Custom Internal Gear Manufacturing: How Precision Internal Ring Gears Are Made

Written by Derek | Hansheng Automation

With the rapid development of the robotics and automation industries, demand for high-precision internal gears continues to grow Internal gears are core transmission components in joint reducers for collaborative robots and precision planetary gear systems.

But finding a truly reliable custom internal gear manufacturer is quite difficult. Although you can look for well-known gear manufacturers, as a purchaser or engineer, you have to consider cost issues. In addition, there is an important reason - the manufacturing difficulty of internal gears far exceeds that of external gears.

• Processing tools need to penetrate deep into the interior of the workpiece for cutting, which is extremely limited in space and prone to tool interference;
• Due to the inability to directly observe the cutting state, the precision control and chip removal difficulty in the internal gear manufacturing process have increased exponentially.

These are the manufacturing difficulties of high-precision precision internal gears. This article will provide an in-depth analysis of the manufacturing process of internal gears from the perspective of a gear manufacturer, and help you understand how to choose a suitable partner.

An internal gear is a cylindrical gear with teeth cut into its inner diameter, designed to mesh with external gears or planet gears. Unlike external gears that mesh on their outer circumferences and rotate in opposite directions, an internal gear meshes with a smaller external gear (a pinion) positioned inside it, causing both to rotate in the same direction.

The structure of the internal gear significantly reduces the center distance between the meshing gears, which can transmit huge torque.Based on the tooth profile and specific engineering requirements, internal gears are generally classified into three main categories:

Internal Spur Gears

Featuring straight teeth parallel to the axis of the gear, these are the most common type, offering straightforward operation for standard speed reduction.

Internal Helical Gears

Cut with angled teeth, these provide gradual tooth engagement. They operate much more smoothly and quietly than spur gears and can carry higher loads, though they are significantly harder to manufacture.

Internal Splines

While technically used for coupling rather than speed reduction, internal splines feature straight ridges inside a bore used to lock a shaft and transmit massive torque without relative rotation.

Internal Gear

External Gear

• For a deep-dive comparison of gear skiving vs hobbing, see our dedicated article

Perhaps you need to understand the manufacturing process of internal gears, which will be more conducive to your decision-making. As a professional precision gear manufacturer, we will use our commonly used gear manufacturing process as an example to explain the four major gear manufacturing processes for you.

 

Gear Skiving

 

For custom, high-precision internal gears, gear skiving is undoubtedly the most advanced and highly recommended process today. This is also the main machining process for our internal gears.

 

• Complex Geometries Made Simple: Skiving it is exceptionally well-suited for high-precision internal gears, particularly those with complex geometries like internal stepped gears, shoulders, or blind-hole structures

 

• Precision Level: The accuracy range is ISO/DIN/GB Level 5 to 7, and with advanced equipment, the accuracy can be stabilized to ISO Level 5. For example, Hansheng Automation use Kashifuji Gear Skiving Machine KPS30 to process internal gears, and the accuracy is stabilized to Level 5.

 

• High Efficiency: Skiving is a continuous rotary cutting process, whereas shaping requires a reciprocating stroke that must decelerate and reverse on every cycle. This fundamental difference means skiving can deliver substantially shorter cycle times on equivalent internal gear profiles - making it the preferred choice for both prototype and volume production.

 

If you require tight tolerances, complex internal features, or production efficiency, our internal gear skiving service is definitely worth your choice.

 

Gear Shaping

 

Gear Shaping was the most commonly used manufacturing process for internal gears prior to the large-scale application of gear skiving. It uses a pinion-like cutter that moves in a reciprocating (up and down) motion while gradually feeding into the rotating gear blank.

 

Key Features:

 

• Flexibility: Shaping is highly versatile and can be used to cut both internal and external gears, as well as splines.

 

• Precision Level: It generally yields precision around GB/ISO Level 7 to 9, making it perfectly suitable for medium-precision requirements.

 

• Design Limitations: Because the shaping tool requires clearance at the bottom of the cut, gears located close to a shoulder or at the bottom of a deep blind bore can be challenging or impossible to produce.

 

For medium-precision internal gears where flexibility matters more than cycle time, internal gear shaping remains a reliable and cost-effective solution.

 

Gear Broaching

 

The broaching process works by pushing or pulling a tool with progressively larger teeth through the gear blank, cutting the entire tooth form in a single pass.Applicable to situations where High Volume Production, Small Modules, and precision requirements are not high.

 

Considerations

 

• Precision Level: Typically achieves ISO/DIN/GB Level 8-11, which is adequate for many applications.

 

• Tooling Cost: Broaches are expensive and custom-made for each gear profile, due to cost issues, it is not suitable for prototyping or small-scale production

 

• Design Limitations: Blind holes cannot be broached; the tool must pass completely through the workpiece.

 

Gear Hobbing – Not for Internal Gears

 

Gear hobbing is the most efficient process for external gears, achieving ISO/DIN/GB Level 6-7 with excellent productivity. It is unsuited for internal gears because the hob's geometry prevents it from reaching inside the workpiece. We mention it here only to clarify that internal gear manufacturing requires different approaches-namely skiving, shaping, or broaching.

 

Gear Skiving

Gear Shaping

Gear Broaching

Chart Summary

At Hansheng Automation, we maintain expertise across all relevant gear manufacturing processes, allowing us to recommend-and deliver-the optimal solution for your specific internal gear requirements. Whether you need prototypes, medium-volume runs, or mass production, we have the technology and experience to meet your precision and budget goals.

Explore our internal gear manufacturing capabilities

The machining process mentioned in the previous chapter determines the dimensional accuracy of the gear, while its service life, load-bearing capacity, and reliability are determined by the material itself. Different materials may have different heat treatment processes, therefore, choosing the appropriate gear material and the correct heat treatment process are also very important.

Common Materials for Custom Internal Gears

gear material selection directly impacts performance, cost, and longevity. Here are the materials we most frequently recommend for internal gear manufacturing. There are many other materials, which will not be listed here.

• 20CrMnTi (Alloy Carburizing Steel): This material is the mainstream choice for robot joints and precision planetary reducers. It has excellent core toughness and, after appropriate treatment, has extremely high surface hardness.

• 42CrMo (Quenched and Tempered Steel): This material has ultra-high structural strength and fatigue resistance, mainly used for heavy-duty conditions.

• 304/316 Stainless Steel: For internal gears used in the medical device sector, aerospace valves, or food processing equipment, these stainless steel grades are mandatory to ensure strict corrosion resistance and hygiene.

• Engineering Plastics (e.g., POM, Nylon, PEEK): Engineering plastics are highly suitable for lightweight, low load applications, as they can reduce noise and self lubricate.

Critical Gear Heat Treatment Processes

Gear heat treatment is where raw material transforms into a high-performance component. The most common gear heat treatment methods include:

Managing Heat Treatment Distortion in Internal Gears

Herein lies one of the greatest problem in internal gear manufacturing: heat treatment inherently causes metal to warp. Thin-walled internal ring gears are particularly notorious for "going out of round" (becoming oval-shaped) during rapid quenching.

It requires specialized fixturing during the quenching process and precisely calculated allowances. Furthermore, any minimal distortion that does occur must be corrected through post-heat treatment processes.

This is where advanced gear skiving truly shines, as specialized solid carbide skiving tools can execute "hard skiving" on heat-treated internal gears, shaving away the microscopic distortions to restore the gear to a flawless GB/ISO Level 5 precision.

Decoding ISO/DIN/GB Gear Precision Grades

Gear precision standards (ISO 1328, DIN 3961, GB/T 10095) establish a common language for communicating accuracy requirements. At Hansheng Automation, we routinely manufacture precision internal ring gears to ISO/DIN/GB Level 5 and above. But what do these grades actually mean in practice?

• Not sure which grade your application requires? Our engineers can review your drawing and recommend the right precision class - send us your specs

The Hansheng Quality Guarantee

To guarantee our precision, Hansheng Automation relies on a strict, data-driven QA protocol. We overcome these spatial limitations using dedicated, state-of-the-art Gear Inspection Machines and advanced Coordinate Measuring Machines (CMMs) equipped with specialized, articulated micro-probes. This high-end metrology equipment allows us to accurately map the internal tooth profile, pitch deviation, radial runout, and helix angle with sub-micron accuracy.

We don't just promise Level 5 precision; we prove it. Every batch of custom internal gears manufactured by Hansheng is delivered with a comprehensive, transparent dimensional inspection report. When the parts arrive at your facility, your assembly team will have absolute confidence that every single internal ring gear perfectly matches your engineering drawings.

At Hansheng Automation, we believe the custom gear order process should be transparent, responsive, and collaborative. Here is exactly how we work with you, from the first drawing to the final delivery.

Step 1: Submit Your Requirements

We accept all major engineering formats

• 2D Drawings: PDF, DXF, DWG (with critical dimensions and tolerances clearly noted)
• 3D Models: STEP, IGES, SLDPRT, X_T
• Specifications: Even if you don't have a complete drawing, our engineers can work from your performance requirements

What we need to know

• Gear type (spur internal, helical internal, internal spline)
• Key dimensions (module, number of teeth, pitch diameter, face width)
• Material requirements
• Precision grade (ISO/DIN/GB Level)
• Quantity and timeline expectations

Step 2: Engineering Review + Quotation

Once we receive your files, our engineering team conducts a thorough feasibility review:

• Process Selection: We determine the optimal manufacturing approach-gear skiving, shaping, or broaching-based on your geometry and precision requirements.
• Material Confirmation: We verify material suitability and recommend alternatives if needed.
• Heat Treatment Planning: For case hardened internal gear applications, we plan the heat treatment sequence and allowances.
• Inspection Method: We confirm how each critical dimension will be measured and verified.

you receive

• A detailed quotation with clear pricing
• Proposed lead times
• Any design-for-manufacturing recommendations (optional, no charge)
• Process documentation outlining how your gear will be made
• No hidden fees, no surprises.

Step 3: Sample Manufacturing and Approval

For prototype internal gear requirements or first-time production, we strongly recommend a sample approval stage:

• Single Pieces or Small Lots: We can produce as few as 1-5 pieces for validation.
• Full Inspection Report: Samples are inspected comprehensively, with all gear parameters documented.
• Dimensional Approval: You verify that the sample meets your print requirements.
• Functional Testing: If needed, we can coordinate with your assembly team for real-world validation.

Step 4: Batch Production

Once samples are approved, we move to production. Our facility is equipped to handle:

Key production parameters:

• Module Range: 0.1 – 5.0
• Outside Diameter Range: 3mm – 300mm
• Achievable Precision: ISO/DIN/GB Level 5 (for applicable geometries)

During production, we maintain:

• In-process inspections at critical stages
• Process documentation for full traceability
• Regular communication on progress and any potential issues

Step 5: Final Inspection + Shipping Report

• Comprehensive Inspection Report: Actual measured values for all critical gear parameters (profile, lead, pitch, runout)
• Material Certification: Traceability to original mill test reports
• Heat Treatment Documentation: Hardness verification, case depth (if applicable)
• Certificate of Conformance: Signed engineering release
• Shipping options:

          1. Standard freight (air or sea, depending on volume and timeline)

          2. Express courier (DHL, FedEx, UPS) for urgent requirements

          3. Customer pickup (welcome to visit our facility)

When auditing potential partners, you should grade them against these five critical criteria:

• 1. Do They Offer Dedicated Gear Skiving Capabilities?
• 2. What is the Pedigree of Their Equipment?
• 3. Is Material and Heat Treatment Managed Effectively?
• 4. Can They Provide Transparent Inspection Reports?
• 5. Are They Willing to Support Low-Volume and Prototype Orders?

Using the above five key points to evaluate partners, the manufacturers you find and the quality of the products delivered will meet your requirements. As a professional precision gear manufacturer, Hansheng Automation has profound engineering and technical strength, advanced equipment, and flexible production capabilities, which can turn your internal gear design into reality.

What size and precision of custom internal gears can Hansheng Automation produce?
As a custom internal gear manufacturer, we specialize in producing gears with modules ranging from 0.1 to 5.0, and tip diameters from 3mm up to 300mm. Thanks to our advanced equipment, including the Kashifuji Gear Skiving Machine KPS30, we consistently achieve ultra-high precision grades of ISO/DIN/GB Level 5.

Is gear skiving really better than gear shaping for internal gears?
In most high-precision applications, yes. Our internal gear skiving service offers significantly faster production cycle times and superior surface finishes compared to traditional shaping. Furthermore, skiving is highly effective for cutting complex internal structures, such as gears located extremely close to an internal shoulder or inside blind holes, where traditional shaping tools struggle with clearance.

Besides gear manufacturing, do you produce complete precision reducers?
Yes. With over a decade of experience, we don't just supply individual gears; we engineer complete, high-precision transmission systems. We manufacture planetary gearboxes with ultra-precision backlash (≤ 1 arc min for P0 class), customized hollow rotary tables (precision ≤ 10 arc sec), cam indexers (custom precision ±15 arc sec), and harmonic drives (custom backlash ≤ 5 arc sec).

Can you handle secondary CNC machining or casting for the gear housings?
Absolutely. We provide comprehensive, one-stop manufacturing solutions. Beyond gear cutting, our in-house capabilities include precision CNC turning and milling (accuracy up to ±0.005mm), ultra-mirror finish machining (accuracy up to ±0.002mm), and full casting services (Sand, Lost Foam, and Gravity casting) for large components up to 3800x1500x1200mm.