Vitrified bond CBN and diamond grinding tools have been used successfully for about 25 years and have been gaining an ever-increasing share of the market. The basis for the success of what is now the most important bond system for CBN are the outstanding grinding characteristics of these usually porous structures. This is possible due to the fact that they can be profiled and dressed readily, their outstanding wear characteristics and the high material removal rates that can be achieved [1]. The structure of a vitrified bond grinding wheel can be characterised conventionally in a three-component system by the following elements:

â—† the abrasive (or its chemical and physical properties),

â—† the bond,

â—† the structure.

The abrasives are not usually produced by the toolmaker, as this is not his area of expertise. A great many suppliers of abrasives offer a wide range in terms of quality and properties. The vitrified bonds are the main key technology of the tool manufacture. These are normally built up of frits,feldspar, kaolin and various clays. The fired bonds mainly contain the following cations: Al3+, Fe3+, Ti4+, Ca2+, Mg2+, K+ and Na+, and less commonly Li+ as well. For health and technical reasons, modern bond systems do not contain lead.

Ultimately the chemical and physical properties are a result of the combination of the raw materials, the grit sizes used and the thermal treatment, i.e. firing [2]. The structure results from the ratio of the volume proportions of the abrasive (usually a secondary abrasive with CBN/diamond), the proportion of bond and the remaining pore volume [3-5]. The aim for a new generation of grinding tools is to optimise the individual factors and their interactions in order to considerably increase the total variability of the three-component system and thus

achieve cost reductions for the user. HPB technology The result of the development of HPB technology for vitrified bond grinding wheels described below is a very promising concept with the potential for achieving significant productivity improvements for the user. With the new bond system the following factors can be optimised with a variable weighting specific to the process in question:

â—† reduced grinding pressure and therefore improved workpiece geometry,

â—† longer dressing intervals,

â—† improved process times.

Any improvement in respect of grinding pressure is extremely beneficial in terms of cutting performance and workpiece

geometry particularly for use in internal cylindrical grinding and the relatively high forces that occur in this process [5]. The individual parameters of the threecomponent system of a vitrified bond CBN wheel have been investigated in detail with regard to the properties of the

abrasive, the bond and porosity. Properties of the abrasive The progressive fracturing of the CBN grit and the generation of new cutting edges is an important factor for the successful machining of a workpiece [7]. In this process the fundamental material-specific grit properties are already modified on account of the heat treatment in the vitrified production process. Particular consideration was given to these effects in order to improve the machining characteristics with the use of HPB technology.

The following characteristics must be taken into account and weighted accordingly in the composition of the HPB specification. Crystal morphology is a determining factor for the physical, thermal and chemical properties of the CBN grit [8]. A corresponding fracture strength and chip size result from this as a function of the grit' s thermal history. Both factors have a profound effect on the machining characteristics of the whole tool.

The future

While HPB technology has already proved itself widely in internal cylindrical grinding in many applications in production grinding, applications in double surface grinding, superfinishing and external cylindrical grinding are currently being tested.

The subsequent aim is to transfer all the detailed analysis work and the further development of the total grinding wheel system to vitrified bond diamond wheels. In Part 2 in the next issue, the characteristics of new concepts for dressing with vitrified bond diamond tools will be presented.