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How to measure a screw

Measuring a screw accurately involves determining several key dimensions: length, diameter, thread pitch, and head size. Here’s a step-by-step guide on how to measure each of these:

1. Length

  • Measurement Method: Use a ruler, caliper, or measuring tape.
  • How to Measure:
    • For screws with flat heads (e.g., pan head, hex head), measure from the flat underside of the head to the tip of the screw.
    • For screws with countersunk heads (e.g., flat head, oval head), measure from the top of the head to the tip of the screw.

2. Diameter

  • Measurement Method: Use a caliper for the most accurate measurement.
  • How to Measure:
    • Measure the outer thread diameter (the major diameter). This is the distance across the widest part of the threads.
    • Common diameter measurements for screws include M (metric) sizes like M4, M6, etc., or inch sizes like 1/4", 5/16", etc.

3. Thread Pitch

  • Measurement Method: Use a thread pitch gauge or a ruler for an approximate measure.
  • How to Measure:
    • Metric Screws: Measure the distance between adjacent threads. For example, an M6 screw with a 1mm thread pitch will have threads spaced 1mm apart.
    • Imperial Screws: Count the number of threads per inch (TPI). For example, a 1/4"-20 screw has 20 threads per inch.

4. Head Size

  • Measurement Method: Use a caliper or ruler.
  • How to Measure:
    • Measure the diameter of the screw head.
    • For hex screws, measure the distance across the flats (the parallel sides of the hexagon).

Tools You’ll Need:

  • Caliper: For precise measurements of diameter, thread pitch, and head size.
  • Ruler or Measuring Tape: For measuring length.
  • Thread Pitch Gauge: For accurately determining the thread pitch.

Example:

Let’s say you have a screw, and you want to measure its dimensions:

  1. Length: Place the screw on a ruler, measure from the base of the head to the tip. If it reads 25mm, then the screw length is 25mm.
  2. Diameter: Use a caliper to measure across the threads. If it reads 5mm, then the diameter is M5.
  3. Thread Pitch: Use a thread pitch gauge, align it with the threads. If it matches the 0.8mm gauge, then the thread pitch is 0.8mm.
  4. Head Size: Measure the diameter of the head using a caliper. If it reads 9.5mm, then the head size is 9.5mm.

By following these steps, you can accurately measure screws and ensure you have the correct dimensions for your needs.