Designing threads might seem simple, but many CNC parts fail because the threads were not planned properly. A poorly designed thread can strip, misalign, or make assembly difficult. This results in wasted time and increased costs. Engineers and designers often wonder which thread type, pitch, or depth works best for a particular material or application.
This guide shows how to design threads that are strong, accurate, and easy to machine. You will learn how to select the correct thread type, ensure proper engagement, and avoid mistakes that can slow production or damage parts. Following these steps helps you make reliable CNC parts more efficiently.
Good thread design forms the base of every assembled component. We will break down the key ideas so you can create threads that work well and are simple to machine.
Fundamentals of Thread Geometry
Before designing threads, it helps to understand how their geometry works. Threads are more than just spiral grooves. They have specific features that determine their shape, strength, and fit.
Key Elements of a Thread
A 3D thread might look simple, but it is defined by several geometric features that must work together precisely:
- Primary Diameter: This is the largest diameter measured across the thread crests. For example, a 1/4 “-20 UNC thread has a primary diameter of 6.35 mm (0.25 in).
- Minor Diameter: This is the smallest diameter measured across the thread roots. For a 1/4 “-20 UNC thread, it is about 5.16 mm (0.203 in). The minor diameter determines the core strength of the fastener.
- 정점: The distance between one thread crest and the next. In metric threads, M6 × 1.0 has a 1.0 mm pitch. In Unified threads, 1/4 “-20 means 20 threads per inch (TPI), roughly 1.27 mm pitch.
- Thread Angle: The angle between the thread flanks. Metric and Unified threads use 60°, Acme threads use 29°, and Whitworth uses 55°. This angle affects how the load is shared and the stress on the thread flanks.
- Pitch Diameter: The diameter at which the thread ridge width equals the groove width. This is the most critical dimension for proper fit and engagement. For instance, an M10 × 1.5 6H thread has a pitch diameter tolerance of ±0.13 mm.
- Crest and Root: The crest is the top of the thread, and the root is the bottom. Sharp roots can cause stress points, so standards like ISO and ANSI set minimum root radii to prevent cracks under repeated loads.
내부 스레드와 외부 스레드
Threads can be internal (female) or external (male). They share the same geometry, but machining methods and strength differ.
- External Threads are made on bolts, shafts, or studs. They can be cut or rolled. Rolled threads are standard in high-strength fasteners. Rolling increases fatigue strength by 20–30% and improves surface finish, making assembly smoother.
- Internal Threads are formed in drilled holes using taps or 스레드 밀링. For example, an M8 × 1.25 thread usually requires a 6.8 mm drill hole. A hole that is too small makes tapping harder, while a hole that is too large weakens the thread.
Thread Standards and Classifications
Thread standards define how threads are measured, labeled, and manufactured. They ensure compatibility between parts made by different manufacturers.
Unified Thread Standard (UN and UNC/UNF)
The Unified Thread Standard (UN) is widely used in the United States, Canada, and several other countries. It defines both coarse and fine thread series, ensuring consistent dimensions across industries.
- UNC (Unified National Coarse) threads have fewer threads per inch (TPI), which makes them stronger and easier to assemble. For example, a 1/4 “-20 UNC screw has 20 TPI, meaning 20 threads in one inch. The coarse design offers improved resistance to stripping in softer materials, such as aluminum.
- UNF (Unified National Fine) threads have more threads per inch. A 1/4 “-28 UNF screw, for instance, has 28 TPI. The fine pitch allows for a greater clamping force and is ideal for hard materials or applications that require precise adjustments.
- UNEF (Unified National Extra Fine) threads are less common but used where limited thread engagement space exists, such as in aerospace or precision instruments.
ISO Metric Threads
The ISO Metric Thread System is the most widely used global standard, governed by ISO 68-1 and ISO 965-1. It’s common in Europe, Asia, and many international markets. Instead of inches, it uses millimeters for all measurements.
Metric threads are identified by the letter M, followed by the nominal diameter and pitch. For example:
- M8 × 1.25 means an 8 mm primary diameter and a 1.25 mm pitch.
- M10 × 1.5 is a 10 mm diameter with a 1.5 mm pitch.
Metric threads use the same 60° thread angle as Unified threads but apply a different tolerance system, noted as 6g for external threads and 6H for internal threads in standard applications.
Designing Threads for Manufacturability
Good thread design is more than picking the right size and pitch. Threads also need to be easy to machine, strong enough to hold loads, and consistent in quality.
Minimum Wall Thickness and Thread Engagement
For holes or thin-walled parts, keeping enough wall thickness is crucial. Thin walls can flex during machining and may bend or crack under load.
A general rule is to maintain a wall thickness of at least 1.5 times the thread’s primary diameter. For example, an M8 thread (8 mm diameter) should have walls at least 12 mm thick. For softer materials like aluminum, increase the thickness to 2× the diameter for more stability.
Thread engagement refers to the portion of the thread length that actually carries the load. Most applications need 1 to 1.5 times the thread diameter for full strength.
Avoiding Undercuts and Sharp Transitions
Undercuts and sharp corners can create weak points and areas of stress. Threads should transition smoothly into the unthreaded section to reduce cracking, especially under repeated loads.
Use a runout relief or thread relief groove to ensure a clean thread termination. A good guideline is 1.5 times the thread pitch for the relief width, with the depth slightly larger than the thread root diameter.
Avoid sharp internal corners near threads. Use fillets or chamfers instead. A 0.5–1.0 mm radius usually spreads stress evenly. Smooth transitions also help CNC tools exit cleanly, improving surface finish and reducing burrs.
Selecting Proper Thread Depth
For Unified and Metric 60° threads, the standard depth is 0.613 × pitch. For example, an M6 × 1.0 thread has a theoretical depth of 0.613 mm. In practice, machinists often reduce the depth by 5–10% to improve tool motion and ensure a proper fit for gauges.
Deep threads in 막힌 구멍 can trap chips and coolant, potentially damaging tools. To prevent this, leave a flat relief at the bottom of the hole or use thread milling for improved chip clearance.
For blind holes, keep the full thread length at least 1.5 pitches shorter than the hole depth. This provides space for chips and prevents tool crashes.
Common Thread Design Mistakes to Avoid
Even experienced designers can make small mistakes that cause significant problems in CNC threading. Recognizing these common mistakes enables you to design threads that are accurate and easy to machine.
Overly Tight Tolerances
Many designers believe that tighter tolerances equate to better quality. That is not always true. Tight tolerances increase machining time, wear tools more quickly, and can make assembly more challenging.
For most parts, standard fits work well: 2A/2B for Unified threads and 6g/6H for Metric threads. Tighter fits are only needed for precise alignment or high-pressure sealing.
For example, an M10 × 1.5 thread with a 6H fit already holds accuracy within ±0.13 mm. Trying to tighten it to ±0.05 mm usually adds cost without improving performance. It also raises rejection rates during inspection.
Tight tolerances can cause parts to seize during assembly, especially when coatings such as 아연 도금 또는 아노다이징 처리 are applied. These treatments can add 5–15 µm per surface. Practical tolerances reduce tool stress, improve consistency, and cut rework.
Insufficient Thread Engagement
Thread engagement is the portion of the thread that carries the load. Too little engagement can make threads strip or loosen.
A general rule: 1× the nominal diameter for steel, 1.5× for softer metals like aluminum or brass. For example, an M8 × 1.25 thread should engage at least 8 mm in steel or 12 mm in aluminum.
Most of the load is carried by the first few engaged threads — over 80% in the first three threads. Adding more engagement rarely improves strength but increases machining time and tool wear.
For thin-walled parts, ensure there is sufficient material behind the thread to prevent cracking. For plastics or inserts, consider helical inserts or threaded bushings to strengthen the thread.
Ignoring Tool Access or Clearance
Threads need space for tools to cut and exit smoothly. Poor access can cause collisions, incomplete threads, or damage to nearby features.
CNC taps and thread mills need axial and radial clearance. For blind holes, leave 1.5 thread pitches of unthreaded space at the bottom for chip clearance. Without it, chips can jam the tool, causing it to break.
For example, threading an M10 × 1.5 blind hole requires the hole depth to be at least 1.5 mm deeper than the diameter of the whole thread. For external threads near shoulders, add a runout groove at least 1.5× pitch wide and slightly deeper than the thread root.
Check tool paths in your 3D model. Features too close to threads can block access. Adding a slight chamfer or relief at the entry helps the tap or cutter start cleanly and reduces burrs.
Specifying Threads on Technical Drawings
A well-designed thread is only as good as its documentation. Proper callouts, tolerance definitions, and inspection notes ensure consistent results across every production batch.
Clear and Unambiguous Thread Callouts
Thread callouts describe the type, size, pitch, and class of fit. They should follow international standards such as ASME Y14.6 or ISO 965-1. Clear callouts help machinists quickly understand which tools and processes to use.
For Unified threads, the format is typically:
1/4-20 UNC-2B
- 1/4 → Major diameter (inches)
- 20 → Threads per inch (TPI)
- UNC → Thread series (Unified National Coarse)
- 2B → Class of fit (internal thread)
For Metric threads, the format is:
M10 × 1.5 – 6H
- M10 → Major diameter (mm)
- 1.5 → Pitch (mm)
- 6H → Internal thread tolerance class
Specify whether the thread is through or blind. Notes like “THRU” or “DEPTH 15 mm” prevent confusion during CNC programming.
Include surface finishes or plating in the callout if needed. For example:
M8 × 1.25 – 6H, AFTER ZINC PLATING
This ensures machinists account for coating thickness when cutting the thread.
Defining Thread Class Tolerances for Fit and Function
Thread tolerances control how tightly parts fit together. Choosing the right class can significantly impact assembly, load capacity, and manufacturability.
Unified threads range from 1A/1B (loose) to 3A/3B (tight):
- 1A/1B: Loose fit for quick assembly, ideal for non-critical or dirty environments.
- 2A/2B: Standard fit for most industrial parts.
- 3A/3B: High-precision fit for aerospace, instrumentation, or hydraulic applications.
A grade number and a letter identify metric threads. For example:
- 6H (internal) / 6g (external): Standard general-purpose fit.
- 5H/5 G: A tighter fit for high-precision parts.
- 7H/7g: Looser fit for easier assembly.
For instance, M12 × 1.75 – 6H/6g provides a good balance of precision and ease of assembly, with a pitch diameter tolerance of about ±0.15 mm. Avoid specifying overly strict tolerances unless necessary, as they increase tool wear and inspection time without yielding significant performance gains.
Communicating Inspection and Quality Requirements
Inspection ensures the quality of threads and proper part fit. Clear inspection notes on drawings maintain consistency during quality control.
Standard inspection requirements include:
- Go/No-Go gauge testing: Verifies functional fit of internal and external threads.
- Pitch diameter measurement: Ensures correct engagement. Measured with a three-wire method or thread micrometer, usually within ±0.02 mm for precision parts.
- 표면 마감: Important for smooth engagement. Typical values range from Ra 1.6 µm to Ra 3.2 µm, depending on the application.
- 육안 검사: Checks for burrs, incomplete threads, or tool marks.
For critical components, include notes like:
THREADS TO BE INSPECTED WITH CLASS 2B GO/NO-GO GAUGE
또는
100% THREAD INSPECTION REQUIRED FOR FIRST ARTICLE
Inserts: Alternatives to Threading
Sometimes traditional threads are not the best choice. Thin walls, soft materials, or high-strength requirements can make tapping difficult or unreliable. Thread inserts provide a strong and durable alternative. They increase load capacity, extend part life, and reduce wear on CNC tools. Inserts also make maintenance and replacement easier.
Benefits of Using Inserts
- Improved Strength: Inserts let a small hole in a soft material carry the same load as a larger steel thread.
- 내구성: They resist stripping even after repeated assembly, especially in high-vibration parts.
- Flexibility: Damaged inserts can be replaced without needing to replace the entire part.
- Machining Efficiency: Threads cut into inserts reduce tool wear compared with cutting directly into the base material.
디자인 고려 사항
- Match the hole diameter to the insert manufacturer’s recommendations.
- Maintain enough wall thickness around the insert to avoid cracking during installation.
- Clearly specify insert type, installation method, depth, and orientation on technical drawings.
Thread inserts are a practical solution for soft materials, thin walls, or high-load applications. They combine strength, reliability, and ease of assembly, making them a solid alternative to conventional threading in CNC parts.
Take Your Thread Design to the Next Level
Designing threads for CNC machining doesn’t have to be complicated. By focusing on thread geometry, engagement, tolerances, and manufacturability, you can create parts that are both strong and precise, as well as easy to assemble. Avoid common mistakes such as overly tight tolerances, insufficient engagement, and poor tool access to save time, reduce costs, and ensure reliable performance.
If you need expert help optimizing your thread designs or require precision CNC parts, 지금 팀에 문의하세요. We can review your drawings, suggest improvements, and help turn your designs into high-quality, manufacturable components. Don’t let poor thread design slow your projects — get professional guidance now.
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