The honing operation was performed on a WFL MILLTURN turn-mill machining centre.
A long blind bore in 15-5 PH (PH13-8Mo), a precipitation-hardening stainless steel, required a very smooth, load-bearing surface as the basis for a subsequent DLC coating. In addition to a final Ra below 0.1 µm, the height of individual profile peaks had to be limited. Positive peaks can locally overload the coating and therefore had to be removed in a controlled manner.
The solution combines robust pre-honing with two matched fine-honing and plateau-honing stages. A coolEXact tool performs the initial stock removal, followed by coolEX tools that progressively generate the required surface structure. Trials assessed machining, cleaning and coolant as one system. Optimisation of the honing stones ultimately reduced loading and stabilised the process for production use.
| Component | long blind bore with run-out at the bore end |
| Diameters | approximately Ø38.1 mm, Ø56.8 mm and Ø62.5 mm |
| Bore depth | approximately 530 to 550 mm |
| Material group | precipitation-hardening stainless steel |
| Hardness range | approximately 37 to 46 HRC |
| Machine system | WFL MILLTURN |
| Coolant | water-miscible emulsion, approximately 11% in the trial |
| Final target | Ra < 0.1 µm and Rp approximately ≤ 0.12 µm |
| Subsequent process | DLC coating |
The published diameters have been slightly altered to protect the customer's identity. They describe the scale of the application but are not manufacturing dimensions.
The bore combines a high length-to-diameter ratio with high material hardness. It also has a blind end and only a short run-out. The tool must therefore cut evenly over the full depth while reliably removing chips and abraded material from the contact zone.
A largely peak-free plateau surface was crucial for the subsequent DLC coating. A low arithmetic mean roughness could already have been achieved while individual profile peaks still exceeded the target. Process development therefore evaluated Ra, Rz, Rp and material ratio together.
Early trials also revealed a clear pressure threshold: the finest honing stage cut reliably only with sufficient contact pressure. At too low a pressure, it produced little smoothing. At the same time, an overly aggressive first step could create an unnecessarily rough starting structure and burden the following stages.
| Stage | Task | Tool principle | Typical intermediate target |
|---|---|---|---|
| 1 | Pre-honing and form correction | coolEXact | Ra < 0.8 µm |
| 2 | Fine machining | coolEX | Ra < 0.3 µm |
| 3 | Plateau and peak machining | coolEX | Ra < 0.1 µm and limited Rp |
The first stage provides stable stock removal and creates a uniform basis. The second stage reduces roughness depth without machining the bore for an unnecessarily long time. In the third stage, the priority is not merely a small Ra value, but the removal of the remaining positive peaks.
The trials showed that the finest stage cut effectively at approximately 30 to 35 bar. Spindle speed, stroke speed, cleaning and coolant condition were also coordinated during process design. The surface was thus produced by a controlled sequence of clearly separated tasks rather than by one extremely fine machining step.
In a structured trial series, five workpieces were measured at three axial positions. After the third honing stage, Ra values across all measuring positions were approximately 0.05 to 0.09 µm. Rz was approximately 0.46 to 0.91 µm. The Ra < 0.1 µm roughness target was therefore achieved reproducibly.
Rp values in the same trial ranged from approximately 0.11 to 0.21 µm. This result was important for further development: it confirmed the very low overall roughness while also showing that the highest profile peaks required more targeted reduction. From the second trial onwards, the material ratios of the finished surfaces were predominantly between approximately 70 and 95%.
A comparison with and without intermediate cleaning demonstrated sensitivity to loaded honing stones. Without cleaning, the first stage in particular became significantly less effective. The cause was identified as the interaction between the highly lubricating coolant and the honing-stone specification. Optimisation of the honing stones improved debris transport out of the contact zone, substantially reduced loading and achieved both the required stock removal and specified surface.
The three honing stages were then retuned as a system: the first stage became less aggressive, the second prepared the surface uniformly, and the third provided targeted peak removal. Development was completed successfully; the approved design subsequently proved itself in production use.
Why are three honing stages required? Stock removal, fine machining and peak removal place different demands on abrasive grit, bond and contact pressure. Separating these tasks allows each stage to perform its function without unnecessarily burdening the next.
Why is Ra not sufficient as an acceptance value? Ra averages the entire profile. Individual positive peaks may remain even at a low Ra value. Rp and the plateau characteristics are therefore particularly relevant for a subsequent DLC coating.
What was the effect of optimising the honing stones? The optimised honing stones were less susceptible to loading, improved debris transport and thereby stabilised stock removal and surface quality.
What role does cleaning between stages play? The measurement series showed that contaminated or loaded honing surfaces can significantly reduce stock removal. A defined cleaning condition is therefore part of process control, not merely maintenance.
Was the solution tested only in trials? No. Following optimisation, the tool design and process were approved and confirmed in production use.
The demanding surface in the long blind bore was not achieved with a single honing operation. The key was separating pre-honing, fine machining and targeted peak removal. Measurement data confirmed Ra values of approximately 0.05 to 0.09 µm while also demonstrating that Rp had to be optimised separately.
Matched contact pressure, controlled cleaning and optimised honing stones brought loading under control. The finally approved three-stage design creates a very smooth, load-bearing plateau surface and was confirmed for production use.