Application of Measuring Instruments in Mechanical Manufacturing Facilities
1、 Classification of measuring instruments
A measuring instrument is a fixed-form device used to reproduce or provide one or more known values. Measuring tools can be classified into the following categories based on their usage:
Single-value measuring tool: A tool that reflects only a single value. It can be used to calibrate and adjust other measuring instruments or as a standard quantity for direct comparison with the measured object, such as measuring blocks, angle measuring blocks, etc.
Multi-value measuring tool: A tool that can reflect a set of similar values. It can also calibrate and adjust other measuring instruments or compare directly with the measured quantity as a standard, such as a line ruler.
Specialized measuring tools: Tools specifically designed to test a specific parameter. Common ones include smooth limit gauges for inspecting smooth cylindrical holes or shafts, thread gauges for determining the qualification of internal or external threads, inspection templates for determining the qualification of complex-shaped surface contours, functional gauges for testing assembly accuracy using simulated assembly passability, and so on.
General measuring tools: In China, measuring instruments with relatively simple structures are commonly referred to as universal measuring tools, such as vernier calipers, external micrometers, dial indicators, etc.
2、 Technical performance indicators of measuring instruments
Nominal value
The nominal value is annotated on a measuring tool to indicate its characteristics or guide its usage. It includes dimensions marked on the measuring block, ruler, angles marked on the angle measuring block, and so on.
Division value
The division value is the difference between the values represented by two adjacent lines (minimum unit value) on the ruler of a measuring instrument. For instance, if the difference between the values represented by two adjacent engraved lines on the differential cylinder of an external micrometer is 0.01mm, then the division value of the measuring instrument is 0.01mm. The division value represents the minimum unit value that a measuring instrument can directly read, reflecting its accuracy and measurement accuracy.
Measurement range
The measurement range is the range from the lower limit to the upper limit of the measured value that the measuring instrument can measure within the allowable uncertainty. For example, the measurement range of an external micrometer is 0-25mm, 25-50mm, etc., while the measurement range of a mechanical comparator is 0-180mm.
Measuring force
Measuring force refers to the contact pressure between the measuring instrument probe and the measured surface during contact measurement. Excessive measurement force can cause elastic deformation, while insufficient measurement force can affect the stability of contact.
Indication error
The indication error is the difference between the reading of the measuring instrument and the true value being measured. It reflects various errors in the measuring instrument itself. The indication error varies at different operating points within the instrument’s indication range. Generally, measuring blocks or other standards with appropriate accuracy can be used to verify the indication error of measuring instruments.
3、 Selection of measuring tools
Before taking any measurements, it’s important to choose the right measuring tool based on specific characteristics of the part being tested, such as length, width, height, depth, outer diameter, and section difference. You can use calipers, height gauges, micrometers, and depth gauges for various measurements. A micrometer or caliper can be used to measure the diameter of a shaft. Plug gauges, block gauges, and feeler gauges are suitable for measuring holes and grooves. Use a square ruler to measure the right angles of parts, an R gauge for measuring R-value, and consider the third dimension and aniline measurements when high precision or small fit tolerance is needed or when calculating geometric tolerance. Finally, a hardness tester can be used to measure the hardness of steel.
1. Application of Calipers
Calipers are versatile tools that can measure the inner and outer diameter, length, width, thickness, step difference, height, and depth of objects. They are widely used in various processing sites due to their convenience and accuracy. Digital calipers, with a resolution of 0.01mm, are specifically designed for measuring dimensions with small tolerances, providing high accuracy.
Table card: Resolution of 0.02mm, used for conventional size measurement.
Vernier caliper: resolution of 0.02mm, used for rough machining measurement.
Before using the caliper, clean white paper should be used to remove dust and dirt by using the outer measuring surface of the caliper to hold the white paper and then naturally pulling it out, repeating 2-3 times.
When using a caliper for measurement, ensure that the measuring surface of the caliper is parallel or perpendicular to the measuring surface of the object being measured as much as possible.
When using depth measurement, if the object being measured has an R angle, it is necessary to avoid the R angle but stay close to it. The depth gauge should be kept perpendicular to the height being measured as much as possible.
When measuring a cylinder with a caliper, rotate and measure in sections to obtain the maximum value.
Due to the high frequency of calipers being used, maintenance work needs to be done to the best of its ability. After daily use, they should be wiped clean and placed in a box. Before use, a measuring block should be used to check the accuracy of the caliper.
2. Application of Micrometer
Before using the micrometer, clean the contact and screw surfaces with a clean white paper. Use the micrometer to measure the contact surface and screw surface by clamping the white paper and then pulling it out naturally 2-3 times. Then, twist the knob to ensure quick contact between the surfaces. When they are in full contact, use fine adjustment. After both sides are in full contact, adjust the zero point and then proceed with the measurement. When measuring hardware with a micrometer, adjust the knob and use the fine adjustment to ensure the workpiece is quickly touched. When you hear three clicking sounds, stop and read the data from the display screen or scale. For plastic products, gently touch the contact surface and screw with the product. When measuring the diameter of a shaft with a micrometer, measure in at least two directions and record the maximum value in sections. Ensure both contact surfaces of the micrometer are clean at all times to minimize measurement errors.
3. Application of height ruler
The height gauge is primarily used for measuring height, depth, flatness, perpendicularity, concentricity, coaxiality, surface roughness, gear tooth runout, and depth. When using the height gauge, the first step is to check if the measuring head and various connecting parts are loose.
4. Application of feeler gauges
A feeler gauge is suitable for measuring flatness, curvature, and straightness
Flatness measurement:
Place the parts on the platform and measure the gap between the parts and the platform with a feeler gauge (note: the feeler gauge should be tightly pressed against the platform without any gap during measurement)
Straightness measurement:
Rotate the part on the platform once and measure the gap between the part and the platform with a feeler gauge.
Bending measurement:
Place the parts on the platform and select the corresponding feeler gauge to measure the gap between the two sides or middle of the parts and the platform
Verticality measurement:
Place one side of the measured zero’s right angle on the platform, and place the other side tightly against the right angle ruler. Use a feeler gauge to measure the maximum gap between the component and the right angle ruler.
5. Application of plug gauge (needle):
Suitable for measuring the inner diameter, groove width, and clearance of holes.
When the diameter of the hole in the part is large and there is no appropriate needle gauge available, two plug gauges can be used together to measure in a 360-degree direction. To keep the plug gauges in place and make measuring easier, they can be secured on a magnetic V-shaped block.
Aperture measurement
Inner hole measurement: When measuring the aperture, penetration is considered qualified, as shown in the following figure.
Attention: When measuring with a plug gauge, it should be inserted vertically and not diagonally.
6. Precision measuring instrument: anime
Anime is a non-contact measuring instrument that offers high performance and precision. The sensing element of the measuring instrument does not directly contact the surface of the measured medical parts, so there is no mechanical force acting on the measurement.
Anime transmits the captured image to the computer’s data acquisition card through projection via a data line, and then the software displays the images on the computer. It can measure various geometric elements (points, lines, circles, arcs, ellipses, rectangles), distances, angles, intersection points, and positional tolerances (roundness, straightness, parallelism, perpendicularity, inclination, positional accuracy, concentricity, symmetry) on parts, and can also perform 2D contour drawing and CAD output. This instrument not only allows the contour of the workpiece to be observed but can also measure the surface shape of opaque workpieces.
Conventional geometric element measurement: The inner circle in the part shown in the figure is a sharp angle and can only be measured by projection.
Observation of electrode machining surface: the anime lens has the magnification function to inspect the roughness after electrode machining (magnify the image by 100 times).
Small size deep groove measurement
Gate detection: During mold processing, there are often some gates hidden in the slot, and various detection instruments are not allowed to measure them. To get the gate size, we can use rubber mud to stick on the rubber gate. Then, the shape of the rubber gate will be printed on the clay. After that, the size of the clay stamp can be measured using the caliper method.
Note: Since there is no mechanical force during anime measurement, anime measurement shall be used as far as possible for thinner and softer products.
7. Precision measuring instruments: three-dimensional
The characteristics of 3D measurement include high precision (up to µm level) and universality. It can be used to measure geometric elements such as cylinders and cones, geometric tolerances like cylindricity, flatness, line profile, surface profile, and coaxial, and complex surfaces. As long as the three-dimensional probe can reach the place, it can measure geometric dimensions, mutual position, and surface profile. Additionally, computers can be used to process the data. With its high precision, flexibility, and digital capabilities, 3D measurement has become an important tool for modern mold processing, manufacturing, and quality assurance.
Some molds are being modified and currently do not have 3D drawings available. In such cases, the coordinate values of different elements and the irregular surface contours can be measured. These measurements can then be exported using drawing software to create 3D graphics based on the measured elements. This process enables quick and precise processing and modification. After setting the coordinates, any point can be utilized to measure the coordinate values.
When working with processed parts, it can be challenging to confirm consistency with the design or detect abnormal fit during assembly, especially when dealing with irregular surface contours. In such cases, it is not possible to measure geometric elements directly. However, a 3D model can be imported to compare the measurements with the parts, helping to identify machining errors. The measured values represent deviations between actual and theoretical values, and can be easily corrected and improved. (The figure below shows the deviation data between the measured and theoretical values).
8. Application of hardness tester
The commonly used hardness testers are the Rockwell hardness tester (desktop) and the Leeb hardness tester (portable). The commonly used hardness units are Rockwell HRC, Brinell HB, and Vickers HV.
Rockwell hardness tester HR (desktop hardness tester)
The Rockwell hardness test method uses either a diamond cone with a top angle of 120 degrees or a steel ball with a diameter of 1.59/3.18mm. This is pressed into the surface of the tested material under a certain load, and the material’s hardness is determined by the indentation depth. The different hardness of the material can be divided into three different scales: HRA, HRB, and HRC.
HRA measures hardness using a 60kg load and a diamond cone indenter, and is used for materials with extremely high hardness, such as hard alloy.
HRB measures hardness using a 100kg load and a 1.58mm diameter quenched steel ball, and is used for materials with lower hardness, such as annealed steel, cast iron, and alloy copper.
HRC measures hardness using a 150kg load and a diamond cone indenter, and is used for materials with high hardness, such as quenched steel, tempered steel, quenched and tempered steel, and some stainless steel.
Vickers hardness HV (mainly for surface hardness measurement)
For microscopic analysis, utilize a diamond square cone indenter with a maximum load of 120 kg and a top angle of 136° to press into the material’s surface and measure the diagonal length of the indentation. This method is appropriate for assessing the hardness of larger workpieces and deeper surface layers.
Leeb hardness HL (portable hardness tester)
Leeb hardness is a method for testing hardness. The Leeb hardness value is calculated as the ratio of the rebound velocity of the impact body of the hardness sensor to the impact velocity at a distance of 1mm from the surface of the workpiece during the impact cnc manufacturing process, multiplied by 1000.
Advantages: The Leeb hardness tester, based on the Leeb hardness theory, has revolutionized traditional hardness testing methods. The small size of the hardness sensor, similar to that of a pen, allows for handheld hardness testing on workpieces in various directions at the production site, a capability that other desktop hardness testers struggle to match.
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