How Self-Drilling Screws are Made

A necessity for the production and manufacturing of self-drilling screws is the use of high quality and high tensile strength metal wire. This single factor ensures that the screws will perform according to their specifications and be able to maintain a firm tight connection.

Wire

Self-drilling screws begin as wire that comes in different diameters depending on the gauge of screw. The wire for self-drilling screws is chosen such that it is harder than the material that it will join. The most common type of wire is 410 stainless steel, which is corrosion and rust resistant. Heated treated versions, up to 40 Rockwell C, ensure that the drill point is able to pierce harder resilient metals.

The first step in the production process is to straighten the wire before it is cut to the correct length.

Heading

The wire is fed into the heading machine where it is cut to length, transferred to a die, and preformed.

Cold Heading

Once the wire is cut to the required length, it is passed to the die. There are various types of dies used to form the head of a screw.

One Die

A one die machine has one die but uses two punches to form the screw. The die is a mold that has the shape of the screw. A punch delivers a forceful blow forcing the blank wire into the die. The force applied by the die and punch creates the head of the screw.

Multiple Blow

The multiple blow process is the same as the single one die process. Instead of two blows being applied, several blows are used to form the screw head.

Threading

There are several methods for forming screw threads, which include subtracting by cutting or grinding, deformation by rolling, molding, or casting, and additive methods. A key factor for self-drilling screws is the drill bit head that can have point sizes of numbers 2, 3, 4, and 5 with the higher the number indicating the longer the tip

To create the thread pattern, using the die method, a deformation process, the workpiece is placed between variable dies that have grooves to create the target thread depth. As pressure is applied, the workpiece is rolled, which leads to the thread pattern being pressured into the wire.

Self-Drill Screw Point

The drill point of a self-drilling screw is in the form of the point of a drill bit and is designed to drill the beginning hole and form the mating threads. Unlike a self-tapping screw that requires a pilot hole, self-drilling screws do not require a pilot hole due to their drill bit points. The construction of their point makes them more versatile and able them to work with a wider range of materials.

For drilling into thicker heavy duty materials, points of self-drilling screws come with wings that help enlarge the hole and create an easier path for the threads.

Heat Treatment

In the last step of the manufacturing process, the completed self-drilling screws are racked vertically or in bulk for heat treatment and are sent through a quenching furnace where the screws are heated to remove contaminants. They are then oil quenched to lock in their hardness.

Different metals require the use of different methods for the heat treatment process. Carbon steel, for example, is case hardened where chemicals are added to the metal surface and heated at high temperature. Since case hardening reduces the machinability of a self-drilling screw, it is performed as the last step of the manufacturing process.

Coating and Plating

The coating and plating process is completed after heat treatment and involves applying a layer of zinc, chromium, nickel, and other materials to the surface of the screw. Designed to be a protective coating against rust and corrosion, coating and plating changes the major and pitch diameters of a screw several times and requires precision and accurate assessment of the process.