Investigation of the accuracy of CNC machines. Lathes High Precision CNC Machining High Precision

When it comes to machine tools or other numerically controlled systems, concepts such as positioning accuracy, positioning resolution, positioning repeatability and part repeatability cannot be avoided. These concepts are very closely related, and confusion often arises among aspiring machine tool builders and CNC operators. Academic definitions and methods for calculating these parameters are indicated in the corresponding GOST, and this article will explain their basic differences for non-specialists. Let's start with the simplest characteristic.

Positioning Resolution

Positioning resolution (discreteness) is a value that shows how accurately you can specify a movement in your CNC system.

Let's look at an example. Let's say a stepper motor with a step of 1.8 degrees (200 steps / rev) is installed on the Y-axis of a machine under Mach3 control and a driver with a step division of 1/16, which is connected to a 1605 ball screw with a step of 5 mm per revolution. Mach3 operates in STEP / DIR mode - it sends discrete pulses to the controller, which are then interpreted into motor steps. One STEP impulse will cause the movement of the motor shaft, which will correspond to the movement of the ideal axis, without backlash and errors, by 1 / (200 * 16) * 5 = 0.0015625 mm. This is the resolution of the Y-axis positioning - the position along the axis in the control program will always be a multiple of this value, and you cannot specify a movement to a point with the Y coordinate = 2.101 - the control program will "round off" this value, depending on the settings, either to 2.1 or to 2.1015625 ...
Naturally, all this does not mean at all that by sending one STEP impulse, in fact, we will get a displacement of 0.0015625 mm, because there are many factors that contribute to the error - from the error in the positioning of the motor shaft to the backlash in the running nut. Here it is appropriate to move on to the following characteristic:

CNC axis positioning repeatability

If we send the axis to the same point from different positions, then each time we will receive a little different result due to mechanical errors - the axis will stop at some distance from the required point. The repeatability shows how wide the spread of this distance is, or more precisely, the repeatability is directly proportional to the standard deviation of the positioning error. In a word, repeatability - characterizes the magnitude of the "spread" positioning errors relative to some average value. The repeatability depends mainly on the transmission backlash and the resulting elastic deformations, and in fact is quite uninformative, since says only whether the positioning error is stable or not, but does not tell anything about its magnitude. It is possible to build a completely inaccurate machine with excellent repeatability.

CNC axis positioning accuracy

Axis positioning accuracy is a generalized value that shows the limits of the real coordinate of the axis after positioning is completed. When they say "machine accuracy", it is usually the positioning accuracy that is meant. Accuracy depends on repeatability, but includes not only the magnitude of the "spread" of the positioning error, but also its average value, i.e. is a more versatile characteristic. Accuracy indicates how large the positioning error of an axis can be. Precision is the main characteristic of the machine. Often, mid-range and hobbyist machine tool makers simply indicate some "machine accuracy" without specifying a "coverage factor" - that is. proportionality coefficient, because the accuracy of, say, 0.05 mm, measured for 3σ and for 1σ is a big difference: in the first version, positioning with an error of no more than 0.05 mm will occur in 97% of cases, and in the second only in 32%. (if you are interested , where the interest is taken from, you are here).

Accuracy is the main characteristic of the machine with t. Sp. positioning of the working tool, and depends on a large number of factors, including the main ones - backlash of guides and gears, misalignment of the guide axes and their non-perpendicularity. Anyone who has ever tried to cut a large rectangle from plywood or other sheet material knows how an error in fractions of a degree when marking right angles can lead to a mismatch in the lengths of the sides by several millimeters, and sometimes even centimeters, so special attention is paid when assembling the machine with CNC. The rigidity and workmanship of the bed and gantry also have a direct impact on the precision of the machine.

Repeatability and accuracy of manufactured parts

The most important parameters. The calculation methods and their essence are similar to the positioning characteristics of the same name, however, it is not the position of the axis that is measured, but the dimensions of the finished parts. It is these parameters that show how suitable the machine is for work and what quality parts can be made on it. However, they depend on even more factors - beating at the end of the spindle cutter, the perpendicularity of the spindle installation, and the actual materials being processed and cutting modes. Therefore, usually manufacturers often indicate the accuracy of manufacturing a part - purely theoretical, "calculated", sometimes - having nothing to do with reality. For middle-class machines, the manufacturing accuracy of 0.2 mm for 3σ can be considered satisfactory, 0.1 mm - good, 0.05 mm - excellent, less than 0.05 mm - excellent, this can be observed only on a few units of economy-class machines.

General information about CNC machines. Design features CNC machines Precision and quality of processing on CNC machines. CNC machines must ensure high accuracy and speed of working out the displacements of the specified UE and also maintain this accuracy within the specified limits during long-term operation.

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Ministry of Education and Science of the Russian Federation

Federal Agency for Education

State educational institution

higher professional education

"Komsomolsk-on-Amur State Technical University"

IKP MTO

Department of TM

Individual assignment

on the topic "research of the accuracy of CNC machines"

2015

Introduction ………………………………………………………………………… ... 3

1 General information about CNC machines. ………………………… ........................... 4

2 Design features of CNC machines ……………………………… 8

3 Accuracy and quality of processing on CNC machines ………………… ... …… ..13

Conclusion ……………………………………………………………………… .17

List of sources used ………………………………………… ... 18

Introduction

CNC machines must ensure high accuracy and speed of working out the displacements specified by the UE, and also maintain this accuracy within the specified limits during long-term operation. The design of CNC machine tools should, as a rule, ensure the alignment different types processing, automation of loading and unloading of parts, automatic or remote control of tool change, the ability to integrate into a common automatic system management. High processing accuracy is determined by the precision of manufacturing and the rigidity of the machine. In the designs of CNC machines, short kinematic chains are used, which increases the static and dynamic rigidity of the machines. For all executive bodies, autonomous drives are used with the minimum possible number of mechanical transmissions. These drives must be fast.

The accuracy of CNC machines increases as a result of eliminating gaps in the transmission mechanisms of drives, reducing friction losses in guides and mechanisms, increasing vibration resistance, and reducing thermal deformations.

1 General information about CNC machines.

It is customary to understand the control of a machine tool as a set of influences on its mechanisms that ensure the execution of a technological processing cycle, and under a control system, a device or a set that implements these influences.

Numerical control (CNC) is a control in which a program is set in the form of an information array recorded on any medium. Control information for CNC systems is discrete and its processing in the control process is carried out by digital methods. Control of technological cycles is almost everywhere carried out using programmable logic controllers, implemented on the basis of the principles of digital electronic computing devices. CNC systems are practically replacing other types of control systems.

By technological purpose and functionality CNC systems are divided into four groups:

Positional, in which only the coordinates of the end points of the position of the executive bodies are set after they have completed certain elements of the working cycle;

Contour, or continuous, controlling the movement of the executive body along a given curvilinear trajectory;

Universal (combined), in which the programming of both movements during positioning and the movement of executive bodies along the path, as well as changing tools and loading and unloading workpieces is carried out;

Multi-circuit systems that provide simultaneous or sequential control of the operation of a number of units and mechanisms of the machine.

According to the method of preparing and entering the control program, the so-called operating systems CNC (in this case, the control program is prepared and edited directly on the machine, in the process of machining the first part from a batch or simulating its processing) and systems for which the control program is prepared regardless of the place where the part is machined. Moreover, the independent preparation of the control program can be carried out either with the help of computer technology, which are part of the CNC systems of this machine, or outside it (manually or with the help of a programming automation system).

Numeric systems program control(CNC) is a set of specialized devices, methods and tools necessary for the implementation of CNC machines. A CNC device (CNC) by machine tools is a part of the CNC, made as a whole with it and carrying out the issuance of control actions according to a given program.

V international practice the following designations are accepted: NC-CNC; HNC-type of CNC with program assignment by the operator from the remote control using keys, switches, etc .; SNS CNC device with memory for storing the entire control program; CNC control of a stand-alone CNC machine, the contents of a mini-computer or processor; DNS-management of a group of machines from a common computer.

For CNC machines, the directions of movement and their symbols are standardized. The ISO-R841 standard is considered to be the positive direction of movement of a machine element when the tool or workpiece moves away from one another. The reference axis (Z axis) is the axis of the work spindle. If this axis is rotary, then its position is chosen perpendicular to the plane of the attachment of the part. The positive direction of the Z-axis is from the device for fastening the workpiece to the tool.

The use of a particular type of CNC equipment depends on the complexity of the part manufacturing and the serial production. The less serial production, the more technological flexibility the machine should have.

In the manufacture of parts with complex spatial profiles in a single and fine serial production the use of CNC machines is almost the only technically justified solution. It is advisable to use this equipment if it is impossible to quickly manufacture the tooling. In serial production, it is also advisable to use CNC machines. Recently, autonomous CNC machines or systems of such machines have been widely used in the context of readjustable large-scale production.

A fundamental feature of a CNC machine is the work according to a control program (UP), on which the cycle of operation of the equipment for processing a specific part and technological modes are recorded. When changing the part being processed on the machine, you just need to change the program, which reduces by 80 ... 90% the labor intensity of the changeover compared to the labor intensity of this operation on manually controlled machines.

The main advantages of CNC machines:

The productivity of the machine is increased by 1.5 ... 2.5 times in comparison with the productivity of similar machines with manual control;

The flexibility of universal equipment is combined with the accuracy and productivity of an automatic machine;

The need for skilled machine operators is decreasing, and the preparation of production is being transferred to the sphere of engineering labor;

The terms of preparation and transition to the manufacture of new parts are reduced due to the preliminary preparation of programs, simpler and more universal technological equipment;

Reduces the cycle time of parts manufacturing and reduces the stock of work in progress. automated production primarily in mechanical engineering.

2 Design features of CNC machines

CNC machines have extended technological capabilities while maintaining high reliability of work. The design of CNC machines should, as a rule, ensure the combination of various types of processing (turning-milling, milling-grinding), ease of loading blanks, unloading parts (which is especially important when using industrial robots), automatic or remote control of tool change, etc.

An increase in processing accuracy is achieved by a high manufacturing accuracy and rigidity of the machine, which exceeds the rigidity of a conventional machine for the same purpose, for which the length of its kinematic chains is reduced: they use autonomous drives, and, if possible, reduce the number of mechanical transmissions. CNC machine drives must also provide high speed.

The elimination of gaps in the transmission mechanisms of the feed drives, the reduction of friction losses in the guides and other mechanisms, the increase in vibration resistance, the reduction of thermal deformations, the use of sensors in machine tools also contributes to an increase in accuracy. feedback... To reduce thermal deformations, it is necessary to ensure a uniform temperature regime in the mechanisms of the machine tool, which, for example, is facilitated by the preliminary heating of the machine tool and its hydraulic system. The temperature error of the machine can also be reduced by correcting the feed drive from the temperature sensor signals.

Basic parts (beds, columns, skids). Tables, for example, are box-shaped with longitudinal and transverse ribs. Basic parts are made cast or welded. There is a tendency to make such parts from polymer concrete or synthetic granite, which further increases the rigidity and vibration resistance of the machine.

Guideways of CNC machine tools have high wear resistance and low friction force, which allows you to reduce the power of the follower drive, increase the accuracy of movements, and reduce the misalignment in the follower system.

To reduce the coefficient of friction, the sliding guides of the bed and the caliper create in the form of a pair of sliding "steel (or high-quality cast iron) -plastic coating (fluoroplastic, etc.)"

Rolling guides have high durability, are characterized by low friction, and the coefficient of friction is practically independent of the speed of movement. Rollers are used as rolling elements. The preload increases the rigidity of the guides by 2 ... 3 times; regulating devices are used to create the tightness.

Drives and converters for CNC machine tools. In connection with the development of microprocessor technology, converters are used for feed and main motion drives with full microprocessor control - digital drives are electric motors operating on direct or alternating current. Structurally, frequency converters, servo drives and main start and reverse devices are separate electronic control units.

Feed drive for CNC machines. As a drive, motors are used, which are synchronous or asynchronous machines controlled by digital converters. Brushless synchronous (valve) motors for CNC machines are made with a permanent magnet based on rare earth elements and are equipped with feedback sensors and brakes. Asynchronous motors are used less often than synchronous ones. The feed motion drive is characterized by the smallest possible clearances, short acceleration and deceleration times, low friction forces, reduced heating of the drive elements, and a large control range. The provision of these characteristics is possible due to the use of ball and hydrostatic screw drives, rolling guides and hydrostatic guides, backlash-free gearboxes with short kinematic chains, etc.

The main motion drives for CNC machine tools are usually AC motors for high powers and DC motors for low powers. Three-phase four-band asynchronous motors are used as drives, which perceive large overloads and operate in the presence of metal dust, shavings, oil, etc. in the air. Therefore, they are designed with an external fan. Various sensors are built into the motor, for example a spindle position sensor, which is necessary for orientation or providing an independent coordinate.

Frequency converters for controlling asynchronous motors have a regulation range of up to 250. Converters are electronic devices based on microprocessor technology. Programming and parameterization of their work is carried out from built-in programmers with a digital or graphic display. Optimization of control is achieved automatically after entering the motor parameters. The software provides the ability to configure the drive and put it into operation.

The spindles of CNC machine tools are precise, rigid, with increased wear resistance of the journals, seating and locating surfaces. The design of the spindle becomes much more complicated due to the devices built into it automatic mode and tool clamping, sensors for adaptive control and automatic diagnostics.

The spindle supports must ensure the accuracy of the spindle for a long time in variable working conditions, increased rigidity, small temperature deformations. The accuracy of the spindle rotation is ensured, above all, by the high precision of the bearings.

The most commonly used spindle bearings are rolling bearings. To reduce the effect of clearances and increase the rigidity of the supports, bearings are usually installed with a preload or the number of rolling elements is increased. Plain bearings in spindle mandrels are used less often and only in the presence of devices with periodic (manual) or automatic clearance control in the axial or radial direction. In precision machine tools, aerostatic bearings are used, in which compressed air is located between the shaft journal and the bearing surface, thereby reducing wear and heating of the bearing, increasing the accuracy of rotation, etc.

The positioning drive (i.e. moving the working body of the machine to the required position according to the program) must have high rigidity and ensure smooth movement at low speeds, high speed of auxiliary movements of the working bodies (up to 10 m / min and more).

Auxiliaries of CNC machine tools include tool changers, chip collectors, lubrication systems, clamping devices, loaders, etc. This group of mechanisms in CNC machine tools is significantly different from similar mechanisms used in conventional universal machine tools. For example, as a result of an increase in the productivity of CNC machines, there was a sharp increase in the amount of coming off chips per unit of time, and hence the need to create special devices for removing chips. To reduce the loss of time during loading, devices are used that allow you to simultaneously install the workpiece and remove the part while processing another workpiece.

Automatic tool changers (magazines, auto-operators, turrets) should ensure minimum time spent on tool change, high reliability in operation, stability of the tool position, i.e. the constancy of the overhang and the position of the axis during repeated tool changes, have the necessary magazine capacity or turrets.

The turret is the simplest tool changer: setting and clamping the tool is done manually. In the working position, one of the spindles is driven by the main drive of the machine. Turret heads are installed on turning, drilling, milling, multi-purpose CNC machines; from 4 to 12 instruments are fixed in the head.

3 Precision and quality of processing on CNC machines.

Quality in a broad sense is understood as a set of significant features, properties, and characteristics of the object in question as a whole, characterizing it as such and distinguishing it from other objects. V industrial production the quality of products (according to the most recent editions of GOSTs) is the degree to which its characteristics meet the requirements. Accordingly, the concept of product accuracy is introduced as a measure of conformity to a sample (usually specified by a drawing and technical conditions production). Accuracy of dimensions, shapes and mutual arrangement of the elements of the product is the main characteristic of its quality.

The quality of products is influenced by a number of factors, which are usually divided into external and internal. External factors is the level of demand and requirements of consumers, as well as legal standards. Internal factors include the material base of the enterprise, the qualifications of personnel and the characteristics of the equipment that manufactures products. Thus, meeting external demand and obtaining competitive advantage on the market is impossible without collateral and permanent work to improve the quality of products manufactured by the enterprise.

Processing quality assurance problems.

Milling is one of the main methods of cutting workpieces. As in other cases, milling on machine equipment is associated with the inevitable appearance of inaccuracies in processing. Among the reasons for the occurrence of errors in the size and shape of the product are:

1. the degree of accuracy (perfection) of the milling machine;

2. errors of basing (installation, fastening) of the workpiece;

3. wear of the cutting tool (as well as errors during its installation / fastening);

4. elastic and thermal deformations of the "machine-fixture-workpiece" system during processing;

5. residual internal stresses in the workpiece.

In addition to the above, it is possible to single out the "human factor", i.e. staff qualifications. For manually operated machines, this factor has a decisive influence on the quality of the products. When milling on modern automated CNC machines, this factor (contrary to common misconception) plays an even greater role, only in a slightly "offset" form. Here, the main work of the adjusters and operators is carried out during the preparation of the machine for work, its programming, trial "run", as well as subsequent periodic maintenance. Directly in the process of processing, the influence of the "human factor" on the quality of products when processing on CNC milling machines is minimized, but it is still not completely excluded.

Processing quality on modern machine tools with CNC.

Most of the causes of errors in the processing of products described above are almost completely eliminated or minimized when using modern CNC milling machines:

1. A high degree of accuracy - due to the perfection of the mechanical design and the widespread use of electronic components - reaches values of the order of 0.05-0.01 mm and does not decrease during operation (there is no accumulation of so-called "floating errors").

2. Inaccuracies in the positioning of the workpiece do not have a decisive effect, since most machines have the ability to correct the "zero point" (initial positioning of the cutting tool), and some models are equipped with special sensors that determine the dimensions of the workpiece and automatically correct their "tool zero". Auxiliary systems for fixing the workpiece on the worktable (both standard clamps and complex type "vacuum table") allow you to place and securely fix workpieces of almost any geometry. And the control program of the machine allows the workpiece coordinates to be read from any convenient point (thus, the choice of the main design bases is greatly simplified).

3. The advent of CNC machines capable of milling at high speed has spurred the corresponding development of cutting tools. At the moment, diamond-coated tungsten carbide cutters are becoming more common. Characterized by low dimensional errors and low vibrations, modern cutters successfully resist wear and provide high quality surface treatment. For fastening the cutters in the chuck of the machine, collet chucks, simple in design and reliable in operation, are used. In this way, the risk of incorrect / insecure installation and clamping of the tool is also minimized.

4.Modern CNC machines, as a rule, are characterized by an increased rigidity of the structure that can effectively withstand vibrations (even when machining on high speeds) and minimize the deformation of the "machine - fixture - workpiece" system. This eliminates tool drift during machining and improves the milling quality. Reliable cooling systems (both the machine spindle and the cutter itself) help maintain a constant thermal regime and ensure that high accuracy rates are maintained even with prolonged stressful processing.

Another important advantage of an automatic CNC machine is the constancy of processing characteristics, which means that there are no significant differences in the accuracy of individual parts within the processed series.

Conclusion

Based on the above, it can be seen that modern CNC equipment allows achieving high accuracy. However, the reserve for improving the quality is far from exhausted and to a greater extent lies in the perfection of control programs. That is, it again depends on the "human factor" - the skill and talent of researchers working to identify new technological opportunities.

List of sources used

1 Gzhirov R.I. CNC machining programming/ R.I. Gzhirov .- : Mechanical Engineering, 1990 .-- 592 p.

2 Shurkov V.N. Production Automation Basics/ V.N. Shurkov, 1989 - 240 p.

3 Kharchenko A.O. CNC machines and flexible equipment production systems / A.O. Kharchenko.-: "Professional", 2004. - 304 p.

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Working in automatic or semi-automatic mode, a CNC machine must first of all ensure the accuracy of the manufactured parts, which depends on the total error. The total error, in turn, is made up of a number of factors:

Machine precision;

Precision control system;

Errors in setting the workpiece;

Tool setting errors per size;

Errors in setting the machine to size;

Tool manufacturing errors;

Dimensional wear of the cutting tool;

The rigidity of the AIDS system.

The accuracy of the machine is understood, first of all, its geometric accuracy, i.e. unloaded accuracy. There are machines of four classes of accuracy: H (normal), P (increased), B (high), A (especially high). When checking machine tools for compliance with the accuracy standards, the accuracy of the geometric shapes and the position of the base surfaces, the accuracy of movements along the guides, the accuracy of the position of the axes of rotation, the accuracy of the machined surfaces, and the roughness of the machined surfaces are revealed.

The accuracy of CNC machines is additionally characterized by the following specific manifestations: the accuracy of the linear positioning of the working bodies, the value of the dead zone, i.e. lag when changing direction of movement, return accuracy, stability of reaching the set point, accuracy in circular interpolation mode, stability of tool position after automatic change.

It should be noted that for CNC machines, the stability of the exit of the working bodies to a given point is often more important than the accuracy of the machine itself. To maintain the accuracy of the machine for a long time of operation, the norm geometric accuracy in the manufacture of the machine, in comparison with the normative ones, they are toughened by 40%, thereby reserving a wear margin.

The precision of the control system. The accuracy of the control system, first of all, is associated with the operation in the interpolation mode - the mode in which the system is controlled simultaneously by several axes. Deviations associated with the operation of the interpolator do not exceed the sample price. For modern machine tools with a unit impulse price of 0.001-0.002 mm, the error is insignificant, but manifests itself in the form of deviations of microgeometry, i.e. roughness.

Errors that do not depend on the operation of the interpolator, but appear in the interpolation mode, can be very significant. Their cause is a systematic error in the transmission of motion by feed drives. These errors occur in the kinematic chain: feed drive motor - gearbox - lead screw - sensor. When moving along one axis, such errors appear in the form of uneven movement of the working bodies and practically do not affect the processing result. However, when moving along several axes, uneven movement even along one axis leads to processing errors in the form of waviness of the processed surface.

Errors in the installation of workpieces. The installation error is determined by the sum of the positioning and fixing errors. The positioning error arises due to the misalignment of the installation base with the measuring one. On CNC machines, it is possible to achieve higher accuracy when, in one setting, the measuring bases and all surfaces, the dimensions of which are measured from these bases, are processed.

When fixing the workpieces, it can be displaced under the action of the clamping forces. The displacement of the workpiece from the position determined by the installation elements of the device occurs due to deformations of the individual links of the chain: the workpiece, the installation elements, the body of the device. Due to the non-uniformity of the surface quality and the instability of the specific loads, it is impossible to compensate for the resulting deformations using tool compensation.

Tool setting errors per size. When setting the tool to size outside the machine, irrespective of the accuracy of the device used, errors occur. These deviations are determined by the error of the device itself and the error of fixing the tool adjusted to the size. This error is compensated for after a test run.

Errors in setting the machine to size. The adjustment of the machine to the size consists in the coordinated installation of the adjusted to the size of the cutting tool, the working elements of the machine and the base elements of the device in a position that, taking into account the phenomena occurring during the processing, ensures that the required size is obtained. The error in setting up the machine arises due to the fact that it is impossible to position the working elements of the machine and tools exactly in the calculated position. To ensure the required manufacturing accuracy, the fitter uses trial runs. By adjusting the installation dimension is meant the restoration of the installation dimension that has changed due to dimensional wear of the tools or thermal deformation of the system. In order to reduce the number of readjustments during the processing of a batch of parts, it is necessary to choose the correct installation dimension. It is recommended to select the installation size so that it is 1/5 of the field away from the lower or upper limit of the tolerance field. Closer to the lower border, tools should be adjusted when processing external surfaces, and closer to the upper one when processing internal surfaces.

Tool manufacturing errors. In profile turning, the surface is formed by various points lying on the rounded part of the cutter. Modern CNC controls allow programming tool radius compensation. If this is not possible, the radius of curvature at the tip of the cutter must be taken into account when drawing up the machining program. It must be remembered that for the cutting tool is made with a certain permissible error, which must also be taken into account when programming the processing.

Dimensional wear of the cutting tool. During processing, the cutting tool is subject to wear, which in turn affects the machining error. The wear criterion is the size of the wear plate along the flank face. Tool wear introduces a systematic error in the initial setup, i.e. the actual size of the machined surface is outside the tolerance range, after a certain time interval, readjustment is required. The readjustment period depends on the wear rate of the tool. Tool wear compensation (readjustment) can be automatic or manual. With manual correction, the operator makes changes to the setup after a certain time interval, and with automatic correction of the size, the CNC system performs the program.

The rigidity of the AIDS system. Elastic deformations. As noted earlier, the AIDS system is an elastic system. The stiffness of an elastic system is understood as its ability to resist deforming action. With insufficient rigidity under the action of cutting forces, the deformation of the AIDS system occurs, which causes errors in the shape and size of the processed surface. The higher the loads (i.e., the greater the cutting forces), the higher the errors associated with insufficient rigidity of the system. To reduce the indicated errors, it is necessary to reduce the size of the metal layer removed in one pass. It should be noted that CNC machines usually have a cruelty 40-50% higher than universal equipment, which allows processing in fewer passes.

Thermal deformations and deformations from internal stresses of the workpiece. During the operation of the equipment, all elements and units of the machine are heated. These deformations are quite significant, for example, heating a steel rod 1 m long by 1 ° C leads to its elongation by 11 µm.

Thermal deformations occur intensively in initial period operation of the machine after which the amount of deformation is stabilized and does not affect further work... Changes occurring in the initial period can significantly affect the accuracy of processing, therefore, it is necessary to warm up the machine before processing parts. Prolonged stoppages of equipment should also be avoided.

The heat generated in the cutting zone will heat the workpiece, especially during multi-pass roughing at high cutting speeds. In this case, its deformation occurs. In order to obtain high accuracy, it is necessary to ensure cooling of the workpiece before finishing. For these purposes, processing using coolant is used, and when processing several workpieces (on multi-purpose) machines, a rational processing scheme is also used, in which a time delay is carried out to stabilize the temperature. In addition, high-precision machines are installed in thermo-constant rooms.

The workpieces are characterized by internal stresses generated by uneven cooling of individual parts of the workpiece during their manufacture. Over time, internal stresses are leveled out, and the workpiece is deformed. The deformation process proceeds especially actively after the removal of the surface layers with the highest stresses. To reduce the effect of such deformations, roughing and finishing deformations should be separated, and to obtain high-precision parts, natural or artificial aging should be performed between roughing and finishing operations.

The accuracy of machines in an unloaded state is called geometric. Depending on the accuracy characteristics, CNC machines are subdivided in ascending order of accuracy into four classes: normal H; increased P; high B; especially high A.

Machine tools increased accuracy differ from machine tools normal accuracy in the main by more accurate performance or selection of parts, as well as individual features of installation and operation at consumers. They provide machining accuracy on average within 0.6 deviations obtained on normal precision machines. CNC machines high accuracy class B provide machining accuracy within 0.4, and class A machines - within 0.25 deviations obtained on machines of normal accuracy. Class B and A machines are obtained as a result of a special design, their assemblies and elements, as well as high manufacturing accuracy.

When checking the accuracy standards of machine tools, they establish * the accuracy of geometric shapes and the relative position of the supporting surfaces that base the workpiece and the tool; accuracy of movements along the guides of the working bodies of the machine; the accuracy of the location of the axes of rotation and trajectories of movement of the working bodies of the machine, carrying the workpiece and the tool, relative to each other and relative to the reference surfaces; the accuracy of the processed surfaces of the sample; roughness of the processed surfaces of the sample.

Accuracy check

The accuracy of CNC machines is additionally revealed by the following specific checks: the accuracy of the linear positioning of the working bodies; the size of the dead zone, that is, the lag in the displacement of the working organs when changing the direction of movement; the accuracy of the return of the working bodies to their original position; the stability of the exit of the working bodies to a given point; the accuracy of working out the circle in the circular interpolation mode; stability of the position of the tools after automatic change.

During inspections, both accuracy and stability are revealed, that is, the multiple repetition of the arrival of the working bodies in the same position, and often stability is more important for achieving machining accuracy on CNC machines than the accuracy itself.

The general admissible error when positioning the working bodies is Δ p = Δ + δ.

Based on the permissible deviations, the greatest error in working out the movement, for example, 300 mm long along the axes X and Y for a class P machine it will be 17.2 microns, and for a class B machine - 8.6 microns.

To maintain the accuracy of the machine for a long time of operation, the standards of geometric accuracy for almost all checks during the manufacture of the machine, in comparison with the normative ones, are tightened by 40%. Thus, the manufacturer reserves a wear margin in the new machine.

Metal processing with high (precision) precision requires a special approach for the manufacture of machine tools. All precision machine tools are divided into classes according to the degree of extreme accuracy with which they are capable of processing parts:

Class A machines (especially high precision).
Class B (high precision equipment).
Class C (special precision machines).
Class P machines (increased processing accuracy).

Precision equipment provides processing of parts of ideal geometric shape, particularly accurate spatial arrangement of the axes of rotation. The machines allow obtaining surface roughness up to the eleventh class of cleanliness. Manufacturing parameters, at certain conditions, reach the values typical for the first class of purity.

To achieve such indicators, it is necessary to use machine-tool assemblies and assemblies manufactured according to the relevant standards, with minimal errors in their production. The bearings used are of particular importance. High quality hydrodynamic and aerostatic bearings are used on precision metal cutting machines.

During the operation of metalworking equipment, a large amount of heat is generated, affecting both the machine components and the workpieces. At the same time, both those and others experience mechanical deformation, leading to a decrease in manufacturing accuracy. In high-precision machine tools, the function of active heat removal is implemented, which prevents geometric deviations of machine elements and parts. Reducing unwanted vibration also contributes to precision manufacturing.

Fundamentals of the theory of high-precision metal processing

A modern metal-cutting machine can be viewed as a kind of system of three components: measuring, computing, and executive. None of them is perfect, each introduces errors in manufacturing accuracy.

The accuracy of the measuring part depends on the readings of the sensors used. The measurement accuracy is increased with the use of more advanced sensors - measuring devices. Today, such devices are capable of tracking sizes down to a few nanometers.

The executive accuracy directly depends on the components and assemblies of the machine. The higher the parameters of the components of the equipment, the smaller the final error will be.

The errors of metalworking machines include:

Geometric, depending on the quality of manufacture of machine components and their assembly. The accuracy of the positioning of the working tool and the workpiece relative to each other during processing depends on this.
Kinematic errors depend on the correspondence of the gear ratios in the mechanisms of the machine. Kinematic chains have a particular effect on the precision of manufacturing gear elements and threads.
Elastic errors are determined by machine deformations. In the process of cutting, there is a deflection, under the action of the arising forces, the relative position of the tool and the workpiece. In precision machines, to combat such manifestations, they create especially rigid structures.
Temperature... Uneven heating of machine units leads to a loss of initial geometric accuracy, reducing the quality of workmanship.
Dynamic errors are explained by the relative fluctuations of the working tool and the workpiece.
Manufacturing and installation errors cutting tool.

Motors, gearboxes contain moving parts with backlash, sliding surfaces undergo wear over time - all this directly affects the quality of processing. Such a concept,

how the positioning accuracy of the "machine-workpiece" system directly depends on the executive accuracy.

Some are capable of machining parts with an accuracy of 0.0002 mm at a spindle speed of 15,000 rpm. Such indicators also have a downside. The cost of the equipment is significantly higher compared to conventional machines. This is a consequence of the use of the latest high-tech technologies in the manufacture of machine tools. An example is the use of aerostatic guides, where the caliper with the working tool slides at a distance of several microns from the surface. That is, it is actually in the "air".

A modern precision grinding machine is an automated complex that allows you to process parts with an accuracy of 0.01 mm. Serves for sharpening tools made of diamonds, hard alloys, tool steel. Ultra-precision grinding machines are capable of processing the inside and outside surfaces of a part in one set. Precision drilling machine has a rigid structure, is equipped with a digital display showing the drilling parameters.

Common to all types of precision machine tools is the use of friction gears in drives. This increases the quality of workmanship, simplifies the kinematic chains. Higher efficiency reduces the cost of work.