The original fixture for the rough boring process of automotive engine connecting rods utilized V-block positioning combined with a horizontal lateral clamping structure. Because the clamping force could not be effectively applied to the positioning datum surface, the workpiece experienced micro-movement under heavy cutting loads. This led to quality defects such as tool chipping, abnormal chip evacuation, and off-center boring of the connecting rod's small-end hole. To address these issues, a new positioning and clamping structure was designed based on a three-point inclined conical envelope principle. This design converts horizontal clamping force into both a radial gripping force (acting on the outer diameter of the small-end blank) and a vertical pressing force, thereby integrating automatic centering with active constraint. Shop-floor application results demonstrate that this structure effectively enhances fixture clamping rigidity and repeatability, nearly doubles tool service life, completely eliminates the off-center boring defect, and ensures the stable, efficient operation of the production line.
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I. Introduction
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In the manufacturing process of automotive engine connecting rods, the rough boring operation is a critical preliminary step that ensures the quality of subsequent finishing processes. The rough boring stage primarily involves the initial shaping of the connecting rod's large-end and small-end holes. Machining stability at this stage directly impacts the positional accuracy of the hole system, coaxiality, and surface integrity, placing high demands on the positioning reliability and dynamic rigidity of the fixture system.
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II. Problem Background
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The AF20 operation is a key rough boring station on the connecting rod machining line. It primarily performs the initial boring of the large-end and small-end holes, providing a uniform machining allowance for subsequent semi-finish and finish boring operations. To boost efficiency, this process employs aggressive cutting parameters, resulting in significant material removal and substantial fluctuations in cutting forces. In actual production, connecting rod scrap caused by off-center boring of the small-end hole occurs frequently (see Figure 1). These defects manifest as deviations in hole center position and coaxiality between the large and small holes, severely compromising subsequent assembly precision and engine operational reliability. Image
Figure 1: Connecting rod with off-center boring
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III. Analysis of the Original Rough Boring Fixture Structure and Its Limitations
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Root cause analysis revealed a fundamental flaw in the original fixture's design, which utilized V-blocks for positioning combined with lateral clamping via horizontal hydraulic cylinders. While V-blocks are commonly used in machining for the automatic centering of cylindrical workpieces [1]-relying on symmetrical inclined surfaces to constrain the outer diameter and establish center alignment in the X and Y axes-their effectiveness hinges on a critical prerequisite: a clamping force acting perpendicular to the V-block surfaces is required to press the workpiece firmly against the V-bottom, thereby eliminating clearance and establishing a stable contact interface [2]. In the original AF20 fixture (see Figure 2), the clamping action was performed by two independent hydraulic cylinders driving flat clamping pads; the clamping force was applied horizontally against the non-locating side of the connecting rod's big end. Although this force prevented lateral sliding caused by cutting forces, it failed to generate a normal force component directed toward the V-shaped locating surfaces.
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a) Fixture schematic
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b) Actual fixture
Figure 2: Connecting rod fixture
Due to the lack of effective clamping pressure against the V-surfaces, the connecting rod relied solely on friction to maintain contact with the V-blocks after clamping. Under the high-load conditions of rough boring, cutting forces (particularly the thrust force) were transmitted through the connecting rod body to the V-contact zone, creating a tendency for localized sliding or rolling. Consequently, the V-blocks could not provide a sustained, stable reaction force, leading to a shift in the actual positioning datum. More critically, friction between the clamping pads and the side of the connecting rod during the clamping process could induce slight rotation or displacement of the part against the V-surfaces, thereby disrupting the initial centering alignment. This positioning mode-theoretically sound yet practically ineffective-fundamentally violated the basic fixture design principle of "synergy between positioning and clamping."
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IV. Chain Reaction of Machining Instability and Process Consequences
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Positioning instability directly triggered a series of process anomalies. First, during the cutting process, the connecting rod undergoes slight vertical oscillation due to a lack of rigid constraint. Under high-speed rough boring conditions, this movement is sufficient to cause instantaneous fluctuations in the depth of cut. This unstable cutting action significantly intensifies localized stress concentration at the tool tip, accelerates the wear of the carbide insert, and makes micro-chipping highly likely (see Figure 3). Once the cutting edge is damaged and its geometry altered, the chip formation shifts from the intended short, flake-like chips to continuous, long ribbon-like chips [3].
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Figure 3: Micro-chipping of the rough boring tool
Such long chips are difficult to evacuate promptly via the chip removal system [4] and easily become entangled with the V-block, clamping block, or the connecting rod body itself (see Figure 4). During subsequent automated loading and unloading cycles, residual chips can lift the workpiece [5], causing the actual clamping height to deviate from the set value and resulting in an overall shift of the bored hole axis (see Figure 5). Furthermore, entangled chips can interfere with sensor signals or obstruct robotic arm movements, triggering equipment alarms and shutdowns. Shop-floor data indicates that prior to optimization, the tool in the AF20 process required replacement every 60–80 parts, and unplanned downtime due to chip entanglement occurred 4–6 times per shift, severely impacting the production line's continuous operation and product quality consistency.
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Figure 4: Chip entanglement on the rough boring tool
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Figure 5: Positioning deviation caused by chip accumulation
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V. Design Concept of the Novel Three-Point Inclined Taper Positioning Structure
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To fundamentally resolve the issue of unreliable positioning, the new fixture abandons the traditional separate design of a V-block combined with lateral clamping (see Figure 6) in favor of an integrated structure that combines positioning and self-locking clamping functions. This structure features three inclined support blocks arranged at 120° intervals around the outer diameter of the connecting rod's small-end blank; each support surface shares the same taper angle, collectively forming an inwardly converging conical positioning cavity. When the small end of the connecting rod is placed into this cavity, the three inclined surfaces simultaneously contact the blank's outer diameter, establishing a spatial three-point centering constraint (see Figure 7). Image
Figure 6: Traditional V-block
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Figure 7: Novel three-point inclined-plane taper positioning block
The key innovation lies in the reconfiguration of the clamping force transmission path. A clamping block driven by a horizontal hydraulic cylinder not only clamps the side of the connecting rod's big end but also pushes the entire rod axially toward the bottom of the tapered cavity. As the clamping stroke progresses, the small end of the connecting rod is gradually "wedged" between three inclined planes; the resulting normal reaction forces increase, resolving into a radial gripping force and a vertically downward clamping component. This mechanism achieves two core functions: first, it enables highly repeatable automatic centering through three-point contact; second, it efficiently converts horizontal clamping force into active clamping pressure against the positioning datum surface, ensuring the connecting rod remains firmly seated against the positioning surface throughout the machining process and eliminating the possibility of micro-movement.
Furthermore, the tapered structure inherently accommodates dimensional tolerances. Even with a ±0.3 mm tolerance in the diameter of the connecting rod's small-end blank, the three-point contact maintains stable constraint, preventing localized deformation caused by single-point overloading. The support blocks are manufactured from high-carbon chromium bearing steel (GCr15) and undergo quenching and precision grinding; the surface hardness exceeds 60 HRC, offering excellent wear resistance and ensuring positioning accuracy during long-term use.
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VI. Process Validation and Performance Improvement Results
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Following the implementation of the new fixture, machining stability for the AF20 process improved significantly. Chip morphology reverted to regular, short segments, ensuring smooth chip evacuation without entanglement and completely eliminating the quality risk of off-center boring. Equipment operational efficiency also increased; the frequency of unplanned downtime dropped from 4–6 incidents per shift to 0–1, and single-shift production capacity rose by approximately 20%.
The core value of this three-point inclined-plane taper positioning structure lies in re-establishing the fixture design principle that "positioning surfaces must be effectively clamped." Through geometric innovation, clamping force is directed toward the positioning datum, achieving an integration of positioning and clamping functions. This solution requires no additional drive components or complex control systems; it offers low modification costs and a short implementation cycle. It is suitable for various rough machining scenarios where a cylindrical blank serves as the positioning datum, such as the rough turning of crankshaft main journals or the rough boring of piston pin holes.
Against the backdrop of the manufacturing industry's pursuit of high quality, high efficiency, and low costs, this type of fixture optimization-grounded in fundamental engineering principles-offers a replicable and scalable technical paradigm for addressing real-world production line challenges. Its successful implementation demonstrates that a deep understanding of the coupling relationships between the process, fixture, and cutting tool-combined with a return to the essentials of mechanical design-can often yield significant performance leaps through simple structural designs.
VII. Conclusion
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The optimization of the AF20 rough boring fixture did not rely on high-tech smart components; instead, it resolved a long-standing process bottleneck by re-evaluating the positioning mechanism and restructuring the clamping logic. The successful application of the three-point inclined taper positioning structure not only improved the quality and efficiency of connecting rod machining but also provided a valuable reference for fixture design in similar operational contexts. Looking ahead, such fundamental process improvements-combining reliability, cost-effectiveness, and robustness-will continue to play an irreplaceable role in intelligent manufacturing systems.
