A two-stage honing concept was designed for a component family with bore diameters ranging from Ø17 to Ø32 mm. Xstep® performs the rough-honing operation and corrects size and geometry. coolEX® then generates the final diameter and required surface in the finish-honing operation.
Process optimisation showed that the result depends on more than the tool design alone. Correct referencing of the honing stones, sufficient guidance in the bore and suitably positioned strokes were decisive. After these points had been adjusted, the bore geometry was significantly improved, particularly at Ø32 mm.
| Characteristic | Configuration |
|---|---|
| Bore diameters | Ø17, Ø18, Ø20, Ø21, Ø22, Ø25 and Ø32 mm |
| Process sequence | Rough honing and finish honing |
| Preferred tool sequence | Xstep® for rough honing, coolEX® for finish honing |
| Measurement sequence | Tactile inspection after boring and measurement after each honing stage |
| Rough-honing tools | Xstep® with four honing stones; six honing stones at Ø32 mm |
| Finish-honing tools | coolEX® with four honing stones; six honing stones at Ø32 mm |
| Rough-honing tool coupling | DB28 |
| Xstep® machine interface | HSK-A63 |
| Finish-honing interface | HSK-C63; new Ø18 mm version with SZ40 × 50 |
| New Ø18 mm version | coolEX® 4L18/35S-115/231 B15 |
| Ø18 mm abrasive specification | B030-2005-A-040 |
Xstep® technology uses electromechanical step feed. A pull-push rod actuates the tool, a load cell measures the feed force and a linear sensor monitors the feed travel. The feed can either simulate hydraulic expansion or move the honing stones towards the target size in fine discrete steps after a defined referencing cycle. This allows the rough-honing stage to be matched specifically to geometry errors and distributed stock allowance. Four-stone versions were specified for Ø17 to Ø25 mm, while a six-stone tool with a larger guiding surface was used for Ø32 mm.
Finish machining is performed with coolEX® honing tools that are hydraulically actuated by the metalworking fluid. The diameter-specific tools generate the final size and surface. A new Ø18 mm version was subsequently added to the tool family.
Measurement-supported process sequence: After boring, the bore first undergoes tactile inspection. An intermediate measurement follows Xstep® rough honing. The final result is measured again after coolEX® finish honing. This sequence clearly separates the effects of pre-machining, geometry correction and surface generation.
Feed strategy I – controlled expansion: The tool advances from its home position until the defined machining force or motor torque is reached. During the machining time, the control maintains this variable through suitable feed increments. Xstep® then retracts the honing stones fully to their home position. The required feed travel compensates for honing-stone wear.
Feed strategy II – discrete feed to target size: Xstep® first references the honing stones against the bore wall with a defined force and stores the reference position. The system then retracts the stones to a safe position. Starting from the reference position, machining proceeds in finely resolved feed increments until the specified size is reached. Wear compensation can adjust the effective feed travel.
Reference the honing stones correctly: An unsuitable approach strategy when referencing against the bore wall caused the honing stones to load up and the machining force to rise sharply. Correcting the referencing strategy prevented this effect.
Ensure sufficient tool guidance: At Ø32 mm, the diameter to be finished was initially located unfavourably between two ribs. The honing stones therefore received insufficient guidance. Revising the pre-machining operation improved tool support and the resulting bore quality.
Match stroke positions to the measurement planes: Differences in diameter between the measurement planes were substantially reduced by targeted adjustment of the stroke positions. The optimisation was carried out step by step over several components.
Distribute stock removal between the stages: Rough honing performs the geometry correction. Additional stock removal in the second stage can then be used to refine the surface specifically.
Before honing, one measured bore showed an S-shaped diameter deviation of up to 25 µm. Despite this unfavourable initial geometry, geometrically conforming components were produced after honing. The documented assessment attributes the significant improvement in geometry to the stepwise expansion of the Xstep® tool.
At Ø32 mm, a good honing result was measured after correct referencing and revised pre-machining. An additional adjustment of the stroke positions improved agreement between the individual measurement planes. Stock removal in the coolEX® finishing stage was increased to optimise the surface.
A documented surface measurement on sample D064 confirms a finely machined functional surface. The measurement used an evaluation length of 4.8 mm and a cut-off filter of 0.8 mm.
| Surface parameter | Measured value |
|---|---|
| Ra | 0.185 µm |
| Rz | 1.821 µm |
| Rmax | 2.614 µm |
| Rpk | 0.181 µm |
| Rk | 0.522 µm |
| Rvk | 0.427 µm |
Why are two tool stages used? Xstep® performs the larger share of the size and geometry correction. coolEX® can then be adjusted specifically for the final size and surface.
Why is the combination of Xstep® and coolEX® preferred? The mechatronic Xstep® feed supports controlled geometry correction during rough honing. The hydraulically actuated coolEX® tool then performs the finishing operation. Measurements between and after the stages make the effect of each operation traceable.
Which Xstep® feed strategy suits the process? For force- or torque-controlled machining, Xstep® simulates hydraulic expansion. For size-based machining, the system can reference the honing stones first and then feed them towards the target size in discrete steps.
What caused the honing stones to load up? The honing stones were approached unfavourably against the bore wall during referencing. The problem was resolved by revising the approach strategy.
Why is the position of the ribs important at Ø32 mm? If the machining zone lies unfavourably between two ribs, the tool lacks uniform guidance. Revised pre-machining provided better support for the honing stones.
How was the bore geometry improved? In addition to corrected referencing, the stroke positions were matched to the axial measurement planes. This made the geometry along the bore length significantly more uniform.
The two-stage concept combining Xstep® and coolEX® covers a component family from Ø17 to Ø32 mm. Measurements after rough and finish honing make the contribution of each stage transparent. The documented process optimisation shows how referencing, pre-machining, tool guidance and stroke position interact. Even an S-shaped diameter deviation of up to 25 µm before honing was corrected effectively.