Machine tools, numerical control (CNC), Postprocessors. CNC Router Selection Guide CNC Router Precision

When it comes to machine tools or other numerically controlled systems, references to such concepts as positioning accuracy, positioning resolution, positioning repeatability, and part repeatability cannot be avoided. These concepts are very closely related, and confusion often arises among beginner machine tool builders and CNC operators. Academic definitions and methods for calculating these parameters are specified 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) - a value showing how accurately you can set the movement in your CNC system.

Let's look at an example. Let's say the Y-axis of a Mach3 machine has a stepper motor with a step of 1.8 degrees (200 steps/rev) and a driver with a step division mode of 1/16, which is connected to a ball screw 1605 with a step of 5 mm per revolution. Mach3 works in STEP/DIR mode - it sends discrete pulses to the controller, which are then interpreted into motor steps. One STEP pulse will cause the motor shaft to move, which will correspond to the movement of an 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 will not be able to set the movement to a point with coordinate Y = 2.101 - the control program will "round" this value, depending on the settings, either up to 2.1 or up to 2.1015625 .
Naturally, all this does not mean at all that by sending one STEP pulse, we actually get a displacement of 0.0015625 mm, because there are many factors that introduce an error - from the positioning error of the motor shaft to the backlash in the travel 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 get a slightly different result due to mechanical errors - the axis will stop at some distance from the required point. The repeatability indicates how large 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 amount of "scatter" positioning errors about some average value. The repeatability depends mainly on the transmission backlash and the resulting elastic deformations, and in fact is quite uninformative, because tells only whether the positioning error is stable or not, but does not say anything about its magnitude. You can build a completely inaccurate machine with excellent repeatability.

CNC axis positioning accuracy

Axis positioning accuracy is a generalized value showing the limits within which the real coordinate of an axis can be after positioning is completed. When they say "machine accuracy", they usually mean positioning accuracy. Accuracy depends on repeatability, but includes not only the magnitude of the "scatter" of the positioning error, but also its average value, i.e. is a more general characteristic. Accuracy indicates how large the axis position error can be. Accuracy is the main characteristic of the machine. Often, manufacturers of medium and hobby class machines simply indicate a certain "machine accuracy", without indicating a "coverage factor" - i.e. coefficient of proportionality, because the accuracy, say, 0.05 mm, measured for 3σ and for 1σ is a big difference: in the first variant, positioning with an error of no more than 0.05 mm will occur in 97% of cases, and in the second, only 32% (if you are interested) , where the interest is taken from, you 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, among the main ones are the backlash of the guides and gears, misalignment of the guide axes and their non-perpendicularity. Everyone who has ever tried to cut a large rectangle out of plywood or other sheet material knows how a fraction of a degree error when marking right angles can lead to a mismatch in the lengths of the sides of several millimeters, and sometimes centimeters, so special attention is paid when assembling the machine with CNC. The rigidity and workmanship of the bed and portal also have a direct impact on the accuracy 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 - the runout at the end of the spindle cutter, the perpendicularity of the spindle installation, and the actual materials being processed and cutting conditions. Therefore, manufacturers usually indicate the accuracy of manufacturing a part - purely theoretical, "calculated", sometimes - having nothing to do with reality. For middle-class machines, 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 only be observed on a few units of economy class machines.

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The stiffness criterion in machines, along with the strength criterion, is one of the most important. Its role is constantly growing, on the one hand, due to increased requirements for accuracy, on the other hand, due to the lagging growth of the elastic modulus of materials from the growth of their strength characteristics. In machine tool building, the stiffness criterion has a special great importance, since along with geometric and kinematic accuracy, the rigidity of the machines determines the accuracy of the machined parts.

Machining accuracy refers to the degree to which the shape and dimensions of the part correspond to the shapes and dimensions specified by the drawing. Their full correspondence can be perfect detail with absolutely precise dimensions and geometrically correct surfaces. However, the real details never exactly correspond to the given ones, there are always deviations. Therefore, it is customary to characterize accuracy by the magnitude of the error, i.e., the deviation of the real part from the given one. Accordingly, there are errors in the shape of parts and dimensions. The form error represents the error in the relative position of the surface of the part. This may not be rectangularity, not flatness and not straightness of the edges, as well as their not being parallel. Cylindrical parts can be made conical, oval, barrel-shaped.

Given that a significant range of parts is made from hard-to-cut materials, in connection with which specific gravity processing errors caused by insufficient rigidity in the balance of machine tool accuracy increases.

The rigidity of the machine system along a given axis is understood as the ratio of the component of the cutting force along this axis to the elastic movement in the same direction from the resultant cutting force. Elastic deformations lead to incorrect contact of parts and to a sharp deterioration in their joint work. The most important condition for the good operation of bearings, gears and worm gears is the smallness of the load concentration, determined by the elastic deformations of the shafts.

Determining the stiffness index is also an urgent task for the input control of newly purchased metal-cutting equipment and for assessing the quality of machine tools after repair and modernization.

The nodes of a working machine are exposed to cutting forces, friction, inertia; forces caused by the weight of workpieces and technological equipment; forces that arise when clamping workpieces. Under the action of these forces, elastic deformations of the parts included in the assembly and deformations of the joints occur. Accordingly, a distinction is made between intrinsic and contact stiffness.

The machine nodes that carry the workpiece and the tool are the main nodes that determine their relative position during processing under the action of the above forces, and determine the accuracy of the machined parts. Therefore, the rigidity of the main units determines the rigidity of the machine as a whole.

For machines of the CNC turning group, GOST establishes as an indicator of rigidity the relative movement under load of a mandrel fixed on the spindle relative to the turret.

With the static method of testing for rigidity, the loads acting on the mandrel in the spindle and the turret are simulated approximately, since this does not create a torque and the axial component of the cutting force.

The system is loaded with force P in a plane perpendicular to the axis of rotation of the spindle, at an angle of 60° to the direction of the transverse feed.

When testing lathes for rigidity, artificial loading is performed, simulating the resulting component of the cutting forces Pz, Py, Px. Static loading is created by a special device, the design and technical characteristics of which must correspond to the type and size of the machine.

Relative displacements are measured with a dial indicator (MIG) with a division value of 1 μm and a measurement range that is 1.5–2 times the maximum allowable value of these displacements.

Bibliography

1. Tests and research of metal cutting machines: guidelines To laboratory work/ comp. Yu. V. Kirilin. - Ulyanovsk: UlGTU, 2012. - 48 p.
2. Machine tools and automata. Textbook for universities. Ed. A.S. Pronikova - M.: Mashinostroenie. 1981
3. Internet resources.

FACTORS DETERMINING THE ACCURACY OF THE TURNING

CNC MACHINE

Dots, V.V. DO DONOV, Assoc. Yu.V. NIKULIN

The article deals with the formation of the accuracy of lathes. Experimental methods for estimating the accuracy of rotation of the spindle assembly by the parameters of its circular trajectories with and without application of workloads are presented; the issues of determining the accuracy of the movement of the machine support, the influence of thermal deformations of the machine on its accuracy are discussed. The scheme of the measuring and testing installation and the results of measuring the parameters characterizing the accuracy of lathes are given.

Questions of precision quality shaping of lathes are examined in this article. Experimental methods of an exactitude estimation of a head slide rotation on parameters of its circular trajectories with and without the application of working loadings are presented. Also questions of running accuracy of a planning tool box, influence of thermal strains of the machine tool on its exactitude are discussed. The scheme of measuring and presetting station and results of measurements on parameters describing the exactitude of

lathes are presented in conclusion.

Improving the quality of machine tools is one of the main problems modern engineering. The technological process of machining must guarantee the specified quality of parts manufacturing in accordance with the established drawings and technological requirements. The most important component, means of implementation technological process- a metal-cutting machine is a complex precision technological machine, which forms the quality indicators of parts processed on it. The quality level of a metal-cutting machine is determined mainly by the requirements for the accuracy of the machined parts - the accuracy of dimensions, shape, relative position, machined surfaces, roughness, waviness. Higher requirements for machines arise in the final processing, which forms the parameters of the rigidity of the workpiece. In view of this, the rigidity indicators of a metal-cutting machine are the main indicators, the implementation of which determines the effectiveness of its application.

Tests of lathes for geometric and kinematic accuracy include checking the accuracy of spindle rotation, the straightness of the guides, the straightness of the movement of the calipers, the correctness of the mutual movement of the machine nodes, the parallelism and perpendicularity of the guides and the spindle axis.

Machine tool tests for static stiffness include the measurement of deformations under the working load of nodes lathe- spindle unit and caliper. Dynamic processes in the machine during cutting are measured when testing the machine for vibration resistance, which has a direct effect on the shape accuracy of the machined part, the waviness and roughness of the machined surface

ness. With increasing requirements for machining accuracy, thermal deformations play an increasingly important role in shaping machining accuracy.

Machining accuracy on lathes is largely determined by the geometric accuracy of the machines, the geometric accuracy of the spindle assembly (SHU),

yes longitudinal and transverse feed, the carrier system of the machine, which mainly determines the accuracy of the relative position of the tool and the part during processing,.

The accuracy of machining on lathes is determined by the complex influence of the subsystems, factors, and components included in the technological system of the machine (Fig. 1).

Rice. 1. Technological system machine tool

The accuracy of metal-cutting machine tools is determined by three groups of indicators: 1) indicators characterizing the accuracy of processing product samples; 2) indicators characterizing the geometric accuracy of machine tools; 3) additional indicators.

The geometric accuracy of the machine is characterized by the following groups of indicators: the accuracy of the trajectories of movement of the working bodies of the machine, carrying the workpiece and tool; the accuracy of the location of the axis of rotation and the direction of the rectilinear movements of the working bodies of the machine, carrying the workpiece and tool, relative to each other and relative to the bases; accuracy of the bases of the day of installation of the workpiece and tool; accuracy of coordinate movements (positioning) of the working bodies of the machine tool carrying the workpiece and tool.

provided by the standards and specifications geometric accuracy checks reflect the effect of machine accuracy on machining accuracy.

Clamping, rotation and processing of the product on a lathe are carried out using a spindle assembly. The lathe is the main subsystem that largely determines the quality of processing: accuracy, surface finish, waviness. Significant Contribution other subsystems and factors also contribute to the formation of the quality of processing: fixture errors, errors in the control room, the accuracy of the machine feed drives, control and measurement systems, workpiece properties.

The maximum accuracy of processing diametrical dimensions on modern lathes is estimated at 0.5. L µm, therefore, when developing the main forming units of a lathe - SHU and longitudinal and transverse feed drives, very stringent requirements are imposed, since their geometric errors must be less than the total processing tolerance.

For the experimental determination of the parameters and characteristics of the circular trajectories of the SHU, which determine the permissible rigidity of turning at the department of machine tools and automatic machines of the Moscow State Technical University. N.E. Bauman developed a measuring installation, the scheme of which is shown in fig. 2.

Test setup layout

Strain Gauge Amplifier

digital voltmeter

digital voltmeter

X Coordinate Table

Y coordinate table

Trajectory

spindle axis

Rice. 2. Schematic of the test setup

The scheme of the test setup (information-measuring channel (IMC) circular trajectories (CT)) includes the following measuring instruments and equipment: sensors D1-D4 (primary non-contact information converters of inductive type); tensometric amplifier type UT4-1; analog-to-digital converter; personal computer for collecting the results of the experiment, processing and displaying them on a graphic monitor, printing and plotting devices; hydraulic load device (HLD), which serves to simulate cutting forces. GNU, consists of two mutually perpendicular loading hydraulic cylinders, mounted on a common bracket in the caliper of the machine being tested.

The test and measurement setup contains two measurement channels: along the X coordinate and along the K coordinate. The main technical characteristics of the test and measurement setup:

measurement range of shifts of the SH axis for each channel, µm ..............................................20

rotational speed range of the control room at which the measurement is carried out,

RPM ............................................... ................................................. .........................±6000

speed of primary converters, ms .............................................................. ..-0.003

maximum measurement error, µm .............................................. ...............±0.5

The accuracy of spindle rotation at idle speed of the machine depends on the mathematical expectation and the standard deviation of the eccentricities for each /-th spindle support from four types of errors: neck runout relative to its axes; runout of the raceway of the inner ring of the bearing relative to the mounting hole; runout of the raceway of the outer ring of the bearing relative to its outer surface; misalignment of the mounting hole for the bearing in the headstock (quills).

deviations

runout of the spindle assembly of the STP-125 lathe gave the following results:

affecting the accuracy of the lathe is the total

cutting forces were set with the help of GNU

Cutting force Ru

Cutting force Ru

125 250 500 1000 2000

(cupboard uneven)

Axis 1 travel

Rice. 3. Dependency graphs

At MSTU im. N.E. Bauman, a stand was developed at the Department of Metal-cutting Machine Tools for measuring circular trajectories (CT) of the spindle assembly (SHU). The STP-125 machine was used as a test object. Pilot tests were carried out on the SHU according to the parameters of the CT,

Carrying out preliminary tests. Test conditions. The tests were carried out on a machine warmed up for 2-3 hours when turning the control valve manually, at idle with a different number of rotations of the control valve, under a load created by a hydraulic load device (HPU). In the latter case, both the number of revolutions n and the value of the load P (Fig. 3) were varied, which radially loaded a special mandrel inserted into the SHU. The radial displacements of the SB were measured along the coordinates A" and Y. Using 4 inductive non-contact transducers operating at a carrier frequency of 5200 Hz, the signal from the inductive transducers was fed to a four-channel strain gauge amplifier, and then, after the ADC and computer, to the graph plotter.

The results of preliminary tests are shown in fig. 4-6. The tests were carried out at idle at n = 100. On fig. Figures 5 and 6 show typical trajectories of the SHU axis displayed on the computer screen.

The accuracy of spindle rotation depends on the accuracy of the manufacture of its parts, the accuracy of the bearings, the quality of its assembly and adjustment. Spindle rotation errors, first of all, are determined by the difference in wall thickness of the bearing rings and different-sized

Rice. 4. Runout of the spindle axis at idle

Fig. 5. Trajectory of the axis of the spindle assembly

Rice. 6. Trajectory of the axis of the spindle assembly

Tew rolling bodies. This error for bearings of small and medium sizes lies within 1 ... 10 microns (depending on the accuracy class and size of the bearing).

The waviness of the tracks and the geometric errors of the rolling elements cause smaller spindle displacements of the order of 0.1 ... 1 μm and are superimposed in the form of high-quality components on the errors from the difference in wall thickness of the rings.

An even higher frequency and lower amplitude of the spindle oscillations are caused by the roughness of the raceways. The addition of these vibrations causes a complex, complex picture of the movement of the spindle axis in space (Lissajous figures, movement of the spindle axis along a hypocycloid or epicycloid with a different number of loops).

A great influence on the accuracy of rotation of the spindles of machine tools, especially high-speed ones, has a residual imbalance, which is determined in [N mm / N] or in the form of eccentricity e in [μm], which determines the actual displacement of the center of gravity of the spindle relative to the axis of rotation. The chuck mounted on the spindle must also be balanced.

It is not possible to display the results of tests at idle when turning the SHU by hand on a computer due to the peculiarities software COMPUTER. However, measurements of the radial run-out of the control valve with the help of sensors showed that its numerical value is within 1.5-2.5 μm in both X and Y coordinates and is slightly less in magnitude than the corresponding radial run-out when measuring the control valve at idle without load.

The tests of the CM runout without load at idle were carried out at various CM speeds: n = 10, 30, 70, 100, 160, 220, 300, 450, 600, 800, 1000, 1300, 2000 rpm (Fig. 7) ,

100 "200" 300 "400 500 600,~700" 8CO 900 "1000" 1100 "1200" 1300

Fig. 7. Runout of the spindle assembly at idle without load at various rotational speeds

Tests have shown that with an increase in the number of revolutions of the SHU, the radial runout increases monotonically up to n = 500-600 rpm, and then the rate of increase in the amplitude of the radial runout tends to some increase. The measurements were carried out with the cartridge in place.

The spindle assembly is a complex mechanical system consisting of several types of elastic elements: bearing, shaft, flanges, bushings, springs connected to each other, acting on each other and forming a single technical device in which complex processes take place, each of which can be described by his mathematical model.

The most significant models are elastic-deformational, dynamic, vibrational, tribological, thermal, fatigue fracture.

The inputs of these models are the design and technological factors in the design and manufacture of the spindle, operating conditions. The output parameters of the models are stiffness, vibration, friction torque, speed, technical resource, heat resistance, fatigue life and other design parameters that characterize, among other things, the geometric accuracy of the machine and the accuracy of processing the part on it.

When testing the CS with the chuck removed with a fixed frequency of its rotation (n = 1000 1/min) and the load that was set by the hydraulic load device, the circular trajectory of the CS slightly expanded in its average diameter (an increase in Ax and Dn) and shifted in the direction of the load

%=n - p; (fig. 8) -

As a result of preliminary tests, the dependence of the amplitude of oscillations of the noise control on the frequency (AFC *) was also determined. The studies were carried out using a special analyzer of the vibration spectrum of the SK4-72 type. The signal came from the displacement sensors to the input of the analyzer, and the frequency response of the oscillations of the control room was plotted at various frequencies of its rotation.

Amplitudes A and B of the frequency response approximately correspond in frequency to the fluctuations in the noise from the stiffness fluctuation caused by the 18 rolling bearings of the front bearing assembly and the vibrations of the toothed drive belt.

When the machine is operating, relative fluctuations occur between the workpiece and the tool, causing certain processing errors. To reduce the level of these fluctuations and

resilience dynamic system the machine tool is used to build the oscillation forms of the spindle assembly and the caliper. The form of oscillations is characterized by a set of ratios of displacements of individual oscillating

points of an elastic system to the displacement of any one point, taken at a certain point in time (taking into account the phase shift) to determine the frequency and direction of vibrations. The operating range of the oscillation frequency is usually in the range from 10 to 500 Hz.

To improve the measurement accuracy, it is desirable to use an excessive number of vibration measurement points. Vibrations are measured, as a rule, in 2--3 mutually perpendicular directions

Rice. 8. Circular trajectory of the spindle assembly under

load

The form of oscillations is measured by vibrometers, which can operate in the modes of measuring vibration displacement, vibration velocity and vibration acceleration. The first mode is used in the low-frequency region (up to 200 Hz), the second mode is preferred for frequencies (100-400 Hz), the third mode is used for higher frequency vibration measurement operating ranges.

The trajectory of any fixed point on the end of the spindle with a sufficiently large approximation reflects the shape cross section workpiece. The degree of this approximation is determined, in addition, by the radial displacement of the tool mounted on the support with a transverse feed and trajectory deviations

caliper from rectilinear movement with longitudinal feed.

The data on the accuracy of the diametrical dimensions of the manufactured part were theoretically determined and experimentally verified (Fig. 9). It depends on the positioning accuracy D position of the cross feed drive, i.e. from the deviation of the actual position of the drive X1 from that specified by the program X with multiple two-way positioning

nii, Methods of mathematical statistics when testing drives are determined by X l and

Arithmetic mean values ​​of the actuator position when positioning in

average ar!

Furthermore, the root mean square deviation of the actual drive position is determined.

X \u003d (X n + X ") / 2; For ■ - the size of the dispersion zone;

/ - ! X + X . | - dead zone that occurs when the drive is reversed

cross feed (Fig. 9).

The maximum value measured on the machine was 5.5 µm. The actual error from D when machining a part will depend on the machining diameter.

to D pos, microns

Rice. Fig. 9. Graph of errors of bilateral positioning of the turret head of the machine tool STP-125 at

transverse movement

1. A test and measurement setup for measuring the parameters of the circular trajectories of the spindle assembly of a CNC lathe has been developed and tested.

2. As a result of testing the STP-125 lathe, the results of the influence of external disturbing influences (cutting forces, spindle displacement) on the parameters of the circular trajectories of the spindle assembly were obtained.

3. An assessment of the influence of errors in the positioning of the transverse support on the accuracy of processing was carried out.

4. The ways and possibilities of diagnosing the spindle assembly and the support group of a CNC lathe are shown.

BIBLIOGRAPHY

1. VDI Richtlinien 2060, Standards for Balancing Rotating Solids. -1980.

2. GOST8-82E, “Machine sweeps for cutting. General requirements for accuracy testing. - M.: Publishing House of Standards, 1982. - 10 p.

3. Pronikov A. S. Program method testing of metal cutting machines. - M.: Mashinostroenie, 1985. - 288 p.

4. Adaptive machine control. / Ed. Balakshin. - M.: Mashinostroenie, 1973. - 688 p.

5. Design and program testing of spindle units of metal-cutting machines / L.I. Vereina, V.V., Dodonov. - M.: VNIITEMR, 1991. - Issue. 1.

6. Figatner A.M. Calculation and design of spindle units with rolling bearings for machine tools. - M.: NIIMASH, series S-1, 1971.

7. Calculation of high-speed spindle units / V.B. Balmont. - M.: VNIITEMR, 1987. - Ser. I. - Issue. 1. - 52 p.

General information about CNC machines. Design features CNC machines Accuracy and quality of processing on CNC machines. CNC machines must provide high accuracy and speed of working out the movements of the given NC and also keep this accuracy within the specified limits during long-term operation.

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Ministry of Education and Science Russian Federation

federal agency of Education

State educational institution

higher professional education

"Komsomolsk-on-Amur State Technical University"

IKP MTO

Department of TM

Individual task

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 used sources…………………………………………...18

Introduction

CNC machines must provide high accuracy and speed of working out the movements specified by the NC, and also maintain this accuracy within the specified limits during long-term operation. The design of CNC machines should, as a rule, ensure the combination various kinds 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 design 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 gears. These drives must have high speed.

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.

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

Numeric program control(CNC) is a control in which the program is set in the form of an array of information recorded on some medium. Control information for CNC systems is discrete and its processing in the control process is carried out by digital methods. Technological cycles are almost universally controlled 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.

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 after they have performed certain elements work cycle;

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

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

Multiloop systems that provide simultaneous or sequential control of the functioning of a number of units and mechanisms of the machine.

According to the method of preparation and input of 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 processing the first part from the batch or simulating its processing) and systems for which the control program is prepared regardless of the place where the part is processed. Moreover, independent preparation of the control program can be performed either using computer technology that is part of the CNC systems of this machine, or outside it (manually or using a programming automation system).

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

IN international practice the following designations are accepted: NC-CNC; HNC is a type of CNC with the operator setting the program from the remote control using keys, switches, etc .; SNS-device of the CNC, which has a memory for storing the entire control program; CNC control offline CNC machine, minicomputer content 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 considers the positive direction of movement of a machine element to be that in which the tool or workpiece moves away from one another. The home axis (Z axis) is the work spindle axis. If this axis is rotary, then its position is chosen perpendicular to the plane of fastening of the part. The positive direction of the Z-axis is from the part holder to the tool.

Usage specific type equipment with CNC depends on the complexity of manufacturing the part and serial production. The smaller the serial production, the greater the technological flexibility the machine must have.

In the manufacture of parts with complex spatial profiles in a single and small-scale production the use of CNC machines is almost the only technically justified solution. This equipment is advisable to use if it is impossible to quickly produce equipment. 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 conditions of reconfigured large-scale production.

The principal feature of the CNC machine is the work according to the control program (CP), on which the equipment operation cycle for processing a specific part and technological modes are recorded. When you change the part processed on the machine, you just need to change the program, which reduces the laboriousness of changeover by 80...90% compared to the laboriousness of this operation on machines with manual control.

The main advantages of CNC machines:

The productivity of the machine is increased by 1.5...2.5 times compared to 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 reduced, and production preparation is transferred to the field of engineering work;

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

The cycle time for parts is reduced and the stock of work in progress is reduced. Production, creating flexible automated production primarily in mechanical engineering.

2 Design features of CNC machines

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

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

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

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

The guides of CNC machines have high wear resistance and low friction force, which makes it possible to reduce the power of the servo drive, increase the accuracy of movements, and reduce the mismatch in the servo system.

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

Rolling guides have a 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; adjusting devices are used to create an interference fit.

Drives and converters for CNC machines. 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 running 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 equipped with feedback sensors and brakes. Asynchronous motors are used less frequently than synchronous motors. The feed movement drive is characterized by the minimum possible gaps, short acceleration and deceleration times, low friction forces, reduced heating of the drive elements, and a large control range. Ensuring these characteristics is possible due to the use of ball and hydrostatic screw gears, rolling and hydrostatic guides, backlash-free gearboxes with short kinematic chains, etc.

Main motion drives for CNC machines are usually AC motors - for high power and direct current - for low power. The drives are three-phase four-band asynchronous motors that perceive large overloads and operate in the presence of metal dust, chips, oil, etc. in the air. Therefore, an external fan is provided in their design. Various sensors are built into the engine, such as a spindle position sensor, which is necessary for orientation or providing an independent coordinate.

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

The spindles of CNC machines are precise, rigid, with increased wear resistance of the necks, seating and locating surfaces. The design of the spindle is much more complicated due to the built-in automatic mode and tool clamping devices, sensors with adaptive control and automatic diagnostics.

The spindle supports must ensure the accuracy of the spindle for a long time under variable operating conditions, increased rigidity, and small temperature deformations. The accuracy of spindle rotation is ensured primarily by the high precision manufacturing of the bearings.

Rolling bearings are most commonly used in spindle bearings. To reduce the influence 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 frequently and only if there are devices with periodic (manual) or automatic clearance adjustment in the axial or radial direction. In precision machines, aerostatic bearings are used, in which there is a space between the shaft neck and the bearing surface. compressed air, due to this, wear and heating of the bearing is reduced, rotation accuracy is increased, 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).

Auxiliary mechanisms of CNC machines include tool changers, chip collectors, lubrication systems, clamping devices, loading devices, etc. This group of mechanisms in CNC machines differs significantly from similar mechanisms used in conventional universal machines. For example, as a result of an increase in the productivity of CNC machines, there was a sharp increase in the amount of chips coming off per unit of time, and hence the need arose to create special devices for chip removal. 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, autooperators, turrets) must provide minimum costs tool change time, high operational reliability, tool position stability, i.e. constancy of the overhang size and position of the axis during repeated tool changes, have the necessary magazine capacity or turrets.

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

3 Accuracy and quality of processing on CNC machines.

Quality in a broad sense is understood as a set of significant features, properties, features of the object under consideration as a whole, characterizing it as such and distinguishing it from other objects. IN industrial production product quality (according to the latest editions of GOSTs) is the degree of compliance of its characteristics with the requirements. Accordingly, the concept of product accuracy is introduced as a measure of compliance with a sample (usually specified by a drawing and production specifications). The accuracy of dimensions, shapes and relative position of the elements of the product is the main characteristic of its quality.

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

Problems of ensuring the quality of processing.

Milling is one of the main methods of processing workpieces by cutting. 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 can be identified:

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 in its installation/fixing);

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

5. residual internal stresses in the workpiece.

In addition to the above, one can single out the “human factor”, i.e. personnel qualification. For machine tools with manual control, this factor has a decisive influence on the quality of products. When milling on modern automated machines With CNC, this factor (contrary to popular misconception) plays an even greater role, only in a slightly “shifted” form. Here, the main work of adjusters and operators is performed in preparing the machine for work, programming it, a 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 during processing on CNC milling machines is minimized, but it is not completely excluded.

Processing quality on modern CNC machines.

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

1. 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 the so-called "floating errors").

2. Inaccuracies in the workpiece location 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 work table (both standard clamps and complex ones such as “vacuum table”) allow you to place and securely fix workpieces of almost any geometry. And the control program of the machine allows counting the coordinates of the workpiece from any convenient point (thus, the choice of the main design bases is greatly simplified).

3. The emergence of CNC machines capable of milling at high speed has intensified the corresponding development of cutting tools. At the moment, diamond-coated carbide cutters are becoming more and more common. With small dimensional errors and low vibrations, modern milling cutters successfully resist wear and provide high quality surface finishes. Collet chucks, which are simple in design and reliable in operation, are used to fasten the milling cutters in the machine chuck. In this way, the risk of incorrect/unreliable mounting and fixing of the instrument is also minimized.

4. Modern CNC machines, as a rule, are distinguished by increased rigidity of the structure, which can effectively withstand vibrations (even when machining on high speeds) and minimize the deformation of the “machine fixture workpiece” system. This eliminates the withdrawal of the tool during processing and improves the quality of milling. 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 during long-term stressful machining.

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

Conclusion

Based on the above, it can be seen that modern CNC equipment makes it possible to achieve high accuracy. However, the reserve for improving quality is far from being exhausted and to a greater extent lies in the improvement 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 .- : Mashinostroenie, 1990. 592 p.

2 Shurkov V.N. Basics of factory automation/ 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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On this complex equipment, all kinds of parts are made of metal, plexiglass, acrylic or plastic, wood. Their versatility lies in the fact that they are well suited for cross planing, the formation of the most complex surfaces, in particular, curved ones; carry out selections of the crest, tongue, folds, groove, slots and moldings.

Description of the machine

The standard equipment of the machine includes:

• heavy and powerful base;
• Desktop;
• , with the simultaneous presence of the spindle shaft;
• a set of several tools for cutting materials;
• front disc brake.

The design of machine tools today includes many important devices that ensure the accuracy of processing and ease of use. It is important to know about them so that the choice of a CNC milling machine is meaningful and correct.

Don't forget the spindle!

One of important qualities in the operation of the spindle shaft electric motor - the ability to rotate it smoothly and evenly. When assembling, bearings of the highest (accuracy class) are selected, and the collet must have increased tolerances for runout and size.

There are main types of spindle cooling systems:

1. Liquid (it is based on the circulation of water or antifreeze in a closed circuit). One of the advantages is reliable heat dissipation. Among the disadvantages is a complex design, because the coolant must be placed in the tank.
2. Air (such cooling consists in forcing air through slots-air intakes in the spindle cavity). Among the advantages of the system - compactness and simplicity. There is also a minus - filters, especially for equipment processing solid wood, must be changed often, they become contaminated with dust.

When choosing a spindle for a CNC machine, you should pay attention to the indicators indicated in the technical data sheet (power and speed during milling), which depend on how hard the materials are processed. For example, for sheet plywood, the required processing power is 800 W; over an array of hardwood, light metals - copper, brass and aluminum, plastic works more powerful machine - 1500 W; and the stone is processed at a power of 3000 - 4000 watts.

Now in equipment for milling, imported spindles are mainly used:

1. Italian - high quality, running at high speed, with smooth rotation and low runout, mainly air-cooled and high price.
2. Chinese has a solid cylindrical body, which is closed at the ends with covers, and bearing assemblies are used to hold the shafts. Among the advantages - the design has a sufficient level of rigidity and minimal vibration, insensitivity to the presence of chips and dust, affordability. Unfortunately, Chinese-made spindle models have a high probability of marriage, it can be difficult to replace bearings. And for models that have water cooling, there is a weak anti-corrosion resistance of internal parts.

Types of milling machines

Choosing such equipment, one must proceed from how it fits the purpose. The Russians have a choice:

• high-speed CNC automatic machines that cut and cut metals, process parts made of cardboard and wood, cope with two-layer plastic and acrylic, PVC, plexiglass and gypsum, natural stone - granite and marble;
• models (milling and engraving) working with sheets (maximum dimension 2000 x 4000 x 200 mm);
• engravers (from 2D modeling to 4D);
• narrow-profile machines that work with one kind of material - varieties of stone, plywood, wood, stainless steel or aluminum;
• small portable CNC models. For example, a milling machine model with "Desktop 3D" is used for milling printed circuit boards, MDF and processes products extremely accurately.

In the line of equipment series for professionals, you can give preference to vertical and horizontal machining centers with program control; large three-, four- and five-coordinate CNC milling engravers who produce in Taiwan.

They are considered quite reliable and buyable (after Germany and Japan - in third position). In addition, it is profitable to purchase them both for individuals and enterprises, due to the presence in Moscow and Tula of service centers that supply equipment, cutting tools, adjust equipment and train personnel.

ATTENTION: It is not difficult to distinguish a machine from Taiwan: it has a one-piece cast bed (the material of manufacture is Brazilian fine-grained cast iron). In addition, the machine is equipped with American or Japanese bearings, imported spindles.

And if the customer is looking for a high-precision jewelry machine, best model for this - P 0403 from the manufacturer Vector.

furniture equipment

Woodworking and furniture manufacturing, workshops manufacturing windows, doors and facades will not be able to function without equipment of wide functionality - CNC woodworking machines.

In recent years, retro-style furniture has become fashionable - with elegant carved armrests, legs and other details. In this case, the technology of automated cutting of a pattern is used on a milling machine, on which numerical control. It provides high precision and quality when complex wood milling is performed and a carved element is created.

With the help of such equipment, it is possible to establish the production of:

• wooden furniture facades and decorative consoles;
• balusters, curly legs and slotted elements;
• embedded carved details;
• symbols, figurines, figurines and frames of various shapes for paintings and mirrors.

Those who are on a budget may buy an inexpensive Chinese standard CNC router - CC-M1, especially for. In the manufacture of facades, engraving decor and bas-relief - usually a lot of dust. Therefore, choose the complete set, where there is a vacuum aspiration for dust absorption. This model has it.

What are the best milling machines? No one will give a definite answer. But there is still more trust in software working equipment. Each master has his own approach to choosing the right technique.

And the CNC router is good, which has higher accuracy, lower power consumption, more convenient to use, reliable in any working situation.

We can formulate three tips for the right choice:

1. Specify in advance with company managers all the data about the model; materials with which the machine works. If there is a video, watch it. This will help you decide.
2. Consult prior to purchase regarding the functionality of the equipment and the range of tasks performed. A the best option- sign up for a demonstration of the operation of the CNC machine and do not be shy to ask questions during operation.
3. When the desired model is selected, be careful at the time of purchase: check the purchased equipment for a complete set of nodes. There must be a program control unit for the machine; cords with connectors of the appropriate configuration, and disks with software. Usually the software is installed by the specialists of the firm selling the machine during its adjustment.

Conclusion

Basically, we tried to help a person facing a choice. We figured out how to choose a milling machine (the thing is expensive, and will work with the owner for more than one year - with metal or wood). At least now there is plenty to choose from. I hope that readers will use this information to purchase a working tool.