Thread types: The practical engineer's guide

Introduction

At university we learn how to design parts to optimize design efficiency - less material, higher performance. In the real world the details, such as threads, can cause you to waste time and lose money.

There is little practical literature targeting threads in the context of design for manufacturing. Most articles you will find talk about the actual machining of the parts and are catered towards machinists. This article seeks to be a practical guide for engineers designing physical products.

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What a thread is made up of

Threads can be either internal or external and typically fit together. They are called out (or specified) in a technical drawing by defining the following parameters.

Thread parameters:

• Thread series: thread profiles (such as UN and Metric)




• Thread type: within the thread series you have different types of thread types, such as UNC and UNF for UN, and M and MJ for Metric.




• Nominal diameter/major diameter/thread size: indication of thread size. Refers to the major diameter of the thread




• Minor diameter: should always be the roughly the same as the drill size (hole size)




• Pitch: distance a point moves linear (parallel to the axis) in one revolution




• Depth: how far deep into a hole the thread goes




• Allowances: allowable tolerances of the thread fit




• Class fit/tolerance fit/allowance: defines the tolerance range of thread dimensions including major, minor, and pitch diameters of internally or externally threaded parts




• Drill size: defines the size of the hole to be drilled for the thread to be tapped. This is a definition you will see in a lot of thread sizing tables. It is essentially the same size as the minor diameter.

Holes: blind vs thru holes

There are two types of holes where you can define threads: blind holes and thru holes.

• Thru hole: a hole that goes all the way through the surface. Therefore, it's depth is the thickness of that particular wall.




• Blind hole: a hole that does not go all the way through the surface. Depth therefore needs to be specified separately.

Overview of thread types

There are various different thread types available, the most common are metric and unified.

• Metric threads (M): Metric threads are the metric international standard that follows ISO. This is one of the most common threads.




• Unified threads (UNC/UNF/UN/UNEF): Imperial standard, typically consists of coarse (UNC) and fine (UNF) threads. UN is used for a set of specific thread series which have uniform or constant threads-per-inch.




• Pipe threads (National pipe) (NPT/NPS/NPTF): typically used for threaded pipe fittings.




• Multi-start threads: Multi-start threads consists of two or more intertwined threads running parallel to one another.




• British Standard Pipe (BSPT): typically used for threaded pipe fittings.




• Thorlabs: Branded threads specs to create adapters for Thorlabs’ optical equipment.




• ACME: a type of trapezoidal threads.

The tolerance position determines the type of tolerance to be used. The main ones are g and h.

• G (internal) or g (external) tolerances define a tolerance changing with pitch diameter


• H (internal) or h (external) tolerances define a static tolerance that does not change with pitch diameter

• 6H/6g

• Standard ISO thread: M10 x 1.5 - 6H THRU LH


• Standard ISO thread: M10

Why are these call outs so different?



With metric threads, you can get away with a very simple call out, like in the second example, where you only indicate the thread series (M) and size (10). In this case, the rest will default to standard. Remember that for blind holes, you also must indicate the depth of the thread, as there is no standard to fall back to (see the table below).

Drill size = Hole size required

Unified threads

Class fit - Unified threads (UN)



Remember that class fit defines the tolerance range of thread dimensions, this is always coupled up with an indication of whether the thread is external (B) or internal (A). There are 6 types of class fit for unified threads:

• Classes 1A and 1B - Loosest tolerance, it’s very rare. Usually used for when quick assembly/disassembly is required.


• Classes 2A and 2B - “Medium” tolerance, it’s the most common fit. This is the best option to balance thread performance and convenience.


• Classes 3A and 3B - Very tight tolerance. Used when the threads are critical to the safety of the service of the final product.

Examples:

• .250 - 20UNC - 2A MOD


• 1/4" x 20 UNC


• #4-20 UNC


• .375 - 24UNEF - 2B


• 10-32 UNF-2A


• 0.190-32 UNF-2A

These examples are very different - how do they relate to the formula?



Unified threads have various different ways of being called out, you are better off picking one way and sticking to it, but if ever you need to look at another engineers drawings here’s a list of common ways to call them out broken out by section.

Design tips

1) Keep it common

If there’s one thing that will without fail save you time and money when it comes to manufacturing is to keep things common.

You don’t learn this when you’re getting your degree, and it's only through trial and error that seasoned engineers realise the importance of keeping components as common as possible - even when designing custom products. If there’s an off the shelf solution, don’t over complicate it. This also applies to threads, always seek to choose common series of threads as much as possible and common sizes.

By common I mean globally understood. We know that there are threads such as NPT, BSP, etc. that are also common locally but overseas suppliers tend to have issues with these unless they're specialized. I suggest to always make design choices that maximize the amount of manufacturers eligible to supply them. Why? Because having a diversified supply mitigates risks and gives you more buying power.

• Common series: UN (namely UNC and UNF) and Metric (namely M)


• Common sizes: UN list, Metric list

2) Be wary of hole sizes

One of the most common mistakes we’ve seen when we make parts containing threads (we’ve manufactured more than 6 million parts at the time of writing) is that the thread sizes do not seem to match the hole sizes they are called out in.

This is a problem because it leads to confusion on whether it’s the right thread size and the diameter of the hole is wrong or vice-versa. As manufacturers, we cannot make assumptions, and this confusion leads to a delay in manufacturing time (1-2 days at least).

3) Call out thread depths on blind holes

Another reason for delay is incomplete thread specifications.

Blind holes are holes that do not go through the full wall thickness of the part. If you call out a thread on a blind hole, you must always indicate the depth of the thread.

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