The caliper is one of the most common measuring tool in the toolbox. It has many advantages such as multiple measurement modes, easy operation, durability, strong versatility, and low cost. However, there are still many precautions to use calipers correctly. Let’s start with the error factors that affect the measurement results.
When measuring with calipers, there are many factors that cause errors, such as parallax, errors caused by the structure not conforming to Abbe’s principle, thermal expansion caused by the temperature difference between the caliper and the workpiece and so on.
In addition, because the caliper has no constant pressure device, it is difficult to grasp a suitable and uniform measuring force when measuring, which is another error factor.
1. The structure of the caliperdoes not conform to the Abbe principle
The reading and measuring axis of the caliper do not conform to the Abbe principle because of the different axes. Therefore, when using the caliper, measuring with the root or tip of the claw may increase the measurement error. When measuring, pay attention to the measured workpiece as close to the ruler body part (reading axis) as possible.
2. Read the parallax of the scale
When checking whether the vernier tick mark is aligned with the main tick mark, look directly at the vernier tick mark.
There is a step height between the vernier ruler and the main ruler scale surface, which is easy to cause reading errors. As shown in the figure below, if you observe the vernier scale line from an oblique direction, the error shown by ΔX in the figure will occur. In order to avoid the influence of such factors, the JIS specification stipulates that the step height (H) should not be greater than 0.3mm.
3. The bending of the reference end face
If the ruler surface that guides the sliding of the vernier is bent, it will cause the error shown in the figure below. This error can be expressed by the same calculation formula as the error that does not conform to the Abbe principle.
Example: Suppose the deformation caused by the bending of the guide ruler surface is 0.01mm/50mm, and the outer diameter measuring claw tip is 40mm to calculate: f=40mm×0.01÷50=0.008mm
4. Relationship between measurement and temperature
Caliper is generally made of stainless steel, which has the same thermal expansion coefficient (10.2±1)×10-6/K as ferrous metals. The material of the measured object, the room temperature and the temperature of the workpiece must be considered when measuring.
5. Notes on operation
The caliper claw is very sharp, so the instrument must be operated carefully to avoid personal injury.§
Avoid damaging the scale of the digital caliper, do not engrave the identification number or use the electric pen to leave other information.
Avoid colliding with hard objects or falling on the stool or floor to damage the calipers.
6. Maintenance of sliding surface and measuring surface
Before using the caliper, wipe off the dust and dirt on the sliding surface and the measuring surface with a soft dry cloth.
7. Check and calibrate the origin before use
Clamp a piece of clean paper between the outer claws, and then slowly pull it out. Before using the caliper, close the measuring jaw and make sure that the vernier scale (or display) returns to zero. When using a digital caliper, please reset (zero button) after replacing the battery.
8. Operation after use
After using the caliper, wipe off water and oil thoroughly. Then, lightly smear it with anti-rust oil, dry it before storing.
Waterproof calipers, in order to prevent rusting after use, also need to wipe off the moisture of the calipers.
9. Precautions for storage
Avoid direct sunlight, high temperature, low temperature and storage in high humidity environment.§
If the digital caliper is not used for more than three months, remove the battery before storing.
During storage, do not allow the outer measuring jaw of the caliper to be completely closed.
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