Machining was carried out on a machining centre.
In this application, radial bores in a spheroidal graphite iron radial piston hydraulic motor were to be finished directly on a machining centre. The focus was an approximately Ø75 mm bore open to the component's outer contour. Its interrupted edge and open end make consistent tool guidance more difficult and can promote geometry errors.
The development trial used an expandable coolEXact® midi shell honing tool with four broad shell segments and metal-bonded diamond honing stones. The tool enters the pre-machined bore at a reduced diameter and expands the shells only after reaching the machining position. This allows the bore to be honed on the machining centre despite the difficult access conditions.
| Characteristic | Documented value |
|---|---|
| Material | spheroidal graphite iron |
| Component | radial piston hydraulic motor |
| Bore arrangement | six radial bores |
| Machining diameter | approx. Ø75 mm |
| Tool entry diameter | Ø74.5 mm |
| Surface target | Ra < 0.4 µm |
| Tool technology | coolEXact® midi shell honing tool |
| Shell segments | four expandable segments |
| Abrasive | metal-bonded diamond |
| Machine interface | CAT50 |
| Process status | documented development trial |
The coolEXact® midi shell honing tool has four expandable shell segments. When retracted, its outside diameter is small enough to enter through the Ø74.5 mm pre-machined opening. The shells are expanded to the effective machining diameter only after the tool is in position.
The broad guidance surfaces distribute machining forces around the circumference. Metal-bonded diamond honing stones provide the required stock-removal capability in spheroidal graphite iron. In a bore opening into the outside contour, the stone layout must also prevent one-sided unloading at the open end. Balancing stock removal, guidance and geometry correction was therefore the central purpose of the trial.
Align the workpiece: The component is positioned so that each radial bore is aligned with the machine spindle. The floating fixture supports alignment without introducing unnecessary constraint forces into the bore position.
Enter the tool safely: The shell honing tool enters the Ø74.5 mm prepared opening at a reduced diameter. The four shells are expanded only after reaching the machining position.
Remove material uniformly: The diamond honing stones machine the complete functional surface. The trial documented approximately 90 µm of stock removal, demonstrating that the tooling solution can do considerably more than merely touch or polish the surface.
Check geometry and surface: Surface roughness and the axial geometry profile are evaluated after machining. At the open end in particular, uniform shell support determines whether taper develops.
Optimise the stone layout: The initial trial condition produced approximately 5–10 µm of taper. Revising the distribution of the honing stones on the shell segments compensated for local unloading at the open bore end and reduced taper to below 2 µm.
Trial videos: The two recordings show positioning and entry of the shell honing tool as well as the machining sequence on the machining centre.
| Inspection criterion | Documented result | Assessment |
|---|---|---|
| Stock removal | approx. 90 µm | high removal capability demonstrated in the trial |
| Surface target | Ra < 0.4 µm | specified target |
| Roughness in the initial trial condition | Ra = 0.466 µm | intermediate result before optimisation |
| Achieved roughness | Ra < 0.35 µm | target achieved |
| Taper in the initial trial condition | approx. 5–10 µm | baseline before revising the layout |
| Achieved taper | < 2 µm | after revising the stone layout |
| Tool entry | Ø74.5 mm | entry before shell expansion confirmed |
The initial trial condition confirmed that the large radial bore can be machined with a shell honing tool on a machining centre. After the stone layout was revised, roughness improved to Ra < 0.35 µm and taper was reduced to below 2 µm. The surface and geometry targets defined for this process development were therefore achieved.
Why use a shell honing tool? The expandable shells combine broad guidance and machining surfaces with a smaller entry diameter. The tool can therefore enter the pre-machined bore safely and expand to its working diameter only after it is in position.
What makes the open bore end challenging? Circumferential support decreases at the open end. This can change the local tool load and promote taper or an uneven surface.
Was the surface target achieved? Yes. Ra = 0.466 µm was an early intermediate result. After optimisation, Ra < 0.35 µm was achieved, meeting the Ra < 0.4 µm target.
How can the taper be reduced further? The decisive measure was to revise the distribution of honing stones on the shell segments. This reduced taper from the initial range of approximately 5–10 µm to below 2 µm. Stroke coverage, tool overlap and pre-machined geometry must still be considered together.
Is this already a production process? No. The records document a development trial. Tool life, process capability and production approval are not substantiated.
The trial shows that a coolEXact® midi shell honing tool can machine the open-ended radial Ø75 mm bore of a radial piston hydraulic motor directly on a machining centre. Approximately 90 µm of stock removal demonstrates the capability of the concept. After the stone layout was revised, Ra < 0.35 µm and taper below 2 µm were achieved. The early Ra = 0.466 µm reading remains documented as a traceable intermediate stage in the successful process optimisation.
Related DIAHON section: coolEXact®.