Types of Threads in CNC Machining: A Guide for Engineers

Threads are a fundamental feature in CNC machining, enabling the assembly of mechanical parts with precision and strength. Whether you’re designing components that require bolts, screws, or pipe connections, choosing the right thread type—and calling it out accurately—can make or break your final product.

This guide offers a comprehensive look at different thread types, how to specify them in your technical drawings, and practical manufacturing tips to avoid delays and ensure high-quality outcomes.

What Is a Thread?

A thread is a spiral-shaped groove that wraps around a round shaft or inside a round hole. A thread on a bolt or screw forms by cutting or rolling a series of ridges that wind around the part. A matching thread in a hole (for example, in a nut or in a machine component) locks around those ridges.

Threads have been in use for thousands of years, even if most of us do not notice them every day. We rely on threads whenever we tighten a jar lid, secure a wheel onto a car axle, or adjust the focus on reading glasses. In every case, the thread offers a reliable way to fasten, seal, or move parts.

Key Parameters in Thread Design

Before diving into specific thread types, it’s important to understand the main geometric features that define a screw thread. These features influence how threads fit and function in different mechanical systems.

1. Major Diameter (Outer Diameter)

The major diameter is the widest part of the thread, measured from crest to crest. It determines the general size of the thread and is commonly used when specifying fasteners. For external threads (like bolts), it can be measured with a caliper.

2. Minor Diameter (Inner Diameter)

The minor diameter is the smallest diameter, measured from root to root. It’s critical for evaluating the strength of a thread, especially in tension-loaded applications.

3. Pitch Diameter (Effective Diameter)

Pitch diameter is an imaginary diameter where the thread thickness equals the space between threads. It is essential for determining how tightly two threaded components will fit together. This value is not directly measurable without specialized gauges.

4. Pitch

Pitch refers to the distance between two corresponding points on adjacent threads, measured along the axis of the screw. Fine-pitch threads are often used in precision applications, while coarse-pitch threads are better for general-purpose use.

5. Lead

Lead is the distance a thread advances in one full revolution. In single-start threads, lead and pitch are the same. In multi-start threads, lead is a multiple of the pitch, allowing for faster movement per turn.

6. Thread Angle

This is the angle formed between the flanks of the thread. Different thread profiles use different angles, such as 60° for metric threads or 55° for British threads.

7. Tooth Form

Tooth form defines the shape of the thread—commonly triangular, square, or trapezoidal. The form directly affects the thread’s strength and efficiency.

External vs. Internal Threads

Before diving into thread types, it’s important to distinguish between external threads (e.g., bolts, studs) and internal threads (e.g., tapped holes in nuts or parts). External threads are formed on the outside of a shaft or rod, while internal threads are cut into a hole. Getting these right in your CAD drawings is essential—confusing the two can cause major production issues.

Common Types of Threads Used in CNC Machining

Engineers use a variety of thread forms depending on the region, industry, and design requirements.

Metric (ISO) Threads

Metric threads follow the International Organization for Standardization (ISO) system. Metric threads use the letter “M” followed by the nominal diameter in millimetres (for example, M8, M10, M12). The thread pitch is given in millimetres after the “×” sign if it is not the standard pitch.

• Standard Pitch Examples:
  • M6 × 1.0 (6 mm nominal diameter, 1.0 mm pitch)
  • M10 × 1.5 (10 mm nominal diameter, 1.5 mm pitch)
• Fine Pitch Examples:
  • M8 × 1.25 (8 mm diameter, 1.25 mm pitch)
  • M12 × 1.0 (12 mm diameter, 1.0 mm pitch)

Metric threads are the most common type used around the world. A bolt or nut marked “M10 × 1.5 – 6g” indicates a standard metric thread with a tolerance class of 6g (external) and a pitch of 1.5 mm. Designers usually rely on ISO 965-1 for tolerance classes, where 6H is the default for internal threads and 6g for external threads.

Unified Threads (UNC, UNF, UNEF, UNS)

Unified threads follow an inch-based system recognised in North America and in many international industries. The Unified system uses the basic series UNC (coarse), UNF (fine), UNEF (extra-fine), and UNS (special).

• UNC (Coarse): Fast general-purpose threads that are more tolerant of minor damage. Example: 1/4″-20 UNC (¼ inch nominal diameter, 20 threads per inch).
• UNF (Fine): Tighter tolerance threads with deeper minor diameter for better fatigue strength and better retention under vibration. Example: 1/4″-28 UNF.
• UNEF (Extra-Fine): Even finer thread spacing for small, precise assemblies or where thin materials require small minor diameters. Example: 10-32 UNEF.
• UNS (Special): Used for special applications that demand nonstandard pitch, depth, or tolerances.

Unified threads use a 60° flank angle, just like ISO metric threads. The standard tolerance classes are 2A/2B (medium), 3A/3B (tight), and 1A/1B (loose). The letter “A” indicates an external thread, and “B” indicates an internal thread. If no class is given, 2A or 2B is assumed in most cases.

Pipe Threads (NPT, NPTF, BSP, BSPP)

Piping and fluid transport systems need threads that seal fluids. Standard fasteners leak under pressure, but pipe threads use tapered or parallel forms that compress and seal when tightened.

1. NPT (National Pipe Tapered): NPT threads taper at a 1:16 ratio. A straight thread cannot seal alone. When tightened, the tapered surfaces press together and seal with thread sealant or tape.
2. NPTF (National Pipe Tapered Fuel): NPTF is a dry-seal variation of NPT. It uses close tolerances to seal without additional tape or sealant.
3. BSP (British Standard Pipe Parallel): BSPP threads are parallel. The fitting needs an O-ring or washer to seal.
4. BSPP (British Standard Pipe Parallel): Threads are parallel—they do not taper. A soft sealing washer or O-ring is typically used at the face of the fitting or flange to create a leak-proof seal.

Pipe threads appear on plumbing connections, hydraulic fittings, and pneumatic systems. When a drawing requires pipe threads, the designer must note the standard (NPT, NPTF, BSP, BSPT) and the size (e.g., “1/2 NPTF”). If the drawing leaves out details, the supplier may pick the wrong standard, leading to leaks or misfits.

ACME Threads

ACME threads use a trapezoidal form with a 29° included flank angle. They offer higher strength and lower friction than V-threads under heavy loads. Common uses include:

• Lead screws in milling machines, lathes, and CNC machines.
• Heavy actuators and screw jacks.

Most ACME screws have a single or multiple starts. A double-start ACME screw advances twice as far per turn as a single-start ACME screw with the same pitch. Typical ACME thread series include:

• **2G or 2V (General purpose, coarse)
• 3G or 3V (Close tolerance, finer)
• 4G (Precision)
• 8G or 8V (cost-reduced alternative)**

Designers should specify ACME series, pitch, tolerance class, and coating if needed.

Square Threads

Square threads have vertical sides and a 0° included flank angle. They provide the lowest friction and highest efficiency among common thread forms. Designers use square threads when precision power transmission and high efficiency matter most, such as in:

• Machine presses
• High-precision actuators
• Large ball-screw alternatives in early machinery

Square threads are harder to cut and inspect than ACME, so their use is more limited to very high-load or high-efficiency cases.

British Standard Threads (BSW, BSF)

British Standard threads predate the Unified system. They use a 55° profile angle. Common flavors include:

• BSW (British Standard Whitworth): coarse threads suited to general assembly.
• BSF (British Standard Fine): finer threads for parts requiring more threads per inch.

Despite being older standards, you still find BSW and BSF fittings on antique vehicles and certain industrial equipment. Unless a part must interface with legacy systems, most developers use either metric or Unified threads because the older British standards are harder to source.

Buttress Threads

Buttress threads resemble square threads on one flank and angled threads on the other. The angled flank usually has a 45° included angle. Designers use buttress threads for unidirectional loads, such as in:

• High-load hydraulic jacks
• Large industrial presses
• Screw clamps with heavy axial loads in one direction

By combining one vertical side and one slanted side, buttress threads resist axial forces efficiently while still being easier to machine than square threads.

Knuckle Threads

Knuckle threads feature rounded peaks and valleys and relatively shallow angles. They are used for:

• Applications that require quick engagement and disengagement.
• Electrical connections or delicate fittings where a V-thread might cross-thread easily.

Knuckle threads can also be called “ball threads” or “round threads” in some industries. They provide good dirt clearance in harsh environments because debris cannot lock the rounded root as easily as a V-thread.

Multi-Start Threads

Multi-start threads have two or more distinct thread helices running in parallel. Designers choose multi-start threads in applications where rapid engagement or fast axial motion is needed. Common uses include:

• Quick-lock camera tripods, where users open legs with a few turns of a knob.
• Large turnbuckles or lead screws in adjustable mounts.

In a two-start thread, each thread ridge is offset by 180° around the cylinder. This arrangement doubles the lead for each turn of the nut compared to a single-start thread with the same pitch.

Worm Threads

Worm threads are used on worm gears rather than straightforward screws. They pair with worm wheels to produce high gear reductions. They are not a fastening thread but instead form part of a gear set.

Seller (SAT) Threads

Seller threads, also called Sellers threads, follow a 60° profile with flat roots. They were once common in the United States before UNC/UNF replaced them. If you inherit older equipment, you might still see Seller threads.

“V” Threads

The “V” shape is the classic 60° thread profile used by ISO metric and unified threads. It offers a balance of strength, ease of cutting, and mass-production compatibility.

How to Choose the Right Thread Type

Selecting the correct thread type depends on the material, load conditions, and performance requirements. Here are a few tips:

• Match thread type to material: Some threads work better in metals, while others are ideal for plastic or soft materials.
• Check thread pitch: Fine threads offer precision, while coarse threads provide better grip and resistance to stripping.
• Understand your sealing needs: Use tapered threads for airtight or watertight seals, and straight threads when a mechanical fit is enough.
• Consider manufacturing method: Some threads, like square or buttress, require special tooling or processes like rolling or casting.

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Tips to Design Threads for Manufacturability

• Stick to industry standards: Stick to UN (UNC/UNF) and Metric (M) threads unless your application requires something specialized. Standard threads improve sourcing options and reduce manufacturing costs.
• Specify thread depth for blind holes: Always include exact depth; otherwise, manufacturing delays occur.
• Use standard drill sizes: Custom drills increase cost and lead time.
• Clearly distinguish external/internal threads on drawings: Use capital letters for internal (6H), lowercase for external (6g).
• Choose appropriate class fit: Medium class fits (6H internal/6g external Metric, 2A/2B Unified) balance manufacturability and performance.
• Avoid Over-Engineering: Unless necessary, avoid complex threads like UNEF, ACME, or multi-start unless your part function demands it. Simpler threads are faster, cheaper, and easier to verify.
• Be cautious with hole sizes: Mismatched hole vs. thread size calls cause confusion and delays.
• Consider thread engagement length: Longer engagement improves strength but adds cost/time.

Practical Manufacturing Notes

• Regional preferences: Metric threads dominate globally except in the US where Unified threads prevail.
• Pipe threads require special attention: They often seal fluids by taper, not fit, so follow specific standards.

Blind Holes vs. Through Holes

When tapping threads into a part, be sure to identify whether the hole is blind (does not pass through the part) or through (extends completely through).

• Blind holes require a clearly defined thread depth.
• Through holes are threaded through the entire part wall, typically simplifying the process.

Incomplete thread specifications—especially missing depth values in blind holes—are a common source of manufacturing delays.

How to Note Threads on Technical Drawings

Engineers must provide clear, unambiguous information when they place threads on drawings. Each detail helps the machinist pick the correct tool, avoid mistakes, and maintain quality. The largest mistakes occur when a drawing omits or mislabels a key parameter. The following guidelines help create complete drawings.

1. Label the View That Shows the Thread Clearly

Choose a drawing view that shows the full shape of the hole or rod. For internal threads in a blind hole, place dimensions on the section view or cutaway so that the machinist sees the hole’s depth. For external threads on a shaft or bolt, show the entire length of the thread in the main view or in an enlarged detail view.

2. Specify Series, Size, and Pitch

Every thread note must include:

• Series (e.g., M, UNC, UNF, NPT)
• Nominal Diameter (e.g., 10 mm, 1/4 inch)
• Pitch or TPI (e.g., 1.5 mm, 20 TPI)

For example, an internal metric thread:

• M10 × 1.5

This note tells the machinist to cut a metric thread with a 10 mm major diameter and a 1.5 mm pitch. If the drawing says just “M10,” the machinist uses the common standard pitch for an M10 thread (1.5 mm) unless told otherwise.

For a Unified external thread:

• 1/4-20 UNC

The machinist reads that as 0.250 inch major diameter, 20 threads per inch, coarse thread series.

3. Show Class Fit or Tolerance

Designers use class fits to control how tight or loose a threaded fit must be. If the application demands a standard fit, drawings often leave out the class fit. Manufacturers assume a default medium fit:

• Metric threads: 6H for internal, 6g for external
• Unified threads: 2B for internal, 2A for external

When a tighter fit or special allowance is necessary, the drawing must explicitly state it. Example:

• M10 × 1.5 – 6H × 6g

This note means both the internal hole thread (6H tolerance) and the mating bolt (6g tolerance) must follow the medium fit. The machinist will pick taps and dies labeled for 6H/6g.

For a Unified thread with specified fit:

• 1/4-20 UNC-2B

Here, 2B means a medium internal class fit that works with a 2A external bolt. Assemblies under high stress or vibration might call for 3A/3B to reduce play and resist loosening.

4. Mark Thread Depth (Blind Holes)

Blind holes need a clear depth callout. If you write “M8 × 1.25 THRU,” machinists know to cut the hole fully through the part. If you write “M8 × 1.25 × 12H,” they know to stop cutting threads 12 mm deep. Use the letter “H” after the number to show that the depth refers to threads, not the hole’s overall depth.

Example for a blind enemy:

• M8 × 1.25 × 15 DP RH

This note tells the machinist to cut a blind hole 15 mm deep with an 8 mm major diameter, 1.25 mm pitch, right-hand thread. If you forget “× 15 DP,” machinists might cut a standard blind hole that cannot accept the bolt fully, or they may go too deep and break the tap.

5. Indicate Hand of Thread (If Not Standard)

Whenever the thread hand differs from the standard right-hand form, you must note it directly. Use “LH” to indicate left-hand threads. Many couplings, jam nuts, or rotating shafts may use a left-hand thread to prevent unintentional loosening. An example:

• M12 × 1.75 LH

This thread tightens counterclockwise. If you leave out “LH,” machinists will cut a right-hand thread.

Downloadable Thread Size Charts

Use the following standard thread charts to quickly identify compatible drill sizes:

Conclusion

Understanding thread types and how to design them properly can drastically improve the manufacturability and assembly of your CNC machined parts. Stick to standards, be clear in your callouts, and always double-check hole sizes against thread specs.

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