An "Over-Engineered" Part

A battery pack must be lifted, positioned and lowered on the assembly line, repeatedly. The central lifting sleeve that carries this action is a hollow aluminum part with a flange: the outside has a flange face mating with the case, the top end has a precision internal thread connecting to the lifting fixture, and a smooth end face ring at the top is used for positioning. The traditional approach is pure CNC machining from 6061 aluminum-alloy solid bar—the part works, but costs about RMB 50 (about USD 7) per piece, with a single-piece cycle of about 20 minutes. A single production line needs thousands of them, and the cost accumulates fast.

The problem is not that CNC precision is insufficient, but that this part was "over-machined": about half the metal is turned off from the solid bar, chips flying everywhere, and both material and machine time are wasted on the removed portion. Worse, turning a hollow flange sleeve from solid bar means the tool must plunge deep into a rotating workpiece, generating long stringy chips that tangle around the cutter and slow the feed; the cutting forces also deflect the thin-walled sleeve, requiring multiple light finishing passes to hold the thread and seat tolerances. Each of these extra passes adds cycle time and tool wear without adding any function the customer actually pays for. On a line that ships thousands of battery packs per month, this "machining waste" is not a rounding error—it is a dominant share of the fastener cost on the BOM.

From a design-for-manufacturing standpoint, the part geometry itself was never questioned: the flange, the hollow bore, the hex body and the top thread were all specified by drawing, and the shop defaulted to the easiest process to program—CNC from bar. Nobody sat down to ask whether the shape could be formed instead of cut. That missed question is exactly where the 60% saving hides.

Geometric Review: Born for Cold Heading

Looking at the geometry again: it is a hollow sleeve with a flange, axisymmetric, with no complex cavities. This shape is naturally suited to cold heading (cold forming)—flowing the metal plastically to shape it in dies at room temperature rather than cutting it away bit by bit. Instead of boring 50% of the metal out of solid bar, we cold-head the blank directly into a near-net shape, with material utilization close to 98% and almost no chips.

A Two-Step Process Route

The new route has only two steps, separating "forming" from "precision":

1. Cold-headed blank forming: a multi-station cold-heading machine upsets the basic shape with flange, body and hex mating face in one pass. Fast, extremely repeatable, with material utilization about 98%. The coil wire is sheared to a slug, then progressively forward-extruded and backward-extruded across several stations to build the hollow bore and flange; because each station only increments the deformation, the metal stays ductile enough to flow without cracking—critical for 6061, which work-hardens quickly.
2. Minimal CNC finish machining: only the two faces with mating-tolerance requirements—the top-end precision thread turning and the seat-face finishing—get a few quick CNC passes; all other dimensions are guaranteed by the cold-headed blank. The thread is cut (or rolled) only where it must engage the lifting fixture, and the flange seat is faced only to a flatness that seats against the battery case; everything else—outer diameter, hex corners, bore wall—comes straight off the die.

Three Ledgers: Cost, Efficiency and Strength

The cost reduction is clear in source: first, material utilization rises from about half to 98%, saving bar-stock cost; second, machine time is compressed from 20 minutes to under 4 minutes, and machine time is the heaviest item in the cost structure. The 5× capacity gain means the same line can deliver 5× the volume with no extra equipment or floor space.

The Strength Bonus of Grain Flow

If this is a battery-pack lifting point, strength is non-negotiable. Pure CNC cutting directly from 6061 bar cuts through the aluminum's internal grain flow; severed flow lines leave weak points that easily stress-crack under repeated lifting loads. Cold heading is the opposite: under high pressure the metal is forced to flow along the part geometry, and grain flow lines run continuously around the flange, body and hex face without being cut.

In tensile comparison tests, the cold-headed version showed higher stress-fracture resistance than the pure-CNC version. For a critical lifting point on a heavy battery pack, this is not icing on the cake but a safety margin—while cutting cost, the structure is actually stronger.

The reason is metallurgical. When you turn a part from rolled bar, your cutting tools cross and sever the longitudinal fiber that the rolling process created; the machined surface is left with end-grain exposed, which is far more susceptible to stress-corrosion cracking and to fatigue cracks that initiate at machined notches. Cold heading instead compresses the fibers to follow the part contour—around the flange, up the body, into the hex—so the surface that sees the lifting load is covered by long, continuous fibers under compressive residual stress. In practice this means the same 6061-T6 sleeve can absorb more repeated lift cycles before any crack initiates, and the crack, once started, propagates more slowly because it must fight the compressive stress state at the surface. For a safety-relevant lifting point that the whole pack hangs from, that margin directly translates into field reliability.

DFM Retrospective Checklist

• Is the part axisymmetric, a body of revolution, with flange/stepped/hollow features? The more symmetric, the better it suits cold heading.
• Is batch volume enough to amortize the die cost? For production parts of thousands or more, cold-heading dies pay back quickly.
• Which faces really have mating-tolerance requirements? Reserve CNC for the few key faces such as threads and seat faces.
• Is it a load-bearing part? Cold heading's continuous flow lines also incidentally improve fatigue and stress-fracture performance.