Spiral bevel gears transmit torque between intersecting shafts through teeth whose trace curves along the pitch cone. Because contact begins at one end of the tooth and migrates across the flank, load is shared by several teeth at once, which gives the pair high torque capacity, smooth motion, low noise and modest vibration. These properties make the spiral bevel gear the default final drive element in automotive rear-axle differentials and in aircraft and machine-tool transmissions, and they also make its quality a direct constraint on the reliability of the whole assembly.
Why cut teeth leave capacity unused
The established production route is metal cutting: the tooth space is generated by a rotating cutter or ground by a profile wheel. Cutting removes material rather than displacing it, so the fibre flow of the blank is severed at the tooth root and along the flank. The tooth therefore behaves as a component with discontinuities at exactly the location where bending stress is highest, and fatigue life and load capacity are limited by that geometry rather than by the base material.
Plastic forming avoids the interruption. Material is displaced into the tooth shape, so the fibre flow follows the profile continuously from the root fillet into the flank. Formed teeth combine this favourable flow with strain hardening and a denser surface layer, which raises bending strength and improves wear resistance on the working flank. The blank is also used more efficiently, since there is no chip removal. The practical obstacle is not the principle but the tooling: the spiral flank is far more difficult to fill than a straight bevel tooth, which is why closed cavities were long regarded as unworkable for this geometry.
The flash penalty of open-die extrusion
In a straight bevel gear the tooth can be filled by a simple axial displacement of material. A spiral bevel gear requires the material to flow with a rotational component while the tooth height and thickness vary from the heel to the toe, so an early attempt to use an ordinary closed die resulted in incomplete filling. Production therefore moved to open dies, which allow the surplus volume to escape as flash on both the inner and outer contours. That flash is not a harmless by-product; it drives a chain of process penalties.
- Flash consumes material that never enters the part, reducing yield and working against economical and low-waste production.
- Filling the flash cavity requires additional deformation work, and the resulting flow pattern can leave the tooth tips underfilled at both the heel and the toe.
- Because the flash band is thick and extends over the full diameter, the trimming press must be sized for a much larger load.
- The elevated deformation force is transmitted to the die as a shock load, and dies are prone to cracking and premature failure under that regime.
- After rough extrusion the part must be trimmed and the web pierced, and the resulting temperature drop is large enough that reheating is required before finish extrusion.
| Aspect | Open-die extrusion | Closed-die precision extrusion |
|---|---|---|
| Flash | Inner and outer flash on every part | None; cavity fully enclosed |
| Material yield | Reduced by flash loss | Billet volume retained in the part |
| Deformation force | Raised by the need to fill the flash band | Lower, since no flash cavity is filled |
| Tooth tip filling | Risk of underfill at heel and toe | Improved by placing the tooth insert in the upper die |
| Operations after rough extrusion | Trimming, web piercing, reheating | Surface cleaning only, followed by machining |
| Trimming equipment | Large tonnage required | Not required |
| Die loading | Overload shock, fracture risk | Controlled by prestressed, guided die assembly |
A flash-free process route
The alternative route keeps the same forming sequence but confines the material at every stage, so no flash is generated and no intermediate trimming is needed:
- Visual inspection of the bar stock and verification of its quality, followed by routine physical and chemical testing to establish the actual material parameters.
- Shearing to billet length.
- Heating; for small batches a non-oxidising atmosphere can be dispensed with.
- Blank preparation by upsetting, piercing and ring expansion.
- Rough extrusion.
- Finish extrusion, followed by surface cleaning, machining and heat treatment; the gear is then inspected and released to store.
The decisive change is that rough and finish extrusion are carried out within a single heating cycle. Compared with open-die practice, closed-die finish extrusion eliminates the trimming operation between the two extrusion stages and removes several handling steps, which saves steel, cuts energy consumption and reduces wear on both operators and equipment.
Selecting the forming temperature
The feasibility of the route depends on the temperature at which the billet is deformed. Three criteria govern the choice: the metal must be formed in a range where ductility is good and deformation resistance has fallen appreciably; the range must lie below the point at which violent oxidation begins; and the finishing temperature must allow the material to strengthen on cooling without an unwanted change of microstructure.
For 40CrMnTi gear steel, the temperature–strength relationship follows a non-monotonic curve. Deformation resistance falls as temperature rises, but in the region of about 300 °C the downward trend reverses sharply: resistance and strength rise again while elongation and reduction of area drop, so ductility passes through a minimum. This interval must be avoided during plastic forming. Ductility recovers at higher temperature; by about 600 °C the strength of the steel has dropped markedly, the ductility indices have risen and the metal flows much more readily, which is the condition needed for extrusion. Continued heating, however, brings intense oxidation at about 800 °C; the resulting scale abrades the die cavity and degrades both dimensional accuracy and surface roughness. For high-strength steels in general, extrusion is sometimes placed above the phase-transformation temperature to secure adequate ductility, but for 40CrMnTi the oxidation limit closes that option and the practical window becomes 600–800 °C.
| Range | Material behaviour | Process implication |
|---|---|---|
| Below 300 °C | Deformation resistance decreases as temperature rises, but remains high | Extrusion loads excessive for the die assembly |
| Around 300 °C | Resistance rises again; strength increases; elongation and reduction of area fall | Ductility minimum — avoid |
| 300–600 °C | Ductility recovers gradually; strength still substantial | Useful only for light reductions |
| 600–800 °C | Strength falls markedly; ductility and flow improve | Selected forming window |
| Above 800 °C | Intense oxidation and heavy scale formation | Avoid; scale wears the cavity and destroys accuracy and surface finish |
Die structure for closed-die precision extrusion
Moving from an open to a closed cavity replaces one set of problems with another: the tooling must fill the tooth completely, release the part, and survive the radial pressure of the tooth-forming flow without cracking. A representative design for a screw press addresses these requirements through four constructional choices.
First, the tooth insert is mounted in the upper die. The heel and toe tips are the hardest regions to fill, and placing the tooth form on the moving side shortens the distance the metal must travel into the extremities of the cavity. It also limits the thermal exposure of the insert: contact between the tooth insert and the hot workpiece is brief, so the insert does not reach a temperature at which it would soften, which suppresses the early annealing and thermal fatigue that shorten tooth-insert life.
Second, the die is split horizontally into upper and lower halves, each carrying a prestressed die ring, and the two halves are aligned by a locking guide. Third, the die ring and the core are mounted so that they float, allowing the cavity to self-align with the incoming billet rather than forcing the billet to conform to a fixed cavity. Fourth, the tooth insert itself is a composite: an outer ring and an inner ring are assembled by prestressed interference fit, so the resulting shrink-fit preload opposes the radial splitting force generated during tooth filling.
The assembled behaviour of this design can be summarised as follows:
- Vertical guidance is accurate and no offset occurs between the halves. Before the upper die touches the billet, the locking guide enters freely with no lateral force acting on it, so wear and impact damage to the locking elements are avoided and positional accuracy is maintained over the die life.
- Billet location is fast and repeatable. The billet seats itself against the die and its own shape without manual alignment, which shortens loading time and eliminates off-centre placement.
- Because the tooth insert sits in the upper die, its contact time with the hot extruded part is short, its temperature rise is small, and early annealing and thermal-fatigue failure are suppressed.
- With a floating die ring and floating core, deformation takes place inside an enclosed cavity. The extruded part carries neither flash nor web, so material consumption and extrusion force both fall, and rough extrusion can be followed directly by open-die finish extrusion without piercing, trimming or reheating.
Design limits and production implications
The economics of the route are straightforward: eliminating flash removes the material loss, the trimming press and the reheating step at once, and shortening the thermal cycle reduces energy consumption per part. The constraints are equally clear. A closed cavity provides no flash reservoir to absorb surplus volume, so billet mass and billet temperature must be held within tight tolerances; the die assembly carries the full forming pressure, which makes the shrink-fit preload and the guide clearance critical variables; and the 600–800 °C window is bounded on one side by a ductility minimum and on the other by scale formation, leaving little margin for temperature drift between the furnace and the press. Within those limits, closed-die extrusion converts a cut, fibre-interrupted tooth into a formed, continuous-fibre tooth with better strength and wear behaviour, and it does so with fewer operations than the open-die practice it replaces.