PCD tools grinding

1. Introduction of PCD

At present, the application scope of PCD, PCBN tool expands to automobile, aerospace, precision machinery, home appliances, wood, electronics and electrical industries, for the production of turning tools, boring tools, milling tools and drill, reamer, countersink, saw knife, carving knife, razor, etc.

Although the development of PCD, PCBN tool is so fast, but because of its high hardness caused by the tool grinding difficulties has been troubled by most users, the regrinding of the blade is mainly completed by the original tool manufacturers. Not only the tool price is high, the delivery time is long, and the occupation of enterprises and institutions working capital. Therefore, it is necessary to study the grinding characteristics of PCD and the grinding technology of PCD cutter.

2. Manufacturing process of PCD cutting tools

The production process of PCD cutting tools generally includes polishing, cutting, fixing, grinding, quality inspection, etc. The surface of the blank of PCD superhard material is generally rough (Ra2 ~ 10μm), so we can not be directly used to make the tool. It must be polished to achieve the mirror effect (Ra≤0.01μm). Then the blade is through laser cutting or EDM wire cutting processing into a particular geometric shape and required size. Further mechanical and chemical treatment of the edge and matrix to be cemented joint, the blade is fixedly connected to the matrix by hard brazing of silver base.  Finally, we grind it with a diamond grinding wheel.

PCD cutting tool manufacturing technology is one of the vital, cutting-edge grinding qualities; using a good grinding process will improve the quality of the tool product, reducing the use of the tool cost.

3. Grinding characteristics of PCD material

PCD is made of specially treated diamond and a small binder sintered at high temperature and ultra-high pressure. The disordered diamond grains make PCD have a uniform and extremely high hardness and wear resistance. Many factories made pcd into cutting tools, grinding wheel dressing, geological drilling, measuring instruments, wire drawing tools, sandblasting tools, etc. However, the high hardness and wear resistance of PCD also lead to significant difficulties in processing.

In order to obtain good PCD cutting edge quality, the most ideal machining method is still grinding or grinding with diamond wheel.

The grinding process of PCD is mainly the result of the mixed action of mechanical and thermochemical. The mechanical action is the micro-crushing, wear, shedding or cleavage of diamond formed by the continuous impact of diamond grinding wheel on PCD material. The thermochemical process is the oxidation or graphitization of diamond caused by the high temperature formed by grinding PCD with diamond wheel. As a result of their mixing, the PCD material was removed. Its grinding processing characteristics are:

1) Great grinding force

The hardness (HV) of PCD is 80 ~ 120KN/mm2, second only to single crystal diamond, much higher than carbide. When using the diamond wheel to grind PCD, the initial cutting strength is very high, about ten times of hard alloy (0.4MPa); Specific grinding can reach 1.2×104 ~ 1.4 ×105J/mm3, So grinding force is much higher than grinding carbide.

(2) The grinding ratio is small

Due to the high hardness and wear resistance of PCD (the relative wear resistance is 16 ~ 199 times of hard alloy), the grinding ratio of PCD is only 0.005 ~ 0.033, about 1/1000 ~ 1/100000 of hard alloy. Grinding efficiency is only 0.4 ~ 4.8mm3/min. Therefore, the grinding time is very long to ensure the cutting edge quality and removal amount of cutting tools, and the processing efficiency is very low. In addition, when the hardness, content, and particle size of PCD are different, its grinding time is also other

(3) Particle size has a significant influence

As cutting tool pcd size is mainly divided into three types: rough size (20 ~ 50μm), medium size (10μm or so), and finish size (-5μm). Different grinding sizes process different workpieces as follows,

4 PCD cutting tool grinding requirements for tooling

Based on the above characteristics of PCD grinding, the diamond grinding wheel has much higher requirements on grinding equipment than standard tool grinding machines. There are mainly:

(1) The machine tool has good rigidity in the processing system

(2) The machine tool has a short-range swing mechanism with adjustable stroke and speed

The grinding ratio of PCD is meager. Therefore, using a short-range swing mechanism is beneficial to improve grinding efficiency and cutting-edge quality. General swing distance 0 ~ 50mm, swing speed 20 ~ 60 times/minute.

(3) The tool clip of the machine tool has the function of high precision rotation and online detection device

5. Grinding technology of PCD and PCBN cutting tools

5.1 Selection of grinding technology

One of the purposes of cutting tool grinding is to obtain cost-effective, cutting-edge quality; The key to advanced quality is the choice of grinding wheel granularity. The PCD cutting tool grinding process comprises three processing stages: rough, semi-finish, and high finish. According to the specific situation, rational grinding technology can significantly improve processing efficiency.

5-1-1The cutting edge damage is 0.5mm, optional grinding wheel size 230/270# ~320/340# or electric machining

5-1-2Cutting edge breakage 0.3mm, optional grinding wheel granularity W20-W40

5-1-3Cutting edge breakage 0.1mm, optional grinding wheel granularity W5-W10

5.2 Main points of the grinding process

(1) Spindle accuracy is better; grinding wheel end beat should generally be less than or equal to 0.02mm. Suppose the end face of the grinding wheel beats too much. In that case, the grinding wheel will intermittently impact the cutting edge during grinding, which is easy to make the cutting edge collapse and challenge to obtain the high precision cutting edge.

(2) The grinding wheel should have good dynamic balance. The unbalance of the grinding wheel will cause the vibration of the machine tool and then affect the tool's cutting-edge quality and machining precision.

(3) Vitrified bonded diamond grinding wheel should be chosen first. Because the vitrified bond is prone to micro-cracking in the grinding process, the abrasive particles can be renewed and self-sharpened, which makes the grinding process smooth and is conducive to improving the precision and efficiency of the machined surface. Second, the resin bonded diamond grinding wheel with higher heat resistance is optional.

(4) Timely pay attention to the cutting edge of the grinding wheel, and the granularity of the cutting edge stone should be appropriate. When the PCD cutter is processed with diamond grinding wheel, the grinding wheel will be blocked, passivated, high temperature and fast wear, leading to the reduction of processing speed and the generation of vibration, noise and burn. We can use silicon carbide stone for dressing diamond grinding wheels.

(5) Because diamond is prone to chemical diffusion with iron alloy, which accelerates grinding wheel wear, we should avoid the simultaneous grinding of metal and PCD;

(6) The rotation direction of the grinding wheel must be from the front tool to the back tool face. When the grinding wheel rotates from the front tool face to the back tool face, the grinding force (the sum of tangential and normal force) acts on the cutting edge inward, that is, the tool compression stress, not easy to break the tool; On the contrary, tensile stress, cutting edge is easy to crack. If the grinding wheel with resin bond is better than the metal and ceramic bond, the grinding wheel with resin bond is better than that with vitrified bond.

(7) To ensure cutting-edge quality and improve grinding efficiency, the back Angle of the tool can be divided into a large back Angle and a slight back Angle. Rough-grain grinding wheel is used for crushing the large Angle first because of the large contact surface grinding force and high grinding efficiency; Then, the fine-grained grinding wheel is used to grind the slight back Angle, and the width of the edge belt of the slight back Angle is controlled within 0.1 ~ 0.3mm, and the grinding quality of the small contact surface is good.

(8) As far as possible in clamping to complete the machining of the cutting edge of the tool;

(9) PCD grinding coolant should be preferred water-based grinding fluid. In addition, the cooling in the grinding process should be sufficient, and the flow can not be interrupted to avoid the large consumption (oxidation, graphitization) of diamond (grinding wheel, tool) and the cutting edge damage of the device due to the small amount of grinding fluid or intermittent supply.