Tech Talk
What are "Ceramic Abrasives"?
In recent years advances in machine tool engineering and CNC technology have lead to enormous improvements in cylindrical grinding performance. The concurrent development of ceramic microcrystalline aluminium oxides and synthetic-bond systems have opened new possibilities for exploiting the full performance potential of modern grinding processes. Today, grinding processes using ceramic abrasives easily hold their own against grinding processes with CBN or alternatives such as hard-turning. If grinding wheels with 30% ceramic abrasive content reach high performance levels, why not use wheels with 100% ceramic content? It is tempting to explain this on the basis of the price difference between standard and ceramic aluminum oxides. On a weight basis we are looking at a pricing factor of 8 to 10 times more for ceramic abrasives. This, however, would be very misleading as only a precise conbination of various abrasives with different grinding properties result in an optimal performance. When blending abrasives many, often opposing, properties such as friability, abrasive resistance and relative grinding force have to be brought into balance. How do we explain superior grinding performances which are two to five times higher than those which traditional abrasives such as aluminuium oxide or silicon carbide achieve? The best way to explain this is to look at the very different structures of the abrasive grains and how this structure affects performance. The easiest way to visualise abrasive grains is to imagine the individual grit as bricks fused together with mortar to form a grain. In traditional aluminium oxides and silicon carbides a single of very few bricks make up a single grain. When these grains fracture in use, the consequences are usually severe. The bricks break away in large chunks, resulting in high wear rates and rapid deterioration in performance. In contrast, the bricks that make up ceramic aluminium oxide are much, much smaller, measuring around 0.5 um in diameter. When this abraisve starts to wear away it happens only in small quantities, continually and very slowly. For this reason, the grain is much tougher, retains its shape longer and ensures constant grinding forces. The important thing to remember is that not all ceramic aluminium oxides are created equal. Simplified, conventional ceramic grains consist of "bricks and mortar" only. The revolutionary feature of 3M's Cubitron 321 is its built-in reinforcement. Tiny platelets are present which reinforce the bricks and mortar in the similar fashion to how glass fibres reinforce plastics. Due to this reinforcement it takes more force to fracture and wear away the cutting edges of this ceramic material. For this reason, Winterthur prefers Cubitron 321 for most cylindrical grinding applications. Not only does it require more force to fracture a Cubitron 321 grain, but when it finally fractures, it does so in a different way to a conventional ceramic abrasive. On a conventional ceramic grain, not reinforced by platelets, fracture will occur along an even path, leaving a surface that is relatively smooth. This creates a larger contact area which can increase the frictional drag during grinding, resulting in heat build-up, poor cut and possible thermal damage to the workpiece. Cubitron 321, on the other hand, does not fracture along an even path; as the fracture plane veers off course as it hits the platelets, resulting in an uneven, serrated cutting edge. Left behind is a grain that is much sharper and more aggressive. This shelf-sharpening process is continuous. The grinding wheel as a whole remains sharp and cool cutting resulting in alower grinding energy and costs. What is "Bond"? A grinding wheel doesw not consist of abraisive alone. Bond and pores are also very important. Bond is the glue that holds a grinding wheel together. A basic rule of grinding states that bonding material does not grind. Therefore, the more bond in a wheel specification the higher the danger of grinding burn or micro-cracking. The inverse is also true: the less bonding material, the cooler the wheel will grind. The wheel manufacturer must, therefore, reduce the amount of bonding material without losing sight of edge retention properites and, of course, safety. Vitrified bonds consist in the main of naturally occurring raw materials such as kaolin (porcelain clay), feldspar and frits (glass). By adjusting the quantities of individual bonding materials, the grinding behaviour of a grinding wheel can be controlled: Porcelain clay will increase dampening properties while frits will increase friability. Natural raw materials, however, are subject to quality variations. Given today's automated grinding processes with predertermined machining parameters, we can ill-afford tools that vary from batch to batch. Therefore, Winterthur has replaced all the antural bond materials with fully synthetic components. These new components make up the so-called recrystallised glasses which, to to its high inherent strength, can be used in smaller quantities than standard bonding materials. This allows for a reduction of 10% in over-all bond material, and a simultaneous increase in the porosity by the same percentage, without any loss of wheels strangth. These reduced properties of modern vitrified ceramic wheels. After firing, these wheels are subjected to a very slow but highly controlled (in terms of temperature and time) gradual cooling process during which a recrystallisation process takes place within the bond. Chains of crystals grow which could also be compared to glass fibres within a plastic matrix. These fibres reinforce the bond and give it its superior strength. The ceramic aluminium oxide wheel used for high performance cylindrical grinding offers a real alternative to CBN processes and hard-turning. It is expected that further developments in overall machine tool and CNC technology, developments of new abrasive grains and bond systems will ensure future gains in quality, reduction of cycle times an overall costs. To give a summary of ceramic abrasives' benefits: - Longer life of up to 5 or more times that of conventional abrasive grains, resulting in lower overall costs; - Cooler cutting with fewer rejects from thermal damage; - Less down-time for dressing, giving higher productivity. - Fewer wheel changes, proving time savings. - Ability to work a great range of material from soft to hard. - Ability to use standard wheel speed machines (35 to 50 m/s).
