Calipers with a variety of measuring mode, easy to operate, durable, versatile, and low-cost many advantages, can be said to be the most common tool kit. However, there are a number of caveats to using the caliper properly, starting with the error factors that affect the measurement results.
The error factors
When measuring with calipers, there are many factors causing errors, such as parallax, errors caused by the structure does not conform to Abbe's principle, thermal expansion caused by a temperature difference between calipers and workpiece, and so on.
In addition, due to the caliper's non-constant pressure device, it is difficult to grasp the appropriate and uniform measurement force during measurement, which is also another factor causing errors.
The structure of the caliper does not conform to Abbe's principle
The reading line and measuring axis of calipers do not conform to abbe principle because they are not coaxial, so when using calipers, the root or tip of the measuring claw will increase the risk of measurement error. When measuring, it is necessary to make sure that the measured workpiece is as close to the ruler body as possible (reading the number line).
Read the parallax of the scale
When checking whether the vernier scale is aligned with the main scale divider, look directly at the vernier scale.
The difference height between the vernier scale and the main scale is easy to cause a reading error. As shown in the figure below, if viewed from the inclined direction vernier scale line, produces Δ X is shown in the error in the figure. To avoid this effect, the JIS specification specifies that the difference height (H) should not be greater than 0.3mm.
Bending of the datum end face
The curved surface of the guiding vernier ruler will lead to the error shown in the figure below, which can be expressed by the same calculation formula as the error that does not conform to Abbe's principle.
Example: It is assumed that the deformation caused by the bending of the guide ruler is 0.01mm/50mm, and the outer diameter is 40mm at the tip of the claw. For calculation, f = 40mm×0.01÷50 = 0.008mm
The relationship between measurement and temperature
The body of the caliper is generally made of stainless steel, which has the same thermal expansion coefficient as iron metal (10.2±1)×10-6/K. The material of the measured object, room temperature and workpiece temperature should be taken into account in the measurement.
Operational considerations
Calipers are very sharp, so the instrument must be handled with care to avoid personal injury.
Avoid damaging the scale of the digital display caliper. Do not mark the identification number or use the electric pen to leave other information.
Avoid touching hard objects or falling on stools or floors to damage calipers.
Maintenance of sliding surfaces and measuring surfaces
Before using the caliper, wipe the sliding surface and measuring surface with a soft, dry cloth to remove dust and dirt.
Check and calibrate the origin before use
Clamp a piece of clean paper between your outer PAWS and slowly pull it out. Before using the caliper, close the measuring claw and ensure that the vernier scale (or display) is set to zero. When using digital display calipers, reset after replacing the battery (zero buttons).
Operation after use
After using the caliper, wipe off the water and oil thoroughly. Then, gently rub it with anti-rust oil and let it dry before storing it.
Waterproof calipers, in order to prevent rust after use, also need to wipe away the moisture calipers.
Precautions for storage
Avoid direct sunlight, high temperature, low temperature, and high humidity environment.
If the digital calipers have not been used for more than three months, remove the battery before storage.
Do not allow the caliper's external measuring claw to be completely closed during storage.