The big eye of a connecting rod in 42CrMo4+HH is finish-honed to Ø183 H6 on a horizontal Makino a81 machining centre. The application combines a large split connecting-rod bore with a close final size, geometric requirements and a defined crosshatch. The original surface specification was Ra ≤ 0.8 µm or Rz ≤ 6.3 µm, together with a honing angle of 30° to 45°.

A hydraulically actuated coolEX multi-stone tool with eight honing stones performs the operation through the HSK-A100 spindle. The process development systematically varied abrasive grit, contact pressure, coolant orifice, stroke sequence and the design of the non-cutting zones. The result was a robust production process whose functional window was later widened to Rz 2.5–11 µm. Rz values from 4.5 to 8.3 µm are documented for 49 consecutive production parts.

Across twelve measured parts, pre-machining produced diameters from Ø182.971 to Ø182.988 mm. Cylindricity before honing ranged from 7 to 13 µm and straightness from 1 to 2 µm. Depending on the measuring plane, roundness values between 5 and 12 µm were documented. The honing tool therefore had to generate the surface while also absorbing variations in stock and existing form errors.

The corresponding inspection reports after the early honing trial show diameters from Ø182.993 to Ø183.024 mm. Eleven of the twelve parts were within the Ø183 H6 size range of Ø183.000 to Ø183.029 mm; one part remained 7 µm below the minimum size. Diameter growth ranged from 17 to 51 µm and averaged 33.4 µm. Cylindricity, straightness and roundness were not yet consistently stable in this early trial. Values reached 16 µm, 8 µm and 14 µm respectively.

Additional measurement sheets with separate columns before and after honing illustrate the early process condition. For fully documented parts 1 to 5, the post-honing deviations ranged from +3 to +25 µm within the H6 tolerance of 0 to +29 µm. The corresponding surface values were Rz 5.9 to 6.5 µm. Nevertheless, a groove was noted on another part, and two parts could not be completely cleaned up despite post-honing values within size. Dimensional compliance alone was therefore insufficient for release.

The first trials revealed a clear conflict. Coarse or very sharp abrasive specifications could produce an excessively rough surface and increase spindle load. Finer specifications sometimes reduced roughness below the desired functional range. Pressure adjustment alone was insufficient because abrasive grit, bond, contact length and stroke reversal acted together on surface and torque.

Later surface photographs showed pronounced reversal marks. The surface was also described as noticeably rough even though the measurements were within the inspection range used at that time. This demonstrated that release cannot be based on one Rz value alone. Crosshatch appearance, local measuring positions and machine load must be assessed together.

Eight honing stones for uniform circumferential guidance: The multi-stone principle distributes machining forces around the large bore. Hydraulic actuation allows contact pressure to be influenced through coolant pressure and the tool orifice.

Small steps instead of one fixed parameter set: Several abrasive grits, pressures and orifice diameters as well as short and long stroke sequences were tested. Early runs documented approximately 1 µm of stock removal per stroke. Size build-up and surface development could therefore be observed step by step.

Matching the non-cutting zones to spindle load: Abrasive non-cutting zones increased honing torque at the stroke reversals and could stop the spindle. Steel support pads reduced the spindle load without giving up the guiding portion of the tool. A later development changed the tool to a continuous 100 mm honing stone.

Treating the orifice as part of the tool configuration: A larger orifice lowers hydraulic back pressure, while a smaller one increases it. The documented trials show that changing the orifice influences roughness, but cannot be assessed independently of abrasive grit, bond and run-in condition.

The trial series demonstrates how sensitive the large connecting-rod bore is to the honing-stone specification. A fine variant produced Rz values that were partly below the desired range. A coarser variant initially produced excessive roughness, stabilised after running in and subsequently became finer again. The process therefore had to be assessed over several parts rather than from a single trial.

One documented group of five parts at approximately 17 bar tool pressure and with a 4.5 mm orifice produced 20 individual readings between Rz 4.1 and 7.8 µm. The five part averages were tightly grouped between Rz 4.925 and 5.75 µm. This trial showed that the combination of abrasive specification and reduced pressure could establish a viable surface window.

For later production, the functional tolerance was widened to Rz 2.5–11 µm. The final 49 documented parts were between Rz 4.5 and 8.3 µm. This range describes the approved later production status and must be distinguished from the original maximum requirement of Rz 6.3 µm.

Continued tool development eventually introduced a continuous 100 mm honing stone without separate non-cutting stones. A 3.5 mm coolant orifice is also documented as a later configuration for the Ø183 tool.

Why is the Rz value alone insufficient for process release? Local reversal marks or uneven crosshatch zones can remain visible despite an acceptable individual measurement. Measurements, crosshatch appearance, measuring position and spindle load must therefore be evaluated together.

How does the orifice influence the process? The orifice determines hydraulic back pressure in the tool. A larger diameter reduces pressure, while a smaller diameter increases it. Its effect also depends on abrasive grit, bond, contact length and run-in condition.

Why were steel support pads used in the non-cutting zones? Abrasive non-cutting zones caused excessive honing torque in trials. Steel pads reduced the load at the stroke reversals and stabilised machining on the available spindle.

What pre-machining condition was documented? Across twelve parts, pre-honing diameters ranged from Ø182.971 to Ø182.988 mm. The planned honing allowance was approximately 20–30 µm on diameter.

What do the paired measurements after the early honing trial show? Eleven of twelve parts reached the Ø183 H6 size range. One part remained below the minimum size at Ø182.993 mm. The geometry results show that this early trial condition still required further tuning.

Why were before-and-after values recorded in several directions? Measurements in the longitudinal and transverse directions at three planes show whether the tool removes existing form and stock variations uniformly. The early records demonstrate that one in-size reading does not guarantee a completely cleaned-up surface.

What production result was achieved? After the functional window was widened to Rz 2.5–11 µm, 49 documented production parts were between Rz 4.5 and 8.3 µm.

The Ø182 H6 connecting-rod big eye can be finish-machined directly on a Makino a81 using a hydraulically actuated coolEX multi-stone tool. The decisive factor was not one parameter, but the combined adjustment of abrasive grit, bond, hydraulic pressure, orifice, stroke sequence and contact zones.

The documented process development also shows the importance of a clear time-based assessment. The original target of Rz ≤ 6.3 µm was later replaced by a functional window of Rz 2.5–11 µm. In the approved production status, 49 parts achieved Rz 4.5–8.3 µm. Steel support pads in the non-cutting zones and the later continuous 100 mm honing stone helped bring surface finish and spindle load together over the long term.

Related DIAHON section: coolEX® tool system – hydraulic honing technology.