Spline shafts are key components in mechanical transmissions that achieve precise connections between shafts and hubs. Their manufacturing process must balance precision, strength, and surface quality. The core process involves producing a spline tooth profile (such as rectangular, involute, or triangular) that meets design requirements through multiple steps. The following is a detailed breakdown of the typical spline shaft manufacturing process, covering all steps from raw materials to finished product:
I. Preliminary Preparation: Raw Material Selection and Pretreatment
Spline shafts must withstand torque, impact, and wear. Therefore, the first step is to ensure that the raw material properties meet the required standards. Pretreatment is also required to eliminate internal stresses and lay the foundation for subsequent processing.
1. Raw Material Selection
Select the appropriate material based on the application (e.g., automotive transmissions, machine tool spindles, construction machinery, etc.): Medium-low carbon steel, alloy steel. The raw material is typically hot-rolled round steel, and a 5-10mm machining allowance should be reserved based on the maximum outer diameter of the finished spline shaft.
2. Pretreatment Process
l Cutting: Using a saw (such as a band saw) or CNC cutting equipment, the round steel is cut into the length of a single spline shaft blank (reserving allowance for subsequent machining).
l Annealing/Normalizing: For high-carbon or alloy steel blanks, annealing (holding at 600-700°C followed by slow cooling) or normalizing (holding at 800-900°C followed by air cooling) is performed in a heating furnace. The objectives are: a. Reducing the material's hardness for subsequent cutting; b. Eliminating internal stresses generated by hot rolling to prevent post-processing deformation; c. Refining the grain size and improving the material's mechanical properties.
l Surface Cleaning: Sandblasting, pickling, or grinding is used to remove scale and rust from the blank's surface to prevent impurities that may affect subsequent machining accuracy.
II. Rough Machining: Initial shaping and removal of most excess material
The core of rough machining is to quickly remove excess material from the raw material, reducing the blank to a near-finished "rough" shape. The focus is on ensuring the basic dimensions of the shaft's outer diameter and step (if any), while leaving a uniform allowance (typically 0.5-2mm) for the spline tooth profile.
Turning Process
Contents: ① Machining both shaft end faces to ensure perpendicularity; ② Drilling a center hole (for positioning support in subsequent processes); ③ Turning the shaft's outer diameter, steps, undercuts, and other features, maintaining dimensional accuracy within IT11-IT13 and a surface roughness of Ra12.5-Ra25μm.
Note: If the spline shaft has auxiliary features such as keyways and oil holes, these can be pre-machined on a milling or drilling machine after rough turning (allowing for finishing allowances).
III. Spline Tooth Profile Machining: The Core Process, Determining Connection Accuracy
Spline tooth profile machining is crucial to the entire process. Different processes are selected based on the spline type (rectangular, involute, etc.) and accuracy grade (e.g., Grade 6, 7, or 8 in GB/T 1144). Common methods are categorized as cutting and non-cutting.
(I) Cutting: Suitable for low- to medium-precision, small-batch production.
1. Milling
Principle: A spline milling cutter (a profiled tool matching the spline tooth profile) is used to rotate the workpiece via an indexing head (indexing occurs once after each tooth is milled). Simultaneously, the milling cutter is fed axially, gradually milling the spline tooth grooves.
Features: Low equipment cost and high flexibility, but low efficiency (requiring multiple indexing steps), low tooth profile accuracy (IT9-IT11), and surface roughness Ra6.3-Ra12.5μm. Suitable for small-batch, low-precision spline shafts (such as those in agricultural machinery and light machinery).
2. Slotting
Principle: The workpiece is fixed to the worktable, and the slotting cutter (forming tool) performs a reciprocating up and down cutting motion. Simultaneously, the worktable indexes the workpiece along its circumference (once after each slot) and feeds it axially.
Features: Suitable for machining internal splines (spline holes), less commonly used for external splines. Advantage: Can machine blind splines. Disadvantages: Low efficiency, medium precision (IT8-IT10), and surface roughness Ra6.3-Ra12.5μm.
(II) Non-Cutting Processing: Suitable for High-Precision, High-Volume Production
1. Cold Rolling
Principle: A rough-machined shaft blank (hardness ≤ 25 HRC) is placed between two forming rollers (the roller tooth profile is opposite to the spline tooth profile). The rollers rotate at high speed and apply pressure to the workpiece, causing the metal on the workpiece surface to flow through "cold plastic deformation," extruding the spline tooth profile.
Features: ① Extremely high efficiency (processing time per shaft ranges from a few seconds to tens of seconds), suitable for high-volume production; ② High tooth profile accuracy (IT6-IT8), with a surface roughness of Ra0.8-Ra3.2μm; ③ Metal fibers are continuously distributed along the tooth profile, increasing tooth root strength by 20%-30% compared to cutting. Disadvantages include high equipment cost and the need for custom rollers. This method is suitable for large-scale production of spline shafts with fixed specifications (such as automotive axles and transmission shafts).
2. Broaching
Principle: A spline broach (a long, multi-tooth forming tool) is used. The broaching force of the broaching machine drives the broach axially through the pre-formed hole (for internal splines) or the outer diameter (for external splines) of the workpiece, broaching the entire tooth profile in one operation.
Features: ① High precision (IT6-IT8), surface roughness Ra0.8-Ra1.6μm; ② High efficiency (one-shot forming, no indexing required). A disadvantage is the extremely high cost of the broach (customization is required). This method is suitable for large-volume production and internal or short external spline machining (such as machine tool spindle splines).
IV. Heat Treatment: Improving Strength and Wear Resistance
Spline shafts withstand torque and wear during transmission, so heat treatment is required to enhance surface hardness and core toughness. Common processes are categorized into two types depending on the material:
1. Overall Heat Treatment (Medium- and Low-Carbon Steels)
Quenching and Tempering: ① Quenching ② High-Tempering
2. Surface Heat Treatment (Low-Carbon Alloy Steels)
Carburizing and Quenching + Low-Tempering: ① Carburizing, ② Quenching, ③ Low-Tempering
3. Aging Treatment (Non-Ferrous Metals)
If the spline shaft is made of aluminum alloy (such as in aerospace lightweighting applications), a "solution-hardening + aging" treatment is required: solution heating followed by rapid cooling, followed by aging at 120-180°C to precipitate strengthening phases and increase hardness and strength.
V. Finishing: Ensuring Final Precision and Surface Quality
After heat treatment, the workpiece may exhibit deformation (such as warping and dimensional shrinkage) and may also have scale on the surface. Finishing is required to correct dimensions and improve precision to meet design requirements.
1. Grinding Process
Contents: ① External Cylindrical Grinding: Corrects heat treatment distortion on the shaft's outer diameter to ensure roundness and cylindricity (accuracy IT5-IT7); ② Spline Tooth Profile Grinding: Using a profiled grinding wheel or an involute grinding wheel, the spline tooth surfaces are ground to correct tooth profile errors and improve surface finish, achieving a final tooth profile accuracy of IT5-IT7 and a surface roughness of Ra0.4-Ra1.6μm.
Note: High-precision spline shafts (such as machine tool spindles and aviation transmission components) require multiple grinding cycles, even combined with superfinishing (Ra0.025-Ra0.1μm).
2. Auxiliary Finishing
Keyways, oil holes, and other features require fine finishing using a jig boring machine or CNC milling machine to ensure positional accuracy (e.g., symmetry between the keyway and spline teeth).
Surface Cleaning: Polishing (to remove burrs) and cleaning (to remove grinding fluid and iron chips) are performed to prevent impurities from affecting assembly.
VI. Quality Inspection: Comprehensive Verification to Ensure Compliance
The quality of the spline shaft directly impacts transmission reliability and requires multi-dimensional testing to confirm compliance.
VII. Subsequent Treatment: Rust Prevention and Packaging
Rust prevention methods are selected based on the operating environment, such as: ① General use: Apply anti-rust oil; ② Long-term storage/humid environments: Phosphate treatment + wrapping with anti-rust paper; ③ High-precision use: Vacuum packaging.
Packaging and Warehousing: Packaging is performed in wooden boxes, cartons, or pallets to prevent deformation during transportation. Product information, such as model, specifications, and quantity, is labeled for ease of storage management.
Summary: Process Routes Differ for Different Production Requirements
The manufacturing process for spline shafts is not fixed and requires flexible adjustments based on batch size, precision, and material. A typical route comparison is as follows:
l Small batch, low precision (e.g., agricultural machinery parts): Raw materials → Cutting → Annealing → Rough turning → Spline milling → Quenching and tempering → Finish turning → Inspection → Packaging;
l Large batch, medium precision (e.g., automotive axles): Raw materials → Cutting → Normalizing → Rough turning → Cold rolled spline → Carburizing and quenching → Grinding → Inspection → Packaging;
l High precision, small batch (e.g., machine tool spindles): Raw materials → Cutting → Annealing → Rough turning → Spline broaching → Quenching and tempering → Rough grinding → Aging → Finish grinding → Superfinishing → Inspection → Packaging.
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