A short, pre-reamed Ø38 mm bore in a GGG70 rocker arm had to be finished to size and provided with the required functional surface. Parts of the pre-machined bore were already within the diameter tolerance. The honing stage therefore had to machine small and locally varying stock allowances without impairing the bore geometry.
A coolEX® honing tool with eight compact honing stones and an HSK-A100 interface was used. The compact stone arrangement provides uniform guidance in the bore, which is only 30.5 mm long, and combines size correction with a targeted improvement of the reamed surface.
| Characteristic | Configuration |
|---|---|
| Component | Rocker arm |
| Material | GGG70 |
| Bore diameter | Ø38 G7, tolerance +0.009/+0.034 mm |
| Bore length | 30.5 mm |
| Pre-machining | Reaming |
| Surface after reaming | Rz 4–5 µm |
| Target surface | Rz < 4 µm; preferred Rz 2–3 µm |
| Roundness | 10 µm |
| Straightness | 6 µm |
| Machine | Hermle C52 |
| Tool interface | HSK-A100 |
| Tool technology | coolEX® with eight compact honing stones |
| Tool version | coolEX® 8L38/30-T70/100 B20 SZ32×50 in HSK-A100 |
| Required clearance | at least 27.5 mm behind the component |
The coolEX® tool system uses the metalworking fluid for hydraulic actuation of the honing stones. A version with eight compact honing stones distributed around the circumference was designed for the short Ø38 mm bore. The large number of short stones provides uniform support and assists in machining small, unevenly distributed stock allowances after reaming.
The tool holder is designed directly for HSK-A100. The tool body and compact stone holder form a coordinated unit for automated use on the Hermle C52. A defined clearance is required behind the component to allow the necessary tool overrun.
Evaluate the pre-machined condition: The reamed bore is assessed for diameter, geometry and surface. Because individual areas may already be close to the finished size, the honing stage must operate particularly uniformly and controllably.
Guide the tool securely in the short bore: Eight compact honing stones distribute tool guidance around the circumference. The tool therefore remains stably supported despite the short machining length.
Combine final sizing and surface generation: coolEX® performs the remaining size correction while improving the reamed surface towards the required functional finish.
The initial condition combined a short bore with a small, locally varying stock allowance and a reamed surface of Rz 4–5 µm. The coolEX® concept was matched to the target surface of Rz < 4 µm and to the bore's roundness and straightness requirements.
Documented feedback from the application confirms reliable tool operation. The combination of circumferential guidance from eight compact honing stones, coolEX® actuation and the direct HSK-A100 connection therefore proved effective in the machining sequence.
Why is honing used after reaming? Reaming already produces a good initial bore, but it cannot always achieve the required functional surface and tight geometry requirements throughout the bore. Honing corrects the remaining stock and refines the surface.
Why are eight compact honing stones used? The short bore requires uniform guidance around the entire circumference. Eight short stones distribute support and material removal particularly evenly.
What is the role of coolEX®? coolEX® actuates the honing stones hydraulically through the metalworking fluid and combines size correction with generation of the required functional surface.
Why is clearance behind the component important? The clearance allows the required tool overrun so that the compact honing stones can machine the full functional length uniformly.
Combining upstream reaming with coolEX® finish honing solves the demanding machining task posed by the short Ø38 mm bore in the GGG70 rocker arm. Eight compact honing stones provide uniform guidance and controlled stock removal. The direct HSK-A100 connection supports automated use on the Hermle C52.
Related DIAHON section: coolEX® tool system.