Camshaft Processing Technology Flow
1. Material Selection
1)Ductile Iron: Ductile iron is obtained by treating molten iron with magnesium or magnesium alloys or other spheroidizing agents, resulting in cast iron with spherical graphite. The spherical graphite significantly reduces the segregation and sharp-point effect of graphite on the matrix, giving ductile iron high strength, wear resistance, oxidation resistance, shock absorption, and lower notch sensitivity.
Ductile iron camshafts are generally used in single-cylinder internal combustion engines, such as the S195 diesel engine, using QT600-3 or QT700-2 ductile iron for camshafts, requiring a spheroidization level of 2 (graphite spheroidization rate of 90-95%) and a graphite grain size greater than level 6. The overall hardness of the camshaft is HB230-280.
2)Alloy Cast Iron: Alloy cast iron is produced by adding elements such as Mn, Cr, Mo, Cu to molten iron close to the composition of gray cast iron. This forms an alloy with pearlite, reducing the amount of ferrite. Alloy cast iron camshafts are generally used for high-speed camshafts, such as the CAC480 camshaft, with an overall hardness of HB263-311.
3)Cold Shock Cast Iron: Generally used for low-alloy cast iron surface cold shock treatment, resulting in a white or mottled structure on the outer layer, while the core remains gray cast iron, such as the 372 camshaft. Camshafts made of cold shock cast iron operate under dry or semi-dry friction conditions, bearing significant bending and contact stresses, requiring the material’s surface layer to be wear-resistant and strong, while the core retains some toughness. Cold shock cast iron mainly consists of two categories: chromium, molybdenum, copper cold shock cast iron and chromium, molybdenum, nickel cold shock cast iron. The microstructure of the hardened layer: ledeburite + pearlite (sorbite) cold shock cast iron has a hardness of HRC45-52, with domestic cold shock cast iron currently having a hardness around HRC47.
4)Medium Carbon Steel: Generally used for large engine camshafts, such as the 6102 engine, which is shaped by die forging. Some are also used for motorcycle camshafts, which are easier to shape. After die forging, annealing treatment is usually required to facilitate machining.
2. Selection of Rough Reference
Typically, the rough outer cylindrical surface of the supporting shaft neck and one side is selected as the positioning reference. End face processing: domestic manufacturers generally use milling, while some foreign manufacturers use grinding instead of milling.
1) For rough blanks that are die forgings, especially precision ground die forgings, the precision of the blank is guaranteed by the die, which has high precision and small machining allowance.
2) For rough blanks that are castings, especially precision castings, they not only have good machinability but also have precise machining allowances. Their blank precision is even higher than that of forgings, ensuring reliable positioning.
3) During the camshaft processing, the selection of the rough reference also needs to consider the uniform and reasonable distribution of machining allowance.
3. Selection of Precision Reference
For rough machining, semi-fine machining, fine machining, and finishing of the supporting shafts, timing gear, gear shaft necks, and connecting shaft neck outer surfaces, two centers are used as precision references. For rough machining of camshafts and eccentric wheels, the processed supporting shaft neck and timing gear shaft neck are generally used as positioning references. Before finishing each surface and after heat treatment, the central hole correction process is usually arranged to correct the central hole while positioning with the supporting shaft, commonly using grinding.
4. Process Flow
1)Rough Machining: Rough machining of each supporting shaft neck, timing gear shaft neck, and threaded shaft neck outer circle, turning camshafts and eccentric wheels, etc.
2)Semi-Fine Machining: Rough grinding of camshafts, eccentric wheels, etc.
3)Fine Machining: Fine grinding of timing gear shaft necks and thrust faces, four supporting shaft neck outer circles, fine grinding of camshafts and eccentric wheels.
4)Finishing: Polishing of supporting shaft necks, camshafts, and eccentric wheels.
Camshaft Processing Methods
1. Rough Machining of Camshaft Neck Using Centerless Grinding
There are two methods of grinding using a centerless grinder: Through-type Centerless Grinding and Infeed Centerless Grinding.
1)Through-type Centerless Grinding: Generally used for single grinding wheels, with a single-leaf double hyperboloid guide wheel, pushing the camshaft to move axially, only used for grinding cylindrical shafts.
2)Infeed Centerless Grinding: Involves multiple grinding wheels.
2. Milling End Faces and Drilling Center Holes
The center hole processing serves as the positioning reference for subsequent processing operations. When milling end faces, generally only five degrees of freedom are limited, with two V-blocks limiting four degrees of freedom, and the axial degree of freedom is determined by the front end face of the camshaft’s 3# shaft neck or rear end face. Currently, self-centering positioning clamping is commonly used, with a fine-toothed cutter used for milling. The axial dimension ensures that the rear end face to the rough positioning reference dimension of the blank and the total length of the camshaft, considering the small dimensions of the camshaft pulley shaft neck, a B5 center drill is generally used for drilling the center hole, and the depth of the drilled hole is checked with a φ10 steel ball, ensuring the distance from the ball tip to the rear end face and between the tops of two steel balls, thus ensuring consistency in future positioning.
3. Heat Treatment of Camshafts
The raw materials or unfinished products are placed in air or specific media, heated, held, and cooled in an appropriate manner to achieve the desired mechanical or process properties.
1)Ductile Iron Camshafts Generally Undergo Isothermal Quenching: The cooling medium is a 10# or 20# ingot oil salt bath or alkaline bath, followed by low-temperature tempering at 140°C-250°C, resulting in a structure of black needle-like martensite, with hardness HRC50-54.
2)Alloy Cast Iron and Steel Camshafts Generally Use Intermediate Frequency Quenching: The quenching frequency is 1000-10000Hz, generally selected at 7000Hz.
The principle is: the cam of the camshaft is placed in the heating coil, and due to the skin effect of the current, the cam is heated from the outer layer to the inside, causing the surface layer to transform into austenite to a certain depth, which is then rapidly quenched, with the current 480 camshaft using natural tempering, resulting in needle-like martensite on the cam surface.
3)Surface Heat Treatment of Camshafts: Significantly improves the torsional and bending fatigue strength of parts and surface wear resistance. Induction heating quenching has small deformation, energy-saving, low cost, high labor productivity, and quenching machines can be placed on cold processing production lines, facilitating production management. For the 480 camshaft, the intermediate frequency quenching machine requires cooling for the power supply, transformer, and induction coil, with the cooling water temperature maintained at 25°C-30°C, and the quenching cooling liquid temperature at 53°C-62°C. If the machine itself does not meet the requirements, an additional cooling device must be provided outside the machine for cooling the water.
4. Deep Hole Machining of Camshafts
In machining, holes with L/D > 5 are called deep hole machining. There are several difficulties when drilling deep holes with ordinary twist drills.
1) The drill bit is long and slender, with poor rigidity, making it prone to bending and vibration during processing, making it difficult to ensure the straightness and machining accuracy of the hole.
Solutions:
A. Regularly replace the guide sleeve; extend the guide sleeve to improve the fit precision between the guide sleeve and the reamer; timely maintain the machine tool and adjust the spindle bearing clearance.
B. Increase the reaming or boring process to correct the hole; reduce the main deflection angle; pay attention to correct operation.
C. Select the reamer material according to the processing material, using hard alloy reamers or coated reamers.
2) A lot of chips are produced, and the channels for chip removal are long and narrow, making chip removal difficult.
Solutions:
Use gun drills for drilling; gun drills are single-edged, external chip-removing types, generally suitable for processing φ2-φ20mm holes, with L/D > 100, surface roughness Ra12.5-3.2mm, and precision H8-H10 levels for deep holes. Single-edged external chip-removing deep hole drills were originally used for machining gun barrels, hence the name gun drill, and are the only method for processing φ2-φ6mm deep holes.
3) Cutting fluid is difficult to enter deep holes, leading to high cutting temperatures and poor heat dissipation, causing the drill bit to break easily.
Solutions:
A. High-pressure cutting fluid (7MPa) is sent through small holes in the drill bit to cool, lubricate, and assist in chip removal, and then the chips are removed along with the cutting fluid into a centralized cooling system.
B. The cutting fluid used for deep hole machining of camshafts is generally ingot oil, which, although less effective than emulsion cooling fluids, provides much better lubrication.
C. The drill bit is made of hard alloy and has a welded structure. The cutting amount is generally 0.06-0.1mm/r. To better control the degree of tool wear, the tool adopts radial load feedback, and once the cutting force reaches a certain value, the tool can automatically retract under the action of the CNC system, thus avoiding the breakage of the gun drill and improving the tool’s service life. Worn tools are replaced and re-sharpened before reuse.
5. Processing of Camshafts
Traditional camshaft processing uses pattern processing; generally, each factory has a master pattern for intake and exhaust camshafts, and the number of patterns corresponds to the number of camshafts on the camshaft. This processing is essentially profile processing, and the processing errors of the master pattern will also reflect on the finished camshaft.
Modern camshaft processing uses CNC grinding, which has the following characteristics:
1) A set of CNC devices controls both the continuous variable speed rotation and indexing of the workpiece spindle and the reciprocating motion and lateral feed of the grinding wheel according to the cam profile rise and fall values.
2) The workpiece spindle is driven by a servo motor controlled by the NC device, achieving continuous variable speed transmission, allowing for high-speed, constant linear speed grinding. It has great flexibility, and the CNC device can store data for 20 cam profiles and 9 grinding data.
3) The grinding wheel spindle uses an internal balancing device, replacing the previous hydraulic balancing device and mechanical balancing device, achieving high balancing precision, with almost no vibration of the grinding wheel, improving the grinding precision of the cam profile.
4) Diamond rollers are used for dressing, and during dressing, a sound velocity sensor controls the amount of dressing for each grinding wheel, achieving good grinding wheel dressing precision. After each dressing of the grinding wheel, the NC device can automatically remember and compensate.
5) CBN grinding wheels are used, with the radius of the newly replaced grinding wheel only 4.5-5mm different from that of the discarded grinding wheel, ensuring the consistency of the cam profile.
6. Chemical Treatment of Camshafts
Chemical treatment involves placing metals in certain chemical media, generating a chemical coating layer on the metal surface through chemical reactions to achieve different properties such as decoration, corrosion resistance, and insulation. Chemical treatments typically include oxidation and phosphating.
7. Polishing of Camshafts
The main shaft neck and oil seal shaft neck of the camshaft require a surface roughness of 0.2, so the surface phosphating film of the main shaft neck and oil seal shaft neck must be removed. To ensure the surface roughness of the main shaft neck and oil seal shaft neck, they must be polished. During the polishing process, due to the low frictional heat generation and long heat dissipation time of the abrasive particles, the deformation and burning of the workpiece can be effectively reduced, mainly improving the surface processing precision, resulting in a bright and smooth surface for the camshaft neck, but not improving product dimensions and geometric precision, nor affecting the form and position errors of the parts.
8. Flaw Detection of Camshafts
As the cam contacts the push rod, the surface contact stress is large, and the camshaft surface must be free of any defects, so flaw detection is necessary for the camshaft surface.
Flaw detection is divided into two types: magnetic particle inspection and fluorescent inspection, primarily detecting quenching cracks generated during quenching and grinding cracks generated during grinding.
9. Cleaning of Camshafts
Camshafts require not only surface cleaning but also cleaning of the main oil passages and oil holes to prevent debris such as iron filings from accumulating at the junctions of the main oil passages, and to remove burrs from the oil hole openings.
10. Common Quality Issues in Camshaft Processing and Solutions
1) Inaccurate grinding of supporting shafts after precision turning
Possible issue: Using single board machines for processing, the program can easily get disordered, leading to unstable machining dimensions and excessive runout. Corrections cannot be made during precision turning.
Solution: Replace the equipment for turning supporting shaft necks, change the control system, and regularly inspect single board machines.
2) Cam lift exceeds tolerance
Possible issue: Wear of the machine tool pattern, leading to decreased pattern accuracy.
Solution: Regularly check, dress, and timely replace the pattern. When grinding the cam profile, adopt methods to change the workpiece rotation direction to improve the shape accuracy of the cam curve.
3) Cam base runout exceeds tolerance
Possible issue: Wear of the machine tool pattern, leading to decreased pattern accuracy. Increased clearance between the coupling and guide shaft.
Solution: Regularly check, dress, and timely replace the pattern and coupling guide shaft.
4) Minor ripples on the major working surfaces of the camshaft after fine grinding
Solution: Regularly check and adjust machine tool clearance. Control the feed rate during fine grinding, increase the number of passes, and avoid fire grinding.
Manufacturing Process of Combined Camshafts:
The combined camshaft is a new type of internal combustion engine component developed in the past 20 years, adapting to the automotive industry’s trends of lightweight, high performance, low emissions, and low costs. Currently, an increasing number of automobile manufacturers worldwide are using combined camshafts in high-performance engines, and the improvement of its production process has significant practical implications for the continuous development of China’s automotive industry.
Traditional Integrated Camshaft Processing Technology and Its Disadvantages
Traditional integrated camshaft processing uses casting or forging blanks for cutting processing into finished products, with some directly using bar materials for part processing. To ensure the machining accuracy of the parts, integrated cast or forged camshaft blanks must be supplemented with extensive cutting processing (turning, milling, grinding, polishing) and surface hardening treatments such as quenching, carburizing, and nitriding.
Main Disadvantages of Traditional Integrated Camshaft Processing:
(1) During engine operation, the cam must resist wear, galling, and pitting; the shaft neck must have good sliding performance; the core shaft must have good rigidity, bending, and torsional performance. It is difficult for an integrated camshaft to meet all these requirements simultaneously, and its material utilization is also unreasonable.
(2) It is challenging to manufacture camshafts with dense cam arrangements and compact structures.
(3) Requires extensive processing steps, machine tools, tools, fixtures, and personnel, resulting in long processing times, large footprint, and high costs.
(4) Surface wear hardening treatments of camshafts are prone to deformation, necessitating straightening.
(5) The excess margin of the cam profile is large and uneven, making processing difficult and affecting part quality.
(6) It results in a large amount of material waste, making it difficult to reduce product costs.
(7) Low production efficiency, high material and energy consumption, and low levels of production automation.
Technical Advantages of Combined Camshaft Technology:
The combined camshaft, also known as a combined axle, consists of several parts including a steel pipe (core shaft), shaft neck (bearing ring), cam, pump lobe, plug, and tail piece, which are optimized in materials and lean processing before assembly into a camshaft.
The core shaft of the combined camshaft generally uses cold-drawn seamless steel pipes, while the cams are made of carbon steel or powder metallurgy materials. Carbon steel cams can be subjected to precision plastic forming and high-frequency quenching or carburizing treatment.
The assembly precision of the camshaft (Cmk=1.67) generally can reach the following levels:
Angle: ±0.5° (rough forged cams), ±0.25° (processed cams).
Axial Dimension: ±0.1mm.
Load: Torque > 150N·m, Axial Load > 10kN.
After assembly, the parts are subjected to fine machining according to product requirements.
Compared to traditional camshafts, combined camshafts have the following technical advantages:
(1) Achieve flexible design, flexible production, and agile manufacturing. During assembly, the phase angle and axial position of the cam can be freely adjusted and corrected, facilitating the design and manufacture of new products and shortening the R&D manufacturing cycle.
(2) Favorable for material optimization and selection of camshaft structural forms. Materials for cam, shaft neck, and core shaft can be reasonably selected based on the performance requirements of various parts, ensuring quality requirements.
(3) The use of near-net-shape forming processes for cams can significantly reduce manufacturing costs. Depending on material and shape requirements, processes such as cold precision plastic forming, powder metallurgy sintering, and precision casting can be used to form cams, reducing the cutting processing steps, saving time, and lowering costs.
(4) The application of hollow tube core shafts, material optimization, and precision forming technology can reduce the overall weight of the camshaft by 20% to 40%, saving over 30% in materials.
(5) The most suitable heat treatment and surface hardening technologies can be employed for different parts, significantly improving the manufacturing precision, product quality, and service life of the camshaft.
(6) All components of the camshaft are processed separately and assembled together, reducing assembly time and costs.
(7) Overall stiffness of the camshaft is improved. Its dynamic torque can reach 800-1,000N·m, and it can reduce friction and withstand higher valve impact loads.
(8) The working profiles of the cams can be designed and selected arbitrarily according to product performance requirements. Near-net-shape forming technology can be used for processing cams of various curvatures, facilitating the development and application of new engines.
(9) It can significantly save metal processing equipment and time for camshafts, and the miniaturization of separate parts and equipment reduces equipment investment and floor space.
(10) The arrangement of cams on the core shaft can be more compact, making it superior in the processing of multi-valve overhead camshafts.
(11) Separate processing of components can greatly improve production automation and intensification.
Key Technical Issues in Combined Camshaft Technology:
1. Connection Technology
As engine power and speed continue to increase, the camshaft must bear higher loads and transmit torque, making the connection strength and service life between the cam and core shaft the most important technical indicators of the combined camshaft.
There are various connection methods due to different assembly methods for shaft necks, cams, and other parts, which can be divided into welded, sintered, and mechanical connections. Among them, mechanical connections can be further divided into hot sleeve, cold sleeve, hydraulic expansion, mechanical expansion, and knurling methods.
(1) Welded camshafts are prone to thermal welding deformation during welding, reducing the dimensional accuracy of the camshaft. Special attention should be paid to avoid cracks at the welding points. This method is limited to steel materials.
(2) Sintered camshafts require the cam to be connected to the steel pipe in a liquid phase state during powder sintering forming, and sintering is conducted in a furnace above 1000°C. The overall sintering can cause the shaft to bend and deform, leading to dimensional accuracy errors. Moreover, sintering has performance limitations on materials and requires large sintering furnaces with low thermal efficiency, suitable only for powder material cams.
(3) The expansion method first fits the cam with the steel pipe, and then hydraulic or mechanical expansion is applied from inside the pipe. To facilitate expansion, the wall of the core shaft must be thin enough for plastic bulging while maintaining sufficient thickness for strength and rigidity; the cam must have enough wall thickness to withstand the internal pressure without bursting or deforming; and the special requirements of high-pressure operations make the equipment large-scale.
(4) The hot and cold sleeve interference connection method is widely used in mechanical part production, but it has issues where heat from the cam transfers to the shaft end during operation, causing changes in the interference amount at the start and end of operation, which may lead to failure.
(5) The knurling method has certain advantages in reliability, precision, equipment, and energy consumption.
Additionally, there are examples of using adhesives for bonding, but this method has long curing times and complex processing, requiring special conditions for materials, bonding surfaces, temperature, oil, vibration, and other working environments. There are also methods using locating pins or keys to complete the combination of the core shaft and cam, but assembly is very difficult; some methods may also be used in combination.
Each of the above methods has its issues, so measures should be taken to avoid them, and specific applications should depend on the available technical conditions and product performance requirements.
2. Assembly and Assembly Equipment
The camshaft has high requirements for the phase angle and axial spacing of the cams, so ensuring the assembly precision of the combined camshaft is extremely important. Different connection methods have different requirements for assembly equipment and processes. With the application of hydraulic expansion connection technology, assembly has evolved from initially using simple alignment fixtures to using robotic arms to insert cams and shaft necks into the core shaft, employing specialized or composite molds, where the molds serve as positioning tools for the cams, with the phase angle and axial spacing primarily ensured by the manufacturing precision of the mold cavity.
Currently, assembly processes and equipment are developing towards high efficiency and automation. For example, the combined camshafts produced by Japan’s Hino company are heated to around 150°C before assembly, and the heated cams and shaft necks are sequentially sent to the clamping fixture on the workbench, where a CNC assembly machine assembles the cams and shaft necks according to specified order and axial spacing and phase angle requirements, followed by copper welding connections of the cams. In Germany, EMAG company uses hot sleeve assembly for camshafts, heating the cams and shaft necks to around 200°C before using a CNC assembly machine with a robotic arm to assemble them onto the core shaft at specified positions. Between 1995 and 2000, EMAG company collaborated with Mahle to provide 14 automated production lines for combined camshafts to German companies like Mercedes-Benz, BMW, Audi, and French PSA Citroën. After 2001, they provided 6 production lines for Canadian Linamar, German Mercedes-Benz Smart cars, and heavy-duty trucks from Mercedes-Benz and MAN.
3. Precision Forming of Core Shafts and Cams
The core shaft of the combined camshaft is a high-rigidity precision steel pipe, generally made from seamless hot-formed thick-walled steel pipes or high-precision cold-drawn steel pipes, suitable for various specifications to meet customer-specific dimensions and high concentricity requirements.
Cams are typically made of carbon steel or powder metallurgy materials, and can be formed using net-shape or near-net-shape technologies for lobed cams, such as cold and warm precision forging of steel cams, precision casting of alloy cast iron cams, and sintering of powder metallurgy materials, greatly reducing manufacturing costs and improving production efficiency. Carbon steel cams can undergo high-frequency quenching or carburizing treatment, providing high resistance to galling and pitting. The sintered alloy material Fe2C2P2Ni2Cr2Mo is an ideal material for cam production. Precision powder metallurgy cams are sintered parts that are already shaped on the outer contour, requiring only surface carburizing and nitriding, with only the inner hole needing processing.
When high strength is required for forged cams, bearing steel forgings are used, which need to be hardened overall or surface hardened by induction.
Currently, cam materials have evolved from alloy cast iron to powder metallurgy materials and steel materials, with applications of precision plastic forming and precision sintering technologies becoming increasingly widespread. As the cam profile is asymmetrical, markings are usually provided during precision forming to prevent misassembly and identify intake and exhaust cams.
Current Status and Development Trends of Combined Camshaft Technology Applications
Currently, there are many types of combined camshafts, with lengths ranging from 120mm to 12m and maximum weights of 12,000kg. Companies such as Japan’s Toyota, America’s MTS2SA, TORINGTON, General Motors, and Ford have already applied combined camshafts in production. German companies like Volkswagen, Mercedes-Benz, ThyssenKrupp, and Britain’s T&NT have also developed production processes and equipment for combined camshafts.
China has significant gaps compared to developed countries in assembly equipment, cam materials, rapid forming, and heat treatment, affecting the application development of combined camshafts in China. Currently, Shanghai Volkswagen has a production line for combined camshafts, and Shanghai General Motors uses wear-resistant sintered iron-based alloy cam materials connected to hollow steel pipes, which have been applied in passenger car engines. The German ThyssenKrupp company in Dalian uses axial knurling connection methods, achieving an annual production capacity of 12 million hollow combined camshafts, becoming a global market leader.
Combined camshafts have been developed for over 20 years, with assembly processes and equipment moving towards high efficiency and automation, and applications of precision plastic forming and precision sintering technologies becoming more widespread. There is currently a strong push to develop and apply composite materials, such as ceramic materials, for camshaft production. With improvements in combined camshaft production technology, better performance, lower costs, and diversified technologies will emerge.
Conclusion
The combined camshaft represents the development and upgrading of camshaft manufacturing technology and is key to achieving innovative breakthroughs. It has advantages that traditional camshafts cannot compare with in terms of reducing weight, lowering costs, and significantly saving processes and equipment. Additionally, it offers significant benefits such as arbitrary cam profiles, structural form flexibility, optimized material selection, high overall rigidity and strength, flexible processing, and agile manufacturing. In an era that strongly advocates environmental protection, developing low-energy, pollution-free engines while achieving low costs and lightweight designs, the combined camshaft has unique advantages and a broad application development prospect in automotive, railway, and marine engine fields.
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