Flat Grinding of Technical Ceramics cost effective - fast - precise - By using lapping kinematics and patented grinding tools The historic lapping and also conventional flat grinding is still very popular. But many applications (also single sided) can be substituted by the flat grinding process with lapping kinematics (Pic. 1). Picture 1: Principe of flat grinding with lapping kinematics This process originated from the conventional lapping. The lapping wheels are substituted with grinding wheels and instead of lapping compound a coolant is fed through bores in the upper wheel. Compared with the traditional lapping machines, the grinding machines are equipped with stronger motors. They allow a higher downforce and the whole machine is designed to resist the higher forces (picture 2). Picture 2: PETER WOLTERS AC 700-F To operate such a machine is quite easy. No clamping or gluing of parts is necessary. They just lie in their carriers. For higher output all grades of automation are available, which is an other advantage compared with the dirty lapping process. To switch from one workpiece to an other, you just have to change the carriers and load a new program. 1 / 5
The running costs are lower compared to the conventional flat grinding. This is a result of the higher output and less handling time. The lapping kinematics allows an excellent flatness of the machined parts. This helps save polishing time, if parts have to be polished after grinding, as is necessary for example in the water tap industry (picture 3). 100 80 60 Grinding Polishing Washing 40 20 0 Conventional double side grinding Flat grinding with lapping kinematics Picture 3: costs of machining Al 2 O 3 -parts The technology depends of course very much on the kind of ceramic. Al 2 O 3 for example is easy to machine. High quartz content and low sintering temperatures cause a brittle material removal that creates only a small amount of heat. The grinding wheels stay sharp for a long time. Additionally removed particles cause a constant self sharpening process. Technology becomes more important if for example Si 3 N 4, SiC or ZnO 2 are to be machined. A higher level of covalent linkages cause higher forces to the diamond edges. This creates heat and heat causes wear of the diamond edges. Now the normally brittle dominated material removal changes more and more to ductile material removal. This creates more heat and the wheels dull in a few minutes. For these ceramics Peter Wolters exclusively offers a patented grinding wheel, which was developed by Diametal in Switzerland. In this grinding wheel, many diamonds are agglomerated in a cluster (picture 4), which has a metal-ceramic bonding. These clusters are bonded in a resin bonding (picture 5). 2 / 5
0,5 mm Picture 4: The patented diamond clusters Diamond Cluster Resin Bonding Diamond Picture 5: A zoom in the clusters This system has three major advantages: 1. The diamonds are bonded very strongly. It is virtually impossible to rip them out. 2. Through the metal-ceramic bonding the heat can be conducted very well. 3. The resin bonding wears more than the clusters do. This enables the coolant to reach the cutting zone, even if large parts are machined. 3 / 5
Cutting Edges Cluster Bonding Resin Bonding Pores Taylor Hobson analysis at IWF / ETH-Zurich tracking distance: 4µm area: 0,5 x 0,5mm Diamond-Cluster of Ø120 µm with 3-4 cutting edges Picture 6: a 3-dimensional structure of the wheels surface Picture 6 shows a 3-dimensional surface structure of a cluster grinding wheel. The active cutting edges are white, the metal-ceramic bonding and lower cutting edges are colored red. This is the area that creates heat by friction. The yellow area marks the resin bonding, and the green and blue areas are pores. Even this high tech grinding wheel will get dull fast, if it is undercharged. That is why Peter Wolters developed the adaptive grinding system. While a normal process runs with a constant downforce, the adaptive grinding system controls the removal rate or the torque of the motors, by regulating the pressure. The result is either a constant removal rate or a constant torque. By using this system you avoid overcharging and much wear of a fresh sharpened grinding wheel as well as undercharging of such a wheel during the process. Downforce and removal rate (SiC - 20 x 100 mm - 20pcs) D126 C40 Diamond-Cluster Wheel 500 dan Ae: 400 cm² Self-sharpening caused by double load Correction of elasticity Constant removal rate of 3 µm/sec in the main load seconds Picture 7: A typical process with the PETER WOLTERS adaptive grinding system In picture 7 you can see a typical process. It is started with a constant ramp from 50 to 500daN. During this time the different workpiece heights are equalized. In the main load the machine constantly removes 3µm/sec. For safety reasons the maximum load was limited to 1500daN. The following runs are summarized in picture 8 You can see, that the load increases from run to run. In run five you can see the effect of self sharpening. The following runs require less downforce, until self sharpening reoccurs. 4 / 5
Load 1000 dan Run No 5 Run No 4 Run No 3 Run No 2 500 dan Run No 1 Constant removal rate of 3µm/sec L1 L2 Lk 20s 40s 60s 80s 100s 120s Picture 8: Summarized loads, machined with a constant removal rate Le Process time The cluster grinding wheels are available in FEPA mesh sizes from 181µm down to 25µm.In the following table you can see examples for different grid sizes (table 1). Grid size Material Size [mm] Workpieces per load Removal rate [µm/min] Roughness R A [µm] Machine Flatness [µm] DCL 25 SiC Ø 31 90 120 0,13 AC 700 1,5 DCL 76 SiC Ø39 30 433 0,6 AC 700 2 DCL 181 Si 3 N 4 108x108 6 160 0,2 AC 1200 6 Table 1: Machining examples of different grinding wheels The Diametal cluster grinding wheel technology together with the rigid Peter Wolters machines has already produced outstanding results in precision machining of technical ceramics. By applying the high speeds and downforces of PW-Machines it is possible to use the full potential of the cluster grinding wheels. The adaptive grinding system allows a reliable process and reduces wear. 5 / 5