Metal cutting generates temperatures as high as 800 to 900 ℃ in the cutting zone, where the cutting edge causes deformation of the workpiece material and cuts it off. In continuous turning, heat is generated in a stable linear manner. On the contrary, the milling cutter teeth intermittently cut in and out of the workpiece material, and the temperature of the cutting edge also alternately increases and decreases. The components of the processing system absorb the heat generated during the metal cutting process. Usually, 10% of the heat enters the workpiece, 80% enters the chips, and 10% enters the tool. The outstanding situation is that chips carry away the vast majority of heat, as high temperatures can shorten tool life and damage the processed parts.

The different thermal conductivity of workpiece materials and other processing factors can have a significant impact on the distribution of heat. When processing workpieces with poor thermal conductivity, the heat transmitted to the tool will increase. Processing materials with higher hardness will generate more heat than processing materials with lower hardness. In general, higher cutting speeds increase heat generation, and higher feed rates increase the area of the cutting edge affected by high temperatures.

In intermittent cutting conditions dominated by milling, the selection of tool engagement curvature, feed rate, cutting speed, and cutting edge groove shape all have an impact on the generation, absorption, and control of heat.

1、 Meshing radian

Due to the intermittent nature of the milling process, the cutting teeth only generate heat during a portion of the processing time. The percentage of cutting time for cutting teeth is determined by the meshing arc of the milling cutter, which is influenced by the radial cutting depth and tool diameter.

The meshing arc of different milling processes also varies. In slot milling, the workpiece material surrounds half of the tool, and the meshing arc is 100% of the tool diameter. Half of the processing time of the cutting edge is spent on cutting, so heat quickly accumulates. In side milling, the relatively small part of the tool engages with the workpiece, giving the cutting edge more opportunities to dissipate heat into the air.

2、 Cutting speed

In order to maintain the chip thickness and temperature in the cutting area equal to the value of the tool during full cutting, the tool supplier has developed a compensation coefficient to increase the cutting speed as the percentage of tool engagement decreases.

From the perspective of thermal load, if the meshing arc is small, the cutting time may not be sufficient to produce the low temperature required for a longer tool life. Increasing the cutting speed usually generates more heat, and combining small meshing arcs with higher cutting speeds helps to raise the cutting temperature to the desired level. A higher cutting speed will shorten the contact time between the cutting edge and the chip, thereby reducing the heat transmitted to the tool. Overall, higher cutting speeds will reduce processing time and improve productivity.

On the other hand, lower cutting speeds will lower the machining temperature. Excessive heat is generated during machining, and reducing the cutting speed can lower the temperature to an acceptable level.

3、 Cutting thickness

Chip thickness has a significant impact on heat and tool life. Excessive chip thickness can cause excessive heat and chips, and even lead to cutting edge fracture due to the resulting heavy load. If the chip thickness is too small, the cutting process is only carried out on the smaller part of the cutting edge, and the increased friction and heat can lead to rapid wear.

The thickness of the chips generated during milling will continuously change as the cutting edge enters and exits the workpiece. Therefore, tool suppliers use the concept of "average chip thickness" to calculate the tool feed rate aimed at maintaining excellent chip thickness.

The factors involved in determining the correct feed rate include the meshing arc or radial cutting depth of the tool, as well as the main deviation angle of the cutting edge. The larger the meshing arc, the smaller the feed required to produce the ideal average chip thickness. Similarly, the smaller the meshing arc of the tool, the higher the feed rate is required to obtain the same chip thickness. The main deviation angle of the cutting edge of the tool also affects the feed requirements. When the cutting edge deviation angle is 90 °, the chip thickness ※ is large. Therefore, in order to achieve the same average chip thickness, reducing the main deviation angle of the cutting edge requires increasing the feed rate.

4、 Cutting edge groove shape

The geometric angle and cutting edge of the milling cutter body help to control thermal load. The hardness and surface condition of the workpiece material determine the selection of the tool rake angle. Tools with positive rake angles generate less cutting force and heat, and can also use higher cutting speeds. However, positive rake angle tools are weaker than negative rake angle tools, and negative rake angle tools can generate greater cutting force and higher cutting temperature.

The groove shape of the cutting edge can cause and control cutting action and cutting force, thereby affecting the generation of heat. The cutting edge in contact with the workpiece can be chamfered, passivated, or sharp. The strength of the edge after chamfering or passivation is greater, resulting in greater cutting force and more heat. Sharp cutting edges can reduce cutting force and lower machining temperature.

The chamfer behind the cutting edge is used to guide the chip, which can be either a positive or negative chamfer. The positive chamfer also generates a lower processing temperature, while the negative chamfer design has higher strength and generates more heat.

The milling process is intermittent cutting, and the chip control characteristics of milling tools are usually not as important as in turning. It may become very important to determine the energy required to form and guide chips based on the material of the workpiece involved and the meshing arc. Narrow or forced chip breaking chip control grooves can immediately roll up chips and generate greater cutting force and more heat. Wider chip control grooves can generate smaller cutting forces and lower machining temperatures, but may not be suitable for certain combinations of workpiece materials and cutting parameters.

5、 Cooling

The method of controlling the heat generated in metal cutting is to control the application of coolant. Excessive temperature can cause rapid wear or deformation of the cutting edge, so it is necessary to control the heat as soon as possible. In order to effectively reduce temperature, it is necessary to cool the heat source.

Multiple interrelated factors together form the load in metal cutting processing. During the processing, these factors will interact with each other. This article explores the heat issues in milling processing and their relationship with mechanical factors. Familiarity with the various factors that generate metal cutting loads and the overall results of their interactions will help manufacturers optimize their machining processes and maximize productivity and profitability.