The screw can pose an interesting dilemma when it is needed for a project around the house. The many varieties available send the normal person into dismay on which one to buy. Questions arise on what kind of screw to purchase, what length, and what thread relief are appropriate. The sales clerk at your local home improvement or automotive center will be able to help you select the proper screw if you are unsure.
History of the Screw:
The screw we know today was invented by Vitold Reabchinsky. It went into industrial production during the 1850’s due to the demand of the consumers. All screws were handmade prior to that time period. Modern civilization created the need for wood and steel screws. The wood screws were the first to be mass produced. The creation of the steel screw helped in the booming during the Industrial Revolution. Society would not have advanced forward with as much velocity without the creation of these two screw types.
Screw Pitches and Threads:
Screws are designed to have different pitches, thread relief and thread direction. Most screws are threaded so they turn to the right to be installed. The right handed screw is normally used for woodworking projects. The recommended thread relief for screws is dependent upon the type of project to be completed.
Types of Threads Available:
The British Standard Fine (BSF) thread is based on the British Standard Whitworth. The threads are finer than the British Standard Whitworth a BS 84: 1959 specification. The British Standard Whitworth (BSW) has rounded crests and roots. Specifications are a BS 84: 1959. The Unified thread came after the British Standard Whitworth along with the metric thread.
Classifications of Screw Types:
Class 1A and 1B spin-on-assembly is used when doing rough commercial construction. Classes 2A and 2B are the commercial screws, nuts and bolts used in normal construction. Classes 3A and 3B are used for high-quality work between mating parts. Class 5 screws are used with a wrench for force fitting. They are used primarily for the connection of studs. They were designed to use a high torque method of installation.
What Type of Screw to Buy:
When deciding on the type of screw to buy, it is best read the packaging to determine if the screw will work on the project intended. As a rule of thumb, a screw should be ¾ of an inch longer than the width of the object it will be penetrating. A metric screw is generally only used in mechanical devices. Zinc plated or coated screws work best in places where they will be exposed to water or the elements.
Vardexusa: Thread Milling Handbook PDF
CNC Zone: Screw Threads 101
Google Books: Technical Drawing
Link: Screw Threads PDF