Understanding Cap Screw Counterbore Sizes: A Comprehensive Guide

When using cap screws, counterbore holes are used when the top of a screw or bolt needs to sit flush with or beneath the surface of the material it is inserted into. The size of the counterbore hole depends on the type of screw and the desired fit. For example, an ANSI Metric M2 socket cap head bolt with a normal fit counterbored hole requires a pilot hole diameter of 2.4 mm, a counterbore diameter of 4.4 mm, and a counterbore depth of 2.5 mm. It is considered good practice to countersink or break the edges of holes that are smaller than F (max) to ensure that the heads of screws seat properly and do not deform.

Cap Screw Counterbore Size Characteristics and Values

ANSI Metric Socket Cap Head Fasteners

When using ANSI Metric Socket Cap Head Fasteners, it's important to select the appropriate counterbore size to ensure a secure and flush fit for your screws or bolts. A counterbored hole is typically used when you need the top of a screw or bolt to be flush with or slightly below the surface of the material it is inserted into. This is in contrast to a simple clearance hole, which allows the head of the screw to sit above the surface.

For ANSI Metric Socket Cap Head Fasteners, the specific counterbore dimensions you'll need depend on the size of your screw and the desired fit type, such as a close fit, normal fit, or loose fit. Let's take a closer look at the various factors that will influence your counterbore size selection:

Screw Size:

The size of your screw is a critical factor in determining the appropriate counterbore dimensions. ANSI Metric Socket Cap Head Fasteners typically range from M2.5 to M36 in screw size. The M2 socket cap head bolt, for example, requires a pilot hole diameter of 2.4 mm, a counterbore diameter of 4.4 mm, and a counterbore depth of 2.5 mm for a normal fit.

Fit Type:

The type of fit you require will also influence your counterbore size selection. There are generally three types of fits for counterbored holes: close fit, normal fit, and loose fit. A close fit is typically used for assemblies with only one screw or when precise alignment is crucial. A normal fit is intended for longer screws or assemblies with multiple screws, providing flexibility for variations in the parts. A loose fit allows for even more eccentricity and is suitable for specific customisations or applications.

Hole Spacing and Alignment:

When dealing with drilled head metric hexagon socket head cap screws, the spacing and alignment of the holes is another important consideration. Screws smaller than M3 typically have two drilled holes spaced 180 degrees apart, while screws of size M5 and larger usually have six drilled holes spaced 60 degrees apart, unless specified otherwise by the purchaser.

Material and Finish:

In summary, when working with ANSI Metric Socket Cap Head Fasteners, selecting the right counterbore size involves considering the screw size, desired fit type, hole spacing and alignment, and the specific material and finish requirements for your application. Always refer to ANSI standards and size charts for precise dimensions to ensure a secure and aesthetically pleasing fastening solution.

Nominal Screw Size

When selecting a cap screw, it is important to consider the nominal screw size, which refers to the diameter of the screw. This dimension is crucial in ensuring the proper fit and function of the screw in the intended application.

The nominal screw size is typically denoted in inches or millimetres, depending on the standard being used. For example, in the Imperial system, a common nominal screw size might be #6, indicating a diameter of approximately 0.14" (3.556mm). In the metric system, a typical nominal screw size could be M6, representing a diameter of 6mm.

It is important to note that the nominal screw size is not the exact diameter of the screw thread. Instead, it is a standardised value that falls within a specific range, allowing for variations in the manufacturing process and ensuring compatibility with corresponding components, such as nuts or threaded holes.

To determine the appropriate nominal screw size for a particular application, one must consider factors such as the load-bearing requirements, the material being fastened, and the desired level of torque or tension. Selecting the right nominal screw size helps ensure that the screw can withstand the expected forces without stripping, breaking, or causing damage to the fastened materials.

In addition to the nominal screw size, other dimensions come into play when specifying a cap screw, including the length of the screw, the thread pitch, and the head style. These factors, along with the nominal screw size, are essential in selecting the correct cap screw for a specific application, ensuring proper assembly, and maintaining the structural integrity of the fastened components.

Counterbore Diameter

When choosing a counterbore diameter for a cap screw, it's important to consider the specific requirements of your project, including the desired fit and the hardness of the materials involved.

The counterbore diameter is the width of the cylindrical hole that houses the cap screw's head. It is one of the key dimensions to consider when designing a joint or assembly. Choosing the right counterbore diameter ensures that the screw head sits flush with or slightly below the surface of the material, resulting in a smooth and aesthetically pleasing finish.

For example, let's consider an ANSI Metric M2 socket cap head bolt. To achieve a normal fit, the counterbore diameter should be 4.4 mm. This means that the bolt will have a snug fit, with minimal clearance between the screw head and the counterbore hole. On the other hand, a close fit is typically used when only one screw is required or when multiple screws need to be precisely aligned during assembly. In this case, the counterbore diameter would be smaller to ensure a tighter fit.

It's worth noting that countersinking, or breaking the edges of holes, is considered good practice for certain applications. This is especially true when the hardness of the parts being fastened equals or exceeds the screw hardness. Countersinking helps prevent issues such as improper seating of the screw head or deformation of the screw's fillets, which can lead to fatigue in dynamic loading applications. However, the countersink diameter should not exceed the maximum recommended size to avoid reducing the effective bearing area and introducing potential issues like imbedment or brinnelling.

Countersink Diameter

When working with cap screws, it is considered good practice to countersink or break the edges of holes that are smaller than F (max.) in parts with a hardness that approaches, equals, or exceeds the screw hardness. If the holes are not countersunk, the heads of the screws may not seat properly, or the sharp edges on the holes may deform the fillets on the screws, making them susceptible to fatigue in applications involving dynamic loading.

The countersink or corner relief should not be made larger than necessary to ensure that the fillet on the screw is cleared. The diameter of the countersink does not normally have to exceed F (max.). A larger countersink or corner relief reduces the effective bearing area and introduces the possibility of embedding where the parts to be fastened are softer than the screws, or brinnelling/flaring of the heads of the screws where the parts are harder than the screws.

The countersink angle of a cap screw is typically 82 or 90 degrees. The standard countersink diameter for a #10-24 UNC screw is 0.319 inches, and for a #8-32 UNC screw, it is 0.275 inches.

For an ANSI Metric M2 socket cap head bolt with a normal fit counterbored hole, the required pilot hole diameter is 2.4 mm, the counterbore diameter is 4.4 mm, and the counterbore depth is 2.5 mm.

Normal Fit vs Close Fit

When it comes to cap screw counterbore sizing, there are two main types of fits to consider: Normal Fit and Close Fit. Both serve distinct purposes and are applicable in different scenarios. Here is a detailed comparison of the two:

Normal Fit:

The normal fit, also referred to as the "clearance dia (normal fit)," is intended for screws of relatively long length. It is typically used in assemblies that involve two or more screws. In such cases, the mating holes are produced using conventional tolerancing methods. One of the advantages of the normal fit is that it provides for the maximum allowable eccentricity of the longest standard screws. This means that it can accommodate deviations in the parts being fastened, such as variations in hole straightness, angularity between the axis of the tapped hole and the shank, and differences in the center distances of mating holes. By allowing for these variations, the normal fit ensures a secure and adaptable connection.

Close Fit:

The close fit, on the other hand, is designed for specific scenarios. It is usually limited to holes for screws that are threaded to the head, and it is applicable in assemblies where only one screw is used or where multiple screws are used in conjunction with matched and coordinated tooling. The close fit provides a tighter and more precise connection. It is important to note that the close fit, also known as the "clearance dia (close fit)," is not suitable for screws of relatively long lengths.

Choosing the Right Fit:

The choice between a normal fit and a close fit depends on the specific requirements of the assembly. If the assembly involves longer screws and potential variations in the parts being joined, a normal fit would be more appropriate. On the other hand, if precision and a tighter connection are crucial, and the assembly involves shorter screws or matched tooling, then a close fit would be the preferred option.

Design Considerations:

When designing and engineering an assembly, it is essential to refer to established standards and guidelines, such as the ASME B18.3, which provides specific dimensions and tolerances for drill and counterbore sizes. Additionally, it is good practice to consider countersinking or breaking the edges of holes to ensure proper seating of screw heads and prevent deformation of screw fillets, especially in applications involving dynamic loading.

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