torque compared to other thread formers.
TYPE PT (PLASTITE®) (SHEET METAL)
Thread-rolling screws were developed specifically for use in plastics. They combine a unique trilobular cross-sectional form with deep, wide, spaced threads. Three swaging lobes (similar to those on the Taptite-style) with full relief of the thread form help reduce both driving effort and operator fatigue. The Plastite screw takes full advantage of characteristics of many plastics by allowing plastic material to recover and fill in between the lobes, which establishes maximum resistance to vibrational loosening. In addition, the screw’s deep, coarsely spaced threads provide a heavier shear area and deeper thread engagement in the plastic, further enhancing holding capabilities. The Plastite screw's trilobular design reduces tangential (cylindrical hoop) stress and the friction of root interference, which can frequently cause the bursting of thin-walled bosses. Plastite screws roll-form high quality internal threads with no damage to the molecular structure of the plastic, significantly reducing the danger of material failure. The easy starting, easy driving capabilities of Plastite screws can reduce assembly time and costs. Exceptional holding power eliminates the need for costly inserts or lock washers. The unusually high drive-to-strip ratio of Plastite screws allow the use of both automatic drivers and power tools. Strip-out can be virtually eliminated. Plastite screws come with different combinations of thread angles but 48/2 is by far the most common.
PLASTI-HOLD® (SHEET METAL)
These thread-rolling screws are a broad range of fasteners with deep, widely-spaced threads designed to reduce cracking, material damage, and strip-out in fastening plastics, PVC vinyl, particle board, and other composite materials. Designed for fast start and low torque drive, Plasti-Hold screws have a 45° thread angle and a 10° trailing angle to cut more cleanly through the material and to provide more holding power and less pullout. It is a form of the Plastite screw described above. We are fond of telling our customers to picture this particular fastener as a particle board screw for plastic. (45/10 as opposed 48/2 for Plastite® Style).
HI-LO® (SHEET METAL)
These fasteners (high-low is the generic term) are the solution for fastening in low pressure environments such as plastic, particle board, Masonite, and wood. It’s designed with a double-lead thread where the high thread is sharper than a conventional one. More material is trapped in between the threads which helps to resist pullout. This also allows for a lower driving torque. It is often the perfect screw for low-stress applications with reduced cracking in plastic. It also offers increased pullout resistance. These are almost always blunt point and tend to have a smaller head size than normal.