3 machine tool thermal performance test

3.1 the purpose of the machine tool thermal performance test

Control machine tool thermal deformation is the key to the thermal characteristic test, fully understand the machine tool of the change of environmental temperature, heat source and the temperature change and key points of the machine tool itself response (deformation). Test data or curve describing the thermal characteristics of machine tools, in order to take countermeasures to control thermal deformation and improve the machining accuracy and efficiency of the machine tool. Specifically, should meet the following five objectives:

(1) machine tools surrounding environment test. Measure the temperature inside the workshop environment, it is the space temperature gradient, and the change of the temperature distribution in the day and night alternate, even should measure seasonal change on the influence of the temperature distribution around the machine.

(2) the thermal characteristics of the machine tool itself. As far as possible to eliminate the disturbance of environment conditions, make the machine is in different operating condition, in order to measure the temperature of the machine tool itself, the important point of change and displacement change, recorded in long enough within the period of temperature change and key point displacement, and can also be used infrared thermography phase meter records each time the distribution of the heat.

(3) processing thermal deformation temperature testing, to determine the accuracy of machine tool thermal deformation of machining process.

(4) the above tests can accumulate a large amount of data, curve, will provide design and user control machine tool thermal deformation with reliable criterion, pointed out the direction of the effective measures.

3.2 the principle of the machine tool thermal deformation test

Can be seen from the graph, thermal deformation test to measure the temperature of the several related points first, including the following aspects:

(1) the heat source, including the parts into to motor and spindle motor and ball screw transmission vice, guide rail, spindle bearings.

(2) auxiliary equipment: including the hydraulic system, chiller, cooling and lubrication displacement detection system.

(3) the mechanical structure, including bed, base, board, column and milling head box and spindle.

Between the spindle and the rotary table clamping have indium steel rods, measurement in the X, Y, Z direction configuration for five contact sensor, measurement in various condition of comprehensive deformation, to simulate the relative displacement between the tool and workpiece.

The plot describes the machine running load state, can separate operation test it step by step, to record various load conditions on the influence of thermal deformation. , for example, the main shaft rotate at different speeds, the feed shaft respectively with different feed speed, the cutting condition, whether the lubrication and cooling condition, to join the constant load, load or mixed load changes, also consider joining in the actual operation mode etc.

3.3 test data analysis

Machine tool thermal deformation test should be in for a long time, the continuous data records, through analysis of processing, which reflect the thermal deformation characteristics of the reliability is high. If through many experiments to eliminate the error, it shows the regularity is credible.

Spindle system thermal deformation test records of the curve shown in figure 5. Test of the communist party of China set up five points and the points 1 and 2 in the spindle end and close to the main shaft bearing, point 4, and 5 respectively in the milling head shell near the Z to guide. Test time have lasted 14 h, the top 10 h spindle speed in 0 ~ 9000 r/min range shift alternately, begin from 10 h, spindle for high speed rotation to 9000 r/min. The following conclusions can be obtained from the graph:

(1) of the spindle thermal equilibrium time about 1 h, balance after temperature range 1.5 ℃;

(2) temperature mainly comes from the spindle bearings and spindle motor, the normal variable speed range, bearing the hot state of good performance;

(3) the thermal deformation in X to influence is very small, and (4) Z to expansion deformation, about 10 m, is caused by increased thermal elongation and bearing clearance of the main shaft;

(5) when the rotating speed of the sustained in 9000 r/min, temperature rising sharply, in 2.5 h inside nasty rose 7 ℃ or so, and have continued to rise, Y and Z to deformation of the 29 m and 37 m, show that the spindle on rotating at 9000 r/min, has not stable operation, but can be a short period of time (20 min).

4 machine tool thermal deformation control

By the above analysis, the temperature rise and thermal deformation of machine tool for processing precision of the various influence factors and control measures, we should grasp the main contradiction, focus on one or two measures, achieve the result of get twice the result with half the effort. Starting from four directions: in the design to reduce fever, reduce the temperature rise, structural balance, reasonable cooling.

4.1 reduce fever

Control the heat source is the fundamental measures. In the design to take measures to reduce the heat source of heat.

(1) reasonable selection of the motor power rating. The output power of the motor I P V is equal to the voltage and current of the product, under normal circumstances, the voltage V is constant, therefore, the increase of load, means that the motor output power increases, the corresponding current I also increases, increases the quantity of heat of consumption in the armature current impedance. If we design the choice of the electric machine long time in close to or more than the rated power under the conditions of work, is the temperature of the motor. Therefore, to BK50 type nc needle slot milling machine milling head compared test (motor speed: 960 r/min, the ambient temperature: 12 ℃), the results shown in table 1.

From the above test to get the following concepts: from heat performance, regardless of the spindle motor or feed motor, choosing the rated power, had better choose about 25% computational power than advisable, in actual operation, the output power of the motor and load matching, increase the motor power rating for the influence of the energy consumption is small. But can effectively reduce the motor temperature rise.

(2) the structure to take appropriate measures to reduce the secondary heat source of heat, reduce the temperature rise. Such as spindle structure design, we should improve the coaxial tolerance of bearing before and after, using high precision bearings. Under the condition of the possible, change the slide guide rail to linear rolling guide, or using the linear motor. These new technologies can effectively reduce the friction, reduce heat, reduce the temperature rise.

(3) in the process, the high speed cutting. Based on the mechanism of high speed cutting. When the linear speed of metal cutting is a certain range, by cutting metal to produce plastic deformation, not deformation hot on the chip, cutting energy most into chip kinetic energy was taken.

4.2 structure of the balance, in order to reduce the thermal deformation

On the machine, heat is always exist, further need to concern is how to make heat transfer direction and speed to reduce the thermal deformation. Or structure and have a good symmetry, the heat transfer through the symmetrical direction, make uniform temperature distribution, deformation offset each other and become hot affinity structure.

(1) prestressed and thermal deformation. In the high-speed feeding system, often using the axial fixation on both ends of the ball screw, formation pre - tension stress. This structure for high speed feeding, in addition to improve the dynamic and static stability, to reduce the error of thermal deformation has obvious effect. Ball screw fixed way the influence of thermal deformation of feed drive system as shown in figure 6.

Pre - tension within the total length of 600 mm 35 m axial fixed structure under different feeding speed of temperature rise is close. Both ends fixed structure of the pre - tension accumulated error obviously less than single fixed at the other end free elongation of the structure. Axial at both ends fixed in the structure of the pre - tension, fever caused by temperature rise of the main stress state is to change the inside of the ball screw by the tensile stress become zero stress and compressive stress. So less influence on displacement accuracy.

(2) change the structure, change the direction of thermal deformation.

Using different ball screw axial fixed structure of the Z axis nc needle slot machine spindle slider such as (figure 7). Required in the processing of milling depth error is 5 m. Using screw axial floating structure, bottom in machining 2 h, groove depth gradually from 0 to 0.045 mm. On the other hand, the screw top floating structure, can ensure the groove depth changes (3) the geometry of the machine tool structure, can make thermal deformation to consistent, make of the tip point drift as far as possible. Yasudo from Japan, for example, (Yasda) precision tools company launched YMC430 micro machining center is sub-micron high-speed processing machine tools, the design of the machine on the thermal properties of full consideration, its overall structural configuration as shown in figure 8.

Can be seen from the graph, the first in the machine tool structure on completely symmetrical layout, column and beam structure is integration, the H type, equivalent to double column structure, has the good symmetry. Approximate circular spindle slider in both the longitudinal and transverse is symmetrical.

Three movements of feeding driven by linear motor, the structure is more easy to implement symmetry, two rotor adopts direct drive, try to reduce the friction loss and mechanical transmission.

4.3 reasonable cooling measures

(1) processing of the cooling fluid is the direct influence on the machining accuracy of. Compared to GRV450C double face grinding machine test, the data shown in table 2. Test showed that with the help of chiller, cooling fluid on heat treatment, to improve the machining accuracy is very effective.

Use the traditional way of coolant supply, after 30 min, the size is super poor. After the chiller, can normal processing to more than 70 min. Min in 80 when the workpiece size error is the main reason of grinding wheel to trim (remove metal chips) on the surface of the grinding wheel, grinding machining precision can reply as soon as the original. Effect is very obvious. Also, for the spindle of forced cooling can expect very good effect.

(2) increase the natural cooling area. Such as the spindle box structure add natural wind cooling area, the workshop is in better air circulation, also can have very good heat dissipation effect.

(3) automatic dust discharge in a timely manner. Timely or high temperature real-time chip from the workpiece, workbench and tool parts, will be very beneficial to reduce temperature rise and thermal deformation of key components.

5 prospects and vision

Control machine tool thermal deformation is an important topic in the field of modern precision processing, the factors influencing the machine tool thermal deformation is very complicated. Moreover, modern cutting and high speed, high efficiency, precision, makes the thermal deformation of machine tools problem more outstanding. Caused the wide attention of machine tool manufacturing industry. Machine tool industry at home and abroad scholars made a lot of research, has made considerable progress in theory. Machine tool thermal deformation problem has become one of the machine tool in the study of basic theory.

In this paper, from the Angle of the machine tool design and application of machine tool thermal properties is analyzed influence factor, measurement and analysis method and the design improvement measures are put forward. Thus, we think the optimum design of machine tool thermal properties from the following aspects:

(1) modern high-end machine tools in the design phase, it should pay attention to the design of machine tool in the future application of environmental conditions.

(2) control and configuration of heat source is the key. Control the heat source is mainly refers to control energy consumption and the matching of the power source, the new structure, reduce secondary friction heat and improve the utilization rate of energy.

(3) change the traditional thinking, the cooling, cooling, lubrication, scraps discharge device from "assistant" part of the machine tool position, the ascent to the "important" part position, cannot despise.

(4) pay attention to the structure of the symmetry, and the design of the direction of the thermal deformation, let to minimize the influence of thermal deformation on accuracy, especially we should attach importance to structure the research and application of mathematical model of thermal deformation and thermal deformation control design in order to provide the instruction of the qualitative and quantitative. Source of Chinese nc machine tool network)