With the rapid advancement in mechanical engineering, the production of components for large-scale projects has become increasingly frequent.
This demands not only higher machining efficiency but also adherence to stringent precision standards.
The crankshaft is a long shaft-type component featuring complex spatial surfaces.
Its structure primarily comprises main journals, connecting rod journals, crank arms, and counterweights.
Operating under harsh conditions of high speed, high pressure, and alternating loads, crankshafts face extremely stringent requirements for fatigue strength, dynamic balance, dimensional accuracy, and geometric tolerances.
Therefore, crankshaft machining must shift from traditional methods to adopt five-axis turning-milling composite technology.
This integrated approach combines multiple functions—turning, milling, drilling, tapping, and boring—to enhance machining precision.
Overview of Five-Axis Turning and Milling Technology
As a representative of multi-process machine tools, the five-axis turning and milling center features two spindles: one high-speed rotating turning spindle and one milling spindle equipped with powered cutting tools.
It incorporates X and Z linear axes as coordinate systems.
Five-axis turning and milling technology enables rapid execution of multiple processes—turning, milling, drilling, tapping, boring—on a single machine.
Leveraging the advantages of five-axis interpolation through coordinated X, Z, C, and B-axis movements, it achieves one-pass machining of complex surfaces, such as milling eccentric structures.
This technology offers the advantage of single-setup machining.
After clamping, all features except the reference surface can be machined without repositioning, thereby eliminating potential repeat positioning errors.
Advantages of Five-Axis Turning and Milling Technology in Crankshaft Machining
Precision Optimization
Taking a typical marine crankshaft as an example, the application of five-axis mill-turn technology in crankshaft machining is illustrated in Figure 1.
Crankshafts are large components typically machined from forged steel or cast iron, with rough stock allowances of 20–30 mm.
They feature elongated shafts with multiple cranks and high hardness, presenting significant machining challenges.
Five-axis turning and milling ensure machining precision and effectively controls geometric tolerances for such crankshafts.
This technology eliminates repetitive clamping required in traditional machining.
The relative positions of journals, hole systems, and contours are guaranteed by the machine tool’s motion accuracy, enhancing angular tolerances, parallelism, and concentricity.
Efficiency Enhancement
Employing five-axis turning and milling technology for crankshaft machining significantly boosts production efficiency.
By leveraging five-axis interpolation, it eliminates auxiliary processes like transferring workpieces between machines, re-clamping, and tool setting.
Consolidating operations further reduces the overall machining cycle time.
Comparative studies reveal that five-axis turning and milling technology can shorten total crankshaft machining time by approximately 30%.
Cost Reduction
Although five-axis turning and milling machines entail higher initial equipment investment compared to conventional machines, their integrated design eliminates the need for multiple specialized machines, extensive custom fixtures, additional factory floor space, and labor costs.
Therefore, from a full lifecycle perspective of crankshaft processing, five-axis turning and milling machines offer superior overall cost advantages.
Digitalization and Production Flexibility
Five-axis turning-milling machines enhance production flexibility.
When switching between crankshaft models, only different CNC programs need to be loaded—eliminating the need to replace or adjust numerous specialized fixtures.
This drastically reduces changeover time, making them particularly suitable for R&D prototyping and multi-product co-line production.
Furthermore, the core integrated digital and intelligent technologies of five-axis turning and milling machines establish cutting-edge digital unit modules.
These modules integrate functions such as tool monitoring, data acquisition, and online measurement, laying a solid foundation for unmanned, intelligent crankshaft machining modes.
Specific Applications of Five-Axis Turning and Milling Technology in Crankshaft Machining
Machine Tool Layout
Utilizing five-axis turning and milling technology for crankshaft machining requires rational machine tool layout.
Taking the common HTM125 model five-axis turning and milling composite machine tool as an example, its structure includes a slant bed, column, spindle head, side-mounted headstock, tool holder, tool magazine management, water cooling, temperature control, electrical protection, safety guards, and chip removal devices. Basic technical parameters are shown in Table 1.
♦ Control System and Structural Layout Design
The system architecture employs Siemens 80D. Two spindle heads are positioned on opposite sides of the machine.
The column structure utilizes a side-mounted single-column design with tool storage units on both sides.
Dual tool holders are installed, with feed direction aligned with gravity. This complex configuration effectively meets crankshaft machining requirements.
♦ Machining Capabilities and Auxiliary Systems
During machining operations, the equipment’s control axes enable five-axis simultaneous motion.
For instance, the HTM125 five-axis turning and milling machine features dual spindles positioned on opposite sides of the machine.
This arrangement facilitates synchronized operation between the dual spindles and multiple axes.
Equipped with a center frame and a second spindle, the machine can autonomously move along the axial direction and control the machining process.
This type of five-axis turning and milling machine features composite machining capabilities, enabling deep hole drilling and heavy-duty boring operations.
Equipped with inspection mechanisms and high-pressure internal coolant systems, it prevents machine overheating during processing that could compromise machining quality while facilitating online tool inspection management.
Precision Turning of Main Journal and Pre-Grinding Preparation
The five-axis turning and milling technology has established a new application system in crankshaft machining due to its ability to efficiently and precisely complete critical feature machining in a single setup through multi-axis interpolation and powered tools.
Typically, crankshaft machining requires at least two lathes to turn both ends of the main journals.
After initial machining, the workpiece must be repositioned and swapped to process the opposite end.
This re-clamping introduces potential minor coaxiality errors.
Five-axis turning-milling technology securely fastens the workpiece to the machine spindle using high-precision hydraulic centers and a drive plate.
Once machining commences, the machine automatically switches to turning mode.
The spindle maintains continuous rotation while axially and radially coordinated controls guide the turning tool’s trajectory, achieving precise main journal machining.
The advantage of five-axis turning lies in the workpiece remaining on the spindle throughout processing.
This ensures a constant machining reference for the main journal, enabling effective precision control through unified turning parameters.
This ensures the turned journal achieves exceptional concentricity.
Any potential deviation is influenced solely by the machine’s radial and axial straightness, along with the spindle’s radial runout precision, and remains controllable at the micrometer level.
With the workpiece maintained in the same clamping state, power tools can be used for root milling of the step shoulder on the journal or direct turning of the journal section for installation.
This consolidates processes and ensures crankshaft machining accuracy.
Milling of Connecting Rod Journal Surfaces
Crankshaft machining requires structural segmentation, typically treating connecting rod journals as eccentric components—a challenging process.
Conventional methods rely heavily on costly, complex internal milling machines or turning-drawing machines.
To accommodate diverse crankshaft geometries, specialized eccentric fixtures must be custom-made, resulting in lengthy preparation cycles and limited flexibility.
♦ Five-Axis Turning and Milling for Eccentric Journal Machining
Replacing traditional methods with five-axis turning and milling technology enables machine tool and digital technology integration, eliminating mechanical eccentric fixtures.
Instead, axis linkage technology performs necessary machining operations, achieving precision results while boosting efficiency.
During connecting rod journal milling, the five-axis turning and milling machine automatically enters five-axis interpolation mode to mill the journal.
The spindle controls workpiece rotation, abandoning uniform motion in favor of precise interpolation synchronized with the oscillation frequency of the milling spindle head.
B-axis oscillation ensures the milling cutter axis remains perpendicular to the instantaneous tangent of the connecting rod journal surface.
The B-axis and C-axis are interconnected to form a virtual concentric circle in space, enabling real-time tool orientation compensation during machining.
This facilitates concentric rotational motion of the eccentric connecting rod journal relative to the milling cutter.
♦ Process Flexibility and Production Efficiency Advantages
With the machine tool controlled in X-axis or Z-axis feed mode, the end mill removes material using helical interpolation or layer-by-layer milling.
This initial milling of the connecting rod journal in an “enveloping” manner produces the outer diameter.
Simultaneously, CAM software generates an optimal toolpath to establish stable cutting forces, preventing vibrations during machining and ensuring surface finish quality.
Furthermore, five-axis turning and milling offers excellent flexibility when machining connecting rod journals.
When switching to process another crankshaft model, operators need only re-adjust the machining parameters suited to that crankshaft model, including phase angle and eccentricity.
The machine then automatically adjusts the linkage relationship between the B-axis and C-axis based on these parameters, enabling the lathe to fully adapt to the new crankshaft geometry.
This process eliminates the need for manual hardware fixture changes, shortening changeover time and meeting the demands of multi-variety and small-batch production.
Deep Oil Hole and End-Face Bore Machining
Angled oil holes are critical to crankshafts as key components of the lubrication system, where machining accuracy and surface quality are paramount.
Traditional machining relies on drill jigs, which themselves may wear out and struggle with cooling and chip removal issues during deep hole machining.
Five-axis turning and milling technology enables precise spatial positioning followed by accurate indexing using the B and C axes. This aligns the workpiece and machine tool, ensuring the oil hole axis perfectly coincides with the machine’s Z-axis.
The adjustment process is fully calculated and executed by the CNC system, delivering high positioning accuracy and effectively eliminating errors caused by traditional drill jigs.
After adjusting the workpiece position, the machine tool engages the power tool—a self-rotating milling unit mounted on the turret.
Most five-axis machines feature internal cooling systems, where high-pressure cutting fluid is delivered directly to the drill bit’s cutting edge through the tool holder.
This addresses issues such as short tool life, complex cooling, and difficult chip removal during deep-hole machining, enabling single-pass operations like deep oil hole drilling or reaming while ensuring machining quality.
Following drilling, tapping and threading operations proceed under the same clamping setup. Powered tools perform tapping to ensure coaxial alignment between threads and holes.
For machining end faces, whether bolt holes are radially or circumferentially distributed on flange end faces, C-axis indexing meets hole positioning requirements.
If hole axes are not perpendicular to the end face, B-axis angle adjustment enables vertical tool engagement, preventing drill deflection and ensuring hole positioning accuracy.
Conclusion
Crankshaft machining accuracy and efficiency are critical to ensuring its performance and service life.
Replacing traditional milling with five-axis turning-milling technology delivers superior precision in critical areas such as main journals, connecting rod journals, and deep oil holes, revolutionizing crankshaft component manufacturing.