Date Published:Feb. 20. 2019 Date Updated:May. 13. 2019
Vol. 12 Back to Basics Series! Screw Reference Dimensions
Let's be sure we understand screw reference dimensions, the ABCs of screw knowledge.
Lack of understanding may result in placing an order for the wrong dimensions, making a mess! Therefore, pay close attention.
I will also explain the terminology used for the reference dimensions. This is very important when designing, so it is worth a quick read.
Terms used in screw standards
First I will explain the terms used.
①Pitch (P) ②Effective diameter Effective sectional area ● Calculating the effective sectional area |
③Thread angle
④Male screw valley diameter ⑤Male screw outer diameter ⑥Female screw valley diameter ⑦Female screw inner diameter |
Nominal
This is a symbol used to express screw dimensions, which often uses the outer diameter of the male thread as the reference dimension.*However, the pitch is often omitted for screws where there is only one standard pitch for the same nominal diameter, as with metric coarse screws and miniature screws.
Various ways to show screw type and thread shape
Here we introduce some of the ways to show screw type and thread shape, such as metric coarse threads, metric fine threads, and unified coarse threads.How screw type is shown
Classification | Screw Types | Symbol showing screw type |
Examples of screw types |
(Old JIS) | Thread angle | |
---|---|---|---|---|---|---|
Screws showing pitch in mm |
Metric coarse screw thread | M |
M6 | 60° | ||
Metric Fine Screw Thread | M6×0.75 | 60° | ||||
Miniature screw thread | S | S0.8 | 60° | |||
Metric trapezoidal thread | Tr | Tr10×2 | (TM) | 60° | ||
Screws showing pitch in TPI |
Tapered Pipe Threads | Tapered male thread | R | R1/2 | (PT) | 55° |
Tapered female thread | Rc | Rc3/4 | (PT) | 55° | ||
Parallel female thread (1) | Rp | Rp3/4 | (PS) | 55° | ||
Parallel pipe Threads(2) | Parallel male thread | G (Add A or B) |
G1/2A | (PF) | 55° | |
Parallel female thread | G | G1/2 | 55° | |||
Unified coarse thread | UNC | 1/4-20UNC | 60° | |||
Unified fine thread | UNF | No.10-32UNF | 60° | |||
Whitworth (3) | W | W3/8 | 55° | |||
American National Tapered Pipe Thread (4) | NPT | 3/4NPT | 60° |
(1). Tapered parallel pipe threads are used to join a tapered male thread to a parallel female thread (Rp).
(2). Parallel pipe threads are used to join a male parallel pipe thread to a female parallel pipe thread and show the G symbol in either case, but for male threads there is a symbol showing the class (A or B) added.
(E.g.) Male screws may show G 1/2A whereas female screws show G 1/2.
(3). Whitworth screws originated in the UK but were abolished in the 1968 JIS standards, though some are still in use. They are imperial screws with 55° threads.
(4). The thread angle of American National Tapered Pipe Threads is the same as unified threads at 60°.
Threads forming a 60° triangular shape
Metric coarse thread, metric fine thread, unified coarse thread, and unified fine thread shapes are as in the figure.Threads forming a 55° triangular shape形
Tapered pipe threads and parallel pipe threads have a 55° thread angle. See the figure.Other screws
Here some special thread shapes, such as square threads and sawtooth threads, are introduced for reference.
Square thread
Sawtooth thread
Round thread
Lightbulb thread
That's all for today!