Measuring instruments commonly used in the workshop

1. Classification of measuring instruments

A measuring instrument is an instrument that has a fixed form and is used to reproduce or provide one or more known quantities. Different measuring tools can be divided into the following categories according to their use:
1. Single value measuring tool

A gage that can only reflect a single value. It can be used to calibrate and adjust other measuring instruments or directly compare it with the measured value as a standard quantity, such as gauge blocks, angle gauge blocks, etc.

2. Multi-value measuring tool

A gage that can represent a group of homogeneous values. Other measuring instruments can also be calibrated and adjusted or compared directly with the measurand as a standard quantity, such as a line ruler.

3. Special measuring tool

A gage designed to test a specific parameter. Common ones are: the smooth limit gauge for checking smooth cylindrical holes or shafts, the thread gauge for judging the qualification of internal or external threads, the test template for judging the qualification of surface contours of complex shapes, and the function of simulating assembly passability to test assembly accuracy gauges, etc.

4. Universal measuring tool

In our country, measuring instruments with relatively simple structure are called universal measuring tools. Such as vernier calipers, outer micrometers, dial indicators, etc.

2. Technical performance indicators of measuring instruments

1. The nominal value of the measuring tool

The quantity marked on the measuring tool to indicate its characteristics or to guide its use. For example, the size marked on the gauge block, the size marked on the ruler, the angle marked on the angle gauge block, etc.

2. Graduation value

On the ruler of a measuring instrument, the difference between the magnitudes represented by two adjacent scale lines (minimum unit magnitude). If the difference between the values ​​represented by the adjacent two scale lines on the micrometer cylinder of an outer micrometer is 0.01mm, the graduation value of the measuring instrument is 0.01mm. The division value is the smallest unit value that can be directly read by a measuring instrument. It reflects the reading accuracy and also shows the measuring accuracy of the measuring instrument.

3. Measuring range

Within the allowable uncertainty, the range from the lower limit to the upper limit of the measured value that can be measured by the measuring instrument. For example, the measurement range of an outer micrometer is 0 to 25 mm, 25 to 50 mm, etc., and the measurement range of a mechanical comparator is 0 to 180 mm.

4. Measuring force

In the process of contact measurement, the contact pressure between the probe of the measuring instrument and the surface to be measured is measured. Too much measurement force will cause elastic deformation, too little measurement force will affect the stability of the contact.

5. Indication error

The difference between the indicated value of a measuring instrument and the true value being measured. Indication error is a comprehensive reflection of various errors of the measuring instrument itself. Therefore, the indication error is different for different working points within the indication range of the instrument. Generally, a gauge block or other measurement standard of appropriate precision can be used to verify the indication error of the measuring instrument.

3. Selection of measuring tools

Before each measurement, it is necessary to select the measurement tool according to the special characteristics of the part to be measured. For example, calipers, height gauges, micrometers, and depth gauges can be used for length, width, height, depth, outer diameter, and level difference; micrometers can be used for shaft diameters. , calipers; plug gauges, block gauges and feeler gauges can be used for holes and grooves; right angle rulers are used to measure the right angle of parts; R gauges are used to measure R value; Use three-dimensional and two-dimensional; use hardness tester to measure the hardness of steel.

1. Application of calipers

Calipers can measure the inner diameter, outer diameter, length, width, thickness, level difference, height, and depth of objects; calipers are the most commonly used and most convenient measuring tools, and are the most frequently used measuring tools on the processing site.

Digital caliper: resolution 0.01mm, used for dimensional measurement with small tolerance (high precision).
Table card: resolution 0.02mm, used for regular size measurement.
Vernier caliper: resolution 0.02mm, used for roughing measurement.
Before using the caliper, remove the dust and dirt with clean white paper (use the outer measuring surface of the caliper to jam the white paper and then pull it out naturally, repeat 2-3 times)

When using a caliper to measure, the measuring surface of the caliper should be as parallel or perpendicular to the measuring surface of the object to be measured as possible;

When using depth measurement, if the measured object has an R angle, it is necessary to avoid the R angle but close to the R angle, and the depth gauge and the measured height should be kept as vertical as possible;

When the caliper measures the cylinder, it needs to be rotated and the maximum value is obtained for the segmental measurement;

Due to the high frequency of use of the caliper, the maintenance work needs to be done the best. After every day of use, it needs to be wiped clean and put into the box. Before use, a measuring block is required to check the accuracy of the caliper.

2. Application of Micrometer
Before using the micrometer, you need to use clean white paper to remove dust and dirt (use the micrometer to measure the contact surface and the screw surface to jam the white paper and then pull it out naturally, repeat 2-3 times), more CNC programming knowledge Follow WeChat Public account (NC programming teaching), then twist the knob to measure when the contact surface is in quick contact with the screw surface, switch to fine-tuning.

When the micrometer measures the hardware, mobilize the knob. When it comes into contact with the workpiece, use the fine-tuning knob to screw in, and stop when you hear three clicks, clicks, and clicks, and read the data from the display screen or scale.

When measuring plastic products, the measuring contact surface and the screw lightly touch the product.

When measuring the diameter of a shaft with a micrometer, measure at least two or more directions and take the maximum value of the micrometer measured in sections. The two contact surfaces should be kept clean at all times to reduce measurement errors.

3. Application of height gauge

The height gauge is mainly used to measure height, depth, flatness, verticality, concentricity, coaxiality, surface vibration, tooth vibration, depth, and height gauge. When measuring, first check whether the probe and each connection part are loose.

4. Application of feeler gauge

The feeler gauge is suitable for the measurement of flatness, curvature and straightness
Flatness measurement:

Place the part on the platform and measure the gap between the part and the platform with a feeler gauge (Note: The feeler gauge and the platform are kept pressed without gaps during measurement)
Straightness measurement:

Place the part on the platform and make one rotation, and use a feeler gauge to measure the gap between the part and the platform.
Curvature measurement:

Place the part on the platform, select the appropriate feeler gauge to measure the gap between the two sides or the middle of the part and the platform.

Squareness measurement:

Place one side of the right angle of the zero to be measured on the platform, make the other side close to the square, and use a feeler gauge to measure the largest gap between the part and the square.
5. Application of plug gauge (pin):

It is suitable for measuring the inner diameter, groove width and clearance of holes.
If the hole diameter of the part is large, and there is no suitable needle gauge, the two plug gauges can be overlapped, and the plug gauge can be fixed on the magnetic V-shaped block according to the 360-degree direction, which can prevent loosening and is easy to measure.
Aperture measurement

Inner hole measurement: When the hole diameter is measured, the penetration is qualified
Note: When measuring the plug gauge, it needs to be inserted vertically, not obliquely.
6. Precision measuring instrument: two-dimensional

The second element is a high-performance, high-precision non-contact measuring instrument. The sensing element of the measuring instrument is not in direct contact with the surface of the measured part, so there is no mechanical action of the measuring force; the second element transmits the captured image through the data line to the data acquisition card of the computer by means of projection, and then It is imaged on the computer monitor by the software; various geometric elements (points, lines, circles, arcs, ellipses, rectangles), distances, angles, intersections, geometric tolerances (roundness, straightness, parallelism, verticality) on the parts can be performed (degree, inclination, position, concentricity, symmetry) measurement, and can also perform CAD output for 2D drawing of outlines. Not only can the contour of the workpiece be observed, but also the surface shape of the opaque workpiece can be measured.

Conventional geometric element measurement: The inner circle in the part in the figure below is a sharp angle, which can only be measured by projection.
Observation of electrode processing surface: The lens of the second element has the function of magnification. Roughness inspection after electrode processing (magnification of 100 times the image).
Small size deep groove measurement
Gate detection: During mold processing, there are often some gates hidden in the groove, and various testing instruments cannot measure them. At this time, rubber paste can be attached to the glue gate, and the shape of the glue gate will be printed on the glue. , and then use the second element to measure the size of the glue print to obtain the gate size.
Note: Since there is no mechanical force during the two-dimensional measurement, the two-dimensional measurement should be used as far as possible for thinner and softer products.

7. Precision measuring instrument: three-dimensional

The three-dimensional is characterized by high precision (up to μm level); versatility (can replace a variety of length measuring instruments); can be used to measure geometric elements (in addition to the elements that can be measured by two-dimensional elements, more CNC programming knowledge pay attention to WeChat The official account (NC programming teaching) can also measure cylinders and cones), shape and position tolerances (in addition to measuring the shape and position tolerances that can be measured by two-dimensional elements, it also includes cylindricity, flatness, line profile, surface profile, Coaxiality), complex profile, as long as the three-dimensional probe can touch, its geometric size, mutual position, and surface profile can be measured; and data processing is completed with the help of a computer; with its high precision, high flexibility, and Excellent digital capabilities have become an important and effective tool for modern mold manufacturing and quality assurance.

Some molds are being modified, and there is no 3D drawing file. The coordinate value of each element and the outline of the irregular surface can be measured, and then exported by drawing software and made into 3D drawing according to the measured elements, which can be processed and modified quickly and without error. (After the coordinates are set, you can take any point to measure the coordinates).
3D digital model import comparison measurement: In order to confirm the consistency with the design of the finished parts or find the fit abnormality in the process of assembling the fit mold, when some surface contours are neither arcs nor parabolas, more CNC programming knowledge Pay attention to WeChat public (NC programming teaching), but some irregular surfaces, when geometric elements cannot be measured, you can import the 3D model and measure the parts for comparison, so as to understand the processing error; because the measured value is a point-to-point deviation value, it is convenient to quickly carry out And the effective correction improves (the data shown in the figure below is the deviation between the measured value and the theoretical value).

8. Application of hardness tester

The commonly used hardness testers are Rockwell hardness tester (desktop) and Leeb hardness tester (portable). The commonly used hardness units are Rockwell HRC, Brinell HB, Vickers HV.

Rockwell hardness tester HR (benchtop hardness tester)

The Rockwell hardness test method is to use a diamond cone with an apex angle of 120 degrees or a steel ball with a diameter of 1.59/3.18mm, press it into the surface of the tested material under a certain load, and obtain the hardness of the material from the depth of the indentation. According to the hardness of the material, it can be divided into three different scales to represent HRA, HRB, HRC.

HRA is the hardness obtained with a 60Kg load and a diamond cone indenter for extremely hard materials. For example: carbide.

HRB is the hardness obtained by using a 100Kg load and a hardened steel ball with a diameter of 1.58mm, and is used for materials with lower hardness. For example: annealed steel, cast iron, etc., alloy copper.

HRC is the hardness obtained with a 150Kg load and a diamond cone indenter for very hard materials. For example: hardened steel, tempered steel, quenched and tempered steel and some stainless steels.

Vickers hardness HV (mainly for surface hardness measurement)

Suitable for microscopy analysis. With a load within 120kg and a diamond square cone indenter with an apex angle of 136°, press into the surface of the material, and measure the diagonal length of the indentation. It is suitable for hardness determination of larger workpieces and deeper surface layers.

Leeb Hardness HL (Portable Hardness Tester)

Leeb hardness is a dynamic hardness test method. During the impact process of the impact body of the hardness sensor with the measured workpiece, the ratio of the rebound speed to the impact speed when it is 1mm away from the workpiece surface is multiplied by 1000, which is defined as the Leeb hardness value.

Advantages: The Leeb hardness tester manufactured by Leeb Hardness Theory changes the traditional hardness testing method. Since the hardness sensor is as small as a pen, it can directly test the hardness of the workpiece in various directions on the production site by holding the sensor, so it is difficult for other desktop hardness testers.