Hard Skiving vs Soft Skiving: What's the Difference and When to Use Each

Write by Derek | Hansheng Automation

If you've spent any time sourcing precision gears or evaluating machining suppliers, you've probably heard the term gear skiving thrown around. when you ask two different suppliers whether they do "skiving," and one might mean roughing out a gear blank before heat treatment, while the other means finish-machining a hardened ring gear to final tolerance.

They're both talking about skiving - but they're describing two fundamentally different operations.

The distinction between soft skiving and hard skiving isn't about the machine or even the cutting motion. It comes down to one variable: where in the production sequence the skiving happens - before or after heat treatment.

This article breaks down both processes from a manufacturer's perspective - what each one does, where each one falls short.

If you are not yet familiar with the basic principles of gear skiving, you can check out our article Gear Skiving vs Hobbing

When first exposed to such terminology, it's easy to misinterpret.

"Soft" and "hard" actually refer to the state of the material as the tool cutter into the workpiece.

Soft skiving is performed on a gear blank before heat treatment. At this stage, the material is typically in an annealed or normalized condition, with hardness in the range of HB 180–250. The steel cuts relatively freely, tool wear is manageable, and cycle times are short.

Hard skiving is performed after heat treatment - on a gear that has already been case-hardened or through-hardened to HRC 58–62. You're in the territory of hard machining, where tool material, coating, and machine rigidity become critical variables.

In a nutshell: Soft vs. hard skiving isn't a difference in process - it's a difference in which side of heat treatment the machining happens on.

In most gear production workflows, soft skiving functions as the forming pass -that is, the operation of defining the tooth geometry of the gear workpiece before it enters heat treatment. Since the workpiece material is still in the annealed or normalized state at this stage, it is easier to cut.

What works in its favor

Cutting forces are low-which means tool life is significantly longer than in hard machining.

Cycle times are short, and the per-piece cost reflects that.

nder controlled conditions, soft skiving can achieve ISO Grade 5–6 accuracy (for a detailed explanation of ISO gear accuracy grades, see this technical reference).

limitation: heat treatment distortion

This is the biggest limitation of soft skiving. Because the carburizing and quenching processes generate pervasive thermal stresses and phase transformation stresses within the workpiece, for solid, geometrically symmetrical external gears the impact may be minor, but for thinwalled internal ring gears, they cause highly uneven distortion across the tooth profile.

This means that the accuracy achieved by soft skiving is effectively lost. For this reason, soft skiving is not suitable for applications that demand extremely high precision.

Soft Skiving Applicable Scenarios

• External gears with simple geometry
• The final precision of the gear is ISO 6-7 level, and the deformation after heat treatment is still within the tolerance range.
• Large scale production line using the combination of "soft skiving+gear grinding" process

Not applicable scenario

• High-precision internal gears
• Blind-hole or near-shoulder configurations, where grinding access is physically restricted and there's no recovery path if distortion exceeds tolerance

If soft skiving is the forming pass, hard skiving is the accuracy guarantee - the operation that determines what tolerances the finished part actually holds, regardless of what heat treatment did to the geometry.

The cutting happens after the gear has been fully hardened, typically to HRC 58–62. That means the tooling has to be up to the task: hard skiving uses solid carbide or CBN-coated skiving cutters specifically engineered for interrupted hard cutting.

Core advantage: final dimensional control.

Hard skiing can correct the deformation caused by heat treatment. By machining the tooth profile after the part has stabilized from quenching, you're working with the actual distorted geometry and cutting back to the true target dimensions.

For external gears, this level of post-HT correction can often be handled by gear grinding. But for internal gears is different. Internal gear grinding requires specialized tooling and clear axial access - conditions that simply don't exist for blind-hole ring gears or near-shoulder configurations. In those cases, hard skiving isn't just the best option. For many geometries, it's the only viable finishing process after heat treatment.

This is precisely why hard skiving capability matters when you're evaluating a precision gear supplier - and why Hansheng Automation has made significant investment in the Kashifuji KPS30, a machine platform purpose-built for high-precision hard skiving of internal gears. The applications that demand it - robotic joint actuators, planetary gearbox ring gears, servo-driven systems where noise, backlash, and fatigue life are non-negotiable - require a supplier who can hold tolerance on a hardened internal gear, not just before the furnace.

Where hard skiving advantage

• High-precision internal gears requiring ISO Grade 5
• Blind-hole and near-shoulder ring gear geometries where post-HT grinding access is restricted
• Applications with stringent requirements on transmission accuracy, noise floor, and fatigue life - including collaborative robot joints and precision planetary reducers

Limits

• Tooling cost per edge is substantially higher than soft skiving cutters
• Machine requirements are demanding, high spindle synchronization accuracy, rigidity under interrupted hard cutting, and thermal stability throughout the run
• Not well-suited for large module gears (typically M5 and above)- gear grinding remains the preferred route at that scale

The two processes aren't competing alternatives - they serve different roles in the production sequence.

In high-precision internal gear production, soft and hard skiving aren't competing options. They're two stations on the same production line.

The workflow above shows how they divide the labor. Soft skiving handles the forming pass - cutting the tooth profile from the blank while the material is still easy to machine, and deliberately leaving a small stock allowance of 0.10–0.15 mm per flank. To give hard skiving something to remove after the heat treatment distortion has settled in.

Carburizing and quenching then do what they always do - they harden the gear, and they introduce distortion. For a thin-walled internal ring gear, that distortion can reach 20–50 microns or more, distributed unevenly across the tooth profile. The soft skiving geometry is now out of tolerance.

Hard skiving closes the loop. Cutting on the hardened, distorted part, it works back to the true target dimensions - correcting the geometry that heat treatment altered and bringing the finished gear to ISO Grade 5 final accuracy. Nothing about the final part's quality depends on what it looked like before it went into the furnace.

Understanding the soft/hard distinction changes how you qualify a supplier - because "we do gear skiving" tells you almost nothing on its own.

If your application targets ISO Grade 7 or below - standard industrial gearboxes, non-critical drives, applications where post-HT distortion stays within acceptable tolerance - soft skiving combined with induction hardening or through-hardening is typically sufficient. The process is well-established, tooling costs are lower, and you don't need to pay for hard skiving capability you won't use.

If you're specifying ISO Grade 5–6 internal gears - robot joints, planetary reducer ring gears, anything where transmission error, noise, and fatigue life are design constraints - the question isn't whether your supplier does skiving. It's whether they do it after heat treatment. A supplier with only soft skiving capability cannot reliably deliver final-dimension accuracy on a hardened internal gear. The furnace will undo their work.

When you're talking to a potential supplier, one question cuts through the ambiguity immediately:

• "Do you perform skiving before or after heat treatment - and can you provide an inspection report showing the tooth geometry on the finished, hardened part?"
• If they can answer yes to both, you're talking to the right supplier.

If your application demands ISO Grade 5–6 accuracy on a finished, hardened internal gear - whether that's a robot joint actuator, a planetary reducer ring gear, or a custom blind-hole configuration - the process capability behind the quote matters as much as the price.

Hansheng Automation works across the full production sequence, from soft skiving through hard skiving to final inspection. If you have a gear drawing that requires post-HT dimensional control, we're happy to review it and tell you exactly how we'd approach it.

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Can soft and hard skiving be performed on the same machine?

Not always. Soft skiving has lower requirements for machine tools, while Hard skiving has higher requirements for the rigidity, thermal stability, and spindle accuracy of machine tools.

What happens if hard skiving is skipped after heat treatment?

The part ships with whatever distortion the heat treatment introduced. For internal gears with ISO 5-6 precision requirements, this deformation directly leads to an increase in transmission error and a shortened service life.

Is hard skiving suitable for external gears as well?

Technically yes, but it's rarely the preferred choice. External gears have access to gear grinding after heat treatment, which is faster, more cost-effective at scale, and capable of reaching ISO Grade 4 or better.

What module range is hard skiving suitable for?

Hard skiving is generally well-suited for module ranges up to approximately M4–M5. Beyond that, the chip loads generated by hard cutting at larger tooth depths push against the practical limits of skiving tool geometry and machine rigidity. For larger module gears - M5 and above - gear grinding remains the recommended finishing process after heat treatment.