Abstract

Polycrystalline diamond (PCD) has become an indispensable tool material for efficient machining of various materials. Inmanufacturing of PCD tools the finishing process in order to obtain the required surface and edge quality is mostly done by grinding. Due to the high hardness and wear resistance of PCD grinding is characterized by low material removal rates and high wear of the grinding wheel. The Fraunhofer IPT has developed a novel finishing method for PCD cutting tools, which combines laser processing and grinding. Short pulse laser ablation is used to completely remove a PCD surface layer of about 180 μm. To achieve the required tool quality the remaining 20 μm are removed by subsequent grinding. The obtained results demonstrate that the combination allows reducing the finishing time by more than 50%. Therefore, laser technology has a high potential to make the finishing process more efficient and significantly lower manufacturing costs.

1. Introduction

Continuous innovations in high-strength and lightweight construction materials for various industry fields, e.g.automotive or aerospace, lead to increasing challenges for the tool industry. Polycrystalline diamond (PCD) tools allow efficient machining operations due to their superior wear resistance. Being one of the hardest materials PCD presents enormous challenges throughout the economic manufacturing of high-quality PCD tools, Friemuth (2002) and Kenter (1990).

Polycrystalline diamond is composed of single diamond grains which are sintered and bonded into a cobalt metal matrix. First step in the manufacturing of PCD tools is the fabrication of a PCD disc, consisting of a PCD layer on a tungsten carbide substrate, within a liquid-phase sintering process. Secondly the PCD disc is diced into pieces mostly done by Electrical Discharge Machining (EDM) or Laser Cutting. The cutting methods use thermal energy to separate the PCD disc and lead to low surface qualities of the near surface zone and insufficient dimensional accuracy of the cutting edge. Subsequently these small PCD cutting inserts are soldered onto a supporting tool carrier. Positional inaccuracies of the PCD insert can occur during the soldering process. An overview about the manufacturing process of PCD tools is shown in figure 1.

For high precision machining and long tool life a high surface quality of the cutting area and a defined cutting edge is required which is adjusted throughout a subsequent finishing process. Currently the most widely used finish machining method is wheel grinding. Besides high surface and dimensional accuracy grinding is characterized by low material removal rates, long process times and heavy abrasive disk wear. The process is therefore inefficient and costly. Laser  machining for finishing processes operates without wear but offers no advantages in terms of shorter process times and the system technology is cost intensive in comparison to grinding machines.

The aim of the project “Pro-PKD” is to increase the efficiency of the finishing process through grinding by exploiting the advantages of laser ablation. In the new process concept the PCD material allowance is reduced by an additional laser roughing process prior to grinding. The combined process reduces the grinding effort and therefore consumption of grinding disks and process time.

2. State-of-the-Art

2.1. Grinding in PCD tool manufacturing

The quality of PCD cutting tools is mainly characterized through the surface condition of the cutting edge. The surface quality is adjusted during the finishing process and is therefore highly dependent on the process capabilities and limitations. According to Kenter (1990), with a Knoop hardness of 50 GPa PCD is nearly at the same level as monocrystalline diamond with 57-104 GPa. Tso et al. (2001) analyzed the PCD grinding process and indicated that only vitrified bonded diamond grinding wheels can be used. Ardelt et al. (2004) explains the difference between the grinding wheel and PCD cutting insert by the proportion of diamond in the composition. The cutting insert consists of over 90 percent of connected diamond grains in a cobalt metal matrix (grain sizes of 2-30 μm, depending on the PCD type) whereas the grinding wheel is composed of 25-30 percent diamond in a vitrified and resin bonding (grain sizes of 15-25 μm). Michels (2003) presents that in comparison to the amount of machined PCD in conventional grinding operations the grinding wheel shows an up to one hundred times higher volume of wear due to the differences in the diamond concentration and the bonding type. This is reflected in the low grinding ratio G= 0,01 –0,05 and explains the inefficiency of the finishing process through grinding. Kenter (1990) identified abrasive removal as main driver for machining of PCD during grinding operations. Beyond this Brecher et al. (2013) discussed that no reliable models for the description of the complex machining mechanisms during grinding exist. Friemuth (2002) shows that despite these drawbacks grinding currently remains state-of-the-art technology for finishing operations because the well-established technology provides high qualities and is widely accepted from the end user point of view.

2.2. Laser processing of PCD

Different PCD tool manufacturers use laser technology for the finishing process. Dold et al. (2012) shows that this comparatively new technology can provide equivalent surface qualities of the cutting edge with the advantage of a mostly wearless process. A time-saving benefit and therefore increasing of throughput cannot be observed. The current system technology uses mainly ultra-short pulsed (USP) laser radiation with pulse width regimes of several picoseconds to generate high qualities of the cutting edges. Poprawe (2004) explains that contrarily to grinding operations material removal is characterized by vaporization mechanisms induced through the conversion of radiation energy into thermal energy. Weikert (2005) presents that especially for ultra-short pulsed laser radiation melting zones are considered to be negligible due to the extreme short times of radiation exposure. The finishing process with picosecond laser radiation is comparable to a cutting process in which the material allowance is removed and the cutting-edge geometry is set in a single step. Complex five-axis processing is required to maintain accurate compliance of tolerances. Besides the elaborate axis system, the picosecond laser beam source constitutes an additional cost factor and lead to a cost-intensive overall system. 

The use of short pulse (SP) laser ablation offers advantages for the machining of PCD. Pulse durations of nanoseconds exhibit high material removal rates. According to Westraadt et al. (2007) the material removal of PCD can be linked to the graphitization of diamond. Komlenok et al. (2011) associated the high laser radiation absorption behavior of graphite and therefore energy input into the surface layer of the PCD with the high material removal rate during nanosecond laser processing. The long pulse durations of a nanosecond laser radiation lead to coarse surface qualities and increased cutting edge roughness. Contrary to the more complex picosecond PCD tool manufacturing nanosecond laser treatment requires a subsequent processing to adjust the required high surface quality of the cutting edge.

3. Approach Laser Roughing of PCD

As described above the degree of hardness and the level of wear resistance present enormous challenges throughout the processes involved in manufacturing PCD tools. The novel finishing method presented in this paper is based on a combination of laser processing and grinding. Laser ablation with short-pulse laser radiation is used to completely remove the PCD allowance subsequently to the soldering process. The required surface and cutting-edge quality is achieved by a final grinding process.

The combination process uses the advantages of high removal rates of a nanosecond laser beam to minimize the PCD material allowance swiftly without causing any tool wear. As shown in figure 2 the first step of the combination process is the determination of the PCD allowance by measuring the height difference between the tool carrier and the PCD cutting insert. This determination has to occur for every single PCD tool. The second step is the laser roughing operation in which the material allowance is reduced until a residual grinding allowance. The grinding allowance is required to set the desired surface qualities and the dimensional accuracy of the cutting insert.

The overall approach of this strategy is to increase the efficiency of the well-established grinding process by integration laser technology in the manufacturing process. Only a few micrometers need to be ground in the combination process which enables a reduction of the finish machining effort and a reduction in the consumption of high-cost diamond grinding wheels. Grinding parameters remain unchanged to provide equivalent PCD tool quality to the conventional manufacturing process.

4. System Technology

For the process combination of laser ablation and grinding an extension system for grinding machines has been developed. The “Pro-PKD”-system is based on a modular design consisting of a laser processing unit and an automatic robot handling. All supply units such as laser source, optics cooler, extraction and filter system for laser dust and control components are integrated into the machine base. The laser source is a pulsed nanosecond fiber laser with maximum pulse energy of 1 mJ. For beam deflection a 2-axis scanning system is used with confocal F -theta lens system that allows processing of multiple inserts within a field of 100 mm x 100 mm. During the laser process cutting inserts are positioned in a tray, which is automatically carried to the laser processing unit by the robot system. Furthermore, the robot can put individual PCD inserts into the chuck of the grinding machine after the laser processing.

A laser triangulation sensor is integrated into the laser processing unit to measure the PCD allowance. Depending on the measured PCD allowance, the depth of laser ablation can be controlled by varying the number of process iterations. Per iteration a certain thickness of PCD depending on laser parameters (e.g. 13 microns) is removed. After processing of the first side the inserts are turned by the robot and reinserted into the tray for laser processing of the second side.

5. Process Results

First process investigations were carried out on PCD cutting inserts, type SPGW 120404, with an average grain size of 10 μm. The PCD-allowance, which has to be removed up to a remaining allowance of 20 μm for the finishing process (grinding), is in the range of 200 μm with a manufacturing tolerance of +/- 100 μm. The ablation process is based on a pulse-by-pulse line pattern, whereas the distance between single pulses is 20 μm and the distance between lines is 10 μm. The line length and number of lines was adapted to the PCD size of the flank face.

The removal depth was investigated experimentally by varying the number of iterations. As shown in figure 5 there is an approximately linear trend between number of iterations and removal depth. After ten iterations a removal depth of 130 μm was achieved. In the height profile it can also be shown that the removal depth in PCD is higher than the depth in carbide. However, aim of the laser processing is the removal of the PCD due to its poor machinability compared to carbide.

Figure 6 shows a PCD insert after roughing of both sides. The measured PCD allowance after the soldering process was 190 microns. The remaining PCD allowance for the finishing process by grinding was defined to be 20 microns resulting in a depth of removal of 170 microns. With an average removal per iteration of 13 μm (Figure 5) the number of iterations was chosen to 13. The measured depth of removal after 13 iterations was 166 μm.

For qualitative analysis the surface roughness Rz was optically measured before and after laser roughing. The initial roughness before laser roughing of the EDM machined surface was 8.1 μm Rz. After laser roughing the flank face has a smoothed surface of 2.7 μm Rz. The resulting surface quality is assumed to be mostly dependent on the initial roughness. For EDM pre-machined inserts surface qualities below 1 μm Rz, as requested by tool manufacturers to achieve sharp cutting edges, cannot be achieved by the laser roughing approach. Those surface qualities are generated in a subsequent conventional grinding step.

The processing time for laser roughing of one side of PCD cutting insert type SPGW 120404 as shown in Figure 6 results in 20 seconds. The overall processing time for both sides including handling time sums up to 60 seconds. A conventional machining by grinding takes approximately 8 minutes, whereas finishing of the last 20 μm takes about 2.5 minutes. By combination of laser roughing and grinding the overall processing time can be reduced by more the 50% from 8 minutes to 3.5 minutes.

6. Summary

For manufacturing of PCD-tools a finishing process is necessary in order to remove the PCD oversize resulting from prior manufacturing steps. Currently, grinding is the most widely used finishing process as it provides a high surface quality and tool life time. However, grinding is currently inefficient and costly due to the low ablation rate and high wear of the diamond grinding wheels. Laser machining with ultra-short pulse laser systems is also an industrially established technology for finishing of PCD cutting tools. Laser machining is a technology free of tool wear. However, the processing time cannot be reduced compared to grinding. The project “Pro-PKD” aims to increase throughput and reduce tool wear by a laser ablation step prior to grinding. Aim of the laser process is to remove a major part of the PCD oversize. The consecutive grinding step is used to smoothen the surface afterwards. For this purpose, a laser processing unit has been developed which can be implemented into already existing grinding machines. The laser roughing process of the major PCD oversize takes approximately 20 seconds per side which represents a significant reduction compared to a grinding time of several minutes.

This article is write by Christian Brechera,b, Michael Emontsa, Jan-Patrick Hermania, Thomas Stormsa,*