The caliper, as one of the most commonly used measuring tools in the field of mechanical manufacturing, is simple
to operate and widely applied in outer diameter measurement. However, it still has many inherent technical limitations.
With the increasing precision requirements in modern manufacturing, the drawbacks of caliper measurement of outer
diameter have become increasingly evident, affecting the accuracy and reliability of the measurement results. The
measurement accuracy of a standard vernier caliper is ±0.02 millimeters, while that of a digital caliper can reach ±0.01
millimeters. However, in actual applications, it is affected by multiple factors such as operational skills, environmental
conditions, and characteristics of the workpiece, and the actual measurement error often exceeds the theoretical
accuracy range.
I. The fundamental reason for the limitation of measurement accuracy
1. Human operational errors are the main drawback of using calipers for measuring the outer diameter. The control of
measurement force directly affects the accuracy of the measurement results. The standard measurement force should
be controlled within the range of 8-12 Newtons, but it is difficult to maintain a constant measurement force in actual
operation. A change of 1 Newton in measurement force may result in a measurement error of 0.005-0.01
millimeters. Especially when measuring thin-walled parts or soft materials, excessive measurement force can cause
the workpiece to deform, seriously affecting the measurement accuracy.
2. Reading errors are inherent technical flaws of the vernier caliper. The subjectivity of human eyes when reading the
scale lines leads to inconsistent measurement results. The minimum reading value of the vernier principle is 0.02
millimeters, but the actual reading is affected by factors such as light conditions, observation angles, and the
operator's vision. The reading difference for the same size by different operators can reach 0.03 - 0.05 millimeters.
This kind of human reading error is unacceptable in precise measurements.
3. The limitation of contact area leads to insufficient representativeness of the measurement results. The contact area
between the caliper jaws and the workpiece is usually only a few square millimeters. For workpieces with high surface
roughness or local defects, a slight change in the position of the contact points can result in different measurement
results. The true roundness error and ellipticity error of circular cross-section workpieces cannot be accurately reflected
by a single-point measurement. Multiple-point measurement is required to obtain reliable geometric parameters.
II. The Impact of Environmental Conditions on Measurement Accuracy
1. Temperature variation is an important environmental factor affecting the accuracy of caliper measurements.
The thermal expansion coefficient of metal materials causes dimensions to change with temperature. The linear
expansion coefficient of steel is approximately 11×10^-6/℃. A temperature hange of 10℃ will result in a 0.011mm
change in a 100mm dimension. Fluctuations in the workshop environment temperature, transfer of operator's hand temperature, and residual heat from the workpiece processing all can affect the accuracy of the measurement results.
2. Humidity and pollutants have a negative impact on the measurement accuracy of the caliper, especially in harsh
production environments. Metal cutting fluids and cooling fluids remain on the surface of the workpiece, forming a liquid
film that alters the actual contact state and measurement reference. The thickness of the surface oil film, ranging
from 0.01 to 0.05 millimeters, is directly superimposed onto the measurement result. To obtain accurate
measurement data, the workpiece surface must be thoroughly cleaned.
3. Vibration and shock can cause interference to precise measurements. In the production site, the operation of machine
tools and the movement of heavy equipment can all generate vibrations. As a handheld measuring tool, calipers are
particularly sensitive to vibration interference. When the ibration amplitude exceeds 0.02 millimeters, it will
significantly affect the stability of the reading. During the measurement process, the tremors of the operator's hand
will also be transmitted to the measurement result, affecting the repeatability and accuracy of the measurement.
III. Constraints of Workpiece Characteristics on Measurement Results
1. Significant differences in surface quality affect the reliability of caliper measurements. The peaks and valleys on
a rough surface cause uncertainty in the contact points. When the surface roughness Ra exceeds 1.6 micrometers,
the randomness of the microscopic geometric shape on the surface increases the dispersion of the
measurement results. The measurement error for rough surfaces such as casting surfaces, forging surfaces, and
welding surfaces can reach ±0.1 millimeters, far exceeding the nominal accuracy of the caliper.
2. The complexity of the workpiece's geometric shape limits the application scope of the caliper. Non-standard
geometric shapes cannot be accurately measured. Workpieces with elliptical cross-sections, polygonal cross-sections,
and irregular cross-sections require specialized measurement methods and equipment. The caliper can only measure
local dimensions but cannot comprehensively evaluate the geometric accuracy. For parts with high geometric accuracy requirements, the representativeness of the caliper measurement results is insufficient.
3. The hardness and elastic properties of the materials affect the stability of the measurement results. Soft materials
undergo elastic or plastic deformation under the action of the measuring force. The measurement of materials such as
aluminum alloy, plastic, and rubber requires special control of the measuring force. For materials with a hardness
lower than HB100, the measurement error may exceed ±0.05 millimeters. The rebound characteristics of highly
elastic materials cause the measurement results to change over time, thereby affecting the repeatability of the
measurement data.
IV. Limitations of Measurement Range and Flexibility
1. The measurement range is restricted by the structural dimensions of the caliper and cannot measure large-sized
workpieces beyond the range. The maximum measurement range of standard calipers is usually 150-300 millimeters.
For measuring the outer diameter of large workpieces, specialized large-sized calipers or other measurement
methods are required. The dimensions of special positions such as inner diameters of deep holes, bottom diameters
of blind holes, and widths of grooves cannot be directly measured using ordinary calipers.
2. Constraints on measuring angles and positions affect the feasibility of measuring certain workpieces. Calipers need
to be perpendicular to the surface being measured to obtain accurate results. For complex-shaped workpieces, some
parts may not be accurately positioned by the calipers. A measurement angle deviating by 5 degrees from the
vertical direction will result in approximately 0.4% of cosine error. The measurement of parts in assembly
conditions is often limited by space and cannot achieve the ideal measurement conditions.
3. The insufficient dynamic measurement capability restricts the application of calipers in automated production,
preventing the realization of online measurement and real-time monitoring. Modern manufacturing requires rapid
detection and feedback. However, calipers have slow measurement speeds and require manual operation,
thus failing to meet the production rhythm requirements for high efficiency. Automated production lines need
integrated measurement systems. The manual nature of calipers becomes a bottleneck for production efficiency.
