In precision engineering, ball screws play a crucial role in converting rotational motion into linear motion efficiently and accurately. They are the basic components of all kinds of machinery in all walks of life. For anyone involved in designing, manufacturing or maintaining a system that requires precise linear motion control, it is essential to understand the ball screw making process.
Understanding Ball Screws
A ball screw assembly is a mechanical mechanism that converts rotational motion into linear motion. It is made up of a precision ball screw and a ball nut packed as a unit with recirculating ball bearings.Ball bearings establish the contact between the ball screw and nut.Ball screws can apply or tolerate large thrust loads while generating minimal internal friction. They are made to precise tolerances, making them appropriate for use in applications requiring great accuracy.
The Manufacturing Process of Ball Screws Shafts
The Ball screw manufacturing process consists of many crucial processes, each of which is critical to ensuring that the end product satisfies the specified criteria. Here’s a thorough summary of the process:
1.Raw material selection
- Material Type: Commonly used materials are high-carbon chromium bearing steel (such as GCr15), alloy structural steel (such as 42CrMo, 20CrMnTi) or stainless steel (requires corrosion resistance scenarios). Raw materials need to meet high strength, wear resistance and dimensional stability requirements, and are usually entered into the factory in the form of hot-rolled round steel or forged blanks.
- Material Inspection: Detect chemical composition, mechanical properties (tensile strength, hardness), metallographic structure (avoid inclusions, segregation), and ultrasonic flaw detection to ensure no internal defects.
2.Preparation of blanks
- Forging / Rolling: The steel is hot forged or hot-rolled and formed into a screw or nut blank. Forging can refine grains, improve mechanical properties, and remove surface defects.
- Annealing treatment: The blank is spherical annealed or stress-relieving annealed to reduce hardness (for subsequent processing), eliminate forging stress, and improve cutting performance.
3.Rough Processing
- Round processing of lead screws
- Turning: Turn the outer circle and the center holes at both ends on the CNC lathe (CNC), determine the basic size (diameter, length) of the lead screw, and reserve the subsequent machining allowance.
- Thread pre-machining: For large-lead lead screws, the thread prototype may be milled or turned first to prepare for subsequent precision processing.
- Nut rough processing: Turn the inner hole, outer circle and end face, process the basic shape of the nut blank, and reserve the machining allowance for the internal thread and circulation groove.
4.Heat treatment
- Quenching and backfire
- Overall quenching (for lead screw): Heat the blank to an austenitization temperature (such as GCr15 about 840°C), oil-cooled or salt bath quenching to obtain martensite structure in the surface and core, followed by low temperature back-temperature (180-200°C), improving hardness (HRC58-62) and wear resistance.
- Local quenching (for nut raceway): Perform induction quenching of the inner raceway of the nut to improve surface hardness and maintain core toughness.
- Aging treatment: After quenching, artificial aging is performed (insulated at 120-150℃ for several hours), eliminating internal stress, stabilizing the size, and preventing subsequent processing deformation.
5.Precision processing
- Precision machining of lead screws
- Cylindrical grinding: Positioned with the central hole, grind the outer circle on a high-precision cylindrical grinder to ensure coaxiality and provide a reference for thread grinding.
- Thread processing
Precision grinding: Use thread grinders (such as Schaublin, Japan Koyo) to grind threads, control the pitch accuracy (lead error) through high-precision grinding wheels and indexing mechanisms to achieve IT3-IT6 level accuracy (according to grade requirements).
Wire rolling (for low precision or small lead): Cold rolling molding is carried out by a wire rolling machine, with high efficiency but low accuracy, suitable for ordinary-grade wire screws. - Polishing: Grind or super-finish the threaded surface to reduce the surface roughness (Ra≤0.4μm) and improve the ball rolling smoothness.
- Cylindrical grinding: Positioned with the central hole, grind the outer circle on a high-precision cylindrical grinder to ensure coaxiality and provide a reference for thread grinding.
- Nut processing
- Inner bore grinding: Grind the inner bore of the nut to ensure coaxiality with the ball raceway.
- Raceway processing
Mill or grind the spiral raceways in the nuts, matching the screw threads, forming a ball running trajectory.
Processing circulation groove (internal circulation or external circulation): The inner circulation nut needs to be processed in the reverser groove on the raceway, and the outer circulation nut is equipped with an external circulation conduit. - Surface treatment: Grind the raceway surface to improve accuracy and surface quality.
- Inner bore grinding: Grind the inner bore of the nut to ensure coaxiality with the ball raceway.
- Ball manufacturing
Purchased standard balls (GB/T308) or homemade: through cold heading molding, rough grinding, fine grinding, and polishing of ball mills, diameter tolerances (such as the diameter error of G10 grade balls ≤1μm), and heat treatment (quenching and tempering) and flaw detection detection.
6.Assembly and debugging
Part cleaning: Clean the screw, nut, ball and circulation device with ultrasonic wave to remove oil and metal chips, and avoid impurities affecting the operating accuracy.
Assembly process
- The balls are loaded into the nut raceway in the designed quantity (based on the lead and load) and a closed cycle is formed by a circulation device (reverse or conduit).
- Apply preloading forces (via double nut misalignment, gasket or spring) to eliminate axial clearance and improve rigidity (preloading is designed according to the load applied).
- Test the coaxiality and preload uniformity after assembly to ensure that the nut moves smoothly on the lead screw without lag.
7.Accuracy detection and quality control
- Dimensional accuracy detection
- Lead screw: measure external diameter, thread lead (pitch), tooth angle, radial jump, and use a three-coordinate measuring machine (CMM) or a special lead screw detector (such as Taylor Hobson, UK).
- Nut: Detection of inner hole diameter, raceway curvature radius, and circulation groove position accuracy.
- Performance detection
- Axial load test: Verify the maximum load-bearing capacity and stiffness, and simulate actual working conditions loading.
- Running accuracy test: Detect feed accuracy (positioning error, repeat positioning accuracy), and use a laser interferometer (such as Renishaw).
- Life test: Simulate high-speed or heavy-load operation through the durability test bench to verify fatigue life.
- Surface quality
- Detect surface roughness (Ra value), hardness distribution after heat treatment (Roycemeter) and metallographic structure (micro-hardness meter).
8.Surface treatment and protection
- Anti-rust treatment: Electroplating of finished products (such as galvanizing, chrome plating), spray anti-rust paint or grease to prevent rust during storage and transportation.
- Logo and packaging: Print the model, accuracy level, production date and other information, and use moisture-proof packaging materials (such as PE bags, foam) to avoid bumps and damage.
Applications of Ball Screws
Ball screws are widely used in many sectors due to their great precision and dependability. Among these industries are:
- Automotive: Ball screws are used in automotive applications where precision is critical, such as steering and suspension systems.
- Aerospace: The aerospace industry requires ball screws that withstand high loads and extreme temperatures.
- Medical: The medical industry needs ball screws to create precise movements to avoid hurting patients.
- Robotics: Robotics needs ball screws to create smooth movements for delicate tasks like handling fragile objects.
- Agricultural: Agricultural machines need ball screws to plant seeds and harvest crops accurately.
- Machine tools: One of the main examples that uses ball screws for their high-tech characteristics is machine tools. Balls screws are used to provide linear motion that is required in many different types of machines including lathes, milling machines and other computer numerical control (CNC) machines.
Summary
The manufacturing of ball screws is a comprehensive application of materials science, precision processing and inspection technology. It takes 20-30 processes from raw materials to finished products, and special attention is paid to accuracy and reliability. The process details of different application scenarios (such as precision level for machine tools vs. standard level for ordinary machinery) will vary, but the core goal is to achieve high-precision and long-life transmission performance through strict process control.