Tapped hole vs threaded hole: the same concept or very different?

In the field of machining and product manufacturing, “tapped holes” and “threaded holes” are two terms that appear with high frequency. Both are closely related, but there are clear conceptual boundaries and application differences. Many practitioners or enthusiasts tend to confuse the two, but in fact they define the attributes of “hole” from the dimensions of machining process and structural function respectively.

In this article, we will analyze the differences and connections between tapped holes and threaded holes in terms of technical principles, processing methods and application scenarios to help readers understand these two important concepts accurately.

1. Basic concepts of tapped hole vs threaded hole: essential differences from the definition

(1) Threaded hole: "structural hole" defined by function

"Threaded hole" is a structural concept, referring to all holes with internal threads (internal helical teeth) processed on the inner wall. Its core function is to achieve mechanical connection with external threaded parts such as bolts and screws through internal threads. The existence of threaded holes does not depend on a specific processing technology, but is based on the core feature of "having an internal thread structure". For example:

The hole on a mechanical part (such as a flange or housing) used to install a screw has regular spiral teeth on the inner wall;

The internal threaded hole formed directly by injection molding of a plastic part;

The connection hole formed by embedding a threaded bushing in non-metallic materials such as wood and plaster.

No matter what processing method is used (tapping, milling, turning, molding or inlaying), as long as the inner wall of the hole has a standard or customized internal thread, it can be called a "threaded hole".

Milling of threads

(2) Tapped hole: "processed hole" defined by the process

"Tapped hole" is a process concept, specifically referring to a threaded hole processed by the "tapping" process. "Tapping" is a common method for manufacturing internal threads in mechanical processing. The specific process is:

First, drill a bottom hole (prefabricated hole) slightly larger than the thread diameter;

Use a special tool "tap" to cut or extrude the internal thread on the inner wall of the bottom hole through rotation and axial feed.

The core of the tapping process is "processing internal threads by tapping", so "tapped hole" must be a type of threaded hole, but it only limits the specific processing method of "tapping". For example:

Most internal threaded holes on metal parts (such as carbon steel and aluminum alloy) are usually completed by tapping;

Threaded holes with smaller diameters (such as those below M6) are almost only completed by tapping due to the limitations of processing accuracy and efficiency.

Tapping threads

2. Tapped hole vs threaded hole: an in-depth comparison in four dimensions

Comparison Dimension	Threaded Hole	Tapped Hole
Conceptual Category	Structural concept (focuses on "presence of internal threads")	Process-based concept (focuses on "how internal threads are manufactured")
Manufacturing Methods	Diverse (tapping, milling, turning, forming, embedding, etc.)	Single (must be completed via tapping process)
Material Adaptability	Wide range (metals, plastics, wood, ceramics, etc.)	Primarily for metallic materials (especially ductile materials)
Precision and Surface Quality	Varies with specific manufacturing process (significant differences)	High precision (depends on tap accuracy and processing parameters)

(1) The core definition of tapped hole vs threaded hole: function vs. process.

The core of the definition of threaded holes is the "internal thread structure", which is a result-oriented concept based nctional requirements; the core of the definition of tapped holes is the "tapping process", which is a process-oriented concept based on the process path. The two are the relationship between "result" and "means" - tapped holes are a way to achieve threaded holes, but threaded holes are not necessarily formed by tapping.

For example:

Plastic threaded holes (not tapped) produced by injection molding are threaded holes, but not tapped holes;

Large diameter internal threaded holes (such as those above M100) milled by CNC milling machines are threaded holes, but because they are not tapped with taps, they are not tapped holes.

(2) Tapping holes vs. threaded hole processing technology: Diversification vs. specialization

The processing methods for threaded holes are diverse:

Tapping: It is suitable for metal internal threaded holes with small and medium diameters (usually ≤M50) and medium precision requirements, and is the most commonly used processing method;

Milling/Turning: Using CNC equipment to directly cut out internal threads with thread milling cutters or turning tools, which is suitable for large diameter, high precision or non-standard threaded holes;

Thread milling cutter

Forming: For example, when injecting or die-casting, the internal thread structure is directly designed in the mold to form threaded holes for non-metallic or metal parts in one go;

Inlaid threads: Prefabricated threaded bushings (such as copper nuts) are embedded in low-strength materials (such as wood and plastic) to form threaded holes.

The processing path for tapped holes is unique:

It must go through the two steps of "drilling → tapping" and rely on tap tools. During the tapping process, the accuracy of the tap (such as tolerance grade H7, H8), cutting parameters (speed, feed rate), lubrication conditions, etc. directly determine the accuracy of the threaded hole (such as pitch, tooth angle, surface roughness).

tap

(3) Tapped hole vs threaded hole Materials and application scenarios: Widespread vs. Focused

The application of threaded holes covers almost all engineering materials:

Metallic materials (brass, steel, aluminum, etc.): threaded holes are processed by tapping, turning, milling, etc.;

Non-metallic materials:

Plastics (such as ABS, nylon): threaded holes can be injection molded, or tapped after drilling (suitable for high-strength plastics);

Wood: threaded holes are usually formed by embedding threaded bushings, and direct tapping can easily cause wood to crack;

Ceramics, glass: threaded holes are manufactured by precision processing (such as laser, electric spark), or metal threaded sleeves are embedded.

Tapping holes are mainly for metal materials:

Especially materials with a certain degree of plasticity (such as low carbon steel, aluminum alloy). This is because during the tapping process, the cutting edge of the tap needs to form a thread by cutting or extruding the material:

For high-hardness materials (such as hardened steel and titanium alloy), tapping is difficult and can easily lead to tap breakage, so thread milling is often used instead;

For low-strength non-metallic materials (such as ordinary plastics and wood), direct tapping may cause thread failure due to insufficient material strength, and it is more suitable to insert thread sleeves.

(4) Precision and quality of tapped hole vs threaded hole: process determines characteristics

The precision range of threaded holes is wide, depending on the processing method:

Tapped holes: The precision is usually medium (such as ISO thread tolerance grade 6H, 7H), with a surface roughness of Ra1.6~3.2μm, which is suitable for most mechanical connection scenarios;

Milled or turned threaded holes: The precision can reach 5H, 4H, and the surface roughness is lower (Ra0.8μm or less), which is suitable for high-precision occasions (such as aerospace, precision instruments);

Formed threaded holes: The precision is low (tolerance grade 8H or above), the surface roughness is high, which is suitable for scenarios with low connection strength requirements (such as daily necessities plastic parts). The precision bottleneck of tapped holes mainly comes from the precision of the tap itself and the stability during the processing. For example, when tapping manually, the operator's technique will affect the verticality and pitch accuracy of the thread; while automatic tapping on CNC machine tools can achieve higher processing accuracy by accurately controlling the speed and feed rate.

3. Tapped hole vs threaded hole: common misunderstandings and precautions in practical applications

(1) Misunderstanding 1: All threaded holes are tapped holes

This is the most common cognitive error. As mentioned above, there are many ways to process threaded holes, and tapping is only one of them. For example:

Large diameter threaded holes on automobile engine blocks (such as spark plug mounting holes) are often processed by CNC turning rather than tapping;

The threaded holes on household plastic hangers are mostly formed directly by injection molding and do not require tapping.

(2) Myth 2: Tapping can only process metal materials

Although the mainstream application of tapping is metal materials, under certain conditions, it can also be used for some non-metallic materials:

High-strength engineering plastics (such as POM, PET): After drilling, tapping can be used to form threaded holes, but the tapping speed must be controlled to avoid overheating and melting of the material;

Wooden boards (such as density boards, plywood): Temporary threaded holes can be made by "drilling first and then tapping", but the thread bearing capacity is weak, and it is recommended to embed embedded nuts.

(3) Key parameters for tapping hole design

If you need to process threaded holes by tapping, pay attention to the following when designing:

Bottom hole diameter: If the bottom hole diameter is too large, the thread profile will be incomplete and the strength will be insufficient; if it is too small, the tapping resistance will increase and the tap may even break. The calculation formula is:

Brittle materials (such as cast iron): bottom hole diameter ≈ thread minor diameter (D1 = D - 1.0825P, D is the nominal diameter, P is the pitch);

Plastic materials (such as steel, aluminum): bottom hole diameter ≈ thread minor diameter + 0.1P (reduce cutting resistance).

Hole depth and effective thread length: When tapping, there is a guide cone at the front end of the tap, and the actual effective thread length is about 1~2 pitches shorter than the hole depth. Sufficient depth must be reserved during design.

Material hardness: For high-hardness materials exceeding HRC35, the difficulty of tapping increases significantly, and thread milling or EDM needs to be used instead.

(4) Inspection and quality control of threaded holes

No matter what kind of processing method is used for threaded holes, the following core indicators must be inspected:

Thread accuracy: Use thread plug gauges (go/no-go gauges) to check whether the pitch, mid-diameter, and thread angle meet the standards;

Verticality: Use a square or a three-dimensional coordinate measuring machine to check the verticality of the threaded hole axis and the reference plane to avoid eccentric load during connection;

Surface roughness: High-precision threaded holes (such as sealing threads) need to control surface roughness to prevent leakage or wear.

4. Summary of tapped hole vs threaded hole: Clarify concepts and serve engineering practice

The relationship between tapped holes and threaded holes is essentially a subordinate relationship between "process" and "function" - tapped holes are a way to realize threaded holes, while threaded holes are a structural concept that includes multiple processing processes. The difference between the two lies not only in the definition scope, but also in various aspects of engineering practice such as processing methods, material adaptability, and precision control.

In actual design and manufacturing, clarifying the difference between the two helps:

Rationally select processing technology: decide whether to use tapping, milling or forming processing according to material properties, hole size, and precision requirements;

Avoid design errors: such as blindly designing tapped holes on plastic parts, resulting in thread failure; or ignoring the limitations of tapping in the design of large-diameter threaded holes;

Improve manufacturing efficiency: For small and medium-diameter metal threaded holes, tapping technology is preferred (high efficiency and low cost); for high-precision or non-standard threaded holes, more appropriate methods such as CNC milling are selected.

In conclusion, the analysis of tapped hole vs threaded hole reflects the underlying thinking of “from process to function” in mechanical engineering - only by precisely understanding the connotation and extension of each technical term can we make the optimal decision in design, processing, inspection, etc., and ultimately realize the balance between product performance and manufacturing cost. The balance between product performance and manufacturing cost is finally realized.