The thermal deformation is one of the reasons that affect the machining accuracy

Machine tool by the workshop environment temperature change, heating of the motor and mechanical motion friction heat, heat cutting, and the influence of the cooling medium, caused by the rise of temperature of the machine tool by uneven, result in a change of shape precision and machining precision machine tool.

For example, in a common on the precision of the nc milling machine processing 70 mm x 1650 mm screw, at 7:30 in the morning - 9 milling workpiece compared with 2:00pm at 3:30 - the workpieces, the change of the cumulative error of up to 85 m. While under the condition of constant temperature, the error can be reduced to 40 m.

Again, such as a for double face grinding as 0.6 ~ 3.5 mm thick piece of workpiece precision double grinder, processing when acceptance 200 mm x 25 mm x 1.08 mm steel piece the dimensional accuracy of the workpiece can achieve mm bending in the span of less than 5 m. But continuous automatic grinding after 1 h, size range increased to 12 m, coolant temperature by the boot of 17 ℃ up to 45 ℃. Due to the influence of grinding heat, the spindle axis neck elongation, spindle bearing clearance before. Accordingly, for the machine tool box to add a 5.5 kW cooling liquid chiller, the effect is very ideal.

Practice proved that the machine after the heat of the deformation is the main reason that affect the machining accuracy. But the machine is in the temperature can change at any time in the environment of; Machine itself at work will consume energy, these energy is part of the will is converted into heat in a variety of ways, from the physical change of members of the machine tool, this change and because of the structure is different, the material of the reasons of the difference of differ in thousands ways. Machine tool designers should demonstrate a grasp of hot forming mechanism and temperature distribution law, adopt corresponding measures to reduce the influence of thermal deformation of machining precision to a minimum.

2 the temperature rise and the temperature distribution of machine tools

2.1 natural climate influence

China's vast, most of the area in the subtropical area, the four seasons of temperature change is bigger, changes in temperature is different in one day. Thus, people on indoor (such as workshop) the temperature of the type and extent of intervention is different also, the temperature of the atmosphere vary around the machine. Yangtze river delta region, for example, seasonal temperature variation range about 45 ℃, day and night temperature changes about 5 ~ 12 ℃. Machining workshop is no heating in winter, summer without air conditioning, ventilation is good, but as long as the workshop of machining workshop temperature gradient. And in the northeast of China, seasonal temperature difference can be up to 60 ℃, day and night change about 8 ~ 15 ℃. Late October each year to the next early April for the heating period, the design of the machining workshop with heating, air circulation. Temperature difference between inside and outside the workshop can be up to 50 ℃. So the workshop in winter the temperature gradient is very complex, measure the outdoor temperature 1.5 ℃, time for morning and, therefore, the workshop temperature 3.5 ℃, the results as shown in figure 1. Precision machine tool's machining accuracy in the workshop will be great affected by ambient temperature.

2.2 the influence of the surrounding environment

Environment refers to the machine from close range within the scope of all kinds of layout form of thermal environment. They include the following three aspects.

(1) workshop microclimate, such as the workshop temperature distribution (vertical direction and horizontal direction). When night and day alternate or climate and ventilation changes workshop temperature will be slow.

(2) workshop heat source, such as the sun, heating equipment and the radiation of high power lights, they leave a machine may directly affect the whole or part of the machine for a long time the rise of temperature of the parts. Adjacent equipment at run time heat effect will be made in the form of radiation or air flow temperature rise of the machine tool.

(3) heat: foundation has good heat dissipation effect, especially the foundations of the precision machine tool of avoid by all means near the heating pipe, burst leakage, can be a difficult to find the cause of heat source; Open the workshop will be a good "radiator", to shop the equilibrium temperature.

(4) constant: the workshop take constant temperature facilities to keep precision machine tool precision and machining precision is of great effect, but the large energy consumption.

2.3 machine tool the internal heat affected factors

(1) machine tool structural heat source. Motor fever such as spindle motor, servo motor, cooling lubrication pump motor, electric control, etc all can generate heat. These things is allowed for the motor itself, but for the spindle, ball screw and other components have significant adverse effects, measures should be taken to be isolated. When the input power drive motor, in addition to a few motor (about 20%) into heat energy, the most by the movement mechanism is converted into kinetic energy, such as the spindle and worktable movement, etc.; The inevitable however, there are still part of the conerted to friction heat in the sport, such as bearing, guide, ball screw and transmission box, etc.

(2) the process of cutting heat. In the process of cutting tool and the workpiece part of kinetic energy consumption in the cutting, a considerable number of transformation of the cutting deformation and friction between the chip and tool of hot forming cutter, the spindle and workpiece fever, and by a large number of chip heat conduction to the machine table fixture parts. They will have a direct impact on the relative position between the tool and workpiece.

(3) cooling. Cooling is for machine tool temperature of reverse measures, such as motor cooling, spindle units cooling as well as the basic structure, etc. High-end machine tends to match the electric refrigerator, to be forced cooling.

2.4 the structure of the machine to the influence of the temperature rise

Discussion in the field of machine tool thermal deformation of machine tool structure form, usually refers to the structure, mass distribution, material properties and the heat source distribution, etc. Affect the temperature of the machine tool structure form distribution, direction of heat conduction and thermal deformation direction and matching, etc.

(1) the structure of the machine tool. In terms of overall structure, the machine is vertical, horizontal, gantry and cantilever type and so on, for the thermal response and stability all have bigger difference. Such as the lathe spindle box of gear shift can be as high as 35 ℃, the temperature of the spindle end service, thermal equilibrium time should be 2 h. And inclined bed type is turning and milling machining center, precision machine tool has a stable base. Obviously increase the stiffness of the engine, the main axis adopt servo motor drive, removed the gear transmission part, its average temperature is less than 15 ℃.

(2) the effect of heat source distribution. Machine that heat is usually refers to the motor. Such as the spindle motor, feeding motor and hydraulic pressure system and so on, actually is not completely. Motor fever just bear the load, the current consumption of energy in the armature resistance, otherwise quite part of energy consumption in bearing, ball screw nut and guide mechanism of the friction caused by the power of fever. So a heat source can be called the motor, the bearings, nut, guides and chip called second heat source. Thermal deformation is all of these heat the influence of the comprehensive.

A pillar mobile vertical machining center in Y to feed movement of the temperature and deformation as shown in figure 2. Y to feed table when not exercise, so little impact on the thermal deformation of X to the. In the column, the farther from the Y axis guide screw of the point, the temperature rise is smaller.

The machine in the case of the Z axis moves as shown in figure 3. Two image contrast, further shows the heat source distribution on the influence of thermal deformation. Z axis feed from X to the farther, the thermal deformation influence smaller, column on the nearer the Z axis motor nut, the greater the temperature and deformation.

(3) the influence of mass distribution. Mass distribution there are three aspects of the influence of thermal deformation of machine tool. Firstly, refers to the quality size and concentration degree, usually refers to the speed of change in heat capacity and heat transfer, change the equilibrium time; Secondly, by changing the quality of decorate a form, such as all kinds of steel plate is decorated, raise thermal stiffness of the structure, in the case of the same temperature, reduce the thermal deformation effect or maintain relative deformation smaller; Thirdly, is refers to by changing the layout in the form of quality, such as the structure of external decorate heat steel plate, in order to reduce the rise of temperature of the machine components.

(4) the material properties, the influence of different materials have different thermal parameters (such as specific heat, thermal conductivity and coefficient of expansion), under the influence of the same quantity of heat, the temperature and deformation are different.