Understanding Screw Sizes: A Guide To Reading Measurements

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Reading screw sizes can be confusing, as they are written in an alphanumeric code. For example, 4-30 x .10 and M5-.4 x 15 are screw sizes. The first letter of the size indicates the largest diameter, or the measurement of the screw on the thread. Unified screw diameters are measured in inches, while metric threads use the letter M to denote measurements in millimetres. The second number indicates the number of threads per unit of distance, also known as the thread pitch. The third number generally refers to the length of the screw. Understanding screw sizes is important to ensure you select the right screw for your project.

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Unified Thread Standard (UTS)

The Unified Thread Standard (UTS) is the main standard for bolts, nuts, and other threaded fasteners in the United States and Canada. It defines a standard thread form and series, along with allowances, tolerances, and designations.

UTS has the same 60° profile as the ISO metric screw thread used in the rest of the world. However, the characteristic dimensions of each UTS thread, namely the outer diameter and pitch, are defined as inch fractions rather than millimetre values. This is the key difference between UTS and ISO.

UTS threads are symmetric and V-shaped, with a 30° flank angle. The outermost 1⁄8 and the innermost 1⁄4 of the height of the V-shape are cut off from the profile. The pitch is the distance between thread peaks and is usually expressed in terms of threads per inch (TPI), which is the reciprocal of the pitch value.

UTS threads are single-start threads, meaning the pitch is equal to the lead, which is the axial distance the screw advances during a 360° rotation.

A standard UTS designation includes the major diameter, followed by the TPI, and an indication of the thread's coarseness: Unified Coarse Thread (UNC), Unified Fine Thread (UNF), or Unified Extra Fine Thread (UNEF). For diameters of 1/4" or above, the diameter is included in the designation; for smaller diameters, a number is assigned. For example, a #1-64 UNC bolt has a major diameter of 0.0073" and a TPI of 64.

UTS threads are used for mass production and applications requiring rapid installation, such as in the aerospace and automotive industries. They are also suitable for tapped holes in hard materials, thin walls, and thin materials.

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Metric system

The metric system is used globally, except in a few countries like the US, and is the most common system for screw sizes. Metric screws are specified as diameter, pitch, and length in millimetres. Metric screws are designated with an "M" followed by the nominal diameter, such as "M8," which indicates an 8-millimetre diameter. The pitch, or distance between threads, is represented by a number like "1.0," indicating a 1-millimetre pitch. Length is also given in millimetres. For example, "M8-1.0 x 20" denotes an 8mm diameter, 1mm pitch, and 20mm length.

Metric screws are classified as coarse or fine, referring to their thread pitch, with coarse being the most common and assumed if not specified. Fine threads have a higher thread count per unit of measure. The length of a metric screw is measured from the underside of the head to the tip, similar to imperial screws.

Metric fasteners are manufactured according to standards set by organisations like the German Institute for Standardisation (DIN) and the International Organisation for Standardisation (ISO). These standards ensure consistency and compatibility in screw sizes and applications.

When identifying metric threads, it is important to determine if the thread is parallel or tapered. A caliper can be used to measure the outer diameter of a parallel thread in millimetres, giving an exact thread size but not the pitch. The pitch can then be determined using a thread gauge or calipers, typically ranging from 1.0 to 2.0 millimetres.

Metric screws and bolts also have markings indicating their strength. On bolt heads, there are two numbers separated by a decimal point, indicating breaking strength and yield point. The higher the number, the stronger the bolt. Nut strength markings may be numerical or clock face-style markings, where the position of the line indicates the grade.

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Screw length

The length of a screw is measured differently for countersinking screws and non-countersinking screws. Countersinking screws are those that can be driven completely into the surface, meaning that the screw length includes the head of the screw. For these screws, the length is measured from the top of the head to the tip. On the other hand, non-countersinking screws, such as truss, round, hex, button, and pan-head screws, do not go all the way into the surface. For these types of screws, the length is measured from beneath the head to the tip.

When reading the callout on a box of screws, the screw length is typically specified at the end of the callout. For example, if you see " .5" at the end, it indicates that the screw length is 1/2 inch long.

It is important to note that the length of a flathead screw is measured to the bottom of the head since these screws are meant to sit flush with the surface.

In addition to the length, the packaging of many screws will also include specifications for the screw diameter or major diameter, gauge size, and various acronyms indicating added features.

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Screw gauge

A screw gauge is an instrument used to measure the diameter of thin wires, the thickness of small sheets such as glass or plastics, and the size of screws. It is also known as a micrometer screw gauge.

The screw gauge has several components, some of which are movable while others are stationary. These include:

  • Sleeve, barrel, or stock: the stationary round component with a linear scale.
  • Thimble: the component that one's thumb turns.
  • Thimble scale: rotating graduated markings.
  • Thimble lock: used to hold the spindle stationary.
  • Spindle: the shiny cylindrical component that moves toward the anvil.
  • Anvil: the shiny part that the spindle moves toward and that the sample rests against.
  • Ratchet stop: a device on the handle that limits applied pressure.
  • Frame: the C-shaped body that holds the anvil and barrel in a constant relation to each other.

Using a Screw Gauge

  • Clean the spindle and anvil with a paper or clean cloth. Tighten the setup slightly until the paper or cloth is jammed, then pull it out.
  • Place the object between the anvil and the spindle, ensuring it does not move.
  • Rotate the ratchet counter clockwise to tighten, ensuring the thimbles zero are parallel to the scale on the sleeve.
  • Tighten the spindle with the object in the middle until the thimble clicks three times.
  • Lock the thimble. The spindle can still be moved at this point, allowing for a more precise measurement.
  • Take the object out cautiously to avoid scratching or shaking the screw gauge.
  • Note the reading before unlocking the spindle. If the setup loosens, re-measure.

There are two main parameters used in every screw gauge: pitch and least count.

Pitch

Pitch is the distance moved by the spindle per revolution. It is calculated as follows:

Pitch of the screw gauge = (distance moved by a screw)/(number of rotations given)

Least Count

Least count is the distance moved by the tip of the screw when turned through one division of the head scale. It is calculated as follows:

Least count of the screw gauge = (pitch of the screw gauge)/(total number of divisions on the circular scale)

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Screw pitch

The pitch is the second number in a screw size written in an alphanumeric code, such as "4-30 x .10". Metric screw sizes denote the pitch in millimetres, so in the example "M5-.4 x 15", the pitch is 0.4mm.

The pitch is important for distinguishing between coarse and fine screws. Coarse screws have a larger pitch than similarly-sized fine screws. Coarse screws are used in general applications, while fine screws are more resistant to damage.

Frequently asked questions

The first letter or number of a screw's size code is its largest diameter, or the measurement of the screw on the thread. Unified screw diameters are measured in inches, while metric threads use "M" to denote the measurement in millimetres.

The second number in a screw size code is either the number of threads per unit of the distance between threads, also known as the "thread pitch". Unified screws give the threads per inch, while metric threads display the number of millimetres per thread.

The third number is the length of the screw. With the exception of flathead screws, which are meant to sit flush with the surface, these are measured to the bottom of the head. Unified screws provide length in inches, while metric screws show the length in millimetres.

"LH" in a screw size code stands for "left-handed". Left-handed screws are threaded in reverse. When you turn a left-handed screw clockwise, it loosens instead of tightens.

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