Parts quality control

Ensuring high quality products is one of the main directions of production development. According to the ISO 8402 standard of the International Organization for Standardization (ISO), quality is a set of characteristics of an object related to its ability to satisfy stated or implied needs.

Needs are divided into functional and latent. The latter reflect the individuality of the object, the emotional opinion about it, prestige. Machine parts must satisfy, mainly, functional needs, that is, fulfill their official purpose. Functional needs are satisfied by two generalized characteristics of the part – accuracy and reliability.

Accuracy the degree of approximation of the true value of the geometric parameter of the part to its nominal value. It determines the compliance of the part with the laws of motion provided for by its official purpose (for example, a lead screw, a gear wheel) or compliance with technological conditions.

Reliability is a property of availability and the properties of non-failure operation and maintainability, and maintenance support that affect it. Reliability is a complex property, which, depending on the purpose of the object and the conditions of its use, may include non-failure operation, durability, maintainability and maintainability, or certain combinations of these properties.

Availability – the ability of an object to perform a required function under given conditions, assuming that the necessary external resources are provided. the property of a part to maintain a working state over time.

Reliability – the ability of an object to perform the required function in a given time interval under given conditions to continuously maintain a healthy state for some time or operating time.

Durability – the property of an object to perform a required function until reaching the limit state under given conditions of use and maintenance.

Maintainability is the property of an object under given conditions of use and maintenance to maintain and restore the state, which consists in adaptability to maintenance and repair.

Persistence – the property of an object to keep within the specified limits the values of parameters that characterize the ability of the object to perform the required functions, during and after storage and (or) transportation.

The reliability of a particular part in the conditions of its official purpose is determined by the basic properties of the material from which it is made, the properties of its surface layer and the accuracy of the machined surfaces intended for connection.

Rice. 7.6. Dependence of surface wear Δh on Ra

For parts operating under wear conditions, the most important characteristics of the surface condition are the arithmetic mean ratio of the profile Ra (Fig. 7.6) and the average pitch of the profile irregularities. For parts operating under conditions of alternating loads, the greatest profile height is the most important. The dependence of fatigue strength σ -1 on R max is shown in fig. 7.7.

The roughness of the machined surface significantly affects the performance properties of the machined parts and, above all, the wear of parts that work on abrasion. The lower the roughness, the larger the actual contact area of the friction surfaces and the lower the friction pressure under given loading conditions. This leads to less wear during the running-in period and to its lower intensity V during the period of normal wear.

Rice. 7.7. Dependence of fatigue strength σ-1 on the maximum profile height Rmax

The wear rate in each period depends not only on Rz (Fig. 7.8), but also on the location of the roughness marks on the mating friction surfaces relative to their motion vector. This dependence is manifested to a greater extent in the case of boundary friction.

Rice. 7.8. Graph of wear intensity V versus wear distance L of friction for various values of Rz

When choosing a method and method of surface treatment, it is necessary to take into account not only the achievement of the required Ra or Rz values, but also the specified location of the scratches.

The roughness of the machined surfaces of the parts also affects such operational properties as fatigue resistance, contact stiffness, fit accuracy, tightness of joints.

The roughness that provides the specified operational properties of the part can be obtained by processing at cutting conditions assigned taking into account experimental dependencies. For example, feed S in mm/min for finishing with through cutters is selected depending on the diameter of the part, the radius at the top of the cutting edge, the main and auxiliary angles in the lead, and the depth of cut.

However, it should be noted that it is very difficult to establish a functional relationship between the operational properties of the part and, moreover, its reliability indicators, on the one hand, and, on the other hand, the quality parameters of the part, in particular Ra, is very difficult. Therefore, quality parameters are assigned either on the basis of operating experience of similar parts, or on the basis of an experiment with prototypes of a given product. Part quality parameters are established as a result of reliability tests of experimental parts manufactured with different quality parameters. Excessively high quality parameters should not be assigned, since their achievement increases not only the technological cost, but also the cost of control operations.

The use of modern control methods (measurements) often affects not only the cost components, but also the increase in strength, reliability and other technical characteristics of products. Measurements and tests by themselves do not improve the characteristics of controlled objects. But based on the results of the control, it is possible to evaluate the actual quality and reliability of the part. You can also perform work to improve the quality of parts, for example, eliminate unacceptable technological deviations and defects. There should be feedback from control to production.

The main means of measuring roughness are profilers-profilometers. The measurement of surface roughness is carried out by touching the surface under study with a diamond needle and is determined by recording the profilogram of the profile of irregularities in a rectangular system by an electrical method. A visual reading is also possible on the scale of the indicating device, graduated by the parameter Ra in min – the arithmetic mean deviation of microroughnesses from the middle line of the profile. Measurements are possible in a wide range of microroughness heights.

When feeling the irregularities of the surface under study, the oscillations of the diamond needle are converted into changes in electrical voltage proportional to these oscillations. The inductive sensor shown in fig. serves as a converter of mechanical vibrations into an electrical signal. 7.9.

Rice. 7.9. Profiler Inductive Sensor

The magnetic system of the sensor consists of a double U – shaped core – 4 with two coils – 3 and an armature – 2, swinging on a support – 10. The armature is connected to a diamond needle through a rocker arm – 1. The sensor coils and the primary winding of the transformer – 6 form a measuring bridge. When the sensor moves, the needle, feeling the irregularities, causes the armature – 2 to oscillate, which changes the air gap between the armature and the core – 4. The magnetic flux and current in the bridge arms change. At the output of the transformer – 6, there is a voltage proportional to the magnitude of the microroughness. The voltage is amplified in block – 7 and is fed to the recorder 9 and the recording device 8.

To assess the roughness, in addition to profilers, optical instruments are used that operate on the principle of the light section of the treated surface. For a rough estimate of the cleanliness of processing, reference samples are used for comparison.

In recent years, measuring heads (MG) – touch sensors – have been widely used to measure the dimensions of precise parts. They can measure the size of the part on the machine after a technological transition or operation, and then the IG is installed on the machine, for example, in a tool turret. The part can be measured outside the machine, and then the IG becomes part of the coordinate measuring machine (CMM). But in both cases, the IG works in conjunction with the CNC system, since it is a touch sensor.

The IG scheme and the principle of its operation are as follows. The scheme of the IG sensor and its implementation are shown in fig. 7.10. The sensor consists of the following main units. In the housing 1, fixed on the movable part of the machine, there is a unit 2 for mounting (suspension) of the measuring tip 3. The tip can have rods 4 connected to each other with contact elements 5. The safety device 6 prevents breakage of the measuring head in case of a large displacement of the tip. In addition, the head has a unit for creating a measuring force and orientation 7, sensitive elements 8 and an electronic power supply and signal processing circuit 9.

When the tip 3 deviates in any of the directions indicated by the arrows, the support disk 2 rotates about an axis perpendicular to the axis of the head, or is displaced along its axis. Together with the disk, the armature of the inductive sensor 8 associated with it is displaced. The change in the electrical signal caused by this is used to determine the moment the tip touches the head of the controlled part. To receive a touch signal, it is sufficient to move the tip by up to 1 µm with a force of up to 0.2 N.

Rice. 7.10. Schematic diagram of the touch sensor

Measurement of the dimensions of parts or shapes of various surfaces outside the processing machine is carried out on coordinate measuring machines (CMMs).

A diagram explaining the principle of operation of the CMM is shown in fig. 7.11, where: 1 – given nominal contour of the part; 2 – actually processed surface; 5 – trajectory described by the body 4 of the MG, which takes into account the radius of the contact tip of the measuring head 3 and interference 8. Deviations of the real contour from the nominal are perceived directly by the measuring head 4 and transmitted to the registration device.

Rice. 7.11. Diagram explaining the principle of operation of the CMM

Automatic CMMs have been developed that implement the differential method by comparing with a measure and controlled by a surface tracking system. In this case, a reference part is used as a “copier”, the surface of which is tracked by the CMM executive body, which carries two heads, one of which is in contact with the surface of the reference part, and the other with the surface of the measured part. The IG in contact with the surface of the sample part is included in the CMM tracking system, which seeks to nullify the signal taken from this head. Then the signal taken from the head interacting with the measured part characterizes the deviations of the real surface from the nominal one.

Rice. 7.12. The principle of operation of the CMM using the example of measurements in the XZ plane

On fig. 7.12 shows the principle of CMM operation on the example of measurements in the XZ plane, where: 1 – printing device; 2 – the program for controlling the movements of the CMM executive bodies and the program for processing the measurement results, which are entered into the CNC control system from the carrier; 3 – control computer that generates control signals for the machine drives and processes the measurement results; 4 – block of digital indication; 5 – quill with a fixed zero head; 6 – zero head; 7 – displacement sensors along X and Z coordinates; 8 – table carrying the measured part; 9 – drives in X and Z coordinates.

The use of the deviation MG in automatic CMMs that implement the zero method of coordinate measurements has made it possible to significantly improve the accuracy of coordinate measurements. An example of such a machine is a CMM manufactured by Opton (Opton, Germany), mod. UMM 500, in which coordinate measurements are carried out by tracking the real surface of the measured part. The UMM 500 is equipped with a 3D component measuring head.

Rice. 7.13. Functional diagram (one coordinate) CMM mod. UMM 500: 1 – table; 2 – measured part; 3 – measuring tip; 4 – IG; 5 – carriage; 6 – engine; 7 – relay switching the operation of the CMM to the tracking mode at x w = 0; 9 – program, which indicates the specified position of the carriage 5; 10 – displacement sensor; 11 – Computer and devices for recording measurement results

On fig. 7.13 shows a diagram explaining the operation of the machine mod. UMM 500 for clarity in one x coordinate. When measuring the surface at given points, the CMM works as follows. In program 9, the position of the carriage 5 is set along the x coordinate, which is compared with the actual position x and a mismatch signal x w = 0 is generated. Relay 8 switches the CMM operation to the “tracking” mode. In this mode, a signal is applied to the engine 6, which is taken from the MG 4. When the condition x ω = 0 is met, which corresponds to the measuring tip 3 touching the measured part 2, a signal is issued to read the information from the sensor 10 and transfer it to the computer to compare the real coordinates with nominal and registration of measurement results. Further, with the help of the CNC system, a transition is made to the next measuring point, information about which is located in the control program.

Optical systems such as interferometers are used to measure particularly precise dimensions. For less accurate measurements, known universal means are used. The quality of control (measurements) affects the quality of parts for their official purpose. In all cases, it depends on the method used, the measuring instruments used, the location of the part and the skill of the operator.

Accuracy is one of the generalized characteristics of the quality of a product and it is influenced by a large number of factors, and these factors also depend on many others, therefore, to manufacture any product is absolutely accurate, i.e. in full accordance with its geometric prototype is impossible, therefore, the measure of the accuracy of the product is taken as the amount of deviation from the theoretical values. These measurements are compared with the deviations allowed by the service designation (TS) of the product.

Thus, the measures of accuracy are: on the one hand, the established permissible deviations, on the other hand, the measured ones, that is, with a known degree of approximation of the actual parameter to the real one.

1. The first indicator of the accuracy of a product is the accuracy of the distance between any of its two surfaces or the accuracy of size .

2. The accuracy of rotation of one surface relative to another serves as the second indicator of the accuracy of the product. Since the product is a three-dimensional body, the accuracy of rotation of one surface relative to another is usually considered in two mutually perpendicular coordinate planes. Turning accuracy refers to the amount of deviation from the required angular position of one surface or part relative to the other in each of the two coordinate planes.

3. The accuracy of the geometric shapes of the product (part) or the correctness of the geometric shapes is the third indicator of the accuracy of the product.

There are three types of deviations of geometric shapes:

a) macrogeometric deviations, which are understood as the deviation of the real surface from the theoretical one within the overall dimensions of the product or surface. For example, the deviation of a flat surface from a planar or cylindrical surface from a geometric cylinder;

b) waviness – represents periodically repeating surface irregularities in sections with a length of 1 to 10 mm;

c) microgeometric deviations (roughness), which are understood as deviations of the real surface from the theoretical one within small areas (about 1 mm). The roughness height is regulated by GOST 2784-79. By setting this or that roughness, thereby assigning a tolerance for micro-deviations from the correct geometric shape.

The assignment of the deviation of the geometric shape of the surfaces of the part to one category or another is conditional. Therefore, it is generally accepted in technology that shape deviations are: a) macrogeometric when the ratio of the length L of the parameter estimate to the height of deviations H is more than 1000; b) waviness at L/H = 50-1000; c) roughness at L/H < 50 mm.

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