Machining was carried out on a machining centre. The big-end bore of a connecting rod for large 6- to 12-cylinder diesel engines in SAE 86B45H is finish-honed to Ø132 H6 on a MAKINO A81 NX after reaming. In addition to the close final size, the 60 mm long through bore requires a surface finish of Rz ≤ 3.2 µm.
A single honing tool could already machine the size and geometry, but did not reliably achieve the required roughness. Dividing the process into rough honing and finish honing separated the main stock removal from the final surface finishing operation. Rz values between 1.9 and 3.0 µm were achieved in the documented process.
| Characteristic | Detail |
|---|---|
| Component | connecting rod for large 6- to 12-cylinder diesel engines, big-end bore |
| Application | finishing after reaming |
| Material | SAE 86B45H, UNS G86451 |
| Bore type | through bore |
| Bore length | 60 mm |
| Final size | Ø132 H6, corresponding to Ø132.000 to Ø132.025 mm |
| Documented size after reaming | Ø131.990 mm |
| Surface finish target | Rz ≤ 3.2 µm |
| Roughness achieved in the two-stage process | Rz 1.9–3.0 µm |
| Documented final size | Ø132.023 mm |
| Documented roundness | 3–7 µm at the evaluated measuring planes |
| Machine | MAKINO A81 NX machining centre |
| Machine interface | SK50, ISO50 to DIN 69871/A |
| Tool | coolEXact 8L132/60S-180 B15 |
| Process | rough honing and finish honing with separate tool sets |
The coolEXact tool uses eight honing stones distributed evenly around the circumference. This multi-stone arrangement supports form correction in the large bore and distributes the machining forces over a broad contact area.
The tool's integrated size control limits expansion at the set final size. The machining sequence can therefore be carried out on the machining centre without transferring the size-determining operation to a separate honing machine.
Different tool sets are used for rough honing and finish honing in the two-stage process. The rough-honing tool removes most of the allowance. The finish-honing tool is matched to the small remaining stock and required surface finish.
Reaming as a defined pre-machining operation: The bore is prepared with a small, uniform honing allowance. In the documented sequence, the reamed size was Ø131.990 mm.
Rough honing for size and form: The first operation removes most of the allowance. In one documented run, approximately 25–30 µm was removed on diameter and the bore was machined to approximately Ø132.020 mm.
Finish honing for the target surface: The second operation removes only a small remaining amount. The documented final size was Ø132.023 mm, while roughness was improved to the Rz 1.9–3.0 µm range.
Include workholding in the process assessment: Measurements showed that workpiece clamping can influence the roundness of the large bore. Clamping force and force introduction are therefore part of the overall process set-up.
Changing to two honing operations resolved the conflict between stock removal and a fine surface. Rough honing removed the required allowance efficiently. Subsequent finish honing improved the surface without exceeding the close H6 final size.
One documented run finished at Ø132.023 mm and therefore within the Ø132.000 to Ø132.025 mm range. The achieved Rz values of 1.9 to 3.0 µm met the maximum requirement of 3.2 µm. Roundness measurements at the evaluated measuring planes were between 3 and 7 µm.
The practical finding is clear: in a large connecting-rod bore, the tool used for high stock removal should not also be expected to provide the complete surface finish. Two matched operations provide suitable conditions for both tasks.
Why are two honing tools used? The rough-honing tool is designed for the main stock removal. The finish-honing tool removes only a small remaining allowance and produces the required surface. Stock removal and roughness can therefore be optimised more independently.
What surface finish was achieved? Rz values between 1.9 and 3.0 µm were documented in the two-stage process. This met the requirement of Rz ≤ 3.2 µm.
How was the final size maintained? The bore was pre-machined with a small allowance. Integrated size control in the coolEXact tool limited expansion; one documented run finished at Ø132.023 mm within the H6 tolerance.
Why is workholding important? Unfavourable force introduction can elastically deform the large big-end bore. Clamping force and support points must therefore be selected so that roundness is retained after unclamping.
How was tool wear assessed? No measurable wear was detected over four machined bores in one documented run. The observation served as an early process check for the tool design used.
Precision honing of the Ø132 mm connecting-rod big eye on a MAKINO A81 NX combines the close H6 final size with a surface finish of Rz ≤ 3.2 µm. Using the coolEXact multi-stone tool and separate rough- and finish-honing operations, Ø132.023 mm and Rz 1.9–3.0 µm were achieved. The decisive factors are a uniform reaming allowance, separate tool sets for stock removal and finishing, and low-distortion workholding.
Related DIAHON section: coolEX® tool system – hydraulic honing technology.