Inside the Cold Extrusion Elongation Die: Forming Ultra-Deep Holes in Slender Parts (2026)

Published: November 2026 | By YONGMAO Engineering Team

An ultra-deep hole no longer forces a choice between slow machining and turning the order down. With a patented cold extrusion elongation die for slender shaft parts, YONGMAO forms deep, continuous bores and long, thin shafts directly from wire at room temperature — keeping the grain flow that machined parts lose, in seconds instead of minutes. This guide walks through how the die works, which machine capabilities make it possible, and why the incoming wire rod decides whether the whole run succeeds.

YONGMAO is the brand of Changzhou Yongmao Special Fastener Co., Ltd., a cold heading fastener factory in Changzhou, China, producing non-standard parts to customer drawings since 1993. You can see the range on our custom cold heading products page.

1. How Does a Cold Extrusion Elongation Die Form Ultra-Deep Holes in Slender Parts?

It forms the bore by backward extrusion across several forming stations: a punch penetrates the blank and metal flows backward around it, opening a deep hole while the outer profile is gathered, pre-shaped and sized station by station. No chip is ever produced. The metal is displaced, not cut — which is the entire commercial and mechanical argument for the process. In 2019 YONGMAO filed the invention behind this capability, granted as Chinese patent CN 4604667: a cold forming process and cold extrusion elongation die designed specifically for ultra-deep-hole, slender parts.

The forming sequence answers why machining struggles here. Drilling a deep bore in a long, thin shaft means cutting away 50–70% of the material, fighting deflection the whole way, and severing the grain flow lines exactly where the head meets the shank — the highest-stress zone of the part. Cold forming does the opposite:

• Backward extrusion opens the bore. Metal flows around the penetrating punch, so the hole and the cavity form in one controlled stroke.

• Multi-station progression respects formability. The blank is sheared, gathered into a cone preform, backward-extruded, then elongated and sized — each station moves a controlled fraction of the volume so no single blow exceeds the material's limit.

• Grain flow stays continuous. Because metal is displaced rather than removed, flow lines follow the head-to-shank transition; cold-headed parts of the same grade typically show 20–30% better tensile and fatigue performance than machined equivalents, with material utilization near 99%.

• Cycle time is seconds, not minutes. One pass through the press line replaces a stack of turning, drilling and boring operations.

The multi station cold heading machine capabilities for complex parts behind this — two- to six-station headers with in-line piercing, trimming and thread rolling — are what let one production flow deliver geometries that would otherwise bounce between a lathe, a drill and a grinder. That is the core of the cold extrusion elongation die for slender shaft parts platform: one press line replacing a chain of machine tools. Practical forming envelope, by long experience: blank length-to-diameter around 3:1 per blow, a head no more than roughly 2.5× the shank diameter in a single blow, single-pass reduction under about 70% for medium-carbon steel, and slender parts in the M16 × 320 mm class with deep hollow cores. One honest boundary: undercuts, off-center features and internal threads still belong to CNC secondary operations. Where machining is genuinely the cheaper route, the YONGMAO engineering team says so before any tooling budget is committed. The same forming principle carries through our hollow thru-axle product line and EV battery fasteners.

✅ Bottom line: A cold extrusion elongation die opens deep bores and slender shafts in seconds while keeping grain flow continuous — the right route once geometry fits the forming envelope and volume supports tooling.

2. Why Do Cold Heading Wire Rod Quality Requirements Decide Deep-Hole Success?

Cold heading wire rod quality requirements spheroidize annealing is the shorthand that matters here: a rounded carbide structure, clean chemistry and capped decarburization decide whether a deep-hole blank extrudes cleanly or cracks at station two. The die does not forgive poor material. Backward extrusion strains the bore wall harder than any other feature on the part, so the coil sitting at the machine entrance is the real first inspection gate.

• Spheroidize annealing is the foundation. It converts lamellar pearlite into rounded carbide, giving the wire the ductility to survive severe wall strain during hole formation without tearing.

• Chemistry and cleanliness are screened, not assumed. Controlled sulfur and phosphorus, freedom from inclusions and seams — a seam in the rod elongates during drawing and opens into a surface crack the moment the blank is upset.

• Decarburization is capped. A soft, carbon-depleted skin lowers surface hardness after heat treatment and becomes a fatigue initiation site on a slender, load-bearing shank.

• Formability is verified by test. Reduction of area in the tensile test, an upset test (compressing a sample to one third of its height with no cracking) and torsion turn counts are the shop-floor screens run before tooling is approved.

• Material families that form well: 10B21 and 20CrMo alloy steels for 10.9-grade hollow parts, low-carbon steels for softer components, 304/316 stainless for corrosion duty, and 7075/6066 aluminum for lightweight axles. Titanium and high-temperature superalloys do not cold head at all.

When a deep-hole run does go wrong, the defect usually points back to the coil: chevron cracks from over-strained extrusion, surface cracks from low ductility or sharp radii, folds from mismanaged metal flow. Every batch at YONGMAO is verified in-house — tensile, hardness, salt spray and metallographic checks — under the ISO 9001 system certified since 2006, so wire condition is documented before it meets the die.

✅ Bottom line: Specify spheroidized, decarburization-controlled wire with certified chemistry before tooling is cut — most deep-hole forming failures start in the coil, not the press.

Frequently Asked Questions

Q: Can every material take an ultra-deep-hole cold forming cycle?

A: No. Ductile carbon and alloy steels, 304/316 stainless and 6xxx aluminum form well; titanium and superalloys crack carbide tooling and stay machined. In practice, meeting the cold heading wire rod quality requirements spheroidize annealing sets — clean chemistry, controlled decarburization — is what lets these alloys run deep-hole cycles at all. The multi station cold heading machine capabilities for complex parts also set limits on undercuts and off-center features, which remain CNC work. YONGMAO reviews each drawing and states plainly when machining is the lower-cost route — before quoting.

Q: How deep and how slender can the elongation die actually go?

A: Proven production covers slender parts in the M16 × 320 mm class with deep hollow cores, and every case is confirmed by a feasibility review on the cold extrusion elongation die for slender shaft parts platform before quotation. Bore depth-to-diameter ratios beyond practical drill limits are routine in ductile alloys; the honest ceiling depends on wall thickness, alloy and the number of stations available.

Q: Do you offer OEM custom cold headed fasteners and small trial orders?

A: Yes. YONGMAO runs OEM programs for custom parts made to drawing — STEP or PDF drawings accepted, feasibility feedback provided, and flexible MOQ on trial runs with full MTR, FAI and salt-spray documentation. Supply runs direct from the press shop, from prototype through volume production.

Get a Free Feasibility Assessment

If a drawing on your desk has a deep bore, a slender shank or a head-to-shank ratio that standard suppliers have declined, send it — STEP or PDF — to YONGMAO. Our engineers will return a free cold-heading feasibility review: whether the part forms, where CNC secondaries are still needed, and what tooling and lead time look like. You lose nothing by asking, and you may cut the part cost by half. Contact YONGMAO direct for custom cold headed fasteners from Changzhou Yongmao Special Fastener Co., Ltd.

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Changzhou Yongmao Special Fastener Co., Ltd.