An aerospace bushing with a through bore in Ti-6Al-4V to AMS 6931 is precision machined. The two component variants have bore depths of 75 mm and 76.1 mm. The nominal size is Ø24.7204 mm with a tolerance of ±0.00635 mm; a tool working length of 80 mm is sufficient for both variants.

DIAHON tested two tool concepts on an OKUMA 5-axis milling machine: an FDhone® fixed-diameter honing tool and a hydraulically expanded coolEXact® midi honing tool. Both concepts are designed for the existing BT40 interface and allow machining in the existing setup.

The FDhone® FD8L24,7/105-T120/215 tool is designed as a fixed-diameter honing tool. Its defined working diameter and the honing stones distributed around the circumference support uniform size and form machining in a compact machining sequence. Further information is available on the DIAHON page about FDhone® technology.

The coolEXact® midi 4L24,7/55-T105/155 tool is hydraulically expanded by the coolant. The integrated size stop mechanically limits expansion at the set diameter. This allows stock variations to be machined while securing the final size within the tool concept. Further information is available under coolEXact® tool system.

Pre-machining: The bore is prepared for honing by drilling and fine boring. In the documented trial, the fine-bored initial diameter was Ø24.697 mm.

FDhone® machining: The fixed-diameter honing tool guides the bore through the machining zone with a defined working diameter. The tool is particularly suitable for a repeatable finishing step when pre-machining is stable.

coolEXact® machining: The hydraulically expanded tool adapts to the available stock during machining. The mechanical size stop limits the working diameter and combines conventional honing kinematics with a defined final position.

Common tool design: The 80 mm working length covers both component depths. Both tools use a BT40 holder and are therefore matched to the existing OKUMA 5-axis milling machine.

During the trial, the fine-bored hole was machined from Ø24.697 mm to Ø24.704 mm with a tolerance of ±0.006 mm. This allowed the dimensional machining of the titanium alloy to be assessed in practice with both tool principles.

FDhone® achieved a surface finish of approximately Ra 0.4 µm. During the first coolEXact® run, a deviating setting resulted in approximately Ra 0.9 µm. After correction, a surface finish of approximately Ra 0.4 µm was also documented. The achieved value is therefore below the specified maximum of Ra 0.8 µm.

Why were FDhone® and coolEXact® tested? FDhone® operates with a defined fixed diameter and is suitable for a repeatable finishing step. coolEXact® is hydraulically expanded and can therefore machine existing stock variations within its mechanical size stop.

Why is the tool working length 80 mm? The two bushing variants have bore depths of 75 mm and 76.1 mm. A working length of 80 mm covers both variants with the same tool concept.

Which pre-machining operation is suitable for the honing process? A dimensionally stable fine-boring operation aligned with the honing axis provides controlled stock. In the trial, the bore was provided at Ø24.697 mm.

Which surface finish was achieved? After process adjustment, a surface finish of approximately Ra 0.4 µm was documented. This is below the maximum requirement of Ra 0.8 µm.

Machining the aerospace bushing shows that both FDhone® and coolEXact® can be used for precision machining of the long Ti-6Al-4V bore. The 80 mm tool design covers both component variants. The narrow size window was machined in practice on the OKUMA 5-axis milling machine and approximately Ra 0.4 µm was achieved after process adjustment.

Related DIAHON sections: FDhone® tool system and coolEXact® tool system.