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TheReasonof VariousTemperature Difference To The Precision Of NC Machine
Aug 07, 2017

2017-08-04 · CNC China Forum 0 keep abreast

The machine is affected by the change of the ambient temperature of the workshop, the heating of the motor and the friction of the mechanical movement, the heat of the cutting and the influence of the cooling medium, resulting in the uneven temperature rise of the machine parts, resulting in the change of the machine shape precision and machining precision.

For example, in a common precision CNC milling machine processing 70mm × 1650mm screw, 7: 30-9: 00 AM milling the workpiece and the afternoon 2: 00-3: 30 compared to the workpiece, the cumulative error changes up to 85m. And in constant temperature conditions, the error can be reduced to 40m.

Another example, a double-face grinding 0.6 ~ 3.5mm thick steel sheet workpiece precision double-face grinder, in the acceptance of processing 200mm × 25mm × 1.08mm steel workpiece can achieve mm dimensional accuracy, Full length less than 5m. But continuous automatic grinding 1h, the size of the range increased to 12m, the coolant temperature from 17 ℃ when the boot up to 45 ℃. Due to the impact of grinding heat, resulting in the spindle shaft elongation, the spindle front bearing clearance increases. Accordingly, for the machine cooler tank to add a 5.5kW chiller, the effect is very satisfactory.

Practice has proved that the deformation of the machine after heating is an important reason for the accuracy of processing. But the machine is in the environment where the temperature changes anywhere; the machine itself in the work will inevitably consume energy, a considerable part of these energy will be converted into heat in a variety of ways, causing the physical changes in the machine components, this change because Different forms of structure, material differences and other reasons vary widely. Machine designers should grasp the formation mechanism of hot and temperature distribution, take appropriate measures to make the thermal deformation of the processing accuracy to minimize the impact.

First, the machine temperature and temperature distribution

The same time as

1, natural climate effects

China is vast, most of the region in the subtropical regions, the temperature changes throughout the year, the temperature difference within one day is not the same. Thus, people on the indoor (such as workshop) the temperature of the way and the degree of intervention is different, the temperature around the machine atmosphere vary widely.

For example, the Yangtze River Delta region seasonal temperature range of about 45 ℃, day and night temperature changes of about 5 ~ 12 ℃. Machining workshop generally no heating in winter, no air conditioning in summer, but as long as the workshop ventilation is better, the temperature gradient of the machine shop is not changed. While the northeastern region, the seasonal temperature difference of up to 60 ℃, day and night changes of about 8 ~ 15 ℃. Every year in late October to the beginning of April next year for the heating period, the machining workshop design heating, air circulation. Inside and outside the workshop temperature difference of up to 50 ℃. Therefore, the temperature gradient in the workshop is very complex, the outdoor temperature of 1.5 ℃, the time is 8: 15-8: 35, the workshop temperature changes of about 3.5 ℃. The precision of precision machine tools in such a workshop is affected by ambient temperature will be great.

2, the impact of the surrounding environment

Machine environment is the environment around the machine within a wide range of the formation of the thermal environment. They include the following three aspects.

(1) workshop microclimate: such as the distribution of temperature within the workshop (vertical direction, horizontal direction). When the day and night alternately or climate and ventilation changes when the workshop temperature will produce a slow change.

(2) workshop heat source: such as solar radiation, heating equipment and high-power lighting, such as radiation, they are closer to the machine can be a long time directly affect the machine as a whole or part of the temperature rise. The heat generated by the adjacent equipment during operation will affect the temperature rise of the machine by way of radiation or air flow.

(3) heat: the foundation has a better heat dissipation, especially the precision machine tool foundation should not be close to the underground heating pipe, once the rupture leak may be difficult to find a reason for the heat source; open workshop will be a good " Radiator ", is conducive to the workshop temperature balance.

(4) constant temperature: the workshop to take constant temperature facilities on precision machine tools to maintain precision and processing accuracy is very effective, but the energy consumption.

3, the machine internal heat factors

The same time as

(1) machine structural heat source. Motor heat, such as spindle motor, feed servo motor, cooling lubrication pump motor, electric control box, etc. can produce heat. These conditions are permissible for the motor itself, but there are significant adverse effects on components such as spindles and ball screws, and measures should be taken to isolate them. When the input electric drive motor is running, most of it will be converted into kinetic energy, such as spindle rotation, table movement, etc., except for a small part (about 20%) converted to motor thermal energy; but there is still a considerable part In the course of the movement into friction heating, such as bearings, rails, ball screws and transmission boxes and other institutions fever.

(2) the process of cutting heat. During the cutting process, part of the kinetic energy of the tool or workpiece is consumed by the cutting work, and a considerable part of the deformation of the cutting tool and the friction between the chip and the tool, the formation of the tool, the spindle and the workpiece heat, and a large number of chips heat conduction to the machine table fixture And other components. They will directly affect the relative position between the tool and the workpiece.

(3) cooling. Cooling is a reverse measure of machine temperature rise, such as motor cooling, cooling of the spindle assembly, and cooling of the foundation. High-end machine tools are often equipped with electric control box chiller, to be forced to cool.

4, the impact of the structure of the machine on the temperature rise

In the field of machine thermal deformation to discuss the structure of the machine structure, usually refers to the structure, quality distribution, material properties and heat distribution and other issues. The structure affects the temperature distribution of the machine, the direction of heat conduction, the direction of thermal deformation and matching.

(1) the structure of the machine. In the overall structure, the machine has vertical, horizontal, gantry and cantilevered, etc., for the heat response and stability are quite different. For example, the gearbox speed of the lathe can be up to 35 ℃, so that the spindle end lift, heat balance time to be about 2h. The slant bed-type precision milling machine processing center, the machine has a stable base. Significantly improve the rigidity of the machine, the spindle using servo motor drive, remove the gear transmission part of the temperature rise is generally less than 15 ℃.

(2) the effect of heat distribution. Machine is usually considered the heat source refers to the motor. Such as spindle motors, feed motors and hydraulic systems, is not complete. The heat of the motor is only in the load, the current consumption in the armature impedance on the energy, and a considerable part of the energy consumed in the bearings, screw nuts and rails and other institutions caused by friction work fever. So the motor can be called a heat source, the bearings, nuts, rails and chips called secondary heat source. Thermal deformation is the result of all these heat sources.

Temperature rise and deformation of a column vertical vertical machining center in Y directional feed. Y is not moving when the table feed, so the thermal deformation of the X to the impact of small. On the column, the farther away from the Y-axis guide lead, the smaller the temperature rise.

The machine in the Z-axis when the situation is further explained the heat distribution on the thermal deformation of the impact. Z axis feed from the X to the farther, so the thermal deformation of the smaller, the column from the Z-axis motor nut closer, the greater the temperature rise and deformation.

(3) the impact of mass distribution. The influence of mass distribution on the thermal deformation of the machine is threefold. First, refers to the quality of the size and concentration, usually refers to change the heat capacity and heat transfer speed, change the time to reach the heat balance; Second, by changing the quality of the layout, such as the layout of various ribs to improve the thermal stiffness of the structure, In the case of the same temperature rise, reduce the thermal deformation of the impact or to maintain a relatively small deformation; the third refers to the quality of the layout by changing the form, such as the layout of the external structure of the heat dissipation plate to reduce the temperature rise of machine parts.

(4) the impact of material properties: different materials have different thermal performance parameters (specific heat, thermal conductivity and linear expansion coefficient), in the same heat under the influence of its temperature rise, deformation are different.

Second, the machine thermal performance test

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1, the purpose of machine thermal performance testing

The key to controlling the thermal deformation of a machine tool is to understand the changes in the ambient temperature of the machine, the heat source and temperature of the machine itself, and the response of the key points (deformation displacement) through the thermal characteristics test. The test data or curve describes the thermal characteristics of a machine in order to take countermeasures, control the thermal deformation, and improve the machining accuracy and efficiency of the machine. Specifically, the following objectives should be achieved:

(1) machine environment around the environment test. Measuring the temperature environment in the workshop, its spatial temperature gradient, changes in temperature distribution in day and night, and even the effect of seasonal variation on the temperature distribution around the machine.

(2) the thermal characteristics of the machine itself test. As far as possible to exclude environmental interference conditions, so that the machine is in a variety of operating conditions to measure the important point of the machine itself, the temperature changes, displacement changes, recorded in a long enough time period of temperature changes and key points of displacement, also The infrared profile can be used to record the thermal distribution of each time period.

(3) the process of testing temperature and heat distortion, to determine the thermal deformation of the machine tool on the accuracy of the process.

(4) The above test can accumulate a large number of data, curves, will provide a reliable criterion for machine design and user control of thermal deformation, pointing out the direction of taking effective measures.

2, the principle of thermal deformation testing machine

Thermal deformation test first need to measure the temperature of a number of related points, including the following aspects:

(1) heat source: including the various parts of the feed motor, spindle motor, ball screw drive, rail, spindle bearings.

(2) Auxiliary devices: including hydraulic system, refrigerator, cooling and lubrication displacement detection system.

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

An indium steel bar was clamped between the spindle and the rotary table, and five contact sensors were arranged in the X, Y, and Z directions, and the integrated deformation in various states was measured to simulate the relative displacement between the tool and the workpiece The

3, test data processing analysis

Machine thermal deformation test to be carried out in a long continuous period of time, continuous data recording, after analysis and processing, the heat distortion characteristics of the high reliability. If the error is removed by multiple tests, the regularity shown is credible.

There are five measuring points in the thermal deformation test of the spindle system, where point 1 and point 2 are at the end of the spindle and near the spindle bearing. Point 4 and point 5 are located near the Z-direction guide in the milling head housing. The test time lasted for 14h, in which the first 10h spindle speed in the range of 0 ~ 9000r / min alternately variable speed, starting from the first 10h, the spindle continued to 9000r / min high-speed rotation. You can get the following conclusions:

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

(2) the temperature rise mainly from the spindle bearing and spindle motor, in the normal range of speed, the bearing's thermal performance is good;

(3) thermal deformation in the X direction is very small;

(4) Z to the telescopic deformation is larger, about 10m, is the spindle of the thermal elongation and bearing clearance caused by the increase;

(5) When the speed is continuously at 9000r / min, the temperature rise rises sharply, rises by about 7 ℃ in 2.5h, and there is a tendency to continue to rise. The deformation of Y and Z reaches 29m and 37m, indicating that the spindle is Speed is 9000r / min can not be stable operation, but can be a short time (20min) run.

Third, the machine thermal deformation control

From the above analysis and discussion, the machine tool temperature rise and thermal deformation of the processing accuracy of a variety of factors, to take control measures, should seize the main contradictions, focusing on one or two measures to achieve a multiplier effect. In the design should be from the four directions: reduce heat, reduce the temperature rise, structural balance, reasonable cooling.

1, to reduce fever

Controlling heat sources is a fundamental measure. In the design to take measures to effectively reduce the heat of the heat.

(1) Reasonable selection of the rated power of the motor.

The output power P of the motor is equal to the product of the voltage V and the current I. In general, the voltage V is constant, so that the increase in the load means that the motor output power increases, i.e., the corresponding current I increases, The heat consumed in the armature impedance increases. If the motor we choose to design for a long time in the near or greatly exceeded the rated power conditions, the motor temperature increased significantly. To this end, the BK50 CNC needle slot milling machine head of the comparison test (motor speed: 960r / min; ambient temperature: 12 ℃).

From the above test to get the following concepts: from the performance of the heat source, regardless of the spindle motor or feed motor, select the rated power, the best choice than the calculated power of about 25% is appropriate, in actual operation, the motor output power and load phase Matching, increasing the motor rated power has little effect on energy consumption. But can effectively reduce the motor temperature rise.

(2) to take appropriate measures to reduce the heat of the secondary heat source, reduce the temperature rise.

For example: the spindle structure design, should improve the coaxiality of front and rear bearings, the use of high-precision bearings. Under possible conditions, change the slide rail to a straight rolling guide or use a linear motor. These new technologies can effectively reduce friction, reduce heat, reduce the temperature rise.

(3) in the process, the use of high-speed cutting. Based on the mechanism of high speed cutting.

When the metal cutting line speed is higher than a certain range, the cutting metal is too late to produce plastic deformation, the chip does not produce deformation heat, cutting energy into the chip kinetic energy is mostly taken away.

2, structural balance, to reduce the heat distortion

In the machine, the heat source is always there, and further need to focus on how to heat the direction and speed of the heat is conducive to reducing heat distortion. Or the structure has a good symmetry, so that the heat transfer along the symmetrical direction, so that the temperature distribution of uniform, deformation offset each other, a thermal affinity structure.

(1) prestress and thermal deformation.

In the higher speed of the feed system, often used both ends of the ball screw fixed to form a pre-tensile stress. This structure for high-speed feed, in addition to improving the dynamic and static stability, the reduction of thermal deformation error has a significant role.

In the full length of 600mm within the pre-stretching 35m axial fixed structure at different feed rate temperature is relatively close. The cumulative error of the fixed pre-tensioned structure at both ends is obviously smaller than that of the single-ended fixed end. In the axial fixed pre-tensioned structure at both ends, the temperature rise caused by heat is mainly to change the stress state inside the screw screw from tensile stress to zero stress or compressive stress. So the impact on the displacement accuracy is small.

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

The use of different ball screw axial fixed structure of the CNC needle slot milling machine Z axis spindle slide in the processing requirements of milling groove depth error 5m. Using the lower end of the screw axial floating structure, within 2h of processing, the groove depth gradually deepened from 0 to 0.045mm. Conversely, the use of screw on the top of the floating structure, you can ensure that the groove depth changes.

(3) the symmetry of the machine structure geometry, can make the thermal deformation to the same, so that the tip of the drift to minimize.

For example, Japan Yasuda (Yasda) precision tool company launched YMC430 micro-processing