Manufacturing Case
Cold-Extruded Upper Assist Steering Shaft
Jinhua Machinery manufactured this drawing-specific upper assist shaft for an automotive electric power steering application using a two-stage cold-forming strategy.
The route formed two external spline features with controlled relative phase and indexed missing-tooth orientation, then redistributed material locally to create the enlarged diameter before downstream precision machining and inspection.
Scope note: This is one manufacturing example. Its dimensions, process route and results are not universal specifications for new RFQs.
Case-specific manufacturing facts
| Application | Automotive electric power steering system |
|---|---|
| Component | Upper assist shaft |
| Material | Medium-carbon steel |
| Starting condition | Cold-drawn |
| Material hardness | HRA 58–64 in the cold-drawn condition |
| Separate heat treatment | Not required for this component |
| Starting stock | Ø17.9 × 194 mm, approximately 383 g |
| Key forming route | Dual-ended spline cold extrusion plus localized diameter forming |
| Downstream processes | Turning, centering, straightening, deep-hole drilling, secondary drilling, finish turning, deburring, grinding, laser marking, cleaning and rust protection |
| Inspection | CMM first/last-piece spline verification, GO/NO-GO spline gauging, dedicated phase inspection and runout control |
Application in an Electric Power Steering System
This upper assist shaft interfaces with a mating lower shaft in an EPS assembly. Each external spline includes an indexed missing-tooth position, and the relative angular phase between the two spline features must support correct assembly with the mating component.
The manufacturing and inspection route therefore preserves the relationship between the splines, rather than evaluating them as two unrelated features. Jinhua Machinery’s role is drawing-based component manufacturing and process control; complete EPS system design, vehicle validation and steering-system safety engineering are outside this case scope.
Manufacturing Challenge
Form both splines
Two external spline features are produced in one cold-extrusion operation.
Control relative phase
The two spline features must maintain their specified angular relationship.
Index missing teeth
Each missing-tooth position must support correct assembly orientation.
Manufacturing Route
The public-safe sequence shows the verified process logic while excluding machine models, press tonnage, die structure, internal instructions and customer dimensions.
Incoming material condition
Incoming cold-drawn stock is checked for bar diameter, hardness and relevant material documentation where applicable. HRA 58–64 is the confirmed hardness in the cold-drawn condition, not a post-heat-treatment result. No separate heat treatment is required for this component.
Why phosphate and soap?
Phosphate-and-soap preparation supports stable material flow and reduces friction during subsequent cold-extrusion operations. It is a process-enabling lubrication step, not a promise of zero tool wear or zero forming defects.
One Operation, Two Functionally Related Splines
Both external spline features are cold-extruded in one operation. Each includes an indexed missing-tooth location, while the two features must maintain their required relative phase. This enables correct mating with the lower shaft and requires controlled workpiece orientation and tooling design.
The critical requirement is not only whether each spline passes its individual dimensional inspection, but whether both spline features maintain their required functional angular relationship.
Forming the Enlarged Diameter Through Material Flow
A second cold-forming operation redistributes material from the smaller starting bar to produce an enlarged local diameter. This controlled material redistribution creates a near-net-shape starting condition and avoids machining the complete component from a substantially larger-diameter bar.
Cold forming does not eliminate downstream machining. It places material where needed so later turning, hole machining and grinding can establish the approved finished geometry.
Larger-bar machining route
Ø23.5 × 188 mm≈630 gCold-forming route
Ø17.9 × 194 mm≈383 gIllustration not to scale.
For this specific component, the cold-forming route reduced the starting-stock mass from approximately 630 g to 383 g, or about 39%. This is not a claim of 39% lower total cost, selling price, waste or universal material utilization.
Downstream Precision Machining
After cold forming, the component proceeds through turning, center preparation, straightening, deep-hole drilling, secondary drilling, finish turning, deburring, grinding, laser marking, cleaning and rust protection. The critical bore is verified using air gauging in the applicable production step.
Forming establishes the dual-spline and enlarged-diameter near-net shape; machining and grinding then establish the remaining finished geometry and relationships required by the approved drawing.
Spline and Functional Relationship Inspection
The inspection plan separates detailed setup verification, routine spline acceptance and functional relationship checks.
First and last pieces
CMM verification of spline features.
Routine production
GO/NO-GO spline gauging at one piece per 48 pieces.
Functional relationship
Dedicated phase gauge plus runout control between spline features.
This provides detailed verification at the beginning and end of the production run. This does not imply that every production piece receives full CMM inspection.
Engineering Takeaway
Cold extrusion can integrate functional spline orientation into the route, not merely create tooth geometry.
Controlled material flow can create a local enlarged feature from smaller starting stock.
Final quality depends on both local spline acceptance and the relationship between the two spline features.
Cold extrusion is part of Jinhua Machinery’s in-house cold-forming capability. Every new component still requires review of its drawing, material, spline standard, functional relationships, production volume and inspection requirements.
Related Manufacturing Capabilities
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