Thread Types Explained
Threads let you fasten parts together. They also seal pipes and move loads. There are five main thread types on CNC parts.
UNC (Unified National Coarse)
UNC threads have a wide pitch. They have fewer threads per inch. They are fast to assemble. They work well in soft metals like aluminum. UNC is the default in the United States.
The coarse pitch makes them forgiving. They handle nicks and dirt better. They also cut faster on a CNC lathe.
UNF (Unified National Fine)
UNF threads have more threads per inch. The smaller pitch gives a tighter fit. UNF bolts are stronger in tension. They resist vibration loosening better than UNC.
Use UNF for thin walls. Aerospace and automotive parts use UNF threads often.
Metric (ISO)
Metric threads use millimeters for size and pitch. They follow the ISO 261 standard. Most of the world uses metric threads.
Metric threads come in coarse and fine pitch. Coarse is the default. You only call out the pitch for fine threads.
NPT (National Pipe Taper)
NPT threads seal pipes and fittings. They taper at 1.7893 degrees. As you tighten the fitting, the taper creates a wedge. This wedge helps seal the joint.
NPT threads need Teflon tape or sealant. The taper alone does not seal against fluids. NPTF (dryseal) threads can seal without tape in some cases.
ACME Threads
ACME threads have a trapezoid shape. They carry heavy loads. Lead screws use ACME threads. They handle thrust forces well.
ACME threads are wider than V-threads. They are easier to machine than square threads. Use them for linear motion.
When to Use Each Thread
The right thread depends on your use case.
- General fastening: Use UNC. It is the default for most parts.
- Thin walls or vibration: Use UNF. The fine pitch grips better.
- Global products: Use Metric. It is the world standard.
- Pipes and fluid fittings: Use NPT. The taper creates a seal.
- Motion or heavy loads: Use ACME. It handles thrust well.
When in doubt, use UNC for inch parts. Use coarse Metric for metric parts. Your shop will have the taps in stock.
Thread Classes
Thread classes set the allowed play between bolt and hole. The ASME B1.1 standard defines three classes.
Class 1A / 1B (Loose Fit)
Class 1 has the most play. It is easy to assemble by hand. This class is rarely used on CNC parts.
Class 2A / 2B (Standard Fit)
Class 2 is the default for most parts. It gives a good balance. Assembly is easy. The fit is tight enough for most jobs. Over 90% of inch threads use Class 2.
Class 3A / 3B (Tight Fit)
Class 3 has very little play. Use it when you need zero wobble. Aerospace fasteners often need Class 3. It costs more because tolerances are tighter.
"A" means external thread (bolt). "B" means internal thread (hole). Always specify both when calling out a threaded assembly.
How to Call Out Threads on Drawings
A thread callout tells the shop what to cut. Each thread type has its own format.
UNC / UNF Callout
1/4-20 UNC-2B — 1/4" diameter, 20 TPI, coarse, Class 2, internal
Metric Callout
M6 x 1.0 — 6mm diameter, 1.0mm pitch. For coarse pitch, write just M6.
NPT Callout
1/4-18 NPT — 1/4" nominal pipe size, 18 TPI, tapered
Always include thread depth for blind holes. Write "THRU" for through holes. Example: 1/4-20 UNC-2B x .500 DEEP.
Common Thread Sizes for CNC Parts
These are the sizes we machine most often. Use them when you can. They cost less because we keep the taps in stock.
- Inch UNC: #4-40, #6-32, #8-32, #10-24, 1/4-20, 5/16-18, 3/8-16, 1/2-13
- Inch UNF: #4-48, #6-40, #8-36, #10-32, 1/4-28, 5/16-24, 3/8-24, 1/2-20
- Metric coarse: M3, M4, M5, M6, M8, M10, M12
- Metric fine: M6x0.75, M8x1.0, M10x1.25, M12x1.5
- NPT: 1/8, 1/4, 3/8, 1/2, 3/4, 1"
Thread Type Comparison
| Thread Type | Standard | Pitch Style | Common Sizes | Best For |
|---|---|---|---|---|
| UNC | ASME B1.1 | Coarse | #6-32 to 1/2-13 | General fastening |
| UNF | ASME B1.1 | Fine | #6-40 to 1/2-20 | Vibration, thin walls |
| Metric | ISO 261 | Coarse / Fine | M3 to M12 | Global products |
| NPT | ASME B1.20.1 | Tapered | 1/8" to 1" | Pipe sealing |
| ACME | ASME B1.5 | Trapezoid | 1/4" to 2" | Linear motion, load |
Need precision threads machined?
Upload your CAD file for an instant quote. Free DFM review included with every order.
Upload CAD for Instant QuoteFrequently Asked Questions
What is the difference between UNC and UNF threads?
UNC threads have fewer threads per inch. They are called coarse pitch. UNF threads have more threads per inch. They are called fine pitch. UNC is easier to assemble. UNF is stronger in tension and resists vibration better.
Can I mix metric and inch threads on the same part?
Yes, but it adds cost. Your shop must switch tools. It also raises the risk of assembly errors. Stick to one system when you can. If you must mix, label each thread on your drawing.
What thread class should I use for CNC parts?
Class 2A/2B is the standard for most CNC parts. It gives a good balance of fit and ease. Class 3A/3B is tighter. Use it when you need zero play. Class 1A/1B is loose and rarely used.
How do I call out a thread on my drawing?
For inch threads, write the size, threads per inch, and class. Example: 1/4-20 UNC-2B. For metric, write M, the diameter, and the pitch. Example: M6 x 1.0. Always include depth for blind holes.
Do NPT threads need sealant?
Yes. NPT threads taper at 1.7893 degrees. The taper creates a wedge fit. But it alone does not seal against fluids. Use Teflon tape or pipe sealant. NPTF (dryseal) threads can seal without tape in some cases.
