01、PCD Tool Features

Polycrystalline diamond (PCD) is a polycrystal formed by mixing micron-sized diamond powder with a small amount of metal powder (such as Co) and sintering at high temperature (1400°C) and high pressure (6000MPa). Compared with other tool materials, PCD has extremely high hardness and wear resistance, high thermal conductivity, low thermal expansion coefficient, small friction coefficient, fast cutting heat dissipation, low cutting temperature, small thermal deformation, and can reduce the roughness of the machined surface. Features. However, because PCD has a strong affinity with iron group elements, it is not suitable for processing ferrous metals and their alloys.

The PCD composite sheet that has been commercialized is prepared by sintering a 0.5-0.7mm thick PCD layer on a cemented carbide substrate, so it has both the high hardness and high wear resistance of PCD and the good performance of cemented carbide. Strength and toughness. PCD tools show excellent cutting performance in high-speed cutting of non-ferrous metals and their alloys and non-metallic materials, and have been widely used in automotive, aviation, aerospace, building materials and other industrial fields. However, the high hardness and high wear resistance of PCD make it difficult to sharpen the tool, which is mainly reflected in the low material removal rate, large grinding wheel loss, low sharpening efficiency, and serrated edge. The sharpening manufacturability of PCD tools has become one of the obstacles to its popularization and application. In order to break through the bottleneck of this process, scholars at home and abroad have carried out a lot of research and development work.

02、Diamond grinding wheel grinding process of PCD tool

Commonly used PCD tool sharpening processes include diamond grinding wheel mechanical grinding, discharge grinding, electrolytic grinding, etc. Among them, diamond grinding wheel mechanical grinding is the most mature in technology and is currently the most widely used. Although this method has low sharpening efficiency and high processing cost, it can obtain good cutting edge quality and complete, smooth front and rear rake surfaces.

2.1 Sharpening processing mechanism

The material removal mechanism of diamond grinding wheel mechanical sharpening of PCD tools is relatively complex. Scholars at home and abroad have conducted a lot of research on this. At present, there are mainly the following viewpoints:

(1) German scholar M. Kenter believes that scribing and sliding action occur in the process of grinding PCD tools with diamond grinding wheels, and the material removal methods are mainly bonding, scribing, tribochemical reaction and surface fracture. He believes that, in the vast majority of cases, PCD material removal is dominated by tribochemical reactions and surface fracture. Due to the high brittleness of PCD materials, cracks are easily induced under the extrusion of diamond abrasive grains, and the cracks expand under the action of mechanical and thermal stress, eventually causing small pieces of PCD material to peel off. At the same time, friction heat will cause PCD to undergo graphitization and other tribochemical reactions. .

(2) GE's K. J. Dunn et al. observed the surface micro-morphology of the sharpened PCD composite sheet with a scanning electron microscope and believed that the failure mechanism of the PCD material was mainly microscopic brittle fracture and fatigue failure.

(3) Academician Aixing and others in my country believe that the removal method of PCD material is: when the grinding wheel is in contact with PCD, the grinding force suddenly increases, and the severe mechanical impact causes cracks or even breakage on the surface of PCD. During the stable grinding period, the abrasive grains of the grinding wheel are squeezed and rubbed on the PCD surface. When the pressure reaches a certain level, cracks will form on the PCD surface; when the friction temperature reaches a certain level, the PCD will undergo graphitization and other chemical reactions.

2.2 Grinding machine conditions

The characteristics of PCD materials determine the requirements for PCD tool grinding machines: they cannot be equivalent to ordinary tool grinding machines, that is, the following conditions must be met:

(1) The grinding wheel spindle and the machine tool as a whole are required to have high rigidity and stability to maintain the constant pressure of the grinding wheel on the PCD material during sharpening.

(2) The grinding wheel frame can swing laterally to ensure that the end face of the grinding wheel wears evenly, and the swing frequency and amplitude of the grinding wheel frame can be adjusted.

(3) Equipped with an optical projection device and a high-precision rotary table.

(4) Using special diamond grinding wheel.

2.3 Sharpening process flow

(1) Selection of sharpening machine tools

Due to the high hardness and wear resistance of PCD tools, the sharpening requirements are high, and it is difficult for general machine tools to meet the sharpening accuracy requirements. According to the characteristics of PCD knives and the necessary conditions for mechanical sharpening, we chose the EWAG RS15 superhard tool grinder produced in Switzerland (see Figure 1). This machine tool is one of the most advanced superhard tool grinders in the world, with the following features:

① The overall rigidity of the machine tool is high and the stability is good;

②The grinding spindle can realize stepless speed regulation (1000--8000rpm);

③The grinding pressure is adjustable (0～400N) and has good stability, and can swing back and forth, and the swing amplitude (0～70mm) and frequency (30～60 times/min) can be adjusted;

④ Equipped with high-precision optical projector and rotary table, the whole process of positioning, grinding and measurement can be completed in one clamping;

⑤ Equipped with digital display device, dimensional accuracy of 0.001ram.

Fig. 1 EWAG RS15 superhard tool grinder

(2) Selection of grinding wheel

The mechanical grinding of PCD tools mainly relies on the tribochemical reaction with the diamond grinding wheel and surface fracture to remove excess material. The particle size, concentration and bonding agent of the diamond grinding wheel have a great influence on the quality and efficiency of PCD tool grinding. Therefore, the correct selection of grinding wheels is very important. After comparison, we selected the European-made Talile (TY-R0LIT) grinding wheel. The diamond abrasive grains of this grinding wheel are implanted into the grinding wheel one by one and arranged in an orderly manner. The grinding rate per minute is 20 to 30 times that of ordinary diamond grinding wheels. Experience has shown that under the premise that the processing quality requirements can be met, the grinding wheel with larger particle size should be selected as much as possible. The particle size of diamond abrasive we choose is: coarse grinding particle size 22um; semi-fine grinding particle size 15um; fine grinding particle size 9um.

(3) Balance of grinding wheel

After selecting the grinding wheel, the grinding wheel needs to be balanced. First, place the grinding wheel balancer on the mirror table for calibration. By adjusting the three balance nuts at the bottom of the balancer, the balance water bubble in the balancer can be located in the center. Then slide the grinding wheel into the grinding wheel adjustment rod, and adjust the three balance screws on the side of the grinding wheel to make them evenly distributed (about 120 degrees with each other at a 63-degree angle), until the balance bubble in the balancer is in the center again, and the grinding wheel is balanced.

(4) Clamping and adjusting the PCD tool by centering the center of rotation and the optical center

First, it is necessary to align the center of rotation of the cross table with the center of the optical projector, and adjust the set screw of the optical projector so that the highest point of the tool tip and a large arc outside the projector are at 270°, Tangent at 0° and 90°; then according to the requirements of the drawing, adjust the clamped PCD tool to the reference line of the optical projector through the cross table, and this position is the sharpening processing position. In order to ensure the machining accuracy requirements of PCD tools, all sharpening dimensions should be completed in one clamping. When grinding PCD tools, all wear should be removed.

(5) Select the grinding wheel swing

Since the diamond grinding wheel is expensive, in order to make full use of the grinding ability of the grinding wheel and prevent the appearance of groove marks, the swing amplitude of the grinding wheel should be reasonably selected according to the size of the PCD tool.

图为砂轮摆幅与刀具的相对位置

(6) Determine the sharpening speed, frequency and other parameters

Through a large number of tests, it is determined that the speed of the sharpening PCD tool is 2300 ~ 2500r/min (150ram grinding wheel), and the grinding wheel rotates in the forward direction. Usually, the higher the grinding frequency, the greater the grinding rate. The PCD tool grinding frequency is 60 times/min. Other parameters: water-based coolant concentration 4%, grinding pressure 2.5Bar (about 167N).

(7) Cleaning of the grinding wheel

After grinding the grinding wheel for a period of time, the gap between the abrasive grains will be filled with grinding debris, resulting in dull grinding surface and reduced processing efficiency (at this time, the grinding wheel often makes a squeaking noise). In order to maintain the cutting ability of the grinding wheel, the cutting surface of the grinding wheel needs to be cleaned frequently by pressing a silicon carbide (SIC) grindstone against the grinding surface of the grinding wheel and rotating it.

(8) Handling of blades before and after sharpening

Before sharpening the PCD tool, you should observe whether there are aluminum chips on the front and rear blade surfaces, and if so, wipe them off with diamond whetstone. After the sharpening is completed, the front end face is ground with a diamond paste. PCD tools that have undergone rough grinding, semi-finishing, fine grinding and grinding can truly meet the requirements of use.

(9) Tool detection

After the sharpening is completed, the dimensional accuracy and edge straightness of the PCD tool can be directly detected on the projector of the machine tool, and obvious defects such as unevenness or zigzag are not allowed. The straightness of the cutting edge can also be checked under a microscope of more than 100 times.