Using a thread pitch gauge

Part 1. Preparation and safety

Objective

• Identify the common types, length, diameter, grade and thread pitch of threaded fasteners.

Personal safety

Whenever you perform a task in the workshop you must use personal protective clothing and equipment that is appropriate for the task and which conforms to your local safety regulations and policies. Among other items, this may include:

• Work clothing - such as coveralls and steel-capped footwear
• Eye protection - such as safety glasses and face masks
• Ear protection - such as earmuffs and earplugs
• Hand protection - such as rubber gloves and barrier cream
• Respiratory equipment - such as face masks and valved respirators

If you are not certain what is appropriate or required, ask your supervisor.

Safety check

• Never use a bolt that has been over-tightened. Its tensile strength is very low and it could break.
• Use the correct tool to tighten or loosen bolts, otherwise you could break them.
• Make sure that you understand and observe all legislative and personal safety procedures when carrying out the following tasks. If you are unsure of what these are, ask your supervisor.

Points to note

• Fasteners are used to secure components or pieces of components together. There are two main types of fasteners: inch and metric. They are not compatible.
• Bolts are identified in four ways:
  • Length
  • Diameter
  • Thread Pitch
  • Tensile Strength
• A bolt’s length is the distance from under the head of the bolt to the far end of the thread. Inch bolts can come in sizes such as 1 1⁄4", 3 1⁄2", etc. Metric bolt sizes might be 25mm, 40mm, etc.

• The bolt diameter is the thickness of the bolt shank. This will be 1⁄4" or 1⁄2", etc if it is an inch bolt, or 6mm, 8mm, 10mm, etc if it is a metric bolt.
• Fine threads can achieve a greater tightening force than coarse threads.
• Coarse threads are used in softer materials because they have a greater grip on the material.
• The measurement of thread pitch for UNC and UNF bolts is described in the number of threads-per-inch (TPI).
• A UNF bolt may measure 1⁄2" x 3" x 20. That is, the bolt is 3" long, has a shank diameter of 1⁄2" and the threaded area has 20 threads in every inch of thread. A UNC bolt that measures 1⁄2" x 3" x 13 will have the same dimensions but have only 13 threads for every inch of thread.
• The length and shank diameter of metric bolts is measured in the same way as UNF and UNC bolts but the measurements are in millimeters, rather than in inches or fractions of an inch. The difference lies in how the thread pitch is measured. Metric bolts define their pitch by the distance between each thread. There are still fine and coarse threads but this time the bolt dimension may be 6mm x 40mm x 1.0 or 1.25 in the case of a fine thread. A coarse threaded bolt of a similar size will have the dimensions of 6mm x 40mm x 1.75 or 2.0.
• The suitability of a bolt for an application is determined by its tensile strength and its yield strength. The tensile strength is defined as the maximum stretching stress a bolt can withstand without breaking. The yield strength is the maximum stress a bolt can withstand and still return to its original form.
• There are two standards of bolt grading in use. The Society of Automotive Engineers (SAE) and the American National Standards Institute (ANSI) apply the ANSI standard. This grading applies to the strength of the bolt. The second is the International Standards Organization (ISO) grading for tensile strength and yield strength of the bolt.
• A bolt graded by the ANSI standard is identified by the number of lines arranged around the head of the bolt. The minimum value of tensile strength is defined as 2. A bolt of this value has no lines on its on its head.
  • 0 lines = Grade 2 tensile strength
  • 3 lines = Grade 5
  • 5 lines = Grade 7
  • 6 lines = Grade 8

  

• A high grade-value = a high tensile value.

• The ISO standard uses two numbers on the head of the bolt. The first number indicates the tensile strength; the second number signifies the yield strength.
• If a bolt is marked 8.8, it has a tensile strength of 800 MegaPascals (MPa) and a yield strength value of 640 MPa, 80% of its tensile strength. A marking of 10.9 indicates a tensile value of 1000 MPa with a yield strength of 900 MPa, 90% of its tensile strength.
  • 4 = 400 MPa
  • 5 = 500 MPa
  • 8 = 800 MPa
  • 10 = 1000 MPa
  • .5 = 50%
  • .6 = 60%
  • .7 = 70%, etc.

• Always use a bolt suitable for the application. If a bolt with too high tensile strength is used and not tightened to its designed value, it may fail. That is because high tensile bolts have less resistance to fatigue than bolts with a lower tensile value.

Part 2: Step-by-step instruction

1. Choose a pitch gauge
   To determine the thread pitch of a particular fastener, you need to use a thread pitch gauge.
2. Check gauge markings
   Open out the pitch gauge set and examine the markings on the toothed blades. The markings will be either in inch units or in metric units. The numbers stamped on the toothed blade of an inch gauge set indicate the number of threads per inch of thread length. For example: 16 threads to the inch. Look at the numbers on the blade of a Metric pitch gauge set. The numbers indicate the width between each thread in millimeters. For example: a thread pitch of 1.5 millimeters.
3. Measure a known size
   Choose a fastener of a size you know. Say, 3/8inch U-N-C bolt. Using your inch gauge set, select each blade and hold the toothed edge against the thread of the bolt. Continue trying the blades until you find one that matches exactly the thread on your bolt. Check the number on the blade; it should read 16. That is, 16 threads per inch.
4. Measure an unknown size
   Now choose a fastener whose size you do not know. If it is a metric bolt, select the metric thread pitch gauge. Repeat the procedure with the blades against the thread of the bolt, until you find a perfect match. Check the number on the blade; it will tell you the thread pitch of this fastener in millimeters.
5. Correctly store gauge
   When you have finished, be sure to fold all of the blades back into their casing before putting the gauge set away. This is to protect the blade teeth from damage.