Threads are one of the most common features on turned and milled parts – and one of the most common sources of drawing questions. This guide explains the thread types you will meet most often and how to specify them clearly.
Thread basics
A thread callout tells the machinist the type, size, pitch (distance between threads) and tolerance class. Missing any of these usually leads to questions or assumptions. Three terms are worth knowing:
- Nominal (major) diameter – the outside diameter of an external thread, e.g. 10 mm for M10.
- Pitch – the distance from one thread crest to the next, in mm for metric threads.
- Thread angle – 60° for metric and NPT threads, 55° for BSP threads.
Metric threads (M)
Metric ISO threads are the most common threads on machined parts in India. They are written as M + diameter, with the pitch added for fine threads.
| Size | Coarse pitch (mm) | Example fine pitch (mm) |
|---|---|---|
| M6 | 1.0 | 0.75 |
| M8 | 1.25 | 1.0 |
| M10 | 1.5 | 1.25 / 1.0 |
| M12 | 1.75 | 1.5 / 1.25 |
| M16 | 2.0 | 1.5 |
| M20 | 2.5 | 1.5 |
If no pitch is given (e.g. “M10”), the coarse pitch is assumed. For fine threads, always write the pitch – for example M10×1.25.
Thread tolerance classes
The most common classes are 6H for internal threads and 6g for external threads – a medium fit that suits most applications. A full callout looks like M10×1.5 – 6H.
Pipe threads: BSP and NPT
BSP (British Standard Pipe)
BSP threads use a 55° Whitworth thread form and are widely used on fittings in India and many other countries. They come in two main types:
- BSPP – parallel, marked G (e.g. G 1/2). Sealing is usually done with a washer or O-ring on a face.
- BSPT – taper, marked R for external and Rc for internal tapered threads. The taper itself helps make a seal.
NPT (National Pipe Taper)
NPT threads are American pipe threads with a 60° thread angle and a taper of 1 in 16. They are common on equipment for US-standard systems.
Important: BSP and NPT threads look similar but are not interchangeable. Always state the exact standard on the drawing.
How threads are made on CNC and VMC machines
- Single-point threading on a CNC lathe – the standard method for external threads and larger internal threads on turned parts.
- Tapping – common for internal threads on both lathes and VMCs; fast and economical for standard sizes.
- Thread milling on a VMC – useful for larger internal threads, special pitches or where a broken tap would be costly.
How to call out threads on a drawing
- State the standard, size and pitch – e.g. M12×1.5, G 3/8, 1/4 NPT.
- Add the tolerance class for metric threads – e.g. 6H or 6g.
- Give the full thread depth for blind holes, and the drill depth if it matters.
- Specify left-hand threads clearly with “LH” – right-hand is assumed otherwise.
- Mention any thread gauge requirement, e.g. “Check with GO/NO-GO gauge”.
For more design guidance, read 10 design tips for VMC machined parts and how to prepare a drawing for a CNC quote. Need threaded components machined? Send us your drawing.
Frequently Asked Questions
What is the difference between BSP and NPT threads?
Both are pipe threads, but they are not interchangeable. BSP (British Standard Pipe) uses a 55° thread angle and comes in parallel (G) and tapered (R/Rc) forms. NPT (American National Pipe Taper) uses a 60° thread angle with a 1 in 16 taper.
What does M10 x 1.5 – 6H mean?
It is a metric internal thread with a 10 mm nominal diameter, 1.5 mm pitch and tolerance class 6H, which is the common medium-fit class for internal threads.
Are threads tapped or machined on CNC machines?
Both methods are used. Internal threads are often tapped on a VMC or lathe; external threads are usually cut by single-point threading on a CNC lathe; and thread milling is used for larger or special internal threads.
CNC and VMC machining specialists in Metoda GIDC, Rajkot. Questions about your part? Send us your drawing.
