The ever-increasing demands on components in terms of dimensional, geometric and positional tolerances, and also the objective of reducing costs through continuous process improvement, can be met only through close cooperation between the tool manufacturer and the

machine builder. With flat honing, new systems are increasingly enabling further increases in material removal rates to be achieved through faster cutting speeds. This article describes the optimising of a flat honing system, also called 'Clean lapping with planetary kinematics' [1],

with which today virtually any material, from soft plastic through (sintered) metals and ceramics to high-strength PCD composites, can be machined in a reliable process using suitable grinding and conditioning tools.

The state of the art

The twin-disc principle of the lapping process with planetary kinematics (loose abrasive grit) has been established for a long time. Conventional machines used in this area have a so-called C-frame construction. Because of the use of oils and unbonded lapping pastes the process is relatively cleaning-intensive, which is critical both with regard tocost per part as with regard to possible disposal problems.

The introduction of tools with bonded abrasive, e.g. diamond or cubic boron nitride (CBN), is therefore to be seen as a major innovation for the development of the process generally.

The first applications involved the use of grinding tools with resin and metal bonds. However, the process forces resulting from the large contact areas with the parts led to many difficulties with both the machine and the process. As only few changes were made to the machines parameters and control systems to make them specifically suitable for flat honing, very little progress was made for a long time.

It was only with the use of vitrifiedbond tools that significant progress was made compared with conventional lapping methods [3]. A further step forward was in tool design in the form of individual pellets, which as well as the concentration calibrates the number of cutting edges engaging the workpiece due to the pattern with which a wheel is covered. For a long time however relatively little development work has been carried out on the vitrified bond of the CBN and the diamond itself. Generally, existing systems of other processes were just slightly modified and adapted. Furthermore, until now the conditioning of the grinding tools has not been a very reliable process. The usual process for sharpening diamond wheels for example is the use of soft corundum stones [4]SiC rings mounted on the rotor discs, or loose abrasive. All these methods require a great deal of experience, take a long time and because of the fact that they

depend on the operator's skills, they are only reliable to a limited extent.

The basic principle of the workpiece tool arrangement and also the relative movement between the two due to the planetary kinematics are shown in Fig 1a and Fig 1b. The wear patterns that are produced on the tool as a result of this

are shown schematically in Fig 2. The limit of any process is determined by the weakest link in the chain. An innovative improvement in the process is therefore only possible with a new design of the system as a whole, which can only be successful with coordinated development both of the machine and the tool.

Machine development Any new machine concept must take the following basic factors into account:

â—† a stable torsionally stiff machine concept

â—† a wide range over which the cutting speed can be varied

â—† it must be possible to adapt (increase) the force that can be applied to the workpiece materials

Prospects

The general trend in favour of vitrifiedbond high-performance tools should continue in the area of flat honing as well, not least due to the machine concepts which are now much more suitable. In future, the combination of vDD technology

with the conditioning of vitrified-bond diamond tools for the flat honing of, for example, ceramics, will also be the

subject of detailed tests. The application examples that have been described show clearly that it is only through close cooperation between

the machine builder and the tool supplier that the total machining system can be fully optimised. Both innovative machine concepts and also new approaches in the areas of tool development, carried out in close coordination one with the other, enable the user to increase productivity significantly. â—†