What are Blind Holes in Engineering Drawing and Machining?

A blind hole is a hole that is closed at one end and does not completely pass through the part. Some valves, such as triple offset butterfly valves, have blind holes at the upper and lower valve stems. When selecting flange bolts, special attention must be paid to the bolt length and the drilling standards for the threaded holes.

This article will introduce what a blind hole is and explain the notation used to indicate blind holes on engineering drawings.

What are Blind Holes?

Before proceeding, it is essential to clearly understand the concept of a blind hole.

In engineering drawing and machining, a blind hole (also known as a pocket or closed hole) does not pass through the entire thickness or length of the workpiece material but terminates within the material. Such holes are typically machined to a specific depth using methods like reaming, milling, or drilling without completely penetrating the material. Therefore, a blind hole is closed at the bottom, providing a recess or space for fasteners, screws, pins, slots, and other components.

Blind Hole Callout Symbol in Engineering Drawing

The annotation symbols for blind holes are shown in the following figure:

The “Φ” symbol indicates the actual diameter value of the hole. The downward arrow symbol (↧) represents the depth to which the hole is threaded (tapping depth).

How to Drill a Blind Hole?

Drilling a blind hole requires careful consideration of the hole depth, proper tools, and techniques to ensure accurate and efficient results. Here’s a step-by-step guide on how to drill a blind hole using different methods:

Hand Drill Method:

• Use a hand drill with a sharp drill bit suitable for the material.
• Wrap a piece of tape around the drill bit to serve as a depth indicator or use a hand drill with a built-in depth stop.
• Drill slowly and steadily, ensuring the drill is perpendicular to the surface.
• Periodically withdraw the drill bit to clear chips and check the depth.

Drill Press Method:

• Use a drill press with a suitable drill bit and depth stop.
• Secure the workpiece on the drill press table.
• Set the drill press speed and adjust the depth stop to the desired depth.
• Drill the hole, ensuring the drill press base is perpendicular to the workpiece surface.
• Clear chips regularly to prevent clogging and overheating.

CNC Mill, CNC Drill, or Lathe Method:

• Use a CNC mill, CNC drill, or lathe with precise control over drilling parameters.
• Program the machine to drill the blind hole to the required depth and diameter.
• Ensure proper tool selection and toolpath programming for optimal results.
• Use cutting fluid to lubricate and cool the drill bit, and evacuate chips continuously.

Regardless of the method used, it’s crucial to:

• Determine the hole depth based on the optimal thread size engagement of the fastener and the tap depth.
• Choose the appropriate drill bit and cutting speed for the material being drilled.
• Use cutting fluid to lubricate the drill bit and evacuate chips to prevent overheating and tool damage.
• Periodically clear chips from the hole to prevent clogging and ensure smooth drilling operation.

By following these steps and considerations, you can effectively drill blind holes with the desired precision and accuracy, ensuring reliable fastener engagement and structural integrity in your workpiece.

Choosing the Right Tap for a Blind Hole

Choosing the right tap for a blind hole is crucial to ensure proper thread engagement and a secure fastening. Here’s a guide on how to select the appropriate tap for tapping blind holes:

• Matching Thread Size and Drill Size:

• Each thread size corresponds to a specific drill size that must be used to create the appropriate hole diameter for tapping. For example, an M10 bolt would require an 8.8 mm drill bit to allow the tap sufficient material to cut a full thread into the hole.
• Refer to drill tap tables available our or in machining handbooks to determine the correct drill size for the desired thread size. See our guide on Screw Tap Drill Size Chart

• Bottoming Taps:

• Bottoming taps with a flat bottom are best suited for blind holes because they can thread close to the bottom of the hole without the taper getting in the way. Taper taps are not ideal for blind holes as they may not reach the bottom, leaving incomplete threads.
• Ensure that the bottoming tap matches the thread size of the hole being tapped.

• Chip Evacuation:

• During tapping, it’s essential to evacuate chips continuously to prevent chip buildup, which can lead to tap breakage.
• Backing out the tap periodically to remove chips is essential, especially in blind hole tapping.

• Alignment:

• Keep the tap perfectly aligned with the central axis of the hole to ensure straight and accurate threading.
• Misalignment can result in crooked or damaged threads, compromising the integrity of the fastener connection.

• Roll Taps (Optional):

• Alternatively, roll taps can be used to eliminate chip evacuation issues. Roll taps cold form the threads by pressing them into the hole walls instead of cutting them.
• Roll taps are particularly useful for high-volume production environments where chip removal can be challenging.

By following these guidelines and selecting the appropriate tap for your blind hole, you can ensure smooth and efficient tapping operations with reliable thread quality and strength.

Blind Hole vs Through Hole vs Counterbore

Blind holes are drilled partially into a material, stopping short of penetrating the entire thickness. They are commonly used for creating threaded openings or accommodating fasteners like screws or bolts. Tapping blind holes can be challenging due to chip buildup, especially with bottoming taps.

Through holes extend completely through the material’s thickness, allowing for fasteners to pass entirely through. They are often used for bolts, dowel pins, or other fasteners requiring access from both sides. Tapping through holes is typically easier as chips can escape from the opposite side, reducing the risk of tap breakage.

Counterbores are cylindrical recesses machined into a material, usually at the entrance of a hole. They provide a flat seating surface for the head of a fastener, allowing it to sit flush with or below the material surface. Counterbores are often used in conjunction with both blind and through holes to accommodate fasteners while providing a neat and flush appearance.

Use

Counterbore: Used for installing bolts or other connecting components.

Through hole: Besides fastener installation, it has various uses and allows appropriately sized objects or liquids to pass through.

Blind hole:

1. Used to connect two or more parts, requiring threading inside the hole and used in conjunction with through holes and bolts.
2. Used for installing locating pins to locate parts.

For more information, see our guide on Deep Hole Drilling Techniques: Applications in Machining.

Conclusion

When processing blind holes, it is necessary to choose appropriate processing methods and parameters based on specific application scenarios and requirements to ensure the quality and performance of blind holes.

This article introduces blind holes, explaining what they are and discussing their applications in engineering and machining processes. If you have any further questions about blind hole drilling, feel free to contact BOYI professional team for assistance throughout the entire process. BOYI offers a wide range of manufacturing capabilities, including CNC machining services and mold manufacturing services, to meet all your prototyping and production needs. Visit our website for more information or to request a free, non-obligatory quote.

FAQ

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Catalog: CNC Machining Guide