Basics of Cutting Heat And Cutting Temperature In Metal Cutting

Cutting heat and the resulting cutting temperature are important physical phenomena in the process of metal cutting.

A large amount of cutting heat increases the temperature of the cutting area, 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 automatic production, so it is of great significance to study the change law of cutting heat and cutting temperature for production time.

(1) Generation and emission of cutting heat

Elastic deformation and plastic deformation of the cut metal layer occur under the action of the tool, which is a source of cutting heat.

At the same time, the friction work consumed between chip and rake face, workpiece and rake face will also be converted into heat energy, which is another source of cutting heat (see Fig. 1-24).

generation and emission of cutting heat

Fig. 1-24 generation and emission of cutting heat

If the friction work on the flank and the work consumed by the feed motion are ignored, and assuming that all the work consumed by the main motion is converted into heat energy, the cutting heat generated per unit time can be calculated as follows:

Where

  • Qc — cutting heat generated in every second, unit: J / S;
  • Fz — main cutting force, unit: N;
  • vc — cutting speed, unit: m / s.

(2) Cutting temperature and its measuring method

Cutting temperature generally refers to the average temperature of the contact area between the tool and the workpiece.

There are many methods to measure cutting temperature, as shown in Fig. 1-25.

At present, the commonly used methods to measure cutting temperature are thermocouple method and photothermal radiation method.

The following will be described separately.

measurement method of cutting temperature

Fig. 1-25 measurement method of cutting temperature

1. Thermocouple method

Thermocouple method is divided into natural thermocouple method and artificial thermocouple method (see table 1-5).

Table 1-5 comparison between natural thermocouple and artificial thermocouple

Temperature measurement methodPrinciple of temperature measurementAdvantageDisadvantage
Natural thermocoupleAs shown in Fig. 1-26, the tool and workpiece are used as the two poles of the thermocouple to form a thermoelectric circuit for measurement.During cutting, the cutting tools and workpieces with different chemical compositions form the hot end of the thermocouple under the action of high cutting temperature, and the leading out end of the workpiece and tool forms the cold end of the thermocouple.There must be thermoelectric potential in this circuit. Measure or record its value with an instrument, and then according to the thermoelectric potential and temperature relationship curve of the thermocouple composed of tool and workpiece marked in advance, the cutting temperature in the contact area between tool and workpiece can be obtained.The cutting temperature measured by the natural thermocouple method is the average temperature of the cutting area.Using this method to study the variation law of cutting temperature is relatively simple and reliable.Every time a tool material or workpiece material is changed, it needs to be calibrated again to get a new calibration curve.Moreover, the temperature of the specified point in the cutting area cannot be measured by the natural thermocouple method.
Artificial thermocoupleTwo kinds of pre calibrated metal wires form a thermocouple, the hot end is welded to the temperature point to be measured of the tool or workpiece, and the cold end is connected with a potentiometer or millivoltmeter in series through wires.According to the indicated values on the table and the thermocouple calibration curve, the temperature on the welding point can be measured.As shown in Fig. 1-27, it is a schematic diagram of measuring the temperature of a point in the tool rake face (figure a) and workpiece cutting area (Fig. b) by artificial thermocouple method.It can measure the temperature of the specified point on the cutting area.Using the artificial thermocouple method, the temperature can only be measured at a certain point at a certain distance from the rake face, but not directly on the rake face.It also destroys the temperature field of the measurement area.
schematic diagram of temperature measurement by natural thermocouple method

Fig. 1-26 schematic diagram of temperature measurement by natural thermocouple method

  • 1. Copper tip
  • 2. Copper pin
  • 3. Lathe spindle tail
  • 4. Workpiece
  • 5. Cutting tools
Schematic diagram of thermocouple manual temperature measurement method

Fig. 1-27 Schematic diagram of thermocouple manual temperature measurement method

  • a) Measure the temperature of the rake face
  • b) Measure workpiece temperature

If you want to know the temperature on the rake face, you need to calculate it by using the principle and formula of heat transfer.

The cutting temperature distribution of cutting tool, chip and workpiece obtained by using artificial thermocouple method and heat transfer calculation (see Fig. 1-28).

cutting temperature distribution

Fig. 1-28 cutting temperature distribution

a) Temperature distribution in tool, workpiece and chip processing conditions: tool P10, vc = 600m / min

b) Temperature distribution on the rake face of the tool processing conditions: workpiece 30Mn4, ap = 3mm, f = 0.25mm/r

The distribution law of cutting temperature can be seen from fig. 1-28:

(1) The highest temperature on the rake face is not at the cutting edge, but at a certain distance from the edge.

The greater the plasticity of the workpiece material, the farther it is from the edge, and vice versa.

This is because there is an accumulation process of heat along the rake face, which is also where the tool wear is serious.

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

2. Photothermal radiation method

In addition to the thermocouple temperature measurement method, the cutting temperature can also be determined by observing the changes of metallographic structure of cutting tools or parts before and after cutting, but these two methods are not intuitive, and the workload of observation and analysis is large.

Recently, more and more infrared thermometers or light energy batteries are used to measure the cutting temperature.

The thermal imager uses the infrared principle to measure the cutting temperature.

It is a detection device that detects the infrared heat through non-contact, converts it to generate thermal image and temperature value, and then displays it on the display, and can calculate the temperature value.

As shown in Fig. 1-29, the field diagram of temperature measurement with thermal imager is shown. Fig. 1-30 shows the cutting temperature field collected by the supporting software of the thermal imager.

The software can collect the cutting process with infrared image.

When the accurate emissivity is determined, set the playback speed, adjust the playback position, and measure and draw the temperature distribution in the acquisition and cutting process.

field temperature measurement with thermal imager

Fig. 1-29 field temperature measurement with thermal imager

measured temperature field

Fig. 1-30 measured temperature field

3. Relationship between chip color and cutting temperature

In production practice, the approximate temperature of the tool tip can be judged by the color of the chips during cutting.

Taking turning carbon structural steel as an example, with the increase of cutting temperature, the change process of chip color is as follows: silver white → yellow white → golden yellow → purple → light blue → dark blue.

Among them, the cutting temperature reflected by silver white chips is about 200 ℃, the cutting temperature reflected by golden chips is about 400 ℃, and the cutting temperature reflected by dark blue chips is about 600 ℃.

(3) Main factors affecting cutting temperature

1. Workpiece material

The higher the strength and hardness of the workpiece material, the more work consumed during cutting, the more cutting heat generated, and the higher the cutting temperature.

The greater the thermal conductivity of the workpiece material, the more heat is transmitted through the chip and the workpiece, and the faster the cutting temperature decreases.

2. Tool geometric parameters

When 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.

When the current angle is greater than 20 ° ~ 25 °, the influence of rake angle on cutting temperature will be reduced.

As the main deflection angle decreases, the cutting width increases, the heat dissipation area increases and the cutting temperature decreases, as shown in Fig. 1-31 and Fig. 1-32.

effect of rake angle on cutting temperature

Fig. 1-31 effect of rake angle on cutting temperature

effect of main deflection angle on cutting temperature

Fig. 1-32 effect of main deflection angle on cutting temperature

3. Cutting parameters

The cutting parameter that has the greatest influence on the cutting temperature is the cutting speed, followed by the feed rate, and the back feed rate has the least influence.

This is because when the cutting speed VC increases, the amount of metal involved in deformation per unit time increases, which increases the power consumed and the cutting temperature;

When f increases, the chip thickens and the heat carried by the chip increases, so the increase of cutting temperature is not obvious;

When ap increases, the generated heat and heat dissipation area increase at the same time, so it has little impact on the cutting temperature, as shown in Fig. 1-33.

effect of cutting parameters on cutting temperature

Fig. 1-33 effect of cutting parameters on cutting temperature

  • a) Influence of cutting speed
  • b) Effect of feed rate
  • c) Influence of back knife amount

4. Other factors

When the wear on the flank of the tool increases, the friction between the tool and the workpiece increases, which increases the cutting temperature.

The higher the cutting speed, the more significant the impact of tool wear on the cutting temperature (see Fig. 1-34).

Pouring cutting fluid has an obvious effect on reducing cutting temperature, reducing tool wear and improving the quality of machined surface.

The lubrication of cutting fluid can reduce friction and reduce the generation of cutting heat.

relationship between flank wear and cutting temperature under different cutting speeds

Fig. 1-34 relationship between flank wear and cutting temperature under different cutting speeds

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