In engineering and manufacturing, different types of holes are designed for various purposes, such as fastener installation, fluid passage, weight reduction, and aesthetic considerations.
At its essence, a hole is an opening or recess that has been created in a material. This feature is essential for housing fasteners, allowing for the passage of fluids or electrical connections, or even reducing weight in structural components. The design and execution of these holes are governed by strict engineering principles to ensure that they meet the necessary functional and safety requirements.
This article explores the most common types of holes used in machining, injection molding, and other manufacturing processes. By understanding these fundamentals, readers will gain valuable insights into how meticulously designed holes contribute to the overall success of engineering projects.
Primary Types of Holes
Engineers classify holes into several primary categories based on their characteristics and intended applications. Common types include:
Through Hole
A through hole is a type of opening that is drilled or machined to completely pass through the material, thereby creating a continuous passage from one side of the workpiece to the other. It is sometimes referred to as a “thru-hole” in various engineering contexts.
Through holes are essential in many engineering applications due to their versatility. They are frequently utilized for:
- Electronic Components: For example, through holes are commonly drilled in printed circuit boards (PCBs) to mount electronic parts, providing both mechanical support and electrical connectivity.
- Assembly Purposes: They allow for the insertion of fasteners or pins, thereby joining different parts together.
- Fluid or Air Passage: In some designs, through holes facilitate the movement of fluids or air, contributing to cooling or lubrication processes.
Counterdrilled Holes
A counterdrilled hole is a specialized hole featuring a recess above the main drilled hole. It serves a function similar to a countersunk hole, but instead of a conical shape, the recess is cylindrical, allowing a fastener or component to sit deeper within the material.
Simple Hole
In engineering, a simple hole is essentially a circular opening that has been removed from a material. Despite its straightforward appearance, there are several variations tailored to different applications.
On engineering drawings, a simple hole is denoted by the callout symbol ‘Ø’. Alongside this symbol, the drawing will specify both the diameter and the precise depth of the hole to ensure accurate fabrication.
Simple holes are incredibly versatile. They serve a range of purposes, such as:
- Clearance: Providing space for moving parts or ensuring proper alignment.
- Location: Acting as reference points for assembly or as part of a positioning system.
- Assembly: Creating openings for fasteners like bolts or screws, enabling secure connections between components.
By defining and detailing simple holes with clarity on technical drawings, engineers ensure that these features meet the functional and structural requirements of the overall design.
Blind Hole
A blind hole does not pass all the way through the material. It has a specified depth, with the bottom enclosed. It is typically produced using methods such as drilling, reaming, or milling, ensuring that the cavity stops at a predetermined depth, preserving part of the material’s integrity.
Unlike through holes, there isn’t a dedicated callout symbol for blind holes on engineering drawings. Instead, they are specified by indicating both the hole’s diameter and its depth, or by noting the remaining thickness of the material beyond the hole. This detailed specification ensures that the fabrication process achieves the correct depth without compromising the design.
Blind holes serve a variety of roles in engineering, including but not limited to:
- Residual Stress Measurement: In some applications, blind holes are strategically employed to assess residual stresses within a component.
- Fastener Accommodation: They are often used where a through hole is impractical, such as in parts where maintaining one continuous surface is critical.
- Threading Operations: In modern manufacturing, CNC milling machines frequently produce blind holes by running a thread milling cycle. Threading these holes can be achieved by various methods, including:
- Conventional Tapping: A standard process for cutting threads.
- Single-Point Threading: A method that uses a single cutting tool to form the thread.
- Helical Interpolation: A CNC technique that creates threads through a helical motion, offering precision and flexibility.
Counterbored Hole
A counterbored hole is an enlarged cavity machined into a material that allows the head of a fastener—such as a bolt or socket-head screw—to sit flush with or below the surface of the workpiece. This is useful in applications requiring a smooth surface after fastening.
Countersunk Hole
A countersunk hole has a conical recess at its opening, accommodating flat-head screws so they sit flush with or below the surface. The angle of the countersink is typically standardized, such as 82° or 90°. This conical design allows the head of a countersunk bolt or screw to sit flush with—or even below—the surface of the workpiece, ensuring a smooth finish and reducing potential interference with adjacent components.
On engineering drawings, countersunk holes are typically identified by the ‘﹀’ symbol. This callout is accompanied by precise dimensions indicating both the diameter of the simple hole and the specific geometry of the conical recess, ensuring that the countersunk fastener fits perfectly.
Interrupted Hole
An interrupted hole is a specialized type of hole in which another feature—such as a recess, pocket, or notch—intersects the intended drilling path. This interruption results in a discontinuous material profile along the hole’s axis, hence the term “interrupted.”
The creation of an interrupted hole involves a unique drilling operation that consists of a sequence of coaxial holes with intentional gaps. These gaps represent sections where the material is absent (i.e., empty space) due to the intersection with another feature.
Interrupted holes are typically employed in complex components where multiple features must coexist. They are useful in designs that require:
- Integration of Multiple Functionalities: For example, combining structural support with the accommodation of other components or fluid passages.
- Space Optimization: Allowing designers to incorporate intersecting features without compromising the overall design integrity.
- Customized Machining Requirements: The intermittent nature of the material requires precise control during the drilling process to maintain the intended geometry and tolerances.
Spotface Holes
A spotface hole is a shallow, machined recess that ensures a fastener, such as a bolt or screw, sits on a smooth and level surface. It is similar to a counterbored hole, but its primary function is to create a uniform seating area rather than accommodate a recessed fastener head.
On engineering drawings, spotface holes are represented by the counterbore symbol (‘⌴’) with the letters ‘SF’ inside it. This notation indicates that the hole has been spotfaced to ensure flatness, rather than fully counterbored for a recessed fastener.
Tapered Hole
A tapered hole is a hole in which the diameter gradually decreases from one end to the other, forming a conical shape rather than a cylindrical one. Unlike standard holes with uniform diameters, tapered holes have varying diameters along their depth, making them ideal for specialized engineering applications.
Screw Clearance Holes
A screw clearance hole is a hole that is slightly larger than the diameter of a screw thread, allowing the screw to pass through the hole without engaging the threads. This type of hole is essential for allowing fasteners, such as screws or bolts, to pass through one part and be secured to another without interference from the material surrounding the hole.
A common method for determining the hole size is:
- Add the diameter of the screw thread and the diameter of the screw head.
- Divide the sum by two.
This gives the hole size that allows the screw to pass through without the head interfering with the material. In many cases, CNC milling machines are used to create screw clearance holes with precise diameter specifications.
Threaded Holes
A threaded hole is a hole that has internal threads, allowing a screw or bolt to be securely fastened. While they are similar to tapped holes, threaded holes are typically made differently, often using a cutting tool or a CNC machine to form the internal threads. Threaded holes are designed to accommodate threaded fasteners, providing a reliable method of securing components.
Tapped Holes
A tapped hole is a hole that has internal threads cut into its surface, allowing a machine screw to be securely threaded into it. This process, known as tapping, is commonly used when a traditional nut and bolt cannot be used, and the fastening needs to be directly integrated into the material itself.
On engineering drawings, a tapped hole is denoted by the ‘M’ symbol (for metric threads) in place of the usual diameter symbol. The callout will specify both the diameter of the hole and the thread size (e.g., M6 for a metric screw with a 6mm diameter).
Tapped Holes vs. Threaded Holes
| Aspect | Tapped Holes | Threaded Holes |
|---|---|---|
| Process | Threads are cut using a tap tool inserted into a drilled hole. | Threads are milled into the material using a milling tool. |
| Tooling | Requires a tap and tap drill. | Requires a milling tool and a CNC machine. |
| Material Compatibility | Best for soft to medium materials. | Suitable for harder materials and complex profiles. |
| Precision | Less precise compared to thread milling. | Offers higher precision and custom thread profiles. |
| Customization | Limited to standard threads. | Can produce custom, complex thread profiles. |
| Application | Common in mass production with standard threads. | Used in high-precision applications (e.g., aerospace, automotive). |
| Cost & Speed | Typically faster and more cost-effective for standard threads. | Slower but offers greater control for complex threads. |
Drilled Hole
Drilled holes are created using a drill bit and are the most common type in manufacturing. These can be further refined with secondary machining processes.
Punched Hole
A punched hole is created by using a sharp tool or die to pierce through a material, typically sheet metal or other thin materials. The punching process is widely used for its speed and efficiency, making it ideal for high-volume manufacturing applications.
The punching process involves placing a sheet of material under a punch press machine. A sharp punch tool is then forced through the material, creating a hole. The die beneath the material helps shape the hole as the punch moves through. This method is particularly effective for materials that are not too thick and is commonly used in industries that require fast, cost-effective production of holes in metal, plastic, or composite materials.
What Is a Hole Callout in Engineering?
A hole callout is an annotation found on engineering drawings that provides detailed specifications about a hole. This includes dimensions, tolerances, and other critical manufacturing information. The callout ensures consistency and precision during production, serving as a universal reference for fabricators and quality inspectors.
How to Choose the Right Hole Type in Machining?
The choice depends on several factors, including the material, the intended purpose of the hole, and the overall design of the part or assembly.
- Is the hole for passing components through, securing parts, or creating threads?
- What material is the part made of, and what is its thickness?
- What diameter, depth, and shape are needed?
- Will the hole be used for fastening or assembly?
- What is the most efficient and accurate way to create the hole in the material?
- What level of precision is required for your application?
- Does the hole need to be flush, or is there a specific design requirement?
Here are the key considerations and steps to help you choose the correct hole type:
- Thin materials (like sheet metal) are often punched or drilled, depending on the required hole size and shape.
- Thicker materials (such as metal plates) may require drilling, reaming, or milling.
- A counterbored or countersunk hole is required when the fastener head must sit below or flush with the surface.
- A tapped hole is chosen when internal threads are needed for a screw to securely connect with a part.
- Consider the depth of the hole: blind holes are ideal for certain depth requirements, while through holes are needed when passing through multiple layers.
- If the purpose is to create a threaded connection, tapped or threaded holes are necessary.
- For bolts or screws, decide whether you need clearance (screw clearance holes) or whether the fasteners will sit below the surface (counterbore or countersink holes).
- For large volumes, methods like punching may be the most efficient. For smaller quantities or more precise holes, CNC machining or drilling might be better.
- Punched holes may be used when the surface finish is less important, but speed and cost-effectiveness are priorities.
Conclusion
Understanding the different types of holes and their applications is crucial in mechanical design, manufacturing, and assembly processes. Each type serves a distinct purpose, from facilitating fastener installation to optimizing fluid or airflow. By selecting the appropriate hole type based on function, shape, and manufacturing method, engineers and designers can enhance product performance, durability, and ease of assembly.
At BOYI, we specialize in high-precision CNC machining services, delivering top-quality components tailored to your specifications. Whether you need custom hole designs, complex geometries, or high-tolerance machining, our expert team ensures accuracy, efficiency, and reliability in every project. Contact us today to bring your designs!
FAQ
A countersink hole is selected when a fastener with a conical head (such as a flat-head screw) needs to sit flush with the surface, offering a smoother finish compared to a counterbore hole.
A threaded hole has internal threads, which allow screws or bolts to be screwed into the hole for secure fastening.
A blind threaded hole is similar to a threaded hole but does not go all the way through the material. It is typically tapped from one side to allow for fasteners to be screwed in without passing through the entire material.
This article was written by engineers from the BOYI team. Fuquan Chen is a professional engineer and technical expert with 20 years of experience in rapid prototyping, mold manufacturing, and plastic injection molding.
