Process and tool
The shape accuracy (cylinder shape) is adjusted automatically, depending on the process, without measurement control. The final diameter is determined by the one-piece tool.
The one-piece and thus maximally rigid tools with precisely dressed dimensions define the final diameter practically independent of the ambient conditions, such as temperature. There can be no “evasion” of the tool into the cross hole due to a change in the surface pressure between the work piece and the tool. The tool, which is stiff in diameter, can be “driven” through the bore and extraordinarily good cylinder shapes can be achieved. For internal grinding and conventional honing, the two-stroke reversal points have a significant influence on the cylinder shape. These must be permanently monitored and adjusted. A relatively large allowance can also be removed (conical tool section) and the wear per part is minimal (cylindrical tool section). Due to the single-layer coated tool, no spontaneous changes occur (e.g. grit breakage).
The feed of the tool is force-controlled. This prevents, for example, the elastic expansion of a thin-walled component. The tool is not under- or overloaded and unproductive “air grinding” is avoided. Too small or too large raw bore diameters are detected and can be intercepted and tool breakage prevented.
The work piece holder is an important component. The work piece must not be deformed during fixing and the work piece position is usually determined by the tool axis.
A central feature of the Microcut Honing System is the minimal dispersion of the machining results, so that no parts are out of tolerance due to dispersion. In conventional honing or internal grinding, the shape must be permanently controlled by a measuring and control loop through the stroke position and length and the dimension through the infeed of the tool. This complicates a stable process and is associated with great technical effort and costs. It is also important to note that the uncertainty of a measurement close to the machining process, especially of the form, is considerable.
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As shown schematically above, the Microcut honing process is continuous and not erratic, within a diameter tolerance of, for example, +/-0.001 mm. The green curve for the Microcut honing process shows a continuous minimum wear of the tool and thus a smaller bore.
The wear of the tool and thus the number of parts in the diameter tolerance of the Microcut Honing System depends on the material to be machined, the bore length, the allowance and the tool geometry. This can vary from < 100 parts to several thousand parts with a tolerance window of 1 µm. With a larger tolerance window, it can even be tens of thousands of parts. The Microcut machining process is not temperature sensitive, i.e. even large temperature fluctuations or the start of production have practically no negative influence on the dimensional accuracy of the bore diameter.
The “cold” micro machining process removes damaged edge structures (e.g. caused by spark erosion, hardening) and additionally compresses the bore edge zone (residual compressive stresses). The surface structure can be defined and reproduced as often as required, typically a roughness value of Ra 0.1 µm (N3) is achieved with machining tools with bonded grain. In hard materials such as carbide, a mirror-smooth surface can be produced by using loose grits and special tools.
Some important points are summarized below:
Spool sleeves with cross bores have very high requirements for the cylinder shape (roundness and straightness) of the bore and also on the surface in various industries, such as aviation or automotive.
Spool sleeve with cross bores and highest accuracy requirements.
Form of a slide sleeve with cross holes: cylindrical shape CYLt = 0.23 µm
As the Microcut Honing tools are one-piece, 350 mm long and coated, the tool is in contact with the hole and the full length of the hole. This results in the best possible straightness correction of the bore. In addition, the advantage is also visible in the roundness, as the surface pressure between the tool and the bore wall is very homogeneously distributed. In the case of a tool used for conventional honing or internal grinding (short honing tool or grinding pin), which is shorter than the length of the bore, the forces change much more due to the transverse bore, which is ultimately reflected in the shape (especially the straightness of the bore wall) of the finished bore.
With hydraulic control components, burr formation in the cross bores is a relevant issue. The deburring process with brushes can be controlled to a limited extent and is cost-intensive due to brush wear. In contrast to conventional honing and grinding, the Microcut Honing System does not generate a typical flake burr, but a symmetrical micro burr with a solid root, which usually does not need to be removed.
Work piece clamping technology
When machining thin-walled work pieces (sleeves), the work piece clamping technology and the control of the machining forces are of particular importance. Any deformation of the work piece during clamping has a direct effect on the shape of the machined bore.
Force-controlled machining process
If the feed of the tool is not force-controlled, there is a risk, depending on the cutting ability of the tool, of expanding the work piece during machining, so that the thin-walled sleeve “contracts” again when the tool is not anymore in the workpiece and thus the diameter of the finished machined bore does not follow the tool exactly.