Flat Head Bolt

Product Description

A flat head bolt (also commonly known as a countersunk bolt) is designed to sit flush with or below the surface of the material it is fastening. This is achieved through its unique conical head shape, which fits into a pre-drilled, countersunk hole.

Anatomy and Design

The defining characteristic of a flat head bolt is the countersink angle. This is the angle measured from one side of the conical head to the other.

• Standard Angles: In the US (Unified Thread Standard), 82° is the most common. In metric systems (ISO), 90° is the standard. Specialized aerospace or military applications may use 100°.

• The "Grip" Measurement: Unlike most bolts where length is measured from under the head, flat head bolts are measured by their overall length, including the head, because the head is intended to be fully submerged in the material.

Common Drive Types

Flat head bolts are available in various drive styles depending on the torque requirements and the tools available:

• Socket (Allen): High-torque capability, common in machinery and precision engineering.

• Phillips/Slotted: Standard for general construction and woodworking.

• Torx (Six-Lobe): Excellent for automated assembly as it prevents "cam-out" (the bit slipping out of the head).

Mechanical Properties & Materials

Depending on the application, flat head bolts are manufactured to different strength grades:

1. Alloy Steel (Grade 8 / Class 10.9): Used in industrial machinery where high tensile strength is required. These are often treated with a black oxide finish.

2. Stainless Steel (304 or 316): Ideal for food processing, marine environments, or medical equipment where corrosion resistance is more important than raw strength.

3. Brass or Bronze: Often used in decorative applications or wooden boat building for a combination of aesthetics and sea-water resistance.

Key Applications

• Aerospace: Used on the exterior "skin" of aircraft to maintain a smooth, aerodynamic surface and reduce drag.

• Machinery: Used on conveyor belts, slide tracks, or any surface where a protruding bolt head would interfere with moving parts.

• Consumer Electronics: Found inside laptops and smartphones to keep components secure within extremely tight spatial constraints.

• Flooring and Decking: Used to ensure a smooth walking surface without trip hazards.

Technical Considerations for Installation

• Countersinking: The hole must be prepared with a countersink drill bit that matches the angle of the bolt head. If the angles do not match, the bolt will not seat properly, leading to uneven stress distribution and potential failure.

• Clearance: Because the head is wide, there must be enough material thickness to accommodate the depth of the countersink without "poking through" the other side of the workpiece.

• Stress Concentrations: In high-vibration environments, the conical shape of the head can act as a wedge, potentially causing cracks in brittle materials like certain cast irons or high-carbon steels.