What Is a Machine Screw?

Pauline Gill

Machine screws are uniformly threaded screws of ¼ inch nominal diameter or less that are designed to be threaded into uniformly threaded nuts or threaded holes in the parts they are intended to fasten. Machine screws are available in a wide array of configurations, materials, driver head types and sizes.

As their name implies, machine screws are used to fasten various components together in machines, tools, appliances, electronic devices and vehicles of every description. They are available in virtually every material including steel, stainless steel, brass and even nylon.

Uniform Threads

Uniform threads maintain the exact same diameter throughout the entire length of the threaded section of the fastener. This is as opposed to tapered threads with a pointed end such as on wood screws, which are intended to cut their own thread into the softer wooden or plastic material they are being threaded into.

Machine Screws vs. Bolts

Although machine screws are generally considered to be smaller than bolts, machine screws also tend to be designated as such if the primary tightening of the fastened joint is accomplished by turning the head of the screw. With a bolt, the primary tightening procedure might be accomplished by turning the nut holding the bolt through a hole.

Fastening Applications

Machine screws are versatile in the number of types of joints they can fasten. They can be used for fastening two or more components with through holes and a nut behind all the components. They can be passed through two or more components and then finally threaded into the rearmost component, which is solid and uniformly threaded and acts as the nut in the joint. Two opposing machine screws can also be used with threaded couplings to separate two plates or boards by the distance equal to the length of the coupling. Machine screws are also used for many types of electrical connections and terminal strips. They are also used to fasten many gasketed devices that are sandwiched together such as waterproof motor casings. They can also be threaded into PEM brand fasteners, which are threaded metallic inserts cast right into plastic parts.

Head Types

Machine screws are available in a wide assortment of driver head types, including standard slotted, Phillips head, hex socket head for use with Allen wrenches, Torx six-pointed star, square-drive socket head and security heads, which are easily driven in but very difficult to remove.

Threading Methods

Machine screws may have their threads formed by die cutting, where some material is actually cut out of the grooves forming the thread, or by rolling or roll forming where the thread is rolled into the screw by precise rolling dies under very high pressure where no material is removed.