Grinding with technical ceramic: in conversation with an expert

Mr. Schmitt, you have been advising your customers on the subject of grinding with technical ceramic for years. What are the various forms that are mostly used in your business?

Bernhard Schmitt: We generally use three types of abrasive grain sizes. The cheapest of these is covered with corundum, a comparatively soft material. We use corundum mainly for normal steels, iron and non-ferrous metals. The next harder variant is silicon carbide, which is used on a larger scale for hardened steels. Boron carbide is the hardest ceramic grain we use. We use it for extremely hard steels and alloys, but also for grinding steels that are not as hard. This is particularly the case when time plays an important role in grinding, for example, when we need to grind entire series of steels. In industrial environments, cubic boron nitride is also used for performing grinding work at high temperatures.

Are there cases where the use of boron carbide reaches its limits?

The use of boron carbide does not extend to grinding very hard ceramic parts such as zirconium oxide, which are used for, among other things, orthopedic products such as artificial hip joints. When grinding such hard materials, diamond powder is really the only substance that remains an option – and there are no ceramic alternatives left.        

Another limitation is that using boron carbide makes it very difficult to achieve a mirror finish. If this is expressly not wanted – fine. However, if this is a desired quality, we help it along through a light polishing with diamond grain and thereby get the desired shine. An important point is that when using boron carbide abrasive powders, you have to know that the material will break more easily when subjected to mechanical stress than, for example, diamond. This is particularly important when adjusting the respective pressure that’s transmitted via the disks. If you don’t, the grinding effect of the boron carbide will decrease, and the targeted roughness depth will soon no longer be reached.

Which are the methods you use most often for grinding?

In our area of application, we still do a lot of work by hand. There’s still a need for this, especially in the area of one-off and custom-made products or for maintenance work. Recently this has not been the case in the automotive industry, where instead of re-grinding the valves, the cylinder heads are often completely replaced. We’re often used for maintenance work of this kind in the shipping industry, for example, where it’s always cost-effective to have the usually very large valves manually ground. Manual work is also increasingly in demand for grinding valves in combined heat and power plants.

What does this hand grinding with the use of ceramic look like?

We work mainly with hand lapping machines. To explain: in contrast to grinding, where “bound” grain is used, one speaks of lapping when the grain is unbounded, i.e. loose, especially as a component of a corresponding oil or paste. This paste is then applied individually, for example to grind into a valve. Using this paste, we are then able to work on the valve seat and the valve cone at the same time.

Why are liquid or oily abrasives preferred in such processes?

The use of oils or pastes has three functions or advantages. First of all, of course, there’s the lubricating effect. Secondly, the cooling effect of the liquid prevents the grinding wheels from becoming too hot. Thirdly, the material that’s been ground away can be removed via the liquid. Which abrasive you use is also defined by these three effects. With a paste, we have less performance with regard to cooling and transport, but a high lubricating effect. In the case of oils, the cooling and transportability are more in the middle range, coupled with a high lubricating effect. When using a grinding emulsion, it is exactly the opposite: low lubrication effect but good cooling and transport performance.

What should one pay special attention to when using grinding pastes?

The main difference here is the hardness of the material to be ground. In the selection of the paste, this will be responsible for ensuring that the target roughness depth can actually be achieved. A second important aspect concerns how rust-sensitive the material is that you use. If it’s highly rust sensitive, you should use oil-soluble pastes and not water-soluble pastes. Other applications, again, are very sensitive to the oil residues left behind by oil-soluble pastes. Here one should then use water-soluble pastes.

As far as the respective application is concerned, do certain rules apply when using pastes?

No, there are actually no clear rules or no-gos here. Depending on the individual case, there are simply too many parameters that can send the requirements for the specific application in one direction or the other. Which paste I use in which composition essentially depends on the body to be ground and its chemical composition, as well as on the grinding process itself and what you want to achieve with regard to the triple effect of lubrication, cooling and transport. With new applications in particular, there’s no one correct way – instead, it’s a matter of carefully working your way toward the desired result.