When evaluating CNC hobbing machines, axis count is one of the first specifications listed — and one of the least well explained. A 7-axis horizontal hobbing machine and a 6-axis vertical hobbing machine can produce the same gear to the same DIN class. So what does the seventh axis actually add, and when does it matter? This article explains each axis’s function, the practical production differences between 6-axis and 7-axis configurations, and the application scenarios where the additional axis changes outcomes.
EP-100: 7-axis horizontal configuration — the U-axis hydraulic tailstock is the seventh axis that distinguishes it from 6-axis vertical machines
All modern CNC hobbing machines control at least six axes. These axes are standardised across machine builders:
| Axis | Function and Production Role |
|---|---|
| B — Hob Spindle Rotation | Rotates the hob at the calculated cutting speed. On direct-drive machines, the motor directly drives the spindle without intermediate gearing, eliminating backlash in this axis. |
| C — Workpiece Table Rotation | Rotates the gear blank. The speed ratio between B and C is maintained by the electronic gearbox and determines the number of teeth generated on the workpiece. |
| X — Radial Feed | Moves the hob toward or away from the workpiece centre. Used to set depth of cut and to retract the hob before axial return at the end of each pass. |
| Y — Tangential Feed | Moves the hob along its own spindle axis, perpendicular to the cutting direction. This is the hob shift axis — used to distribute wear across the hob length to maximise tool life. |
| Z — Axial Feed | Feeds the hob across the face width of the gear blank during the cutting pass. Speed and direction determines the axial feed rate per workpiece revolution. |
| A — Helix Angle Rotation | Tilts the hob spindle relative to the workpiece axis to match the gear’s helix angle. On CNC machines this is set automatically from the part program; physical change of helix angle is not required. |
The seventh axis — universally designated U — is a controlled hydraulic tailstock axis. On a 6-axis machine, the tailstock (if present) is positioned manually to support the workpiece, then locked in place for the cutting cycle. On a 7-axis machine, the tailstock position is CNC-controlled: the U-axis moves the tailstock to the correct position automatically as part of the part program, then applies the correct support pressure throughout the cut, and retracts automatically at the end of the cycle.
EP-100 7-axis system: the U-axis hydraulic tailstock is positioned automatically by the CNC program and applies controlled support pressure throughout the hobbing cycle
The U-axis tailstock matters most for shaft-type workpieces — gears on shafts, output shaft pinion assemblies, and cluster gears where the workpiece is long relative to its diameter (L/D ratio greater than approximately 2:1). Without tailstock support, slender shaft gears deflect under the radial hobbing force, producing a gear that is barrel-shaped in lead (positive crowning) rather than flat. The 7-axis controlled tailstock applies a precise counter-force that opposes this deflection and keeps the shaft straight throughout the cut.
For disc-type gears — where the gear blank diameter is substantially larger than its face width — tailstock support is typically not required. The blank sits on the C-axis table and is held by the clamping fixture without any tendency to deflect under hobbing loads. For this large population of gear types, a 6-axis vertical machine is fully appropriate and the U-axis adds no production benefit.
On a 6-axis machine with manual tailstock, the operator must manually position and lock the tailstock for each new workpiece type. In a multi-variant production environment with frequent changeovers, this adds a manual step to every setup. On a 7-axis machine, the tailstock position is a parameter in the part program — changeover to a new shaft length is automatic as part of the program call, with no operator adjustment required. For production cells with robot loading, the U-axis automated tailstock is often essential: a robot cannot reliably position a manual tailstock between cycles.
| Scenario | 6-Axis Vertical | 7-Axis Horizontal |
|---|---|---|
| Disc-type gear blanks (L/D < 1) | Fully suited — no tailstock required | Capable but horizontal config less natural for disc blanks |
| Shaft-type gears (L/D 1–2) | Possible with manual tailstock setup | Preferred — U-axis automates tailstock support |
| Long shaft gears (L/D > 2) | Not recommended without modifications | Purpose-designed — U-axis critical for deflection control |
| Automated cell with robot loading | Suitable for disc gear automation | Essential for shaft gear automation — U-axis enables full automation |
| Multi-variant production with frequent changeover | Manual tailstock = operator step per changeover | U-axis = tailstock positioning in part program |
EP-200 and EP-350: 6-axis vertical configuration — optimised for disc-type gear blanks where the face width is small relative to the blank diameter
The axis count does not directly determine gear accuracy. A well-built 6-axis machine with direct-drive B and C axes and an active thermal compensation system will produce more accurate gears than a poorly built 7-axis machine without these features. The U-axis adds production flexibility for shaft workpieces — it does not improve the fundamental gear generation accuracy, which is determined by the B/C synchronisation, hob quality, and machine thermal stability.
Where the 7-axis configuration indirectly improves accuracy for shaft gears is through the controlled tailstock support force. A manually positioned tailstock applied with inconsistent force will produce inconsistent workpiece deflection between parts and between operators — leading to variation in lead accuracy that appears as a random component rather than a systematic error. The U-axis controlled support eliminates this variation source.
The 7-axis machine in the EP range (EP-100) is a horizontal machine — the workpiece axis is horizontal. All 6-axis machines in the EP range (EP-150 through EP-500) are vertical machines. This orientation difference is at least as significant as the axis count difference for production outcomes:
Horizontal: chips fall into a hopper below the workpiece — suited to the inclined-bed design. Vertical: chips fall by gravity away from the cutting zone around the blank perimeter — better for larger module production where chip volume per pass is substantial.
Horizontal machines contain coolant more easily around the enclosed cutting zone. Vertical machines benefit from gravity-assisted drainage but require careful guarding design to prevent coolant from reaching the table bearing.
Horizontal orientation is natural for shaft-type gears loaded from the end. Vertical orientation is more natural for disc-type gear blanks loaded from above — aligning with robotic arm approach direction in most automation concepts.
Horizontal configurations are typically used for smaller module ranges (M0.5 to M6). Vertical configurations scale more naturally to larger module production (M2 to M14+) where the heavier workpiece loads are better managed vertically.
The decision between 6-axis and 7-axis hobbing machine configurations is determined primarily by workpiece geometry, not by accuracy requirements. Use this decision guide:
Your primary gear family includes shaft-type workpieces with L/D > 1.5, you run a multi-variant mix of shaft lengths requiring frequent changeover, or you are building an automated cell with robot loading of shaft gears.
Your primary gear family is disc-type gear blanks, you are producing medium to large module gears (M4+), your workpiece diameters exceed Phi 150mm, or gravity-assisted chip evacuation is a priority in your production environment.
Many gear production facilities operate both configurations — horizontal 7-axis machines for shaft gear families and vertical 6-axis machines for disc gear families — with both platforms sharing the same FANUC control system to minimise training requirements and spare parts inventory. Contact our engineering team for a multi-machine cell recommendation.
Yes, with a manual or semi-automatic tailstock. The limitation is that manual tailstock positioning adds a setup step for each changeover and introduces operator-dependent variation in support force. For low-variety shaft gear production with infrequent changeovers, a 6-axis machine with manual tailstock is entirely practical.
For shaft gear automation with robot loading, yes. The robot loads the shaft into the chuck, and the U-axis moves the tailstock to the support position automatically as part of the cycle — eliminating the need for the robot to also position the tailstock. For disc gear automation, the U-axis is not required.
No — accuracy is determined by machine build quality, drive system type, and thermal stability, not axis count. Both machines use direct-drive B/C axes and FANUC 0i-MF Plus control. The EP-150 adds an active thermal deformation prevention system that specifically benefits shift-long production consistency. The axis count difference reflects the workpiece family each machine is designed for, not their accuracy capability.
Unsure whether a 6-axis vertical or 7-axis horizontal hobbing machine better suits your gear family? Share your workpiece drawings and production mix for a configuration recommendation.
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