Loading and welding of single crystal diamond tools

Abstract: This paper introduces three methods and their advantages and disadvantages of mounting a single crystal diamond cutter head onto the cutter bar.
Key words: single crystal diamond tool, card bonding, damascene, brazing
Clamping and Welding of Single-crystal Diamond cutters

Zhang Jingmin

Abstract: The three technologies to clamp single-crystal diamond cutting heads on toolholders are introduced, and merits and demerits of the technologies are presented.
Keywords: single-crystal diamond cutter clamping bonding process inlaying process welding process

The rigid and reliable mounting of the single crystal diamond cutter head on the cutter bar is a key step in the manufacture of diamond cutters, and the quality of the fixture directly affects the performance of the diamond cutter. At present, there are three main types of card loading methods, namely, bonding method, mosaic method and brazing method.

1. Bonding method
The bonding method uses an organic binder such as epoxy resin or 502 glue to bond the diamond cutter head to the groove on the cutter bar that matches the size of the diamond cutter head. This method is easy to operate and does not require special equipment and fixtures to achieve a certain bond strength. However, since the temperature resistance of the organic binder generally does not exceed 200 ° C, the high temperature generated during the grinding of the cutter head or the use of a large cutting amount can cause the binder to fail, so the bonded diamond cutter is only suitable for Ultra-precision machining with a small amount of cutting, and the diamond cutter head cannot be directly ground on the cutter bar. When the tool needs to be ground, it is necessary to first melt the adhesive with a solvent such as acetone, and then re-bond the diamond head to the tool holder after grinding. In addition, the positional accuracy of the diamond tip during bonding is difficult to guarantee.

2. Mosaic method
The damascene method firstly sinters the diamond cutter head together with the metal powder, and then mechanically fixes the diamond cutter head to the cutter bar through the pressure plate. The mosaic method is currently the most widely used diamond tool loading method. The process is as follows:
(1) Roughly grind the diamond into a large and small shape and determine the position of the blade and the flank.
(2) Place the diamond face down on the cast iron mold shown in Figure 1, fill it with a proper amount of copper-based metal powder (such as 663 copper powder), then insert the upper plunger and pressurize the metal slightly. powder.
  Figure 1 mosaic mold
(3) While slowly heating the mold with the high-frequency induction device, the pressure between the upper and lower plungers is continuously increased until the temperature and pressure reach 750 ° C and 5.5 kg / mm 2 , respectively. After cooling, the sintered block and the diamond cutter head therein are taken out, and a hole corresponding to the shape and size of the diamond cutter head can be obtained on the sintered block.
(4) According to the relative position of the designed blade and the shank, the excess portion on the agglomerate is removed. The machined agglomerates are then brazed to the correct position of the toolholder with a lower temperature copper braze and the rear corners and other parts of the toolholder are precisely ground.
(5) The polished diamond cutter head is placed in the hole of the agglomerate block and the appropriate pressure plate is applied to complete the loading of the diamond cutter. The installed diamond cutter head (shown in Figure 2) only exposes the flank of the tool and the rake face of about 1 mm.
  Figure 2 Clamped diamond cutter
The damascene method can firmly fix the diamond cutter head, and the high temperature resistance is also good, and the flank surface of the cutter head can be directly polished after the card is installed, thereby achieving high angular precision. When grinding, just open the pressure plate, take out the diamond cutter head, and reassemble after grinding and it is very convenient.
The disadvantage of the damascene method is that the process is complicated, and the diamond cutter head can only expose a small part of the rake face, and the rear platen can hinder the chip from flowing out, which easily blocks the chip near the blade and destroys the processed surface. Since there is no chemical bond between the diamond of the insert method and the hole surface of the sintered block, only the pressure is pressed, and slight displacement may occur during the processing, so it is not suitable for the requirements of the superfinishing tool. In addition, the damascene method requires a large volume of diamond, which leads to a lower utilization of diamond, and on the other hand, it is difficult to make a small-diameter tool for small hole machining.

3. Brazing method
Direct welding of diamond tools by brazing has always been a concern. Because diamond has high chemical stability, it is difficult to chemically react with other metals to achieve welding. Therefore, how to reliably achieve the firm connection between diamond and metal has become a technical difficulty and a research topic to be solved in the diamond tool manufacturing industry.
To achieve the brazing of diamond tools, the following problems need to be solved:
(1) Wetting property: When brazing, the brazing material must wet the material to be welded to ensure that the welding site is free from defects. Due to the high surface energy of diamond and poor wettability, it has been found that solders with good wettability on the surface of diamond and metal substrates are the key to successful brazing.
(2) Bonding properties: The brazing filler metal should not only have good wettability, but also chemically react with the material to be welded to form a chemical bond to achieve a welding with sufficient adhesion.
(3) Residual stress: The thermal expansion coefficient of diamond and metal is nearly ten times different, and a large residual stress is formed during the cooling process after welding, which affects the welding strength and even damages the diamond.
(4) Corrosion of diamond: Diamond will oxidize or graphitize at high temperature. Some metals also have an effect on diamond. Therefore, it is necessary to select a suitable working environment and solder to minimize the corrosion phenomenon of diamond.
Numerous studies have found that brazing filler metals containing active elements such as titanium, zirconium, niobium and tantalum can directly wet the diamond surface in a vacuum. These active elements can also react with carbon atoms on the surface of diamond to form stable carbides, forming a chemical bond. The metal molybdenum or cobalt-based cemented carbide has a similar thermal expansion coefficient to diamond, and it can be used as a welding base to not damage the diamond and ensure the brazing strength. Based on this research, diamond brazing technology under vacuum conditions was developed. In recent years, a new technology for brazing diamond in a protective gas atmosphere has been further developed. Compared with the vacuum brazing technology, the protective gas brazing technology has the advantages of low equipment cost, simple operation, short process cycle, and the like, and maintains the welding strength equivalent to vacuum brazing. The emergence of this technology has solved the long-standing problem of diamond brazing technology more satisfactorily.
The brazing material used for the brazing diamond of the protective gas is silver-copper-titanium alloy, and the composition ratios of silver, copper and titanium in the alloy are 68.8%, 26.7% and 4.5%, respectively. The shielding gas is a mixed gas of argon (95%) and hydrogen (5%). The welding is carried out in a semi-open cavity as shown in FIG. The brazing process is as follows:
(1) sufficiently removing oxides on the surface of the diamond and metal substrate;
(2) heating the substrate and the brazing material in a protective atmosphere until the brazing material is melted and uniformly dispersed at a specified position of the substrate, and then cooled;
(3) Place the diamond to be welded in the correct part of the substrate, refill it with the shielding gas, reheat it to the melting temperature of the solder, and then slowly cool to room temperature.   Figure 3 Schematic diagram of the brazing device
The use of brazing to mount a diamond cutter head has the following advantages: high welding strength, shear strength of the welded surface up to 340 MPa, and a diamond cutter head weighing only 0.02 carats can be firmly welded to the cutter bar; Afterwards, the diamond cutter head is further refined to ensure the machining accuracy of the cutter geometric angle; the cutter rake face can be higher than the cutter bar, thereby ensuring smooth discharge of the chips, and making the cutting process and the surface quality of the workpiece more stable and reliable; Improve the rigidity of the diamond tool system.