Introduction
Screws are a really important part of machines, tools, and structures, helping to hold things together with strong, reliable connections. Understanding screw threads—the spiral ridges that wrap around a screw’s body, is important for anyone working with mechanics, engineering, or design. This study covers the basics of screw threads, including how they are measured, drawn, and classified. We’ll explore the difference between single and double threads, how to determine a screw’s pitch, and the steps to accurately sketch a screw in technical drawings. Whether you’re a student, hobbyist, or aspiring engineer, this guide will help you grasp the fundamentals of screw threads in a clear and simple way.
(a) Single and Double Threads; Pitch
A screw thread is the spiral ridge that wraps around a screw. If you follow the edge of a single thread (shown as EFGHJ in Figure 92A), you’ll see that as it makes one full turn around the screw, it moves up by a certain distance. This distance, called the pitch, is the space between two threads, including both the raised part (the thread) and the groove between threads.
If a screw has just one continuous thread going around it, it’s called a single-threaded screw. But if there’s another thread in between (like in Figure 92B), the screw is double-threaded. In that case, the pitch is measured from one point on a thread to the same point on the next thread, covering the width of two threads and two grooves.
A right-handed screw has threads that slant upward from left to right, meaning you turn it clockwise to tighten. A left-handed screw has threads that slant the opposite way and tightens counterclockwise.
(b) Shape and Size of a Screw Thread
The size and shape of a screw thread depend on its cross-section—the way it looks when cut straight through the middle (as shown in Figure 93). A screw’s size is usually described by counting how many threads fit into one inch along its length. For example, if there are 10 threads per inch, it would be written as 10 Th. or 10 Thds.
(c) Measuring a Screw
To measure a screw, you first need to recognize its thread shape, which you can do by looking at its cross-section (see Figure 93). When recording measurements in a drawing, note:
- The thread shape
- The number of threads per inch
- Whether the screw has a single or double thread
The key measurements of a threaded bolt include:
- Total length (including the head)
- Body length (including the threaded portion)
- Threaded portion length
- Body diameter
- Head height and width
(d) Drawing a Screw Thread
Start by drawing a side view (longitudinal section) of the screw (see Figure 94). A full drawing should include the curved shape of the threads, but in simpler drawings, straight lines can be used instead.
Figures 94-97 show four key steps:
- Sketching the basic shape (A)
- Marking the front and back edges of the thread (B)
- Drawing the front of the finished thread (C)
- Showing how the thread fits inside a nut (D)
To draw the screw:
- Mark the diameter and draw the outline of the threaded section.
- Space out the threads evenly along the screw.
- Use a 30° triangle to shape the grooves of the thread.
- Make sure the roots (bottoms of the grooves) align properly.
- Complete the thread sections on both sides of the screw.
- Add details like one full turn of the thread and the edges of the nut.
- The screw’s end is often drawn as a curved arc for a realistic look.
(e) Drawing a Double, Left-Handed Thread (Figure 95)
For a double-threaded screw with 2 threads per inch, each thread is spaced ½ inch apart. The threads on opposite sides of the screw should align directly across from each other. Otherwise, the drawing steps are the same as before.
(f) Drawing a U.S. Standard Thread (Figure 96)
For a 4-thread-per-inch screw, first draw the outline of the screw. Then, draw additional guide lines spaced ¼ of the pitch away from the screw’s edges. These will help place the threads accurately.