Investigation of the accuracy of CNC machine tools. Recommendations for choosing a cnc milling machine Positioning accuracy of a cnc axis

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

Share your work on social media

If this work did not suit you at the bottom of the page there is a list of similar works. You can also use the search button

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 movements specified by the UE, and also maintain this accuracy within the specified limits during long-term operation. The design of CNC machines should, as a rule, provide for the combination of various types of processing, automation of loading and unloading of parts, automatic or remote control of tool change, the ability to integrate into a general automatic control system. 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 effects.

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 practically replace other types of control systems.

According to the technological purpose and functionality, the 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 machine mechanisms.

According to the method of preparation and input of the control program, the so-called operational CNC systems are distinguished (in this case, the control program is prepared and edited directly on the machine, during the processing of the first part from the batch or its processing simulation) and systems for which the control program is prepared regardless of the place of processing details. 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).

Numerical control systems (CNC) are a collection of specialized devices, methods and tools necessary for the implementation of CNC machines. The 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.

In 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 minicomputer or processor; DNS-control 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. Positive direction of the Z-axis is from the device for fastening the workpiece to the tool

The use of a specific 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.

When manufacturing parts with complex spatial profiles in one-off and small-scale 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 from 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 (PC), 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 complexity of the changeover in comparison with the complexity 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;

The duration of the production cycle of parts is reduced and the stock of work in progress is reduced, the creation of flexible automated production facilities, 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 of tool change, etc. ...

An increase in processing accuracy is achieved by 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: autonomous drives are used, and the number of mechanical transmissions is reduced if possible. CNC machine drives must also provide high speed.

Improving accuracy is also facilitated by the elimination of gaps in the transmission mechanisms of feed drives, a decrease in friction losses in guides and other mechanisms, an increase in vibration resistance, a decrease in thermal deformations, and the use of feedback sensors in machine tools. To reduce thermal deformations, it is necessary to ensure a uniform temperature regime in the mechanisms of the machine, which, for example, is facilitated by the preliminary heating of the machine 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 bodies. 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 is significantly 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 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.

Rolling bearings are most commonly used in spindle 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., the movement of 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 size 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. In industrial production, product quality (according to the latest 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 production conditions). 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 are the level of demand and customer requirements, 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 gaining a competitive advantage in the market is impossible without ensuring and constant work to improve the quality of the products manufactured by the enterprise.

Problems of quality assurance of processing.

Milling is one of the main methods of cutting workpieces. As in other cases, milling on machine tools 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, 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 when processing on CNC milling machines is minimized, but it is still not completely excluded.

The quality of processing on modern CNC machines.

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 \u200b\u200bof 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 finishing. 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 setting and securing 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 processing at high speeds) and minimize 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 during 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 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 equipment for flexible manufacturing systems / A.O. Kharchenko.-: "Professional", 2004. - 304 p.

Other similar works that may interest you. Wshm\u003e
12245.		RESEARCH OF THE ACCURACY OF GEODETIC WORKS DURING THE CONSTRUCTION OF BRIDGE STRUCTURES	46.96 KB
	The use of modern geodetic devices for laying out and monitoring the centers of the supports of bridge structures. Application of modern geodetic technologies for laying out and monitoring the centers of bridge supports ...
14532.		Features of the design of technological processes for CNC and GPS machines	14.6 KB
	Features of designing technological processes for CNC and FPS machines When designing technological operations for CNC machines, it is necessary to take into account a number of processing features. The order of surface treatment of workpieces for parts such as shafts is as follows. Roughing and finishing additional surface shapes if there are additional shapes that require roughing. Processing of additional surface shapes that do not require roughing.
19612.		Maintenance of electrical equipment of metal-cutting machines on the example of the mechanical section of workshop No. 37	40.86 KB
	The following is not subject to grounding: and electrical equipment installed on grounded metal structures, while on the supporting surfaces, protected and unpainted places must be provided to ensure electrical contact; b relay housing of electrical measuring instruments, buttons, etc. With significant wear of the contacts of the relay and switches, the contact surfaces are cleaned with a file with a fine notch, while trying to maintain the shape of the contact surface. During the current care, the relay actuation values \u200b\u200bare monitored: current ...
8947.		NORMALIZATION OF ACCURACY OF KEY JOINTS	4.91 MB
	They are used mainly in low-loaded low-speed gears, kinematic feed chains of machine tools in large-sized joints, gearwheels, pulleys of press-forging machines in all critical fixed conical joints in single and small-batch production of products. The shape and dimensions of the sections of the keys and grooves are standardized and are selected depending on the diameter of the shaft, and the type of keyway connection is determined by the operating conditions of the parts to be joined. Parallel keys make it possible to receive both movable ...
8949.		Standardization of the accuracy of gear wheels and gears	2.4 MB
	The kinematic transmission accuracy determines the constancy of the transmission ratio over a complete revolution of the gear. The wheels of these gears in most cases have a small module and operate at low loads and low speeds. The smooth operation of the transmission depends on the fluctuation of the instantaneous gear ratios, that is, on the differences in gear ratios at each moment of engagement, which are repeatedly reproduced during one revolution of the gear wheel.
13583.		Standardization of the accuracy of typical connections and their parts	132.92 KB
	The interchangeability of products, their parts or other types of products is called their property to be equivalently replaced when using any of the many copies of products, their parts or other products with another of the same type. Interchangeability can be complete, incomplete and group with the use of regulation and fit when assembling units and assemblies of machines. The most widely used is complete interchangeability.
8952.		NORMALIZATION OF ACCURACY OF THREADED SURFACES AND CONNECTIONS	1.98 MB
	Thread parameters In general mechanical engineering, the most widely used metric thread. GOST 247052004 establishes the nominal profile of the metric thread and the dimensions of the profile elements Fig. Parameters of metric threads d outside diameter of the outer thread of the bolt; D is the outer diameter of the inner thread of the nut; d2 is the average diameter of the bolt; D2 is the average diameter of the nut; d1 is the inner diameter of the bolt; D1 is the inner diameter of the nut; dз the inner diameter of the bolt along the bottom of the cavity; P step of the profile; H is the height of the original triangle; \u003d 60 angle ...
13010.		Development of a technological process for manufacturing a part of an assembly product using CNC machines and automation equipment	6.58 MB
	For the manufacture of the case, metals or their alloys are usually used: bronze or brass, which can be plated with gold-plated nickel with chromium; stainless steel; titanium; aluminum; precious metals: silver, gold, platinum and plastic; ceramics; titanium or tungsten carbides; a natural stone; sapphire; wood rubber. As a watch glass, transparent plastic, mineral or sapphire glass is usually used ...
5873.		CONSUMER RESEARCH	25.61 KB
	The subject of the study is the characteristics of consumers that determine their behavior when choosing a product. The results of these studies can be a typology of consumers, the allocation of their types of classes of groups, forecasts of changes in capacity and market share, which will allow an enterprise to rationally carry out market segmentation to determine target segments and develop tools for positioning its goods. At the same time, the influence on the behavior of the buyer of such factors as the level of his income, the price of the goods, his operational properties is considered ...
5916.		ACS quality research	87.25 KB
	Analysis of the ACS, establishing the identification of the influence of the structure of the system and its parameters of the initial conditions and input influences on the quality indicators of the control process. The error in processing the input action by the system is a measure of the dynamic accuracy of the system; a quantitative indicator of the quality of regulation, a function formed by the difference between the actual process at the output of the system under study and the required desired reference type of the output function. The priority in stabilization systems are the properties of the system in steady-state modes ...

The criterion of rigidity in machines, along with the criterion of strength, is one of the most important. Its role is constantly growing, on the one hand, in connection with the increased requirements for accuracy, on the other hand, in connection with the lag in the growth of the elastic modulus of materials from the growth of their strength characteristics. In machine tool construction, the stiffness criterion is of particular importance, since, along with geometric and kinematic accuracy, the rigidity of machine tools determines the accuracy of machined parts.

Processing accuracy refers to the degree to which the shape and dimensions of the part correspond to the shapes and dimensions specified by the drawing. A perfect part with absolutely precise dimensions and geometrically correct surfaces can have their full compliance. However, the real details never exactly match the given ones, there are always deviations. Therefore, it is customary to characterize the accuracy by the magnitude of the error, i.e., the deviation of the real part from the specified one. Accordingly, errors in the shape of parts and dimensions are distinguished. A shape error represents an error in the relative position of the surface of a part. It may not be squareness, not flatness or straightness of the edges, as well as their non-parallelism. Cylindrical parts can be made conical, oval, barrel-shaped.

Considering that a significant nomenclature of parts is made of difficult-to-machine materials, in this connection, the proportion of processing errors caused by insufficient rigidity in the balance of machine 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 displacement in the same direction from the resultant cutting force. Elastic deformations lead to improper contact of parts and to a sharp deterioration in their joint work. The most important condition for the good operation of bearings, gear and worm gears is the low concentration of the load, determined by the elastic deformations of the shafts.

Determination of the stiffness index is also an urgent task for the incoming inspection of newly acquired metal-cutting equipment and for assessing the quality of machine tools after repair and modernization.

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

The units of the machine, carrying the workpiece and the tool, are the main units that determine their relative position during machining under the influence 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 machine tools of the turning group with CNC, GOST sets the relative movement under load of the mandrel fixed on the spindle relative to the turret as an indicator of rigidity.

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

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

When testing lathes for rigidity, artificial loading is performed, simulating the resultant components 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 scale division of 1 μm and a measurement range that is 1.5-2 times the maximum permissible value of these displacements.

List of references

Tests and research of metal-cutting machines: guidelines for laboratory work / comp. Yu.V. Kirilin. - Ulyanovsk: UlSTU, 2012.- 48 p.
Metal-cutting machines and automatic machines. Textbook for universities. Ed. A.S. Pronikova - M .: Mechanical engineering. 1981
Internet resources.

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 machining 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, under certain conditions, reach the values \u200b\u200bcharacteristic of 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 deformations, leading to a decrease in manufacturing accuracy. In high-precision machines, 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 imperfect, 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, a deviation occurs, under the action of the forces that arise, 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 technology 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. The precision drilling machine has a rigid structure and is equipped with a digital display showing the drilling parameters.

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

When it comes to machine tools or other numerically controlled systems, you cannot avoid mentioning concepts such as positioning accuracy, positioning resolution, positioning repeatability and part repeatability. These concepts are very closely related, and confusion often arises among novice 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 the Y-axis of a machine running Mach3 has a stepper motor with a step of 1.8 degrees (200 steps / rev) and a driver with a step division of 1/16 that is connected to ball screw 1605 in 5 mm increments 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 \u003d 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 \u003d 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 introduce an 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 get a slightly 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 transmission backlash and arising 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 the "coverage factor" - ie. 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 backlash 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 cutters spindle , perpendicularity of the spindle installation, and indeed of the materials being processed and cutting conditions. 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.