Thread Specifications for CNC: UNC, UNF, Metric and NPT
Short answer: the three thread families you must keep straight are inch (UNC coarse, UNF fine), metric (M-series with a pitch in millimetres), and pipe threads (NPT and BSP, which are tapered sealing threads, not fastening threads). A callout like "1/4-20 UNC 2B" means 1/4 inch nominal, 20 threads per inch, Unified National Coarse, class 2B internal fit. The most common CNC mistakes are mixing up coarse and fine series, using NPT where a straight thread is wanted, and omitting the class of fit. At BQUQ we machine ±0.005 mm, so send a drawing with your thread callouts and get a quote within 12 working hours.
Threads look trivial on a drawing and cause a disproportionate share of scrap and rework. Getting the callout complete the first time is the cheapest quality control there is.
The Three Thread Families
Inch: UNC and UNF
UNC is the coarse series; UNF is the fine series. Coarse threads (fewer threads per inch) are faster to cut and more tolerant of slight damage, while fine threads offer more holding strength per unit length and are used on thin walls and precision adjustments. A "1/4-20" is UNC; a "1/4-28" is UNF.
Metric: M-Series
Metric threads are written as nominal diameter × pitch in millimetres, for example M6×1.0. A "fine" metric thread uses a smaller pitch, such as M6×0.75. If no pitch is given, the coarse series is assumed, which is a common source of mismatch with a mating part.
Pipe: NPT and BSP
| Pipe thread | Angle | Seals by | Where common |
|---|---|---|---|
| NPT (US) | 60° | Taper + sealant | US fluid/pneumatic fittings |
| BSPT (UK) | 55° | Taper + sealant | UK/EU fittings |
| BSPP (parallel) | 55° | Gasket/washer | EU ports, some hydraulic |
NPT and BSPS/P are sealing threads and have specific taper and form. They are not interchangeable with each other, and neither is interchangeable with a straight fastener thread. Crossed standards here cause leaks, not just loose fits.
Callouts and Class of Fit
A complete inch callout looks like: 1/4-20 UNC - 2B. The 2B refers to the class of fit for an internal thread. Classes 1 (loose), 2 (general), and 3 (tight) exist for both internal (B) and external (A) threads. Class 2 is standard. Class 3 is used for precision or high-vibration joints and is harder to produce.
| Class | Fit | Typical use |
|---|---|---|
| 1A/1B | Loose | Quick assembly, dirty environments |
| 2A/2B | General | Most fasteners |
| 3A/3B | Tight | Precision, aerospace, high vibration |
Fastening Thread Reference
| Inch (UNC) | Pitch (TPI) | Metric | Pitch (mm) |
|---|---|---|---|
| 1/4-20 | 20 | M6 | 1.0 |
| 5/16-18 | 18 | M8 | 1.25 |
| 3/8-16 | 16 | M10 | 1.5 |
| 1/2-13 | 13 | M12 | 1.75 |
| Inch (UNF) | Pitch (TPI) | Metric fine | Pitch (mm) |
|---|---|---|---|
| 1/4-28 | 28 | M6×0.75 | 0.75 |
| 5/16-24 | 24 | M8×1.0 | 1.0 |
| 3/8-24 | 24 | M10×1.25 | 1.25 |
| 1/2-20 | 20 | M12×1.25 | 1.25 |
Tapping Drill Sizes
The tapping drill creates the minor diameter the tap cuts into. Getting it right prevents both broken taps and weak threads.
| Thread | Tapping drill |
|---|---|
| M3×0.5 | 2.5 mm |
| M4×0.7 | 3.3 mm |
| M5×0.8 | 4.2 mm |
| M6×1.0 | 5.0 mm |
| M8×1.25 | 6.8 mm |
| M10×1.5 | 8.5 mm |
| M12×1.75 | 10.2 mm |
| 1/4-20 UNC | 5.1 mm |
| 5/16-18 UNC | 6.6 mm |
| 3/8-16 UNC | 8.0 mm |
| 1/2-13 UNC | 10.8 mm |
These are standard sizes for roughly 75% thread engagement. Deeper engagement needs a smaller drill and risks tap breakage; shallower is weaker. The percentages are a design choice, not a default to ignore.
Threading Methods in CNC
| Method | Best for | Notes |
|---|---|---|
| Tapping | Standard holes | Fast; risk of breaking taps in blind holes |
| Thread milling | Large or fragile threads | One tool, many sizes; stronger in hard materials |
| Single-point turning | OD/ID threads on lathes | Accurate, flexible |
| Thread rolling | High strength, volume | Cold-forms the thread, adds fatigue strength |
Thread milling and thread rolling are covered in more depth in our guide to thread machining and our comparison of thread milling vs tapping.
How to Get Threads Right the First Time
Write the full callout: series, pitch, nominal size, and class of fit. State the depth for blind holes and whether a specific tap drill or engagement percentage is required. For pipe threads, specify NPT, BSPT, or BSPP explicitly — do not just say "pipe thread." If a mating component is already fixed, send its thread spec too, so the shop can match it rather than guess. For tolerance context around pitch diameter, see the tolerance guide.
Frequently Asked Questions
Q: What is the difference between UNC and UNF?
A: UNC is the coarse thread series with fewer threads per inch; UNF is the fine series with more. Coarse threads cut faster and resist damage; fine threads give more holding strength and are better on thin walls. They are not interchangeable.
Q: Is NPT the same as a straight pipe thread?
A: No. NPT is a tapered thread that seals by wedging, usually with sealant. BSPP is a parallel pipe thread that seals with a gasket or washer. Using the wrong one leaks.
Q: What does 2B mean in a thread callout?
A: It is the class of fit for an internal thread. Class 2B is the general-purpose standard; 1B is looser and 3B is tighter, used for precision or high-vibration joints. Omitting it lets the shop assume class 2.
Q: Why do my threads feel tight or loose?
A: Usually a class-of-fit or pitch mismatch. Check that the internal and external threads use the same series and class, and verify the pitch diameter, not just whether the nut starts on the fastener.
Q: Does BQUQ cut inch, metric and pipe threads?
A: Yes, on both milled and turned parts, including thread milling for fragile or large threads. Send the drawing with full callouts to sc@bquq.com or WhatsApp +86 13713157787 for a quote in 12 working hours.
Related Resources
- Thread machining guide: methods, tooling, and practice for accurate threads.
- CNC turning parts: turned parts where single-point and rolled threads are common.
- About BQUQ: an ISO9001-certified source factory in Dongguan running CNC, stamping, springs, and heat sinks under one roof.
- Contact us: thread questions and quotes within 12 working hours.
Authored by the BQUQ Engineering Team. BQUQ is an ISO9001-certified source factory in Dongguan, China, running CNC machining, metal stamping, custom springs, heat sink and collet lines under one roof. Send drawings to sc@bquq.com or WhatsApp +86 13713157787 for a quote within 12 working hours. www.bquq.com
