Date Published:Feb. 20. 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.
Effective sectional area
● Calculating the effective sectional area
④Male screw valley diameter
⑤Male screw outer diameter
⑥Female screw valley diameter
⑦Female screw inner diameter
NominalThis 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 shapeHere 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
of screw types
|（Old JIS）||Thread angle|
| Screws showing
pitch in mm
|Metric coarse screw thread|| M
|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）
|Parallel female thread||G||G1/2||55°|
|Unified coarse thread||UNC||1/4-20UNC||60°|
|Unified fine thread||UNF||No.10-32UNF||60°|
|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 shapeMetric 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.
Here some special thread shapes, such as square threads and sawtooth threads, are introduced for reference.
That's all for today!