Threads are among the most common features on CNC machined parts — and among the most frequently mis-specified. A wrong thread callout, incorrect tolerance class, or poorly chosen thread cutting method can mean parts that don't assemble, leak under pressure, or fail in service. This guide covers everything engineers and buyers need to know about CNC thread machining: thread types, standards, production methods, tolerances, and how to specify them correctly.
At KING HAN, threading is a daily operation on our Swiss-type CNC lathes. Small-diameter threaded components — from M1.6 medical screws to M20 hydraulic fittings — are our bread and butter. Here's our comprehensive guide to getting threads right.
The global standard for most engineering applications. Defined by ISO 261 (general purpose) and ISO 1502 (gauging). Designation format:
Common metric thread sizes for small-diameter CNC parts:
| Size | Coarse Pitch | Fine Pitch | Common Application |
|---|---|---|---|
| M2 | 0.4mm | 0.25mm | Electronics, watch components |
| M3 | 0.5mm | 0.35mm | Electronics, sensors |
| M4 | 0.7mm | 0.5mm | Electronics, small mechanisms |
| M5 | 0.8mm | 0.5mm | General fastening |
| M6 | 1.0mm | 0.75mm | General fastening |
| M8 | 1.25mm | 1.0mm | Structural, hydraulic |
| M10 | 1.5mm | 1.25mm | Structural, automotive |
| M12 | 1.75mm | 1.25mm | Heavy-duty fastening |
Dominant in the United States and for products sold into the US market. Defined by ASME B1.1:
Format: 1/4-20 UNC-2A (1/4" diameter, 20 TPI, Coarse, Class 2, external thread)
Used for fluid connections. Critical to specify whether sealing or mechanical joining:
Critical warning: NPT and BSPT threads are NOT interchangeable despite looking similar. NPT has a 60° thread angle; BSPT has a 55° thread angle. Mixing them causes leaks and potential safety hazards.
Keep all thread specs at your fingertips. The CNC Terminology Glossary ($6.99) includes comprehensive thread standard references, pitch tables, and tolerance class explanations for metric, unified, and pipe threads.
The most versatile method. A single-point cutting tool traces the thread profile in multiple passes on a CNC lathe.
On Swiss-type CNC lathes, single-point threading is the primary method for external threads. The guide bushing provides exceptional rigidity, making it ideal for long, thin threaded shafts that would deflect on a conventional lathe.
A helical interpolation operation using a thread mill cutter on a 3-axis or higher CNC mill.
The fastest method for internal threads. A tap — either cut tap or form tap — is driven into a pre-drilled hole.
Thread rolling: A cold-forming process that creates external threads by pressing the workpiece between hardened dies. Produces extremely strong threads (cold-worked grain flow follows the thread profile). Used for high-volume fastener production.
Thread whirling: A specialized process for long, small-pitch threads (especially medical bone screws). A ring of cutting inserts rotates around the slowly turning workpiece. Produces excellent surface finish on materials like titanium and stainless steel. Common on Swiss-type CNC lathes equipped with whirling attachments.
Thread tolerances are defined by ISO 965 using a tolerance class system:
| Class | Type | Fit | Application |
|---|---|---|---|
| 6g / 6H | External / Internal | Standard | General engineering, most common |
| 4g / 4H | External / Internal | Close | Precision applications |
| 8g / 8H | External / Internal | Loose | Coated threads, hot-dip galvanized |
| 6e | External | With allowance | Plated threads (room for coating) |
Key point: If your drawing says "M8×1.25" without a tolerance class, the supplier will default to 6g/6H. If you need tighter (4g/4H) or looser (8g) fits, specify explicitly. This is especially important for parts that will be plated or anodized after machining — the coating thickness can push threads out of tolerance.
| Class | Fit | Application |
|---|---|---|
| 1A / 1B | Loose | Easy assembly, dirty environments |
| 2A / 2B | Standard | Most commercial fasteners |
| 3A / 3B | Close | Precision, aerospace, no allowance |
"M8 thread" is ambiguous in some contexts. While ISO convention treats "M8" as M8×1.25 (coarse), explicitly stating the pitch eliminates confusion — especially for international suppliers who may work across multiple standards.
The strength of a threaded joint depends on engagement length. General rule: engagement length should be at least 1× the nominal diameter (e.g., 8mm engagement for an M8 thread) for steel. For softer materials like aluminum, use 1.5-2× diameter.
If your thread will be plated (zinc, nickel, chrome), the coating adds material to the thread flanks. A 6g external thread may not fit into a 6H internal thread after 10-15µm of plating. Solution: specify 6e tolerance class for the external thread, or call out "thread tolerance after plating."
These tapered pipe threads look identical to the naked eye but have different thread angles (60° vs. 55°). They will screw together partially but won't seal properly. Always specify the exact standard on your drawing.
Internal threads in small holes are limited by tap availability and strength. An M2 internal thread in hardened stainless steel is extremely difficult — the tap is fragile and prone to breakage. Consider thread inserts (Helicoil) for small threads in hard or soft materials.
Avoid back-and-forth on thread specifications with your supplier. The CNC Email Templates ($4.99) include technical specification templates designed for clear communication of thread requirements, tolerances, and inspection criteria.
The primary inspection method for production threads. A thread ring gauge (for external threads) or thread plug gauge (for internal threads) checks both:
For high-volume production, every batch should be checked with calibrated Go/No-Go gauges. See our quality inspection guide for more on inspection methods.
Optical comparators, tool maker's microscopes, or pitch micrometers can verify thread pitch accuracy. Critical for non-standard or custom threads.
For high-precision applications (aerospace, medical), CMM with thread scanning software can measure pitch diameter, lead angle, taper, and profile deviations to sub-micron accuracy.
Thread surface finish affects both sealing performance and fatigue life. For sealing threads (NPT, BSPT), aim for Ra 1.6 µm or better on thread flanks. For load-bearing threads in fatigue applications, rolled threads (Ra 0.4-0.8 µm) significantly outperform cut threads.
Not all CNC shops have proper thread gauging equipment. The CNC Factory Audit Checklist ($7.99) includes specific items for evaluating a supplier's threading capabilities, gauging equipment, and calibration practices.
Trapezoidal profile (29° included angle). Used for lead screws, vises, and linear motion applications. Stronger than V-threads for power transmission. CNC single-point threading is the standard production method.
Asymmetric profile designed for high axial loads in one direction. Used in vise screws, press screws, and artillery breech mechanisms.
Technically not threads, but often specified alongside threading operations. Knurling provides grip for hand-tightened components. Can be straight or diamond pattern, produced by knurling tools on the CNC lathe.
Threads below M2 (M1, M1.2, M1.4, M1.6) require specialized tooling and Swiss-type CNC lathes for reliable production. These tiny threads are common in medical devices, optical instruments, and micro-electronics.
Swiss-type CNC lathes excel at producing small threaded components with exceptional precision. The Swiss CNC Lathe Guide ($12.99) covers threading operations, cycle time optimization, and tooling selection specific to Swiss-type machines.
Properly specified and machined threads are the foundation of reliable mechanical assemblies. Taking the time to get your thread specifications right in the design phase saves costly rework, delays, and field failures down the line.
From M1.6 medical screws to M25 hydraulic fittings, KING HAN produces millions of threaded components annually. Our Swiss-type lathes with live tooling handle single-point threading, whirling, and tapping in one setup.
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