A Ø28 mm bore (pinion gear) in hardened 20MnCr5 is honed after hard turning. The material has a hardness of 60–63 HRC. The bore finish requires a roughness of Ra 0.2 µm.
The application shows how strongly honing depends on the dimensional and surface quality of pre-machining. After hard turning was adjusted, the bore was honed from Ø28.006 mm to Ø28.013 mm. With approximately 7 µm of stock removal on diameter, Ra values of around 0.08–0.10 µm were achieved; the documented run covered 120 parts with stable machining behaviour.
| Characteristic | Detail |
|---|---|
| Application | bore finishing after hard turning |
| Material | 20MnCr5 |
| Hardness | 60–63 HRC |
| Nominal diameter | Ø28 mm |
| Diameter after adjusted hard turning | Ø28.006 mm |
| Diameter after honing in the documented run | Ø28.013 mm |
| Honing stock removal on diameter | approximately 7 µm |
| Roughness after adjusted hard turning | Ra 0.6 µm; Rz 3.6 µm |
| Roughness target | Ra 0.2 µm |
| Roughness achieved in the documented run | Ra approximately 0.08–0.10 µm |
| Documented machining run | 120 parts with stable machining behaviour |
| Machine | DMG/Gildemeister CTV 160 |
| Tool principle | hydraulically expanded honing tool |
| Coolant | water-miscible coolant, FUCHS ECOCOOL R-VHCM |
The hydraulically expanded honing tool finishes the hard-turned bore. The honing stones must effectively remove the small remaining allowance while producing the required fine surface.
Different bond variants were compared. The softer variant used in the documented machining run achieved the desired balance between stock removal and surface finish. Still softer variants, however, did not automatically produce a better machining result.
Tool development therefore includes not only the honing stone but also hydraulic expansion and coolant supply. Microscopic images of the honing stones support comparison of their surface condition at different axial positions.
Match pre-machining: Hard turning is adjusted for a uniform remaining allowance and suitable initial roughness. In the documented run, Ra 0.6 µm and Rz 3.6 µm at Ø28.006 mm were used as the starting condition.
Limit dimensional variation: The pre-machining size must be closely controlled. The service report specifies an accuracy of approximately ±1 µm. Greater dimensional variation increases the amount of material to be removed and can lead to remaining turning marks or a strongly fluctuating machining time.
Match honing: The honing stones are matched to the hardened material and small honing allowance. Hydraulic expansion is checked at the tool so that conditions remain comparable when the process is transferred to the machine.
Include coolant: Concentration, water quality and supply are treated as part of the honing process. A change in coolant conditions is assessed together with cutting performance and the surface.
After hard turning was adjusted, the small remaining allowance of approximately 7 µm on diameter could be removed effectively. The achieved Ra values of around 0.08–0.10 µm were below the required value of Ra 0.2 µm. Stable machining behaviour was observed over 120 parts.
The key finding is directly relevant to production: a suitable honing stone cannot compensate for unlimited variation in pre-machining. If the hard-turning size drifts or the initial surface is too rough, the honing task changes from part to part. A closely controlled pre-machining size and suitable initial roughness provide the basis for a consistent finish.
A resin-bonded honing stone variant was selected for subsequent machining. The selection considers the interaction between stock removal, surface quality and machining behaviour.
Why is the honing allowance so important? Here, honing finishes a bore that has already been hard-turned. A small, uniform remaining allowance limits the amount of stock removal and supports consistent machining behaviour.
What surface finish was achieved? The documented machining run achieved approximately Ra 0.08–0.10 µm. This was below the roughness target of Ra 0.2 µm.
Why is pre-machining closely controlled? Even a few additional micrometres of dimensional deviation significantly change the honing allowance. An accuracy of approximately ±1 µm was therefore required for the pre-machining size.
Is a softer honing stone bond always better? No. The trials showed that the bond must be matched to stock removal and surface finish. A further reduction in bond hardness did not automatically improve the result.
What role does coolant play? It is part of matching the tool and process. Concentration, water quality and supply must be considered when assessing cutting performance.
Honing the Ø28 mm bore in 20MnCr5 at 60–63 HRC combines a small honing allowance with a fine surface. With adjusted hard turning and matched honing stones, Ra values of around 0.08–0.10 µm were achieved with stable machining behaviour over 120 parts. The decisive factor is considering pre-machining, hydraulic expansion and coolant together.
Related DIAHON section: coolEX® tool system – hydraulic honing technology.