The Manufacturing Processes of Bolts, Screws, and Fasteners – Part II

In the previous post we had discussed the various materials that can be used in the manufacturing process of bolts, screws, and fasteners. We also explained the preferred manufacturing methods and processes.  In this post we shall take a look at some other options.

The Optional Four Steps in Fastener Manufacturing

These steps comprise complicated specialized processes to achieve specific results.  However, we will explain each option briefly:

1. Heat Treatment: This process is performed to increase the overall durability of the screw or bolt. There are two operations within heat treatment:
   1. Hardening: Here, the fasteners are heated to a high temperature (about 1652^oF), and then dipped into a cooling media (usually water). This sudden change in temperature hardens the bolt or screw material.
   2. Tempering: This operation is performed to reduce some of the excess hardness/brittleness of the fastener. The metal is heated to a temperature level below its critical point after which the screw is left to air cool.

2. Surface Finishing: This process includes operations that are performed to alter the surface of the bolt or screw. It is done to enhance its function and aesthetic appeal. Surface finishing can include:
   1. Knurling: A lathe is used to create crossed, angled, or straight lines into the metal.
   2. Burnishing/Reaming: A tool is passed along the body of the fastener in a sliding motion. This motion helps enhance the aesthetics of the fastener’s surface finish.
   3. Grinding: A grinder is used to remove any imperfections from the screw to make the surface flat, smooth or even reflective. 

3. Thread Forming and Rolling: Bolts and screws need to be threaded for many applications. Thread forming or cutting is performed to create threads on the inside or outside of the fastener. Thread rolling operations are performed using thread rolling dies, which form various thread sizes on a part without removing material. The die is placed against the area where the threading needs to be made. The fastener is moved in a circular motion, allowing the die to create perfect threads around the body at precise locations. 

4. Protective Coating: The last step involves coating the screws and bolts with a protective coating. This prevents the fasteners from rust or corrosion, and can change the appearance by adding color. 

After all these steps have been completed the custom machined fasteners are then sent for testing and inspection. They are checked for their hardness levels, threading accuracy, impact and tensile strength, torque levels, coating thickness, and mating fit.  If they pass all these tests, they are then packed and shipped for delivery.