CNC Gear Machining: Processes, Accuracy Grades, and How to Source From China

A gear that measures correctly in inspection can still destroy a gearbox in the field. Specify the wrong accuracy grade and the gear runs noisily, wears unevenly, and eventually fails - not because the tooth dimensions were wrong, but because the tooth form errors were never caught. Specify it correctly and the same geometry holds for millions of cycles.

CNC gear machining is the process of cutting and finishing gear teeth using computer-controlled machines to achieve the tooth profile, pitch accuracy, and surface finish required for the gear's intended load and speed. It is not a single operation. Most functional gears go through at least two processes: a rough cutting stage and a finishing stage, with the second stage determining the accuracy grade that ends up on the drawing.

This guide is for engineers, sourcing teams, and procurement teams working with CNC machining factories in China. It covers which cutting process your part needs, how to read gear accuracy grades, the module vs diametral pitch issue that trips up cross-border procurement, and exactly what to put on your sourcing inquiry.

TL;DR / Key Takeaways

• CNC gear machining includes four main processes: hobbing, shaping, grinding, and skiving.
• Gear hobbing is the most common process for external spur and helical gears.
• Gear shaping is used for internal gears and geometries where a hob cannot reach.
• Gear grinding is a finishing step for high-accuracy, low-noise, or hardened gears.
• ISO 1328 and DIN use lower numbers for higher accuracy. Older AGMA Q-class uses higher numbers for higher precision. Always confirm the exact standard.
• Most Chinese gear machining factories work in metric module, not diametral pitch. Confirm the tooth size system before quoting.
• A complete gear RFQ should specify gear type, module or DP, number of teeth, helix angle, accuracy grade, material, heat treatment, and inspection requirements.

 

What Is CNC Gear Machining?

CNC gear machining is the cutting and finishing of gear tooth geometry using computer-controlled machine tools to produce spur gears, helical gears, bevel gears, worm gears, internal gears, splines, and gear shafts to specified dimensional and accuracy tolerances.

What separates gear machining from other CNC operations is the compound geometry of a gear tooth. The involute profile, lead angle, pitch spacing, runout, and surface finish must all fall within the accuracy grade specified on the drawing. An error in any one of these affects how the gear mesh behaves under load.

A gear can pass basic dimensional inspection and still fail in service if the tooth profile, lead, pitch, or runout is not controlled. That is why the accuracy grade matters as much as the visible geometry.

Which Gear Cutting Process Does Your Part Need?

The right gear cutting process depends on your gear geometry, the required accuracy grade, and whether you need rough cutting, finishing, or both. These four processes cover most industrial gear work:

Process	Best For	Achievable ISO Grade	Notes
Gear hobbing	External spur and helical gears, high-volume production	ISO 6-8	Fastest process and widely available; grinding may be required for tighter grades
Gear shaping	Internal gears, cluster gears, gears near shoulders that block hob access	ISO 6-8	Slower than hobbing; essential for geometries hobbing cannot reach
Gear grinding	High-accuracy finishing after hobbing, shaping, or heat treatment	ISO 3-6	Adds cost and cycle time; required for many automotive, turbine, and precision gearbox grades
Gear skiving	Internal gears in a single CNC setup	ISO 5-7	Newer process; faster than shaping for internal gears when the factory has the right equipment

Most production gears are hobbed and then ground if accuracy requires it. If your drawing calls for ISO Grade 6 or better, plan for a grinding step. If ISO Grade 7-8 is acceptable, hobbing alone may be sufficient and significantly cheaper - and the CNC machining cost difference between these two routes can be substantial on larger batches.

 

Which Gear Accuracy Grade Should You Specify?

Gear accuracy grades define how much deviation from perfect tooth geometry is acceptable. The right grade depends on the gear's speed, load, noise requirement, service life, and inspection standard.

• ISO 1328 is the standard most often used by Chinese and European suppliers. It uses grades 0-12, where lower numbers mean higher precision. Grade 0 is ultra-precision metrology; Grade 12 is low-cost agricultural equipment. Most commercial and industrial gears fall between Grade 4 and Grade 9.
• AGMA is common on US drawings, but the direction of the numbering depends on the standard version. Older AGMA Q-class drawings use higher Q numbers for higher precision. Other AGMA formats use different class labels. Always state the exact AGMA standard and class rather than assuming a direct conversion.

Application	Typical ISO Grade	Typical AGMA 2000 Q-Class	Manufacturing Route
Aerospace and turbine gears	ISO 3-5	Q12-Q14	Hobbing + grinding + lapping
Automotive transmission	ISO 5-7	Q9-Q11	Hobbing + grinding
Industrial gearboxes	ISO 6-8	Q8-Q10	Hobbing, with grinding for tighter grades
Agricultural and construction	ISO 8-10	Q5-Q7	Hobbing or shaping
General purpose / non-critical	ISO 10-12	Q3-Q5	Hobbing, milling, or shaping

A practical starting point: if the gear runs continuously in a power transmission application, specify ISO Grade 7 or better and ask whether grinding is required. If it is a slow-moving, intermittent-duty application, ISO Grade 9 with hobbing alone is often sufficient and noticeably less expensive.

Note. The accuracy grade is not just a quality label. It determines the manufacturing route, inspection method, lead time, and price.

 

Module vs Diametral Pitch: Get This Right Before You Send Your Drawing

Module and diametral pitch both describe gear tooth size, but they use different unit systems. Most Chinese CNC gear factories work in metric module, so drawings using diametral pitch should include a confirmed metric conversion.

1. Module is the metric parameter. It equals the pitch diameter divided by the number of teeth, expressed in millimetres. A module 2 gear has larger teeth than a module 1 gear.
2. Diametral pitch is the imperial parameter. It equals the number of teeth divided by the pitch diameter in inches. Unlike module, a higher DP number means smaller teeth.

The conversion is straightforward: module = 25.4 / diametral pitch.

This matters for procurement because most Chinese CNC gear factories use module as their standard. If your drawing specifies diametral pitch, the factory must either convert or work with non-standard tooling - both add cost and the potential for misinterpretation. For cross-border RFQs, specifying the module value directly is the simpler path.

What Materials and Heat Treatment Do CNC Gears Use?

Gear material and heat treatment are chosen together. The material determines strength, toughness, machinability, and cost. Heat treatment determines surface hardness, wear resistance, distortion risk, and whether grinding is required after hardening.

Material	Common Gear Applications	Heat Treatment Options
Carbon steel (C45, 1045)	General industrial, low-to-medium duty	Induction hardening
Alloy steel (42CrMo4, 4140)	Automotive, medium-to-high duty	Case hardening, nitriding, quench and temper
Case-hardening steel (16MnCr5, 8620)	High-load automotive and gearbox gears	Carburizing + quench + temper
Stainless steel (303, 316)	Corrosion-resistant or food-grade environments	Limited hardening options depending on grade
Brass / bronze	Low-load, quiet running applications such as worm wheels	None typically required

Case-hardening achieves high surface hardness, typically 58-62 HRC, while keeping a tough core. Nitriding gives slightly lower surface hardness but causes minimal distortion - important when accuracy grade must be preserved through heat treatment without a grinding correction step. The choice of metal finishing process also matters post-machining for corrosion protection, particularly on steel gears operating in wet or chemical environments.

What to Include on Your RFQ for CNC Machined Gears

Incomplete gear RFQs produce quotes that are impossible to compare, because each factory will fill in the gaps differently. Before submitting, confirm your drawing and RFQ include every item below:

• Gear type - spur, helical, bevel, worm, internal, spline, rack, or gear shaft
• Module or diametral pitch - specify which system and the value; state both if the drawing is mixed
• Number of teeth
• Pressure angle - commonly 20°, but confirm on the drawing
• Helix angle - for helical gears; 0° for spur gears
• Accuracy grade and standard - e.g., “ISO 1328 Grade 7” or the exact AGMA standard and class
• Material and grade - e.g., “42CrMo4” or “4140 alloy steel”, not just “steel”
• Heat treatment - case hardening, nitriding, induction hardening, quench and temper, or none
• Hardness requirement - surface hardness, core hardness, and case depth if applicable
• Surface finish on tooth flanks - Ra value if specified
• Bore diameter, keyway, spline, or set screw details
• Inspection requirements - profile, lead, pitch, runout, and first article report if required
• Quantity and delivery requirement

Gear machining capability varies significantly between factories. A shop equipped to hob standard industrial gears is not the same as one set up for post-grind hardened gears to ISO Grade 5. You can submit your gear drawing for quotes from Haizol's verified gear machining network, where factory profiles can be compared by hobbing, shaping, grinding, and CNC machining capability. New to Haizol? Create a Haizol account to compare capability profiles before committing to a factory.

Frequently Asked Questions (FAQ)

When Should I Choose Gear Hobbing vs Gear Shaping vs Gear Grinding?

Choose hobbing for most external spur and helical gears in production. Choose shaping for internal gears, gears near shoulders, or geometries where a hob would interfere. Choose grinding when the gear is hardened, low-noise, high-speed, or needs a tighter accuracy grade. The practical rule: if the gear runs unhardened in a standard industrial drive, hobbing may be sufficient. If it is hardened and must hold tight tooth profile tolerances in service, grinding is usually required.

What Is Gear Hobbing?

Gear hobbing uses a rotating multi-tooth cutting tool, called a hob, that generates gear teeth progressively as the gear blank rotates in synchronisation. It is the most common and fastest CNC gear cutting process for most external spur and helical gears in production quantities.

What Is the Difference Between Gear Hobbing and Gear Grinding?

Hobbing is the primary cutting operation that forms the gear teeth. Gear grinding is a finishing operation performed after hobbing, shaping, or heat treatment to correct distortion and achieve tighter accuracy grades, lower noise, and better tooth contact. Many high-duty gears require both.

What Is Module in Gear Machining?

Module (m) is the metric parameter describing gear tooth size. It equals the pitch diameter divided by the number of teeth, expressed in millimetres. Chinese and European gear factories generally work in module. If your drawing uses diametral pitch, convert using: module = 25.4 / diametral pitch.

Which Accuracy Grade Do I Need for My Gear?

For automotive power transmission, ISO Grade 5-7 is common. For industrial gearboxes running continuously, ISO Grade 6-8 is common. For agricultural or low-duty applications, ISO Grade 8-10 may be sufficient. If you are unsure, state the application, speed, load, and noise requirement so the factory can recommend a realistic grade and manufacturing route.

Compare the Manufacturing Route, Not Just the Gear Geometry

A gear drawing is not complete enough to quote if it only shows tooth count, module, bore size, and material. Those details define the geometry. They do not define whether the gear will run quietly, carry load, or survive in service.

That is why the accuracy grade and manufacturing route matter. Hobbing alone may be suitable for some ISO Grade 7-9 gears. Hardened, high-speed, low-noise, or high-load gears usually need grinding after heat treatment to control profile, lead, pitch, and runout errors.

Before comparing gear machining quotes, confirm four things: the accuracy standard, manufacturing route, heat treatment, and inspection method. A gear that measures right on basic dimensions is not always a gear that performs right in a gearbox.