Tips on Machining Vol. 27
How to prevent material adhesion to tool tip during low-speed machining

Higher machining efficiency is crucial for saving machining costs. One of the ways to improve machining efficiency is increasing cutting (rotational) speed. And today, tools made of CBN or ceramics, or coated tools are increasingly used to achieve higher cutting speed.
Meanwhile, there are machining operations in which cutting speed cannot be increased. One such example is drilling. Cutting speed at a point close to the center of the drill is lower than that at the outermost, and the speed becomes zero at the chisel (center).
Tapping is another example. The responsiveness of the mechanical (drive) system to the command from the electrical (control) system becomes worse in proportion to the cutting speed. As low responsiveness negatively affects machining accuracy, cutting speed cannot be increased beyond a certain limit.
Also, you cannot increase cutting speed when using a low-rigidity tool or machining a fragile workpiece. These machining operations are inevitably done at low speeds.

In low-speed cutting, some melted portions of a workpiece, which are caused by cutting heat, adhere to the tool tip. This phenomenon is called “adhesion or welding.” Adhesion has a negative effect on finished surface quality and machining accuracy, and causes tool breakage. So, adhesion is one of the greatest enemies of stable machining, and thus needs to be suppressed.
In this issue, we will introduce three key points to prevent adhesion based on our experiment results.

(1) Increase rake angle

The rake angle serves as a tilt angle of a rake face, on which cutting chips slide. Figure 1 and 2 show workpiece surfaces that were cut with a non-coated cemented carbide tool at rake angles of 0°and 20°, respectively. As you can see, the amount of adhesion is large at 0° and is small at 20°. This indicates a large rake angle facilitates chip removal, which leads to reduced adhesion. However, it should be noted that the strength of the tool cutting edge decreases as the rake angle increases, making the edge easy to chip.

Fig. 1: Rake angle 0°

Fig. 2: Rake angle 20°

(2) Minimize friction on race face

Cutting chips slide on the rake face and are ejected to the outside. So, if friction on the rake face is reduced, chip evacuation is facilitated, and as a result adhesion is suppressed. To minimize friction on a rake face, you can use a tool with a mirror-polished rake face or one with coolant supply holes. Coolant with high lubricity is also effective in reducing adhesion.

(3) Use tool/coating materials with low chemical reactivity

Adhesion occurs when the material of the tool reacts chemically with the workpiece. So, a tool made of an unreactive material helps reduce adhesion. For instance, when a workpiece is made of aluminum alloy, a diamond tip or a DLC coated tool should be used. A diamond tip does not cause adhesion even when the rake angle is 0°, as shown in Fig.3.
You can also see from Fig. 4. that a DLC-coated tool can perform machining without adhesion. A cutting tool made of cermet is effective for steel workpieces. Cermet is unreactive with iron and thus causes no adhesion, making itself suitable for use in finishing.

Fig. 3: Diamond Rake angle 0°45 m

Fig. 4:

The three points discussed here are examples of how to prevent material adhesion to cutting tools. The theoretical methods of (1) and (2) were often used in the past, and the chemical approach of (3) is now considered effective as well. We expect the advancement of diamond tips, DLC and coolant will bring more effective solutions against adhesion. If you have any concerns about adhesion, please feel free to contact us.

• Consult with DMG MORI engineers on adhesion