In this application, plasma-coated cylinder bores of approximately Ø90 mm in a four-cylinder engine block are honed in several stages. The development task was not limited to achieving one roughness value. Honing-stone wear, size development, roundness over the bore height and the functional plateau surface all had to be stabilised together.
The tooling consisted of application-specific DIAHON honing stones and holders in a multi-stage honing head. The sequence separated roughing, semi-finishing, finish honing and a short final treatment. This separation revealed which stage actually influenced geometry, diameter and surface structure. The project records do not clearly identify a current standard tool system such as coolEX, coolEXact or coolEX LM, so no such product designation has been added retrospectively.
| Characteristic | Documented value |
|---|---|
| Component | four-cylinder engine block, anonymised |
| Bore | approx. Ø90 mm |
| Functional layer | plasma-coated cylinder running surface |
| Process | roughing, semi-finishing, finish honing, short final treatment |
| Ra target | 0.30–0.65 µm |
| Rpk target | < 0.50 µm |
| Rk target | 0.62–1.60 µm |
| Rvk target | 0.50–2.20 µm |
| Mr1 / Mr2 target | 2–12% / 73–87% |
| R3z reference | 1.8–3.5 µm |
| Roundness limit | 7.5 µm |
| Roundness measurement heights | 10 mm, 68 mm and 140 mm |
Monitor roughing-stone wear: Wear records are kept block by block. This distinguishes abrupt changes in stock removal from normal, gradual wear. Visible damage at the bore entrance is compared with the wear history.
Set semi-finishing for geometry and remaining stock: The intermediate stage prepares consistent geometry and controlled remaining stock for finish honing. The finishing stage therefore does not have to correct major form errors while generating the final surface structure.
Set finish honing for functional parameters: Ra alone does not describe a cylinder running surface adequately. Rpk, Rk, Rvk, Mr1, Mr2 and R3z are therefore assessed together. This shows whether peaks have been removed, the load-bearing core is sufficiently developed and the valleys retain their lubricant function.
Use a short final treatment selectively: A short final stage improves plateau formation without removing unnecessary material. Its effect is assessed from a complete measurement series rather than one favourable individual reading.
Verify roundness over the bore height: Roundness measurements near the bore entrance, at mid-height and in the lower section show whether a process change acts locally or remains stable over the complete functional length.
| Assessment | Initial trial | Optimised trial |
|---|---|---|
| Ra within target | 14 of 27 measurements | 11 of 12 measurements |
| Mr1 within target | 17 of 27 measurements | 12 of 12 measurements |
| Mr2 within target | 26 of 27 measurements | 12 of 12 measurements |
| R3z within target | 6 of 19 valid measurements | 12 of 12 measurements |
The optimised short finishing stage brought nearly all Ra readings into the target range and stabilised the material ratios and R3z. At the same time, the evaluation showed that Rpk, Rk, Rvk and the derived oil-retention volume remained sensitive to the coordination between honing stages. The complete group of parameters, rather than one isolated value, was therefore used to make process decisions.
A later documented condition achieved Ra = 0.566 µm, Rpk = 0.22 µm, Rk = 0.84 µm, Rvk = 2.18 µm, Mr1 = 5.3% and Mr2 = 77%. These key surface parameters were simultaneously within their specified ranges.
The roundness evaluation for the documented production series remained below the 7.5 µm limit at the measurement heights of 10 mm, 68 mm and 140 mm. Wear tracking covered 41 engine blocks with four bores each and made the development of the roughing stones traceable throughout the run.
Why is the process divided into several honing stages? Each stage can be assigned a limited task. Roughing provides the main stock removal, semi-finishing stabilises geometry and remaining stock, finish honing generates the functional surface, and a short final treatment removes remaining peaks.
Why is Ra insufficient for evaluating a cylinder running surface? Two surfaces can have the same Ra value but very different peaks, cores and valleys. Rpk, Rk, Rvk, Mr1 and Mr2 add this functionally relevant information.
How was honing-stone wear evaluated? Adjustment records were collected for 41 consecutively documented engine blocks. Combined with dimensional deviations and photographs of the bore entrance, they showed whether the roughing stones wore gradually or their cutting behaviour changed abruptly.
Why is roundness measured at three heights? The bore entrance, middle and lower section respond differently to tool guidance and stroke position. Three measurement heights show whether geometry remains stable over the entire functional length.
Did the optimised trial already meet every parameter completely? No. Ra, Mr1, Mr2 and R3z improved significantly. Other parameters remained sensitive in the early comparison and required further adjustment. The measurement series was valuable precisely because it exposed these interactions.
Which DIAHON tooling technology was used? The records confirm application-specific DIAHON honing stones and holders in a multi-stage honing head. They do not identify a current standard tool system unambiguously. The case is therefore linked to DIAHON honing technology without adding an unsupported product name.
This case shows how a plasma-coated Ø90 mm cylinder bore can be developed systematically by assigning a clear task to each honing stage. Application-specific DIAHON honing stones and holders, a short adjusted finishing stage and combined evaluation of roughness, material ratio and roundness made the process interactions transparent. In the optimised trial, 11 of 12 Ra readings and all Mr1, Mr2 and R3z readings were within their respective targets. The production measurements also confirmed roundness values below 7.5 µm at three bore heights.
Related DIAHON section: Honing on machining centres.