Understanding Screw Sizes: Industry Standard Measurements

how are screw sizes determined

Screws come in a variety of sizes, and it is important to know how to measure them to ensure you are using the correct size for your project. The three main measurements of a screw are gauge, length, and threads per inch (TPI). The gauge of a screw refers to the outside thread diameter, also known as the major diameter. The length of a screw is measured from the head to the tip of the thread. Finally, the TPI is a measurement of the number of threads in a one-inch section of the screw.

Characteristics Values
Screw Measurement Gauge, Length, and Threads per Inch (TPI)
First Measurement Screw gauge, which refers to the outside thread diameter (major diameter)
Screw Gauge Screws with a major diameter less than 1/4" are labelled #0 to #14. Screws with a 1/4" or larger major diameter are labelled in fractions of an inch.
Decimal Equivalent #1 = .073" and increases by .013" with each increasing size
Other Width Measurements Root diameter (minor diameter) and shank diameter
Shaft Length Distance from the top of the head to the tip for countersinking screws
Shaft Length Distance from the bottom of the head to the tip for non-countersinking screws
Exception Oval-head screws are measured from the widest point of the head to the tip
Threads Per Inch (TPI) Measurement of the number of threads in a one-inch section of the screw
Metric System Measure length in mm from where the screw head sits to the tip
Metric System Measure the width of a thread in mm to get the diameter

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Screw gauge

A screw gauge, also known as a micrometer screw gauge, is a device that uses a calibrated screw for the precise measurement of spherical or cylindrical objects. It is often used in mechanical engineering, machining, and other mechanical trades. The object to be measured is placed between the spindle and the anvil, with the spindle moved towards the anvil until it lightly touches the object. The accuracy of a screw gauge depends on the accuracy of the thread-forms at its core.

The process of measuring using a screw gauge involves several steps:

  • Familiarise yourself with the structure of a screw gauge, including movable and stationary parts such as the sleeve, anvil, spindle, thimble lock, frame, thimble, and ratchet stop.
  • Clean the spindle and anvil by placing a paper or clean cloth between them, tightening the setup, and then pulling out the paper or cloth.
  • Place the object to be measured carefully between the anvil and the spindle, ensuring it is secure and does not move during measurements.
  • Rotate the ratchet counter clockwise to tighten, aligning the thimble zero with the scale line on the sleeve.
  • Tighten the spindle further by rotating the thimble until it clicks three times.
  • Lock the thimble while the object is in place.
  • Remove the object cautiously to avoid compromising the accuracy of the screw gauge.
  • Note the reading before unlocking the spindle; if the setup loosens, re-measure.

The thimble of a screw gauge has various scales, measuring one-tenth of an inch or 0.100, with subdivisions representing twenty-five thousandths of an inch (0.025) and one-thousandth of an inch (0.001). The sleeve typically has whole numbers, with the final number representing thousandths. For example, if the last number is 6, the final representation would be 0.500.

The parameters of pitch and least count are important in screw gauges. Pitch refers to the distance moved by the spindle per revolution, calculated by measuring the distance moved by a screw and dividing it by the number of rotations. The least count is the distance moved by the tip of the screw when turned through one division of the head scale, calculated by dividing the pitch by the total number of divisions on the circular scale.

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Screw length

The length of a screw is determined by measuring the shaft size, which is the portion of the screw that is inserted into the material. The measurement is taken from the tip of the screw to the point where the screw head rests. For countersunk screws, which sit flush with the surface, the measurement is taken from the top of the head to the tip. For non-countersunk screws, the measurement is taken from the bottom of the head to the tip.

It is important to consider the type of screw head when measuring length. For example, flat-head screws are measured from the top of the head to the tip, while round-headed countersunk screws are measured from the bottom of the rounded top. Oval head screws are an exception, with the length measured from the widest point of the head to the tip.

When selecting a screw, it is also crucial to consider the thickness of the materials being attached. Thicker materials may require longer screws to ensure a secure hold. As a rule of thumb, the screw should always go through the thinner piece and thread into the thicker piece. The McFeelys 2/3 Rule for Screw Length suggests that about 2/3 of the screw should be threaded into the secondary piece. However, this ratio can be reduced for thicker materials, with only half of the screw shaft needing to be threaded into the secondary piece.

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Threads per inch

TPI is also referred to as thread pitch, which is the term used to calculate the difference between two threads. For metric fasteners, thread pitch is used instead of TPI. The distance is measured in millimetres.

The thread pitch or TPI is important as it ensures that bolts and nuts couple together properly. If the threading of a bolt and nut are different, they will either seize or strip the threading, resulting in an unusable connection.

In the imperial system, thread counts generally range from 35-40 threads per inch. In the metric system, pitch is usually less than 1mm.

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Diameter

The diameter of a screw is a fundamental measurement that determines its size and suitability for a particular project. It is important to know whether you are measuring in metric or imperial units before you start. Here is how you measure the diameter of a screw in both systems:

Metric System

Place the end of the screw against a ruler or measuring tape and measure the width of the screw's threads in millimetres. This is the standard unit of measurement for screw diameters in the metric system. The diameter is typically the first number listed on the packaging, for example, "5.0" indicates a diameter of 5 mm.

Imperial System

Use a ruler or measuring tape to measure the width of a single thread in fractions of an inch. This measurement is usually represented by a gauge number, which corresponds to a certain fraction of an inch in diameter. For instance, a #0 gauge screw has a diameter of 1/16 of an inch, while a #1 gauge screw measures 5/64 of an inch. You can use a gauge guide to match the "#" gauge number with its corresponding fraction of an inch.

Conversion

If you need to convert between imperial and metric units, you can use a conversion chart. For example, a #8 gauge screw in the imperial system (with a diameter of 0.164 inches) would be approximately 4.17 mm in diameter in the metric system.

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Thread pitch

In the metric system, thread pitch is the distance between threads expressed in millimetres. For example, a thread pitch of 1.5 means that the distance between one thread and the next is 1.5 mm. In general, smaller fasteners have finer threads, so they have a lower thread pitch.

Frequently asked questions

The standard measurements for screws are length, width, and thread count. Length is the distance from the tip of the screw to the head. Width is the diameter of the screw, measured from the outer thread on one side to the outer thread on the other. Thread count is the number of threads per inch.

The length of a screw is measured from the tip of the screw to the head. The type of screw head will determine where exactly to start measuring from. For example, for a flat-headed countersunk screw, measure from the top of the flat head to the tip of the screw. For a round-headed countersunk screw, measure from the bottom of the rounded top.

The width of a screw, also known as the diameter, is measured from the outer thread on one side to the outer thread on the other. For the metric system, this is measured in millimeters, while for the imperial system, this is measured in fractions of an inch.

The thread count of a screw is the number of threads per inch. To measure this, lay the screw next to a ruler and count the number of threads in the space of one inch. For the metric system, this is measured in millimeters, while for the imperial system, this is measured in threads per inch.

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