Quick answer: The most common screw thread types include metric threads, UNC, UNF, NPT, BSP, Acme, trapezoidal, buttress, and special custom threads. Metric, UNC, and UNF are mainly used for fastening. NPT and BSP are used for pipe connections. Acme and trapezoidal threads are used for power transmission and motion. CNC drawings should clearly define thread size, pitch, class, depth, and inspection method.
What Is a Screw Thread?
A screw thread is a helical ridge or groove formed on the inside or outside of a cylindrical feature. Threads allow parts to fasten, adjust, seal, transmit motion, or carry load. In CNC machining, threads can be tapped, thread milled, single-point turned, die cut, rolled, or formed depending on material, geometry, quantity, and accuracy requirements.
Thread problems are common because thread standards look similar but are not always interchangeable. A metric M6 thread is not the same as a 1/4-20 UNC thread. A straight BSPP pipe thread is not the same as a tapered NPT thread. A drawing must remove this ambiguity before machining begins.
Main Screw Thread Types
| Thread type | Typical use | How it is specified | CNC machining note |
|---|---|---|---|
| Metric thread | General international fastening | M size and pitch, such as M6 x 1.0 | Very common for global mechanical assemblies |
| UNC | General inch fasteners | Diameter and threads per inch, such as 1/4-20 | Coarse pitch, practical for many materials |
| UNF | Fine inch fasteners | Diameter and TPI, such as 1/4-28 | Finer pitch, often used where adjustment or vibration resistance matters |
| NPT | Tapered pipe sealing | Nominal pipe size and TPI | Requires taper and sealing consideration |
| BSPP / BSPT | Pipe connections in many international systems | British Standard Pipe callout | Confirm parallel or tapered form before machining |
| Acme / trapezoidal | Lead screws and motion systems | Diameter, pitch, and form | Used for load and movement, not ordinary fastening |
| Buttress thread | High load in one direction | Special thread form and dimensions | Requires careful drawing and tool planning |
Metric Threads
Metric threads are common worldwide. A metric callout such as M8 x 1.25 means the nominal diameter is 8 mm and the pitch is 1.25 mm. Coarse pitch metric threads are common for general fastening, while fine pitch threads can be useful when there is limited engagement length, a need for finer adjustment, or vibration concerns.
For CNC machined parts, metric threads should include depth for blind holes, through-hole notation where relevant, and tolerance class when fit is important. If the part will be anodized, plated, or coated, thread fit may need masking or post-finish checking.
UNC and UNF Threads
UNC means Unified National Coarse, while UNF means Unified National Fine. UNC threads are common for general inch fasteners and are usually more tolerant of dirt, plating, and softer materials. UNF threads have finer pitch and can provide more precise adjustment or better resistance to loosening in some assemblies.
UNC and UNF are not interchangeable. They may have the same nominal diameter but different pitch. A 1/4-20 fastener will not fit a 1/4-28 hole correctly. Drawings should always show the full thread callout.
NPT, BSP, and Pipe Threads
Pipe threads are used for fluid or gas connections, but they are not all the same. NPT is a tapered pipe thread commonly used in North America. BSPP is parallel, while BSPT is tapered. The sealing method may depend on thread interference, sealant, O-rings, washers, or mating hardware.
Pipe thread features require careful planning because the effective depth, gauge position, sealing requirement, and orientation may matter. For CNC buyers, it is safer to provide the mating fitting standard or sample if there is any uncertainty.
Internal vs External Threads
Internal threads are inside holes. They are commonly produced by tapping or thread milling. External threads are on outside diameters and are often produced by turning, thread milling, rolling, or dies. Internal blind holes need extra care because taps and thread mills require clearance at the bottom.
Tapping vs Thread Milling vs Single-Point Threading
| Method | Best use | Advantages | Limitations |
|---|---|---|---|
| Tapping | Standard internal threads | Fast and economical | Broken taps can be costly, less flexible for special threads |
| Thread milling | Large threads, hard materials, blind holes, special sizes | Flexible and lower cutting force | Usually slower than tapping |
| Single-point turning | External or internal lathe threads | Good control for turned parts | Best suited to round components |
| Thread rolling | High-volume external threads | Strong thread form, no chips | Requires suitable material and tooling |
CNC Thread Design Checklist
- Use standard thread sizes whenever possible.
- Show thread size, pitch, class, and depth clearly.
- Identify blind holes and through holes.
- Leave tool clearance near shoulders and hole bottoms.
- Do not specify unnecessary deep threads.
- Confirm thread fit after anodizing, plating, or coating.
- Use inserts when plastic or thin walls cannot provide enough thread strength.
- Use go/no-go gauges for standard thread inspection.
Related CNC Machining Considerations
Threaded features often appear in CNC milled brackets, housings, plates, covers, and fixtures. They also appear in CNC turned fittings, shafts, sleeves, pins, and bushings. If the thread is close to a cosmetic surface, planned finishing from the surface treatment stage can prevent fit issues.
FAQ
What are the most common screw thread types?
Metric, UNC, UNF, NPT, BSP, and Acme are among the most common thread types used in mechanical design and CNC machining.
What is the difference between metric and UNC threads?
Metric threads are specified in millimeters and pitch. UNC threads are inch-based and specified by nominal diameter and threads per inch.
Is NPT the same as BSP?
No. NPT and BSP have different forms and standards. They should not be treated as interchangeable unless the connection is specifically designed for compatibility.
Should I use tapping or thread milling?
Tapping is usually faster for standard internal threads. Thread milling is useful for large threads, hard materials, blind holes, and special thread control.
How do CNC shops inspect threads?
Standard internal and external threads are commonly checked with go/no-go gauges. Critical features may also require depth checks, CMM location checks, or mating part tests.
Need threaded CNC parts?
Send the full thread callout, material, finish, and drawing. MTSPi can review tap, thread milling, turning, and inspection requirements.
Conclusion
Screw thread selection affects assembly strength, sealing, cost, inspection, and production risk. Clear thread standards, practical depth, suitable machining method, and correct gauges help CNC machined parts fit correctly the first time.
