A caliper is a tool used to gauge an object’s dimensions.
Many types of calipers allow you to read a measurement on a ruled scale, a dial, or a digital display. Some calipers are as simple as a compass with inward or outward-facing points but no scale. The caliper tips are adjusted to fit across the points to be measured, and the dimension is read by measuring between the tips with another measuring tool, such as a ruler.
It is employed in a wide range of industries, including mechanical engineering, metalworking, forestry, woodworking, science, and medicine.
The earliest caliper was discovered in the Greek Giglio shipwreck off the coast of Italy. The ship was discovered in the 6th century BC. There was already a fixed and movable jaw on the wooden piece.
Pierre Vernier developed the modern vernier caliper as an improvement on Pedro Nunes’ nonius.
Types:
Inside Caliper
- Inside calipers are used to determine the internal size of an object.
Before fitting, the upper caliper in the picture (on the right) needs to be manually adjusted. This caliper type is fine-tuned by lightly tapping the caliper legs on a convenient surface until they are nearly in contact with the object. A slight push against the central pivot screw’s resistance causes the legs to spread to the proper dimension, giving the measurement the necessary, consistent feel.
An adjusting screw on the lower caliper in the image enables it to be precisely adjusted without removing the tool from the workpiece.
Outside Caliper
- Outside calipers are used to determine the size of an object from the outside.
The same observations and methods that apply to the inside caliper also apply to this type of caliper. These instruments can offer a high level of accuracy and repeatability with little knowledge of their usage and constraints. They are especially useful when measuring over long distances, such as when measuring a large-diameter pipe. A vernier caliper lacks the depth capacity to straddle this large diameter while also reaching the pipe’s outermost points. They are made of high-carbon steel.
Divider caliper
- A divider caliper, also known as a compass, is used to mark out locations in the metalworking industry. The points are sharpened to function as scribers, allowing one leg to be inserted into the dimple made by a center or prick punch while the other leg is pivoting to scribe a line on the surface of the workpiece, thereby forming an arc or circle.
A divider caliper is also used to measure the distance between two points on a map. The two caliper ends are brought to the two points whose distance is being measured. The opening of the caliper is then measured on a separate ruler and converted to the actual distance, or measured directly on a scale drawn on the map.On a nautical chart, distance is frequently measured using the latitude scale that appears on the map’s sides: one minute of arc along any great circle, such as any longitude meridian, is approximately one nautical mile or 1852 meters.
Dividers are also used in the medical field. An ECG (also EKG) caliper measures distance on an electrocardiogram; when combined with the appropriate scale, the heart rate can be calculated.
Oddleg caliper
- Oddleg calipers, Hermaphrodite calipers, or Oddleg Jennys, as shown on the left, are commonly used to scribe a line at a specific distance from the edge of a workpiece. The bent leg is used to run along the edge of the workpiece while the scriber makes a predetermined mark, ensuring a line parallel to the edge.
The uppermost caliper in the diagram on the left has a slight shoulder in the bent leg, allowing it to sit more securely on the edge. The lower caliper does not have this feature, but it does have a renewable scriber that can be adjusted for wear and replaced when it becomes excessively worn.
Vernier caliper
The labeled parts are:
- Outside large jaws: used to measure the external diameter of an object (like a hollow cylinder) or width of an object (like a rod) and the diameter of an object (like a sphere).
- Inside small jaws: used to measure the internal diameter of an object (like a hollow cylinder or pipe).
- Depth probe/rod: used to measure the depths of an object (like a small beaker) or a hole.
- Main scale (Metric): marked every millimeter and helps to measure the length correctly up to 1 mm.
- Main scale (Imperial): marked in inches and fractions.
- Vernier scale (Metric) gives interpolated measurements to 0.1 mm or better.
- Vernier scale (Imperial) gives interpolated measurements in fractions of an inch.
- Retainer: used to block movable parts to allow the easy transferring of a measurement.
The calipers in the diagram show a primary reading of approximately 2.475 cm on the metric scale (2.4 cm read from the main scale plus about 0.075 cm from the vernier scale).
Calipers frequently have a “zero point error,” which means that the calipers do not read 0.000 cm when the jaws are closed. The primary reading must always be subtracted from the zero-point error. Assume the zero-point error of these calipers is 0.013 cm. This gives us a length measurement of 2.462 cm. The vernier, dial, and digital calipers directly read the distance measured with high accuracy and precision. They are functionally identical, but they read the result differently. These calipers are made up of a calibrated scale with a fixed jaw and another jaw with a pointer that slides along the scale. The distance between the jaws is then measured in various ways for the three types.
The simplest method is to read the position of the pointer directly on the scale. When the pointer is between two markings, the user can mentally interpolate to improve the precision of the reading. This would be a simply calibrated caliper, but the addition of a vernier scale allows for more accurate interpolation and is the universal practice; this is the vernier caliper.
Dial caliper
Instead of a vernier mechanism, which requires some practice to use, a dial caliper reads the final fraction of a millimeter or inch on a simple dial.
In this instrument, a small, precise rack and pinion drive a pointer on a circular dial, allowing direct reading without the need to read a vernier scale. Typically, the pointer rotates once every inch, tenth of an inch, or millimeter. This measurement must be added to the coarse whole inches or centimeters read from the slide. Typically, the dial is set up so that it can rotate beneath the pointer, enabling “differential” measurements (the measuring of the difference in size between two objects, or the setting of the dial using a master object and subsequently being able to read directly the plus-or-minus variance in the size of subsequent objects relative to the master object).
A dial caliper’s slide can usually be locked at a setting with a small lever or screw, allowing for simple go/no-go checks of part sizes.
Digital caliper
A popular improvement is to replace the analog dial with an electronic display that displays the reading as a numeric value. Instead of a rack and pinion, these calipers use a linear encoder. Most digital calipers allow you to switch between centimeters, millimeters, and inches. All of them allow you to zero the display at any point along the slide, allowing you to take differential measurements just like a dial caliper. Digital calipers may have a “reading hold” feature that allows for the reading of dimensions after use in awkward locations where the display cannot be seen. Ordinary 6-inch (150 mm) digital calipers are made of stainless steel and have a rated accuracy of 0.001 in (0.02 mm) and a resolution of 0.0005 in (0.01 mm). [8] The same technology is used to make longer 8-inch and 12-inch calipers; the accuracy for longer measurements drops to 0.001 in (0.03 mm) for 100-200 mm and 0.0015 in (0.04 mm) for 200-300 mm.
