Caliper
A caliper is a hand measuring tool designed for measuring distances, allowing the measurement of both external and internal distances and depths. Typically, the measurement accuracy of a caliper is between 0.1-0.02 millimeters. Common items measured with a caliper include perpendicular distances, external and internal diameters of objects, hole depths, or shoulder lengths. Calipers are widely used in various fields such as the metal industry, woodworking, science, and medicine.
When measuring with a caliper equipped with an analog vernier scale, the object to be measured is placed between the measuring jaws of the caliper, and the measurement result is read from the scale, which indicates the full millimeters, and a separate vernier scale shows the tenths of a millimeter (0.1mm). There are also calipers equipped with a dial gauge on the market, where the reading is taken from the position indicated by the gauge's needle on the dial.
Modern digital calipers are easy to read and provide measurement results with an accuracy of a hundredth of a millimeter (0.01mm). However, when using a caliper, one must remember the measuring device's stated accuracy and the measurer's uncertainty, e.g., regarding measuring force, so a reliable measurement result cannot be guaranteed with a digital caliper to 0.01mm accuracy. In machine shops, digital calipers are often IP67 rated, meaning they are completely protected from dust and can withstand immersion up to 30 minutes between a minimum of 15 centimeters and a maximum depth of 1 meter. Thanks to the IP67 protection rating, calipers can also be used in environments where exposure to machining fluid cannot be avoided. Modern digital calipers often also come with wireless data transfer, which makes it easy to create a measurement log, for example, in an Excel-based measurement record.
Calipers are the most common measuring tool in machine shops and are generally considered part of a machinist's personal tools. The most common calipers have a measuring range of less than 200mm, but calipers with a measuring range of several meters are also available. Various versions are available, such as depth calipers, thread calipers, groove calipers, and they are available with various jaw options for different applications, such as measuring pipe wall thickness, internal groove diameters, etc.
The condition of the caliper's measuring jaws and tips can be checked by bringing the measuring jaws together and holding it up to the light to see if any light shows through the measuring surfaces. If no light is visible, the caliper's jaws and tips are in good condition. When using a caliper with a vernier scale, it is also necessary to check that the scale reads zero when the jaws are closed. The condition of the measuring tongue at the end of the caliper is checked by ensuring it is level with the caliper's body when the jaws are closed.
Maintenance of the caliper's sliding and measuring surfaces should be performed before each use by wiping away any dust and dirt with a soft, dry cloth. After use, the caliper should be cleaned of oil and water and lightly oiled. Calipers should not be stored in direct sunlight, extreme temperatures, or a humid environment. The jaws of the caliper should not be left completely closed during storage.
Calipers should be handled carefully to avoid damage to their sharp measuring tips. Identification numbers or other markings should not be engraved on the caliper with engraving tools.
Parts of a Caliper
The parts of an analog caliper with a vernier scale are:
- Frame
- Slide
- Fixed jaw
- Movable jaw
- Internal measuring tips
- Depth gauge tongue
- Main scale
- Vernier scale
- Locking clamp
In addition, calipers may also have various measuring tips, inch-calibrated scales, or a locking screw. Digital calipers contain the same parts except for the scales, which are replaced with a digital display.
Reading a Caliper
Reading an analog caliper is done using the main scale and the vernier scale. First, from the caliper's main scale, read the full millimeters indicated by the zero line of the vernier scale (1). Then, from the vernier scale, read the tenths of millimeters from the point where a line on the vernier scale aligns with a line on the main scale (2). The measurement result of the caliper is obtained by adding the full millimeters and the tenths of millimeters together. Full millimeters 24 + tenths of millimeters 0.4 = 24.4mm
Various types of measurement errors can occur when using a caliper, for example, excess measuring force, thermal expansion due to temperature differences between the caliper and the workpiece, the parallax effect in vernier scale calipers, and the thickness of the internal measuring jaws affecting the measurement of small hole diameters. Errors can also occur when measuring an object at the base or tip of the jaws.
When measuring with manual calipers featuring a vernier scale, it is important to ensure that the caliper is perpendicular to the measurer. If the vernier scale is read at an angle, a reading error may occur due to the parallax effect caused by the height difference between the main and vernier scales.
For internal measurements, the internal measuring tips should be placed as deep as possible and the largest reading noted during measurement. For depth measurements, the smallest reading during measurement is recorded.
Measurement errors can also occur if the movable jaw tilts compared to the fixed jaw due to the caliper's straightness error or excess measuring force. Calipers are generally made of stainless steel, so it's important to note that when measuring materials other than steel, the caliper's coefficient of thermal expansion differs from that of the material being measured.
