The inner diameter and aperture measurement method of precision steel pipes

In the production and inspection of precision steel pipes, aperture measurement plays a crucial role. There are several measurement methods available, each suitable for different accuracy levels and measurement requirements. Below are the commonly used methods for aperture measurement:

1. Direct Measurement (Most Common Method)

Direct measurement of apertures is frequently employed during the production and inspection of precision steel pipes. This method typically involves positioning tools such as vernier calipers, tool microscopes, universal length comparators, horizontal length gauges, optical meters, and pneumatic measuring instruments. Special aperture measuring tools, such as internal diameter micrometers, internal diameter dial indicators, and electronic plug gauges, can also be used.

- Lever Mechanism for Aperture Measurement:
Hand-held aperture measurement tools often utilize a lever mechanism. For example, internal diameter dial indicators and mechanical or electrical internal diameter micrometers fall into this category. The difference between the measured aperture and a calibrated ring gauge is read through the dial indicator or micrometer. These tools generally measure apertures in the range of 10 to 800 mm, with internal diameter micrometers offering precision up to 3 to 5 microns.

- Wedge Principle for Aperture Measurement:
This principle is commonly used in hand-held tools for measuring smaller apertures. Internal diameter dial indicators, for instance, can measure apertures as small as 0.5 mm. When the aperture compression probe moves a conical measuring rod, the aperture size error is displayed on the dial indicator or micrometer. The three-point positioning method is suitable for measuring apertures with diameters above 3 mm. As the measuring rod rotates, it moves forward via a fixed nut, and three measuring heads positioned with a spiral boss on the top of the rod contact the measured hole. The aperture size is then read from the fixed sleeve and differential cylinder scales. A typical tool for this type of measurement is a three-jaw internal diameter micrometer.

- Seat-Type Aperture Gauge:
For high-precision measurements, especially in controlled environments, seat-type aperture gauges based on pneumatic, optical, and electrical principles are used. These are typically employed to measure apertures at a constant temperature close to 20°C. An example of such a tool is the light wave interference aperture gauge, which measures apertures from 1 to 50 mm with an accuracy of±0.5 microns.

2. Indirect Measurement (Used to Resolve Disputes)

Indirect measurement methods are often considered when there is a dispute between the supply and demand sides regarding steel pipe sizes. This method involves first measuring a function related to the aperture, and then converting the result into the aperture size. The main techniques are:

- Three-Point Circle Determination:
In this method, the coordinates of three points on the circumference of the measured hole are recorded. These coordinates are used to calculate the coefficients D, E, and F in the equation x2+y2+Dx+Ey+F=0x^2 + y^2 + Dx + Ey + F = 0x2+y2+Dx+Ey+F=0, from which the aperture size can be determined.

- Rolling Method with Known Diameter Roller:
A roller with a known diameter is rolled against the wall of the measured hole to measure the circumference. Once the circumference is measured, the aperture size can be calculated. This method is typically used for measuring holes with a diameter greater than 500 mm and a continuous surface. The tool used for this purpose is known as a large-diameter measuring instrument, which is also commonly used for measuring the outer diameters of large workpieces. However, this method lacks high precision. Some attempts have been made to improve accuracy using multi-roller measurement and computer simulation, though these methods have limited practical value.

3. Comprehensive Measurement

Comprehensive measurement uses a smooth plug gauge to verify the qualification of the inner hole of a steel pipe or mechanical workpiece. The pass-stop method is employed to determine whether the aperture is within acceptable limits. This method is straightforward and is often used for routine checks, but it will not be explained in further detail here.