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What is a hole in engineering? Holes are an important component of mechanical parts; Holes are among the most common features in mechanical components.

So what are the main types of holes? This article will answer these questions and provide more information. Below, I will outline twenty-three common hole types in engineering and their classifications: through hole, blind hole, stepped hole, tapered hole, simple hole, counterbored hole, countersink hole, spotface hole, deep hole, slotted hole, interrupted hole, overlapping hole, screw clearance hole, pilot hole, dowel hole, vent hole, oil hole, relief hole, inspection hole, tapped hole, threaded hole, counterdrilled hole, and reamed hole.

Structural Holes (Geometric Form)

1. Through Hole

A through hole is a hole that is made to pass completely through a part from one side to the other. The hole has two open ends and does not stop inside the material. It is usually created by drilling, milling, or boring.

What Is the Callout Symbol for a Through Hole?

The symbol for a through hole is the diameter symbol “Ø”. On engineering drawings, a through hole is shown by giving the hole diameter. For example, a through hole with a diameter of 10 mm is written as: Ø20.

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How are Through Holes Used in Engineering?

Through holes are widely used in mechanical and electrical engineering. They are commonly used for bolts, screws, and pins that need to pass completely through a part. Through holes are also used in electronic products, such as the holes in printed circuit boards (PCBs) where component leads go through the board. In many structures, through holes are used when parts must be clamped together from both sides.

2. Blind Hole

A blind hole is a hole that does not go all the way through the part. The hole has only one open end and a solid bottom inside the material. It is made by drilling, milling, or boring to a fixed depth.

What Is the Callout Symbol for a Blind Hole?

A blind hole has no special symbol. A blind hole is defined by its diameter and its depth. The depth shows how far the hole goes into the material.

For example, a blind hole with a diameter of 30 mm and a depth of 12 mm is written as: Ø30 ↓12

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How are Blind Holes Used in Engineering?

Blind holes are used when a hole is needed but the back side of the part must remain solid. They are very common in housings, brackets, and thick plates where there is no space for a through hole. Blind holes are often used for tapped holes, where a screw needs to be fixed into a part without using a nut on the other side.

3. Stepped Hole

A stepped hole is a hole made of two or more sections with different diameters. Each section has a straight cylindrical shape, and the diameter changes in clear steps.

What Is the Callout Symbol for a Stepped Hole?

There isn’t a single symbol for stepped holes. They are defined by listing multiple diameters and depths.

For example, the top section is 20 mm in diameter and 5 mm deep is written as: Ø20 × 5

How are Stepped Holes Used in Engineering?

It is made by drilling or boring multiple times with different tool sizes. Stepped holes are used when different components need to fit into the same hole at different depths. For example, a stepped hole can hold a shaft in the lower section and a bearing or bushing in the upper section. This type of hole is common in gearboxes, motor housings, and mechanical assemblies that require precise positioning of multiple parts.

4. Tapered Hole

A tapered hole is a hole whose diameter changes along its length. One end of the hole is larger than the other. It is made with a taper drill, taper reamer, or special milling tool.

What Is the Callout Symbol for a Tapered Hole?

Tapered holes are usually specified with a taper angle or taper ratio. The drawing specifies a tapered hole with a top diameter of Ø40 mm, a total depth of 80 mm, and a taper angle of 20°.

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How are Tapered Holes Used in Engineering?

It is made with a taper drill, taper reamer, or special milling tool. Tapered holes are mainly used for accurate positioning and self-locking connections, such as taper pins or machine tool spindles. Because the tapered hole is tapered, the parts can be naturally centered and form a tight fit, so it is often used to fix cutting tools or other tool holders.

5. Simple Hole

No standard definition found in mechanical engineering.
The term is used informally to describe a basic cylindrical hole without extra features. There are many types of simple holes; they can be through holes or blind holes.

What Is the Callout Symbol for a Simple Hole?

There isn’t a formal standard symbol. A simple hole is shown using only the diameter symbol.

Example: A hole diameter is 10 mm is written as: Ø10

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How Are Simple Holes Used in Engineering?

Simple holes are often used for basic alignment, weight reduction, or as passage holes for wires, tubes, or small rods. In many designs, a simple hole is used as the starting point before adding other features such as threads, counterbores, or countersinks, commonly used for installing fasteners such as bolts, screws, or rivets.

6. Counterbored Hole

A counterbored hole is a hole with a wider flat-bottom section at the top. The main hole goes straight down, and the upper part has a larger diameter. It is made by drilling a normal hole first and then enlarging the top with a counterbore tool.

What Is the Callout Symbol for a Counterbored Hole?

The symbol for a counterbored hole is “⌴”. The main hole size is written first, and the counterbore size and depth are written after the symbol.

For example: For example: Ø10 ⌴ Ø20 × 15

This means the main hole has a diameter of 10 millimeters. The top section is counterbored to a diameter of 20 millimeters, and the counterbore depth is 15 millimeters.

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How are Counterbored Holes Used in Engineering?

Counterbored holes are used when a bolt or screw head needs to sit below or level with the surface. This is common in machine parts where the surface must remain flat for safety, appearance, or assembly reasons. Counterbored holes are widely used in equipment frames, fixtures, and mechanical covers.

7. Countersink Hole

A countersink hole is a hole with a cone-shaped opening at the top. The shape matches the angle of a flat-head screw. It is made by drilling a hole and then cutting the top with a countersink tool.

What Is the Callout Symbol for a Countersink Hole?

The symbol for a countersunk hole is “⌵”. The callout shows the main hole size, then the countersink size and angle.

For example: Ø10 ⌵ Ø20 × 90°

This means the main hole has a diameter of 10 millimeters. The top opening is countersunk to a diameter of 20 millimeters, and the countersink angle is 90 degrees.

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How are Countersink Holes Used in Engineering?

Countersunk holes are used when a flat-head screw must sit flush with the surface. This is important in parts that slide, rotate, or need a smooth outer surface. Countersunk holes are common in sheet metal parts, panels, aerospace structures, and consumer products.

8. Spotface Hole

A spotface hole is a shallow, flat hole that provides a smooth, flat surface. The flat area is only wide enough to support a bolt head or washer. Similar to a counterbore hole but shallower, it allows fasteners to be installed smoothly.

What Is the Callout Symbol for a Spotface Hole?

The callout symbol for a spotface is the spotface symbol “⌴” (same graphic as counterbore, labeled as SF).

Example: Ø5 SF Ø10

This means The part requires a through hole with a diameter of 5 mm and a spotface with a diameter of 10 mm and a radius of 1 mm (R1).

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How are Spotface Holes Used in Engineering?

Spotface holes are used to create a clean, flat area for a bolt head or washer. They are often used when the original surface is rough, curved, or cast. Spotfacing holes ensure that parts connected to components with rough surfaces can be correctly positioned and subjected to uniform force.

9. Deep Hole

A deep hole is a hole whose depth is much greater than its diameter. In practice, this usually means a depth-to-diameter ratio above 5:1 or 10:1. It is made with special drilling methods such as gun drilling or deep-hole drilling.

What Is the Callout Symbol for a Deep Hole?

There isn’t a special symbol for deep holes. They are specified using diameter and depth.

Example: Ø6 × 80

This means the hole diameter is 6 mm, depth is 80 mm.

How are Deep Holes Used in Engineering?

Deep holes are used in parts where long internal channels are required. They are common in shafts, hydraulic cylinders, injection molds, and oil passages. Deep holes are often used to carry fluids, support long rods, or reduce part weight.

10. Slotted Hole

A slotted hole is an elongated hole that is longer than it is wide. The ends may be round or flat. It is usually made by milling instead of drilling.

What Is the Callout Symbol for a Slotted Hole?

A slotted hole has no symbol. A slot is defined by its width and total length.

For example: 8 × 20 SLOT

This means the slot is 8 millimeters wide, and the total length of the slot is 20 millimeters.

How are Slotted Holes Used in Engineering?

Slotted holes are used when parts need adjustment during assembly. They allow components to move slightly before final tightening. Slotted holes are common in mounting plates, brackets, motor bases, and alignment systems.

11. Interrupted Hole

An interrupted hole is a hole whose inner wall is partly missing or cut away. This happens when the hole intersects with another hole, slot, or surface. The hole is usually created by normal drilling or milling, it is important to note that the holes remain on the same axis even after the material-free zone.

There is no specific symbol on the engineering drawings for marking interrupted holes.

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How are Interrupted Holes Used in Engineering?

Interrupted holes appear in complex parts where multiple features intersect. They are not usually designed on purpose, but result from the part geometry. These holes are common in manifolds, valve bodies, and multi-axis machined parts.

12. Overlapping Hole

An overlapping hole is a hole that partially intersects with another hole. The two holes are not on the same centerline, and part of their circular profiles overlap. This creates a combined geometry where material is removed from the overlapping region.

Overlapping holes are typically created by drilling or milling two separate holes whose positions are intentionally offset.

What Is the Callout Symbol for an Overlapping Hole?

There is no specific symbol for overlapping holes in engineering drawings. In technical drawings, we usually use the diameter and center point of two holes to represent overlapping holes.

How Are Overlapping Holes Used in Engineering?

Overlapping holes are used when a larger or irregular opening is required but produced using standard drilling tools.

Overlapping holes can also appear in manifold blocks, valve bodies, and multi-channel fluid systems. In some cases, they are intentional; in other cases, they may result from design revisions or space constraints.

Because overlapping holes create thin walls or sharp edges between intersections, engineers must carefully evaluate strength and stress concentration in critical parts.

Functional Holes (Assembly Purpose)

13. Screw Clearance Hole

The diameter of the screw clearance hole is larger than the outer diameter of the thread, but smaller than the diameter of the screw head. The screw passes through the hole without gripping the material.

It was originally designed to accommodate a smooth shaft for screws or bolts, not a threaded portion. The thread cannot be secured in this component. It is made by drilling to a size based on standard clearance charts.

What Is the Callout Symbol for a Screw Clearance Hole?

There isn’t a unique symbol. The hole is specified by diameter based on screw size.

Example: Ø6.5 (for M6 screw)

This means the hole allows an M6 screw to pass through freely.

How to choose the clearance hole size?

Clearance hole size = (Screw Diameter + Screw Head Diameter) / 2.

The method for calculating the clearance hole size is to add the screw diameter and the screw head diameter, then divide by two. This method yields the maximum hole diameter that can accommodate the fastener head. Many CNC milling machines can machine clearance holes.

How are Screw Clearance Holes Used in Engineering?

Screw clearance holes are used when a screw must pass through one part and tighten into another part. They are common in almost all bolted connections, such as plates, covers, and structural joints.

14. Pilot Hole

A pilot hole is a small hole drilled before a larger hole.

There isn’t a specific symbol. It is shown as a normal small hole.

How are Pilot Holes Used in Engineering?

Pilot holes are used to guide tools and improve accuracy. They are often drilled before larger holes, tapping operations, or deep drilling. Pilot holes reduce tool wandering and help prevent damage to tools and parts.

15. Dowel Hole

A dowel hole is a hole made to fit a locating pin. The hole is usually drilled and then reamed for high accuracy.

There isn’t a special symbol. It is usually shown as a reamed hole with tight tolerance.

How are Dowel Holes Used in Engineering?

Dowel holes are used for precise alignment between parts. They ensure that two components always assemble in the same position. Dowel holes are common in molds, fixtures, machine bases, and precision assemblies.

16. Vent Hole

A vent hole is a hole used to release trapped air or gas. It is usually small and placed in enclosed spaces.

There isn’t a standard symbol.

How are Vent Holes Used in Engineering?

Vent holes are used to release trapped air or gas. They prevent pressure buildup inside closed spaces. Vent holes are common in molds, sealed housings, fluid systems, and vacuum applications.

17. Oil Hole

An oil hole is a hole used to carry lubricant to moving parts. The hole connects oil sources to bearings or sliding surfaces. It is made by drilling and sometimes intersecting with internal channels.

How are Oil Holes Used in Engineering?

Oil holes are used to deliver lubricant to moving parts. They guide oil or grease to bearings, gears, and sliding surfaces. Oil holes are common in engines, gearboxes, and rotating machinery.

18. Relief Hole

A relief hole is a hole added to reduce stress or avoid tool interference. It is placed at corners, ends of slots, or near sharp features.

How are Relief Holes Used in Engineering?

Relief holes are used to reduce stress and improve machining. They are placed at sharp corners, slot ends, or thread exits. Relief holes help prevent cracking and make tools last longer.

19. Inspection Hole

An inspection hole is a hole made for viewing or measuring internal areas. It allows access for cameras, probes, or gauges.

How are Inspection Holes Used in Engineering?

Inspection holes are used to allow visual or physical access to internal areas. They are used for maintenance, testing, and quality checks. Inspection holes are common in pressure vessels, pipelines, and large mechanical systems.

Process Holes (Manufacturing Method)

20. Tapped Hole

A tapped hole is a hole with internal threads cut into its wall. The hole is first drilled to a specific size and then tapped with a threading tool. This type of hole allows a screw or bolt to fasten directly into the part without a nut.

What Is the Callout Symbol for a Tapped Hole?

A tapped hole has no graphic symbol. A tapped hole is defined by the thread specification instead of a simple diameter.

However, metric threads are usually indicated by the letter \"M\". For other standard threads, such as Uniform National Fine Thread (UNF) and Uniform Coarse Thread (UNC), they are followed by the bore diameter and pitch.

For exampl: M8 × 1.25

The major thread diameter is 8 millimeters, and the thread pitch is 1.25 millimeters, which is the distance between two adjacent threads.

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How are Tapped Holes Used in Engineering?

Tapped holes are used when a screw or bolt needs to fasten directly into a part. They eliminate the need for a nut. Tapped holes are extremely common in machine frames, enclosures, and mechanical assemblies.

21. Threaded Hole

A threaded hole is any hole that contains internal threads. The threads can be made by tapping, thread milling, or thread turning. This term is more general than tapped hole and describes the final condition of the hole.

What Is the Callout Symbol for a Threaded Hole?

There are two forms of marking for threaded holes: the conventional \"Ø\" or \"M\". Here, M represents the metric unit, and the following value indicates the thread length.

A threaded hole can also use imperial thread notation.

For example, an imperial threaded hole is written as: 1/4-20 UNC

This means the thread size is one quarter inch in diameter, and there are 20 threads per inch. “UNC” means it is a Unified National Coarse thread.

How Are Threaded Holes Used in Engineering?

Threaded holes are used to allow screws or bolts to fasten directly into a part without using a nut. They are widely used in machine structures, enclosures, housings, and mechanical assemblies. Threaded holes are especially common in aluminum and steel parts where repeated assembly and disassembly is required.

22. Counterdrilled Hole

A counterdrilled hole combines the features of a countersink hole with an added groove at the top. It is a hole enlarged by drilling a second time at the top. It has a larger upper diameter and a flat bottom.

What Is the Callout Symbol for a Counterdrilled Hole?
There isn’t a separate symbol. It uses the same symbol as counterbore: “⌴”.

Example: Ø10 ⌴ Ø18 × 6

This means a 10 mm main hole, a 18 mm flat enlargement, 6 mm deep.

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How Are Counterdrilled Holes Used in Engineering?

Counterdrilled holes are used when a fastener head needs to sit below or flush with the surface, but the hole is made using a drilling operation instead of a counterbore tool. They are commonly found in older mechanical drawings, cast parts, and basic machined components where high precision is not critical. In practice, counterdrilled holes are often used for bolts or cap screws in frames, fixtures, and mounting plates.

23. Reamed Hole

A reamed hole is a hole that has been finished with a reamer tool. The hole is first drilled slightly undersize and then reamed to final size.

What Is the Callout Symbol for a Reamed Hole?

A reamed hole has no special symbol. The hole size is written with the word REAM to show the finishing process.

For example: Ø10 REAM

This means the final hole diameter is 10 millimeters, and the hole must be finished by reaming to achieve better accuracy and surface quality.

How are Reamed Holes Used in Engineering?

Reamed holes are used when high accuracy and smooth surfaces are required. They are common in bearing seats, dowel holes, and precision fits. Reaming is often used when parts must align tightly with minimal clearance.

Conclusion

Holes are one of the most fundamental and widely used features in mechanical engineering. Although they may appear simple, different hole types serve very different structural, functional, and manufacturing purposes. From basic through holes and blind holes to specialized features such as counterbored, tapped, or overlapping, each design choice affects assembly, strength, accuracy, and production efficiency.

Understanding the differences between hole types helps engineers select the right geometry for fastening, positioning, fluid flow, stress reduction, or precision fitting. Proper hole design also improves manufacturability and reduces machining complexity and cost.

If you require machining parts with holes, GSG-Tech offers a complete solution. Our machining plant provides 5-axis CNC machining service, CNC milling service, and other hole machining services. Let our expertise help you succeed in your projects!

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