Understanding Allen Head Cap Screw Dimensions: A Comprehensive Guide

Socket button head cap screws are used in a variety of applications, from compartment design in overhead spaces to heavy-duty applications. They are known for their wide load-bearing surface, which makes them resistant to loosening. These screws come in various materials, including stainless steel, carbon steel, and alloy steel, and can be customized to meet specific requirements. Understanding the dimensions and standards of socket button head cap screws is essential for their effective use in different projects. This includes considerations such as nominal size, body, head diameter, and length.

Head diameter

The head of a cap screw may be plain or knurled, depending on the manufacturer's preference unless specified by the customer. The maximum head diameter of a knurled screw is measured by drawing a line across the tops of the knurl, while the minimum head diameter is the diameter of the un-knurled portion. If the screw does not have an un-knurled portion, the minimum head diameter is the diameter across the tops of the knurl just above the radius or chamfer at the bottom edge of the head.

The head of the screw should be chamfered, with the chamfer at an angle of 30 to 45 degrees with the plane of the top of the head. The edge between the top of the head and the chamfer may be slightly curved. The bearing surface of the screw should be perpendicular to the axis of the screw, with a maximum deviation of 1 degree.

The edge between the bearing surface and the side of the head may be broken (rounded or chamfered), but the radius or chamfer measured along the bearing surface should not exceed the values given for K. The values for K are not provided, but they likely refer to a standard sizing chart or measurement that can be found in engineering resources.

The concentricity of the head and shank of the screw is also important. The head should be concentric with the shank within 1 percent of the basic screw diameter, with a maximum runout of 0.006 inches when held within one diameter of the head but beyond the fillet. This ensures that the screw is straight and reduces the risk of it bending or breaking during use.

Head chamfer

The head chamfer is a crucial aspect of screw design, and it refers to the angle at the edge of the screw head, where it transitions to the bearing surface. This chamfer, denoted as angle E, should be within the range of 30 to 45 degrees relative to the plane of the top of the screw head. This means that the chamfer forms an angle of 30 to 45 degrees with an imaginary flat surface that is level with the top of the screw head.

While the edge between the top of the head and the chamfer can be slightly rounded, it is important to maintain the specified angle for the chamfer itself. This angle plays a role in ensuring the proper function and appearance of the screw.

The head chamfer is just one of many important dimensions and features of a screw. Other specifications, such as the nominal size, body, head diameter, bearing surface, and thread length, also contribute to the overall performance and application of the screw.

Additionally, the head chamfer interacts with other features of the screw head, such as the drive type, which can be a slot, cross, or hexagonal socket. The chamfer angle and its relationship to the bearing surface can influence how the screw is driven and its ability to be used in confined spaces or areas with limited clearance.

Bearing surface

The bearing surface of a screw is a critical area of its design. The bearing surface is the area under the head of the screw, where it meets the radius and fillet juncture. This area is important as it helps the screw to fulfil its holding assignment. The bearing surface is also part of the screw that is carefully designed to ensure dimensional control.

The bearing surface of a screw is designed to be strong, and in some cases, it can be wider to allow for a smoother, finished appearance. This is the case with low head cap screws, which are used when a wider bearing surface is required.

The bearing surface of a screw is also important in determining the number of screws required for a job. For example, Holo-Krome screws are made from alloy steel and are stronger than conventional screws, meaning fewer screws are required to fulfil a holding assignment.

The bearing surface of a screw is also an important consideration when it comes to the finish of the screw. For example, black oxide-treated screws have a sleek, black appearance that is achieved through a chemical process. This process adds a layer of corrosion and abrasion resistance to the bearing surface of the screw, as well as a desirable finish.

Edge of the head

The edge of the head of an Allen head cap screw is typically flat and chamfered. The head is cylindrical and has a hexagonal cross-section socket. The outer sides of the cylindrical cap are often knurled.

The cap of the screw can be situated either above the surface or recessed into a counter-bored hole. The cap screw is placed above the surface when the screw is used in applications that require a neat, finished appearance.

The edge of the head of an Allen head cap screw is designed to be driven with an internal hex socket drive, which matches a hex key or Allen wrench. The screws are often used in projects with limited space that cannot accommodate traditional wrenches or sockets.

Allen head cap screws are available in a wide range of dimensions and can be easily tightened or loosened with tools from a standard socket set. They are commonly used in machine parts, die fixture, and clamping. They are also used in automotive, boating, and machinery-based equipment.

Concentricity

The head of the Allen head cap screw must be concentric with the shank, maintaining a high degree of accuracy. The specified tolerance for this concentricity is within 1% of the basic screw diameter, with a maximum deviation of 2% total runout or 0.006 inches, whichever is greater. This tolerance range ensures that the head and shank are aligned correctly, preventing potential issues during assembly and use.

Similarly, the socket of the screw should also be concentric with the shank. The tolerance for this aspect is slightly different, with a maximum deviation of 1-1/2% of the basic screw diameter. This translates to a maximum total runout of 3% or 0.005 inches for sizes up to and including 1/2 inch. For sizes larger than 1/2 inch, the total runout can be up to 6%. Maintaining concentricity between the socket and the shank is crucial for a secure and proper fit.

The total runout between the thread, body, and head of the screw is another important consideration. To ensure proper assembly, the screw should fit into a compound hole with specific threaded and counterbored ends. The diameters of these ends, De and Ae, must be concentric with the axis of the thread. The length of the center portion, equal to De, should also be considered, ensuring it aligns with the screw length minus two screw diameters (2D).

Additionally, the underhead fillet of the screw can vary in form, provided it maintains a smooth and continuous concave curve that blends seamlessly into the bearing surface of the head and the screw shank. This fillet is crucial for ensuring stress distribution and preventing cracking or failure at the junction between the head and the shank.

In conclusion, concentricity plays a vital role in the performance and functionality of Allen head cap screws. By adhering to the specified tolerances and requirements, manufacturers can ensure the screws' components are properly aligned, contributing to their overall effectiveness and reliability.

Frequently asked questions