How Cold Heading Cuts the Cost of EV Battery Pack Lifting Sleeves by 60%
Executive Summary
The same 6061 aluminum alloy lifting sleeve drops from about RMB 50 to about RMB 20, takt compresses from 20 minutes to 4, and strength goes up instead. This is not achieved by squeezing price, but by changing the way swarf is produced. This article breaks down the cost structure of replacing CNC with cold heading, the metal flow-line mechanism, and design points of the process route.
Table of Contents
The Gap Between RMB 50 and RMB 20
A central lifting sleeve for an EV battery pack is traditionally made by pure CNC machining from 6061 aluminum alloy solid bar. The process is fine and the part works, but unit cost is about RMB 50 and single-piece takt is about 20 minutes.
After re-examining the geometry, the proposal changed: this is a hollow sleeve with a flange, axisymmetric, with no complex cavities — a shape born for cold heading. After changing the process to multi-station cold-heading blank forming + minimal secondary machining, unit cost dropped to about RMB 20, saving about RMB 30 per piece.
Where the Savings Come From
1. Material utilization
2. Machine hours
Machine hours are the heaviest item in the cost structure. In the pure CNC route, every feature — OD, inner bore, flange, threads, milled faces — occupies machine time, accumulating to nearly 20 minutes. Cold heading forms the main body in one blow, and secondary machining only handles the high-precision threads and mating faces.
| Operation | Pure CNC Route Time | Cold Heading + Secondary Machining Time |
|---|---|---|
| Blank preparation | Sawing bar stock | Wire coil cut to length (seconds) |
| Body forming | Turn OD, drill/bore inner hole, turn flange, about 15 min | Multi-station cold heading, seconds |
| Thread machining | Tapping, about 2 min | Tapping or thread rolling |
| Finishing of mating faces | Finish turning, about 3 min | Finish turning, minimal |
| Total takt | about 20 min | under 4 min |
| Output per shift (est.) | about tens of pieces | 300+ pieces |
3. Equipment and floor space
Cost Structure Comparison Table
The following compares the cost composition of the same part under two routes; the figures are order-of-magnitude references measured on projects.
| Cost Item | Pure CNC Route | Cold Heading + Secondary Machining | Change |
|---|---|---|---|
| Raw material | Solid bar, low utilization | Wire coil, high utilization | Down |
| Blank forming | Cutting, machine time occupied | Cold heading, seconds | Greatly down |
| Secondary machining | All features need machining | Threads and mating faces only | Down |
| Total unit cost | about RMB 50 | about RMB 20 | down about 60% |
Cost Down, Strength Up at the Same Time
This is the part of the case most worth attention: after cost drops by 60%, the part's strength goes up rather than down.
The reason lies in the metal flow lines
Cutting directly from 6061 bar cuts through the grain flow lines inside the aluminum. Where the flow lines are cut, a weak point may appear — especially under repeated stress or heavy lifting conditions.
Cold heading does the opposite. It deforms the metal under high pressure, forcing the grain flow lines to follow the part's geometric contour. The flow lines wrap continuously around the flange, body and hex faces, without being cut. In actual tensile tests, the cold-headed version shows higher resistance to stress fracture than the pure CNC version.
For critical lifting points on a heavy EV battery pack, the strength gain is not a bonus but a safety guarantee. This is also the fundamental reason cold heading remains irreplaceable for load-bearing fasteners and structural parts.
The three orientations of flow lines
| Forming Method | Flow-Line Pattern | Effect on Strength |
|---|---|---|
| Machining (CNC) | Flow lines cut through, end faces exposed | Stress concentration and potential crack sources at cuts |
| Cold heading (extrusion forming) | Flow lines continuous axially, turning with the contour | Uniform strength, improved fatigue and fracture resistance |
| Cold heading (upsetting forming) | Flow lines bend radially to wrap the flange | Flange-root strength markedly better than machined parts |
How to Design the Process Route
What the cold-heading stage completes
- Head / flange forming: Upsetting bends the flow lines to wrap around the flange root — the area most needing strengthening when the flange bears bending moment.
- Body forming: Forward or backward extrusion forms the outer contour, dimensions close to finished.
- Inner bore pre-forming: Leave a tapping pre-form diameter, or directly form a through / blind hole.
- Hex or special features: Form in one station where tooling allows, reducing later milling.
What secondary machining completes
- Internal thread: Precision class, requires tapping or thread rolling, per drawing accuracy class.
- Top mating face: High flatness and roughness requirements; finish turning or grinding.
- Flange bearing face: Ensure perpendicularity to the axis, avoiding off-center load during assembly.
Process control points
| Control Item | Target | Common Problems |
|---|---|---|
| Material condition and lubrication | Wire coil fully annealed, surface lubricant film intact | Insufficient lubrication causes die wear and surface galling |
| Per-station deformation allocation | Each station's deformation within the material's plastic range | Over-large deformation at one station causes cracking |
| Die life management | Periodic wear inspection and batch records | Die wear causes dimensional drift and batch inconsistency |
| Flow-line integrity check | Metallographic section confirms no flow-through or folding | Folding defects become fatigue sources |
| Heat-treatment condition | 6061 solutionized + artificially aged to T6 | Insufficient aging leads to substandard strength |
Advice for Design Engineers
If you are designing or procuring battery pack lifting points, brackets or similar mechanical fasteners, do not reflexively default to CNC. Try a three-step judgment first:
- Look at the geometry: Is the part axisymmetric? Are there deep narrow cavities or complex profiles? Axisymmetric hollow parts, flanged parts and sleeves best suit cold heading.
- Count the volume: Is the full-life-cycle quantity enough to amortize the die investment?
- Identify precision: Which features must reach precision class? Leave those to secondary machining; hand the rest to cold heading.
The hybrid process — cold heading for blank forming, minimal CNC for precision mating faces — is a proven cost-optimization solution. It delivers a safer, stronger part while sharply lowering BOM cost — a combination not often seen in manufacturing.
Closing: The Same Mechanism Behind Material Savings and Greater Strength
Achieving both a 60% cost reduction and a strength gain at the same time relies not on tricks, but on returning to the part's shape itself to choose the right forming method. Cold heading makes the metal flow into the required shape, saving the material that would have been turned into swarf while preserving continuous, intact grain flow lines.
For load-bearing structural parts, these two gains are two sides of the same mechanism: the metal is not cut away, so both material and strength are saved. This is also the fundamental reason cold heading continues to replace pure CNC on EV battery pack load-bearing parts.
