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The relationship between cutting temperature measurement and cutting heat

Reading guide: cutting temperature is an important physical phenomenon in the process of metal cutting, a large amount of cutting heat makes the temperature of the cutting area rise, which directly affects the wear and life of the tool, and affects the machining accuracy and surface quality of the workpiece. Cutting temperature can also be used as a monitoring factor in automated production, so it is of great significance to study the law of cutting heat and cutting temperature change for production time.

(1) Generation and transmission of cutting heat

The cut metal layer undergoes elastic and plastic deformation under the action of the tool, which is a source of cutting heat. At the same time, the frictional work dissipated between the chip and rake face, and between the workpiece and flank face, is converted into heat, which is another source of cutting heat (see Figure 1).

If we ignore the frictional work on the flank face and the work expended by the feed motion, and assume that all the work expended by the main motion is converted into thermal energy, then the cutting heat generated per unit time can be calculated:

Qc=Fzvc

where Qc – the heat of cutting generated per second, in J/s;

Fz–main cutting force, in N;

vc – cutting speed, in m/s.

(2) Cutting temperature and its measurement method

Cutting temperature generally refers to the average temperature of the contact area between the tool and the workpiece. There are many ways to measure cutting temperature, as shown in Figure 2. At present, the most commonly used methods to measure cutting temperature are thermocouple method and photothermal radiation method. Each of these will be explained below.

1. Thermocouple method

The thermocouple method is further divided into natural thermocouple method and artificial thermocouple method.

To know the temperature on the rake face, it is necessary to apply the principles and formulas of heat transfer to make an estimate. The temperature distribution of the tool, chip and workpiece obtained by artificial thermocouple measurement and heat transfer calculation is supplemented by heat transfer calculation.

Distribution of cutting temperature:

(1) The highest temperature on the rake surface is not at the cutting edge, but at a certain distance from the cutting edge, the greater the plasticity of the workpiece material, the farther away from the cutting edge, and vice versa. This is because there is an accumulation of heat along the rake face, which is where tool wear is severe;

(2) The temperature gradient of the bottom layer of the chip is the largest, indicating that the friction heat is concentrated in the contact between the bottom layer of the chip and the rake face.

2. Photothermal radiation method

In addition to thermocouple thermometry, the cutting temperature can also be determined by observing the metallographic structure changes of the tool or part before and after cutting, but these two methods are not intuitive, and the workload of observation and analysis is large. Recently, infrared thermometers or solar cells are increasingly used to measure the cutting temperature. The thermal imager uses the infrared principle to measure the cutting temperature, which is a detection device that detects infrared heat through non-contact and converts it into a thermal image and temperature value, which is then displayed on the display and can calculate the temperature value. Figure 6 is a live demonstration of temperature measurement with a thermal imager. Figure 1-30 is the cutting temperature field map collected by the supporting software of the thermal imaging camera, the software can collect the cutting process with infrared images, set the playback speed, adjust the playback position, and measure and plot the temperature distribution of the collected cutting process under the condition of determining the accurate emissivity.

3. The relationship between chip color and cutting temperature

In production practice, the approximate temperature of the cutting edge can be judged by the color of the chip during cutting. Taking turned carbon structural steel as an example, with the increase of cutting temperature, the order of chip color change is: silvery-white→ yellow-white→ golden yellow→ purple→ light blue→ dark blue. Among them, the cutting temperature reflected by silver-white chips is about 200°C, the cutting temperature reflected by golden-yellow chips is about 400°C, and the cutting temperature reflected by dark blue chips is about 600°C.

(3) The main factors affecting the cutting temperature

1. Workpiece material

The higher the strength and hardness of the workpiece material, the more work is consumed during cutting, the more cutting heat is generated, and the higher the cutting temperature. The greater the thermal conductivity of the workpiece material, the more heat is transferred through the chips and the workpiece, and the faster the cutting temperature drops.

2. Cutter geometric parameters

The rake angle increases, the cutting deformation decreases, the heat generated is less, and the cutting temperature decreases; However, too large rake angle will reduce the heat dissipation volume, and when the front angle is greater than 20°~25°, the influence of the rake angle on the cutting temperature will be reduced. The entering angle decreases, so that the cutting width increases, the heat dissipation area increases, and the cutting temperature decreases.

3. Cutting amount

The cutting amount that has the greatest impact on the cutting temperature is the cutting speed, followed by the feed, and the back eating amount has the least impact, because when the cutting speed VC increases, the amount of metal involved in the deformation per unit time increases, so that the power consumed increases and the cutting temperature increases. When f increases, the chips become thicker, and the heat taken away by the chips increases, so the cutting temperature rises not very obviously. When AP increases, the heat generated and the heat dissipation area increase at the same time, so the influence on the cutting temperature is also small.

4. Other factors

When the amount of flank wear increases the amount of tool flank wear, the friction between the tool and the workpiece is intensified, so that the cutting temperature increases, and the higher the cutting speed, the more significant the influence of tool wear on the cutting temperature (see Fig. 11). Pouring cutting fluid has a significant effect on reducing cutting temperatures, reducing tool wear and improving the quality of machined surfaces. The lubricating effect of the cutting fluid reduces friction and reduces the generation of cutting heat.

 

 

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