In this production application, a large connecting-rod bore in forged C70 steel is precision-honed directly on a machining centre. Starting with approximately 20 µm of machining allowance, the process must reliably achieve the H5 diameter range while controlling roundness, straightness and a load-bearing functional surface.

The solution uses a standardised eight-stone coolEXact® honing tool with µ-matik® fine adjustment. The ML80 system separates the honing tool from the machine holder: the holder can be changed from SK50 to HSK-A100 and back again whenever required. The same tooling concept can therefore be used on different machining centres, giving the user maximum flexibility and a future-proof solution when machines change. The honing stones are distributed evenly around the circumference to support tool guidance in the large bore, while fine, controlled expansion permits precise size corrections and compensation for honing-stone wear.

• Tool technology: standardised eight-stone coolEXact® honing tool
• Size correction: finely controlled µ-matik® adjustment
• Interface system: ML80 with interchangeable SK50 ↔ HSK-A100 holder
• Machine integration: the same tool base can be used on different machining centres by changing the holder
• Process strategy: incremental optimisation with continuous diameter checks

Check the pre-machined condition: Allowance, geometry and surface condition are evaluated before honing. The documented allowance of approximately 20 µm can be adapted to the actual pre-machining result.

Guide and expand the tool: The eight-stone coolEXact® tool is guided in the bore over a large circumferential contact area. The µ-matik® adjuster expands the honing stones in a defined manner and enables small, repeatable size corrections.

Change the machine holder: The standardised ML80 system connects the appropriate holder to the tool base. Changing from SK50 to HSK-A100 and back again enables the honing tool to be used on machines with different spindle interfaces without rebuilding the complete tooling concept.

Control geometry and diameter: The process corrects the diameter within the H5 tolerance range while keeping roundness and straightness within specification. Changes to the motion sequence are introduced incrementally and verified by diameter measurements.

Evaluate the functional surface: In addition to conventional roughness values, the material ratio in the P-profile is evaluated. At a section depth of 2 µm, a material ratio of at least 45% is required. This value describes how much load-bearing material is present at the defined depth.

Production feedback confirms trouble-free operation of the honing tools. Approximately 15,000 to 18,000 connecting rods were machined per set of honing stones. This is an application-specific field value, not a guaranteed tool life; material batches, pre-machining, coolant and process control can influence the result.

• A consistent pre-machining allowance reduces unnecessary honing time and simplifies size control.
• Eight honing stones distributed around the circumference improve tool guidance and support geometry correction.
• The µ-matik® fine adjustment enables small, repeatable expansion steps and facilitates wear compensation.
• The standardised ML80 system separates the coolEXact® tool base from the machine holder. Reversible changeover between SK50 and HSK-A100 increases flexibility for both existing and future machine concepts.
• Surface evaluation should not rely on Ra or Rz alone. The material ratio adds information about the load-bearing structure of the functional surface.
• An excessively fine surface can make geometrical evaluation more difficult. A functionally selected Rz range provides a useful balance between load-bearing capability and measurability.
• Motion sequences should be optimised incrementally, with the diameter verified after every change.

Why is an eight-stone honing tool used? The large number of evenly distributed honing stones improves guidance in the bore and distributes machining forces around the circumference. This supports both diameter and geometry correction.

What is the role of µ-matik®? The fine adjustment provides defined small expansion movements. It helps approach the target size and compensate for honing-stone wear during the process.

What does a material ratio of at least 45% at 2 µm mean? At a defined depth of 2 µm below the reference line, at least 45% of the profile must consist of load-bearing material. This value complements roughness data with a functional assessment of the bearing area.

Why is an Ra value alone insufficient? Surfaces with different structures can have the same Ra value. Profile depth and load-bearing material ratio are also important for function.

Is the stated tool life guaranteed? No. The 15,000 to 18,000 connecting rods per honing-stone set are documented production feedback from this application. Different conditions may produce different tool-life results.

Can the process be used on different machining centres? Yes. With the ML80 system, the holder can be changed from SK50 to HSK-A100 and back again. This allows the standardised coolEXact® tool base to be used flexibly on machines with either interface. Machine kinematics, coolant supply and measurement strategy must still be coordinated for the specific installation.

The standardised eight-stone coolEXact® honing tool with µ-matik® fine adjustment combines stable tool guidance with finely controlled size correction. In the Ø117 mm connecting-rod bore made from forged C70 steel, the system addresses the tight H5 diameter range, geometry requirements of 6 µm each and a material-ratio-oriented functional surface. Documented production experience of approximately 15,000 to 18,000 connecting rods per honing-stone set confirms the practical suitability of the tooling concept under the project-specific conditions. ML80 allows reversible changeover between SK50 and HSK-A100 holders, providing maximum flexibility and keeping the tooling solution ready for future machine concepts.

Related DIAHON sections: coolEXact® and µ-matik®.