cnc manufacturing

Whether you’re designing a small component for an electronic gadget or large-scale machinery, you will inevitably encounter threaded features. The ability to specify, design, and manufacture these features effectively can drastically impact the functionality and longevity of a component or product. In this blog post, we’ll delve into the fundamentals of threading, offering practical tips for engineers.

External vs. Internal Threading

Threads can generally be categorized into two main types: external and internal.

  • External Threading: This refers to threads that are on the outer surface of a component. A common example is the threading seen on a screw. External threads are designed to mate with an internal threaded component, like a nut.
  • Internal Threading: As you might guess, these threads are located on the interior surface of a hole in a component. They are designed to accept an externally threaded part. A nut, which accepts a screw, is a basic example of internal threading.

Practical tip: It’s crucial to ensure compatibility between external and internal threads. They should be designed with the same thread form, pitch, and size to ensure proper engagement.

Key Parameters for Indicating Threads

When specifying or reading about threads, there are a few primary parameters to be aware of:

  • Major Diameter: This is the largest diameter of the thread. For external threads, it’s the diameter from crest to crest, and for internal threads, it’s the diameter from root to root.
  • Minor Diameter: The smallest diameter of the thread. For external threads, it’s the diameter from root to root, and for internal threads, it’s the diameter from crest to crest.
  • Pitch: This denotes the distance between two adjacent thread crests. Alternatively, threads are sometimes specified by their threads per inch (TPI).
  • Thread Form: This refers to the thread’s profile or shape. Common thread forms include Unified National (UN), British Standard Whitworth (BSW), and metric threads.
  • Thread Class: Indicates the tolerance and allowance of threads. It defines how tight or loose the mating parts will fit together.

Practical tip: Always clearly specify these parameters in your designs or when ordering parts. Even a minor deviation can lead to incompatibility and component failure.

Blind Holes vs. Through Holes

A key consideration in the realm of internal threading is the nature of the hole itself:

  • Blind Hole: This is a hole that doesn’t go all the way through the material. It has a distinct depth and bottom. Internal threads in a blind hole must be carefully controlled to ensure the threading doesn’t exceed the depth of the hole.
  • Through Hole: As the name implies, this hole extends completely through the material. Internal threads can run the entire length of the hole, or they can be partial.

Practical tip: When threading a blind hole, always leave some unthreaded space at the bottom of the hole to accommodate the lead of the tap (the tapering part at the start of a tap) and ensure full thread engagement for the mating part. Additionally, ensure proper chip evacuation methods during the threading process to prevent tool breakage.

Threads, while often overlooked, are integral in the assembly and functionality of countless devices and systems. A solid grasp of thread types, their parameters, and application is pivotal for engineers. As always, clear communication and diligent verification are paramount in ensuring successful, functional, and durable threaded connections in any manufacturing endeavor.

Different Thread Types

In the vast arena of manufacturing, threads aren’t one-size-fits-all. Various applications require specialized threads, each having its own set of standards and uses. Here are some common thread types:

  • Unified National (UN): These threads are common in North America and come in several variations including UNC (Unified National Coarse) and UNF (Unified National Fine).
  • Metric Threads: Widely used internationally, these threads are specified based on millimeters and follow the ISO metric screw thread standard.
  • British Standard Whitworth (BSW): Originating in the UK, these threads have a unique profile and are used less frequently today but can still be found in some British applications.
  • British Standard Pipe (BSP): Used for joining pipes and fittings, these threads can be either tapered (BSPT) or parallel (BSPP).
  • Acme Threads: Featuring a trapezoidal profile, these threads are typically used in applications like vices, jacks, and other devices where large loads are moved linearly.
  • NPT (National Pipe Thread): A U.S. standard for tapered threads used on pipes and fittings.
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Calling Out Metric and UN Threads

When specifying threads, clarity is paramount. Here’s how to call out metric and UN threads:

  • Metric Threads: Typically called out by the nominal major diameter followed by the pitch. For example, M10x1.5 where ‘M’ indicates a metric thread, ’10’ is the major diameter in millimeters, and ‘1.5’ is the pitch in millimeters.
  • UN Threads: They are called out by the nominal major diameter, threads per inch (TPI), and the thread series. For example, ¼-20 UNC where ‘¼’ is the major diameter in inches, ’20’ is the TPI, and ‘UNC’ indicates it’s a Unified National Coarse thread.

Recommended Tolerances for Internal and External Threads

The tolerance for threads ensures the fit between mating components. Here are general recommended tolerances:

  • Unified National (UN) Threads: UN threads have classes like 1A/1B (loose fit), 2A/2B (general purpose), and 3A/3B (tight fit). For most applications, the 2A/2B class is suitable.
    • External (e.g., bolts): For a 2A fit, you’d typically have no allowance and a tolerance range that varies with size but is often around ±0.001 inches for common sizes.
    • Internal (e.g., nuts): For a 2B fit, again there’s usually no allowance, and the tolerance might be slightly larger than for external threads, possibly ±0.0015 inches for common sizes.
  • Metric Threads: Metric thread tolerances are denoted by a fit grade and a tolerance position. Common classes include 6g for external threads and 6H for internal threads.
    • External: A bolt with an M10x1.5-6g thread will have specific max and min diameters defined by the standard. The ‘6g’ provides a specific tolerance range for that diameter.
    • Internal: For an M10x1.5-6H internal thread, the tolerance position ensures a good fit with a 6g external thread, but the actual tolerance values might differ slightly.

Delving Deeper into Metric Threads

Metric Threads are a system of screw threads that have been internationally accepted as the standard for most countries, except the United States and Canada which primarily use the Unified Thread Standard. Metric threads are based on millimeters, and their use simplifies the threading process and specifications since it follows a base-10 system.


  • Pitch: Unlike the TPI (threads per inch) of UN threads, metric threads use a direct pitch system. The pitch denotes the distance between threads and is expressed in millimeters.
  • Nominal Diameter: This represents the external diameter of the thread and is also expressed in millimeters.

Class Fit for Metric Threads

Class fit is an essential concept when it comes to metric threads. It refers to the tightness or looseness of the thread fit, based on the manufacturing tolerances.

  • 6g/6H: This is the standard fit, where “6g” is used for external threads like bolts, and “6H” is for internal threads like nuts. In most standard applications, a 6g external thread will mate well with a 6H internal thread.
  • 4g/4H & 5g/5H: These are used less commonly and represent tighter tolerance classes, often used in precision applications.
  • 7g/7H & 8g/8H: These have looser fits compared to the standard 6g/6H and might be chosen for applications where ease of assembly is prioritized over the tightness of fit.

What are Unified (UN) Threads?

Unified National (UN) threads are a standardized thread form that was introduced to unify the existing American National (AN) and British Whitworth thread systems. These threads are predominantly used in the United States and Canada.


  • Pitch and TPI: UN threads are usually specified by threads per inch (TPI), which denotes the number of threads in a linear inch.
  • Diameter: Expressed in inches, it specifies the thread’s external diameter.
  • Thread Forms: The most common types of UN threads include:
    • UNC (Unified National Coarse): This has fewer threads per inch and is used in general applications where quick assembly or disassembly is preferred.
    • UNF (Unified National Fine): More threads per inch than UNC, offering a finer thread. This type is often chosen for applications requiring tighter thread fit or where the risk of cross-threading needs to be minimized.
    • UNEF (Unified National Extra Fine): As the name suggests, this has an even finer thread than UNF and is used in specific precision applications.

Metric Threads

Metric Threads are a system of screw threads that have been internationally accepted as the standard for most countries, except the United States and Canada which primarily use the Unified Thread Standard. Metric threads are based on millimeters, and their use simplifies the threading process and specifications since it follows a base-10 system.


  • Pitch: Unlike the TPI (threads per inch) of UN threads, metric threads use a direct pitch system. The pitch denotes the distance between threads and is expressed in millimeters.
  • Nominal Diameter: This represents the external diameter of the thread and is also expressed in millimeters.

Designing Threads for Efficient Manufacturing

Thread manufacturing is a process that might seem straightforward, but minor design adjustments can vastly improve efficiency. Here are some strategies:

  1. Standardize Where Possible: Opt for standard thread sizes and pitches whenever feasible. This reduces the need for custom tooling and speeds up setup times.
  2. Optimize Depth: For internal threads in blind holes, ensure there’s adequate unthreaded depth at the bottom. This facilitates the tapping process, prevents tap breakage, and ensures full thread engagement.
  3. Radiused Roots: Incorporating a radiused (or rounded) root can increase the thread’s strength and reduce the chances of stress risers which can lead to failure. It also makes the threading process smoother, particularly during rolling.
  4. Avoid Thin-Walled Threaded Sections: They can be challenging to produce and can deform during the threading process. A robust thread profile is more stable and easier to manufacture.
  5. Consider Alternative Thread Forms: In some applications, non-standard thread forms, like buttress or square threads, may be easier to produce or offer better performance characteristics.
  6. Surface Finish: A good surface finish can improve the quality and longevity of threads. Smooth surfaces reduce friction during mating and can prevent premature wear.
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UN Thread Chart

When specifying UN threads, a thread chart can be invaluable. Here’s a simplified version:

Nominal SizeThreads Per Inch (TPI)Coarse (UNC)Fine (UNF)Extra Fine (UNEF)
1/4”201/4-20 UNC1/4-28 UNF1/4-32 UNEF
5/16”185/16-18 UNC5/16-24 UNFN/A
3/8”163/8-16 UNC3/8-24 UNF3/8-32 UNEF

(Note: This is a very simplified chart for illustrative purposes; full charts can contain many more sizes and variations.)

Metric Thread Chart

For metric threads, the chart would typically present diameter and pitch combinations. Here’s a basic representation:

Nominal Diameter (mm)Coarse Pitch (mm)Fine Pitch (mm)
M101.51.0, 1.25

(Note: As with the UN chart above, this is a basic representation; comprehensive charts would have many more sizes and pitch variations.)

UNS Thread Chart (Unified Special)

Unified Special threads (UNS) allow for threads of a non-standard diameter or pitch. These are threads that do not conform to the standard UNC, UNF, or UNEF series but are still within the unified inch standard. UNS is mostly used in specific applications where standard threads aren’t optimal.

UNS Thread Chart:

Nominal SizeThreads Per Inch (TPI)Example
7/16”207/16-20 UNS
9/32”329/32-32 UNS

(Note: The UNS series has numerous potential combinations; the above is illustrative.)

NPT/NPS Thread Chart

NPT (National Pipe Tapered) and NPS (National Pipe Straight) threads are used for joining pipes and fittings.

NPT/NPS Thread Chart:

Nominal Pipe SizeThreads Per Inch (TPI)Type

(Note: This chart is abbreviated. Full versions would include a wider range of sizes.)

BSPP/BSPT Thread Chart

BSPP (British Standard Parallel Pipe) and BSPT (British Standard Tapered Pipe) threads are UK standards, used for joining pipes and fittings.

BSPP/BSPT Thread Chart:

Nominal SizeThreads Per Inch (TPI)Type

(Note: Again, this is a shortened version, and a full chart would provide a wider array of sizes.)

PG Thread Chart

PG threads are a German standard, mainly used for cable glands or fittings. “PG” stands for “Panzer-Gewinde,” translating to “armored thread.”

PG Thread Chart:

PG SizeOuter Diameter (mm)Pitch (mm)

(Note: This is a basic representation. Comprehensive charts have more variations.)

Frequently Asked Questions (FAQs) About Threads

1. What’s the primary difference between metric and UN threads?

  • Metric threads are based on a metric system with sizes and pitches defined in millimeters, while UN (Unified National) threads are based on an imperial system with sizes defined in inches and pitches often defined as threads per inch (TPI).

2. Can I mate a metric threaded bolt with a UN threaded nut?

  • No, metric and UN threads have different pitches and profiles. Mating them can damage the threads and will not ensure a secure fit.

3. What does “Class fit” mean for threads?

  • Class fit refers to the manufacturing tolerances of threaded components. It defines how tight or loose the thread engagement will be. Both metric and UN systems have different class fit designations to specify the exactness of the fit.

4. What is a ‘blind hole’ and how is it different from a ’through hole’?

  • A blind hole is a hole that doesn’t pass entirely through a workpiece, while a through hole goes completely through.

5. Why would I choose NPT over BSPP or vice versa?

  • NPT and BSPP are both pipe thread standards, but NPT has a tapered profile, making it self-sealing. BSPP is parallel and often requires a bonded seal or O-ring. Your choice would depend on the application, regional standards, and desired sealing mechanism.

6. What is the significance of a thread’s “pitch”?

  • Pitch refers to the distance between individual thread peaks. In metric threads, it’s the distance in mm between two threads. In UN threads, it’s usually given as threads per inch (TPI).

7. Why are there different thread standards?

  • Different standards evolved due to regional preferences, industrial needs, and historical reasons. For instance, the US predominantly uses UN threads, while most other countries have adopted metric threads.

8. Are there any general guidelines for selecting thread type and size for an application?

  • It largely depends on the load the thread will carry, the material it’s made of, the assembly and disassembly frequency, and the environment (corrosive, high temperature, etc.). However, always opt for standard sizes and pitches for ease of replacement and availability.

9. When should I use a fine thread over a coarse thread?

  • Fine threads are typically used when higher resistance to loosening from vibration is required, in thinner materials, and for applications needing greater adjustment precision. Coarse threads are more durable and faster to assemble.

10. Why would threads strip or cross-thread during assembly?

  • This can happen due to forceful assembly, misalignment, or using the wrong size/type of mating thread. Ensuring proper alignment, lubrication, and correct component selection can prevent this.