A thermal deformation is affecting one of the reasons of machining accuracy
The machine may be a change in temperature of the plain bearing workshop environment, the motor heat and mechanical movement frictional heating, cutting heat as well as the influence of the cooling medium, resulting in uneven temperature rise of each machine, to cause the changes in the machine shape accuracy and machining accuracy.
For example, on an ordinary precision CNC milling machining φ70mm × 1650mm screw the sleeve bushing morning 7:30-9:00 milling compared to the machining of the workpiece by the workpiece and 2:00-3:30 pm, cumulative error DU bushing changes up 85μm. In isothermal conditions, the error can be reduced to 40μm.
Another example, a double disc grinding 0.6 ~ 3.5mm thick, thin steel workpiece precision double end grinder, in the acceptance process 200mm × 25mm × 1.08mm steel workpiece dimensional accuracy can reach mm curvature in The entire length is less than 5μm. But continuous automatic grinding 1h, the size change range increases to 12μm, and the coolant temperature from the boot when 17 ° C rising to 45 ° C. Due to the impact of the grinding heat, lead to the spindle the journal elongation, spindle front bearing gap increases. Accordingly, for machine tool coolant tank add a 5.5kW refrigerator, the results are quite satisfactory.
Practice has proved that the the machine heated deformed affect machining accuracy. But the machine is in the environment of temperature change anytime anywhere; machine itself will inevitably consume energy at work, considerable part of the energy will be transformed in various ways as heat, causing a physical change of the machine member, this change because The different forms of structure, material differences vary widely. Machine designers should master the formation mechanism and the temperature distribution of the hot, take appropriate measures to minimize the thermal deformation of the machining accuracy.
2. Machine temperature rise and temperature distribution
2.1 natural climate impact
China has a vast, most areas in the subtropical regions, temperature changes throughout the year, within a day temperature changes are not the same. Thereby, also different (eg shop) of indoor temperature of the manner and extent of the intervention, the temperature of the machine around the Atmosphere vary. For example, the seasonal temperature changes in the scope of the Yangtze River Delta region is about 45 ℃ diurnal temperature change of about 5 ~ 12 ℃. The machining workshop general winter without heating in summer without air conditioning, but as long as the workshop better ventilation, little change in the temperature gradient in the machine shop. The Northeast region, seasonal temperature up to 60 ℃, the diurnal variation of about 8 to 15 ° C. Late October to early April the following year for the heating period, the machine shop design heating, lack of air circulation. Workshop inside and outside temperature up to 50 ° C. Therefore, the temperature gradient in winter in the workshop is very complex, when measuring outdoor temperature of 1.5 ° C, 8:15-8:35 am, a the workshop temperature change of approximately 3.5 ° C, the results shown in Figure 1. Precision machining accuracy influenced by the ambient temperature in this workshop will be great.
2.2 surrounding environment
The machine surrounding environment refers to the thermal environment of the machine close range formed by the various layouts. They include the following three aspects.
(1) Workshop Climate: such as the temperature distribution (the vertical direction, the horizontal direction) in the workshop. When alternating day and night, or changes in climate and ventilation the workshop temperature will produce a slow change.
(2) workshop heat source: the sun, radiation heating devices and high-power lights, when they are away from the machine close to direct prolonged impact of the temperature rise of the machine as a whole or part of the components. The heat will be generated at run time by the neighboring devices to radiation or air flow patterns affecting the machine temperature rise.
(3) Cooling: foundation better cooling effect, in particular, is the foundation of the precision machine tools avoid near underground heating pipes, leak once broken, may become a difficult to find the cause of the heat source; open workshop will be a good For radiator favor workshop temperature equilibrium.
(4) Thermostat: workshop take thermostat facilities to maintain accuracy of precision machine tools and machining accuracy is very effective, but energy consumption.
2.3 Factors Affecting the machine internal thermal
(1) machine the structural heat source. Motor heat - such as the spindle motor, feed servo motor, the cooling lubrication pump motor, electric control box can produce heat. These circumstances is to allow the motor itself, but have a material adverse effect on the components of the spindle, ball screw, measures should be taken to be isolated. When the input power to the drive motor operation, in addition to a small part (about 20%) conversion motor heat, most by sports bodies into kinetic energy, such as spindle rotation, bench movement; but inevitably there is still a considerable part transformed into friction heat in the course of the campaign, such as bearings, rail, ball screw and gearbox institutions fever.
(2) the process of cutting heat. A part of the kinetic energy of the tool or workpiece during the cutting process is consumed in the cutting power, a considerable part of the conversion of the deformation of the cutting and the heat of friction between the chip and tool, forming the cutting tool, the spindle and the workpiece heating by a large number of chips heat conduction to the machine bench fixture and other components. They will directly affect the relative position between the tool and the workpiece.
(3) Cooling. Cooling machine temperature rise of reverse measures, such as motor cooling, the spindle assembly cooling, as well as the basis of structural parts cooling and other. The high-end machine tools are often equipped with a refrigerator, electric control box and be forced cooling.
The 2.4 Machine morphology temperature rise
Discussion in the field of machine tool thermal deformation machine morphology, usually refers to the structure, mass distribution, material properties and heat source distribution. Morphology affect temperature distribution of the machine tool, the heat conduction direction, the direction of thermal deformation and matching.
(1) the structure of the machine tool morphology. In the overall structure, vertical, horizontal, gantry and cantilever machine have a large difference in the thermal response and stability. For example, the temperature rise of the lathe headstock gear change can be as high as 35 ℃, the spindle end elevation thermal equilibrium time required 2h. The slant bed-style precision milling machining center, machine tools have a stable base. Significantly improved machine stiffness, the spindle servo motor driver, in addition to the gear portion, its temperature is generally less than 15 ° C.
(2) the distribution of the heat source. The machine usually think that the heat source is the motor. Such as the spindle motor, the feed motor, and a hydraulic system, in fact, is not complete. The heat generation of the motor only in the bear load, the current consumption of energy in the armature impedance fever caused by the friction work otherwise a considerable part of the energy consumed in the bodies of the bearing, the screw nut and guideway. So called the motor once the heat source, the bearings, nuts, rail and chip called the secondary heat source. Thermal deformation is the result of all of these heat sources combined effects.
A traveling column vertical machining center Y to feed the temperature rise in the movement and deformation as shown in Figure 2. Y to feed workbench not for sports, thermal deformation of X to little effect. In the column, a point farther away from the Y-axis rail screw, its temperature decreases.
When the Z-axis movement of the aircraft shown in Figure 3. The comparison of the two figures, further heat source distribution of thermal deformation. Z-axis feed off X farther, so the impact of thermal deformation smaller, the column from the Z-axis motor nut closer the temperature rise and the greater the deformation.
(3) the impact of the mass distribution. The mass distribution of machine tool thermal deformation. First, the quality of the size and the degree of concentration, and usually refers to the change of the heat capacity and the heat transfer speed, to change the time to reach thermal equilibrium; secondly, by changing the form of an arrangement of quality, such as the arrangement of the various ribs, to enhance the thermal rigidity of the structure, In the case of the same temperature rise, reducing the influence of thermal deformation or to maintain relatively small deformation; Third, the means by changing the quality is arranged in the form of, such as arranged in a structure of the external cooling ribs, in order to reduce the temperature rise of the machine components.
(4) the influence of the material properties: different materials have different thermal properties (specific heat, thermal conductivity and linear expansion coefficient), under the influence of heat in the same, and its temperature, deformation varying.
3. Machine thermal performance test
The 3.1 machine thermal performance testing purposes
The key to controlling the machine tool thermal deformation is tested by thermal characteristics, fully understand the machine tool in which the ambient temperature changes, the response of the machine itself, the heat source and the temperature change as well as key point (distortion displacement). Test data or curve describes the the machine thermal characteristics, in order to take measures to control the thermal deformation to improve the machining accuracy and efficiency. Specifically, should achieve the following five purposes:
(1) machine ambient test. Measuring the temperature inside the workshop environment, space temperature gradient diurnal cycles in temperature distribution changes, or even should measure the seasonal variations of the temperature distribution of the machine around.
(2) test the thermal characteristics of the machine itself. Under conditions as much as possible to exclude environmental interference, so that the machine in the various operating state, in order to measure the change in temperature of the important points of the machine itself, the displacement change, recorded in a long enough period of time the displacement of the temperature change and the key points, and also The available infrared thermal phase meter record heat distribution in each time period.
(3) testing process temperature rise thermal deformation, to determine machine tool thermal deformation processing accuracy.
(4) The test may accumulate large amounts of data, the curve for machine design and user control of thermal deformation reliable criterion, pointed out the direction to take effective measures.
3.2 machine tool thermal deformation test principle
The hot deformation test first need to measure the temperature of a number of relevant points, include the following aspects:
(1) heat: including the various parts of the feed motor, spindle motor, ball screw drive vice, rail, spindle bearings.
(2) assist device: including hydraulic systems, chillers, cooling and lubrication displacement detection system.
(3) Mechanical structure: including bed base, skateboarding, columns and milling head box and the spindle.
Indium steel measuring rods are held sandwiched between the spindle and the rotary table 5 touch sensor is arranged in the X, Y, Z directions, and measured under various conditions deformation simulation tool and the workpiece relative displacement between the .
The figure depicts the state of the machine operating load can be divided into steps alone run test, to record various load state of the thermal deformation. For example, the spindle is rotated at different speeds, each feed shaft at different speeds, respectively feed the tool cutting condition, with or without lubrication and cooling state, by adding a constant load, change load or mixed load, but also including the consideration of the added to the actual operation of the manner and other factors.
3.3 Test Data processing analysis
Machine tool thermal deformation test to be carried out in a long continuous time, continuous data recording, after hot deformation characteristics of reliability, as reflected in the high analytical processing. If the error is removed through several experiments, the regularity of the display can be trusted.
Test record spindle system thermal deformation curve shown in Figure 5. The trials were set five measurement points, which point 1, point 2 in the spindle end and near the spindle bearings at point 4, point 5 Z milling head casing near the rail at. Test time lasted 14h, 10h spindle speed alternating variable speed range of 0 ~ 9000r/min from 10h spindle continued to 9000r/min high-speed rotation. Can be obtained from the figure the following conclusions:
(1) about the time when the spindle is in the heat balance of about 1h after balancing the temperature rise range of 1.5 ° C;
(2) temperature rise comes mainly from the spindle bearings and spindle motor in the normal range of variable speed, bearing good thermal performance;
(3) Heat distortion in the X direction impact is small, approximately <0.005mm, Y to the distortion is much larger than the X-directional view of the point 4, the point 5 the temperature rise of <1 ° C, the deformation of the spindle to Y is mainly derived from the milling head box structure, not associated with the screw and guide rail. Casting structure should be improved milling head;
(4) Z to the stretching deformation of about 10μm, the increase caused by the thermal elongation of the spindle and the bearing gap;
(5) When the speed last 9000r/min when temperature rise is a sharp rise in 2.5h surged about 7 ℃, and the trend continues to rise, Y and Z-deformation reached 29μm and 37μm, the spindle in speed is 9000r/min when stable operation, but may be a short period of time (≤ 20min) run.
4. Machine tool thermal deformation control.
By discussion of the above analysis, the impact of temperature rise and thermal deformation of the machine tool for machining precision a variety of other factors to take control measures should seize the main contradiction focus to take one or two measures, achieving a multiplier effect. Design and should be in four directions: to reduce fever, reduce temperature, well balanced, reasonable cooling.
4.1 reduce fever
The heat source is fundamental measures. To take measures to reduce the heat of the heat source in the design.
(1) reasonable selection of the rated power of the motor. The motor output power P is equal to the product of the voltage V and the current I, under normal circumstances, the voltage V is constant, therefore, the load increases, means that the motor output power is increased, i.e., the corresponding current I is also increased, the current calories consumed in the armature impedance increases. If the motor design choices for a long time working in close or greatly exceed the rated power conditions, the temperature rise of the motor is increased significantly. BK50 NC pin slot milling machine milling head comparison test (motor speed: 960r/min; ambient temperature: 12 ° C).
From the tests the following concepts: performance considerations from the heat source, regardless of the spindle motor or feed motor, select the rated power, the best election than the calculated power preferably about 25%, in the actual operation, the output power of the motor and load phase match, increased small motor rated power for energy consumption. But can effectively reduce the motor temperature rise.
(2) structure to take appropriate measures to reduce the secondary heat source heat, reduce the temperature rise. Such as: the spindle structural design, front and rear bearings should improve concentricity, high precision bearings. Conditions will slide rails instead linear rolling guide, or a linear motor. These new technologies can be effective to reduce friction, reduce fever, and reduce the temperature rise.
(3) In the process, the use of high-speed cutting. Based on the mechanism of high-speed cutting. Cutting metal when the metal cutting line speed is higher than a certain range, the time to produce the plastic deformation of deformable heat is not generated on the chip, the majority of the cutting energy converted to chips kinetic energy is taken away.
4.2 balanced structure to reduce the thermal deformation
On the machine, the heat source is always there further need to focus on is how to make the heat transfer direction and speed helps to reduce the thermal deformation. Or structures that have good symmetry, along the symmetry direction of the heat transfer through the uniform temperature distribution, deformation, offset each other, become hot affinity structures.
(1) prestressed and thermal deformation. Both ends of the ball screw in the relatively high speed of the feed system, often axially fixed, pre-tensile stress is formed. This structure for high-speed feed, in addition to increased static and dynamic stability, reducing thermal deformation error has a significant role. Ballscrew fixed manner the influence of the thermal deformation of the feed drive system is shown in Figure 6.
Pre-stretched within the total length 600mm 35μm axially fixed structure is relatively close to the temperature rise in the different feed speeds. The cumulative error of the fixed ends of pre-stretched structure is significantly less than the elongated structures of the single-end fixed to the other end free. Ends axially fixed, pre-stretched structure, the temperature rise caused by heat is mainly change the stress state of the screw inside the - by the tensile stress becomes zero stress or compressive stress. Therefore, the displacement accuracy was less affected.
(2) changing the structure, change the direction of the heat distortion.
The Z-axis spindle of different ball screw axially fixed structure NC needle groove milling carriage, such as (Figure 7). Requirements in the processing of the the milling groove depth error ≤ 5μm. The screw lower end of the axial floating structure, processing 2h groove depth gradually deepened from 0 to 0.045mm. Conversely, the use of the floating structure of the screw upper end, it will ensure that the groove depth changes <5μm.
(3) symmetry of the geometry of the machine tool structure will enable the thermal deformation of the same trend, minimize the tip point drift. Micro-machining center Japan Yasuda (Yasda) Precision Tool Company introduced YMC430 submicron high-speed machine tools, the design of the thermal performance of the machine were fully taken into account, the overall structure of the configuration shown in Figure 8 below.
Seen from the figure, first, taken in the machine structure completely symmetrical layout, the columns and beams is an integrated structure, an H type, equivalent to the double-pillar structure, having a good symmetry. Nearly circular spindle slide, whether in portrait or landscape orientation are also symmetrical.
Three moving axes feed drive linear motor structure easier to achieve symmetry, two rotary axis with direct drive, friction loss and minimize mechanical transmission.
4.3 reasonable Cooling measures
(1) processing of the coolant is directly machining accuracy. GRV450C double end grinders comparative test data are shown in Table 2. The tests showed that: With the refrigerator the cooling liquid to the heat exchange process, is very effective to improve the machining accuracy.
Using a conventional cooling liquid supply, 30min later, the workpiece size ultra-poor. Using the refrigerator, the normal processing to 70min. 80min when the size of the workpiece over poor mainly due to the grinding wheel needs trimming (removal of the grinding wheel surface metal shavings), trimmed as soon as you can back to the original machining accuracy. The effect is very obvious. Similarly, forced cooling spindle can expect to get very good results.
(2) increase the area of natural cooling. For example, in the spindle housing structure to add natural air cooling area, in better airflow workshop, also can play a good cooling effect.
(3) timely automatic chip. Timely or real-time high-temperature chip discharging the workpiece, table and tool part, will be very beneficial to reduce key part of the temperature rise and thermal deformation.
5. Outlook and vision
Control machine tool thermal deformation is an important topic of modern precision machining fields, the factors that affect the machine tool thermal deformation is very complex. Furthermore, modern cutting speed, high efficiency, precision three simultaneously, so that the thermal deformation of the machine even more prominent. Aroused extensive attention in machine tool manufacturing sector. Domestic machine tool industry scholars have done a lot of research, in theory made considerable progress. Thermal deformation of the machine has become one of the basic theory of machine research.
Analyzed from the point of view of machine tool design and application of machine tool thermal performance factors, methods of measurement and analysis and improved design measures. Thus, we believe that the machine thermal performance optimization design from the following aspects:
(1) the design phase of the modern high-end machine, you should pay attention to the design of the machine of the future applications of environmental conditions.
(2) control and configuration of the heat source is the key. Control the main heat source means to control energy consumption and power source match, using the new structure, reducing the heat source of the secondary friction, improve energy utilization.
(3) change the traditional thinking, cooling, cooling, lubrication, chip removal device status from "auxiliary" components of the machine, upgrade to the "important" parts status, we must not underestimate.
(4) emphasis on the symmetry of the structure and the design of the thermal deformation direction, so that the impact of thermal deformation on the accuracy reduced to a minimum, with particular emphasis on the research and application of mathematical model of thermal deformation of structural components for thermal deformation control designed to provide a qualitative quantitative indicator.