5 to 10 micrometers is the tolerance CNC gear cutting holds on a normal production run. Your gear teeth are being shaped to an extreme level of accuracy.
Manual machines cannot do this. You need computer control, rigid setups, and the right cutting process for your gear. Pick the wrong method and you do not just get worse gear. You get a more expensive one, longer lead time and more scrap.
You will read following information in the article:
- How CNC gear cutting works
- Which method to pick and why
- Gear machining materials
- The full manufacturing process from blank to finished gear
- Tolerances, and how to choose a supplier
What is CNC Gear Cutting?
How it works
CNC gear cutting is the process of machining tooth profiles into a gear blank using computer-controlled machines. The blank is a rough cylinder and it gets mounted on a precision arbor. Then the cutting tool removes material along a programmed path until each tooth takes its final shape.
The important part is synchronisation. On a modern CNC hobbing centre, the hob rotation and blank rotation are linked electronically through an Electronic Gear Box. The machine executes the tooth geometry mathematically, so every tooth in a batch of 5,000 comes out the same as the first.
CNC vs. Manual Gear Cutting
Manual gear cutting depends on the operator to set feeds, check alignment, and catch problems. It works for one-off jobs. But it cannot carry tight tolerances across a run, and it is slow.
CNC changes both; tighter tolerances and faster cycle times. Consistent quality from piece one to piece ten thousand. For example, manual machines hold ±0.05 to 0.10 mm. CNC hobbing keeps ±0.020 mm as a standard figure. Gear grinding brings that down to ±0.005 mm.
Achievable ISO Grades by Process:
| Process | Achievable ISO Grade | Use |
| Direct from hobbing | Grade 6–8 | General industrial gears |
| Direct from skiving | Grade 6–7 | Internal ring gears, EV |
| After gear grinding | Grade 3–5 | High-speed, precision gears |
| After honing/lapping | Grade 3–4 | Ultra-quiet, long-life gears |
CNC Gear Cutting Methods
There are six main CNC gear cutting methods. Each method is explained below in the table.
| Method | Best Gear Type | Volume Range | ISO Grade | Speed |
| Gear Hobbing | Spur, helical | 1,000 – 100,000+ | Grade 6–8 | High |
| Helical Milling | High helix >30°, bevel | 200 – 2,000 | Grade 6–7 | Medium |
| Gear Shaping | Internal, cluster | 500 – 50,000 | Grade 7–9 | Slow |
| Gear Skiving | Internal ring gears | 1,000 – 100,000+ | Grade 6–7 | Very fast |
| Gear Grinding | Any | Add-on op. | Grade 3–5 | Slow |
| Gear Broaching | Internal splines only | 10,000+ | Grade 7–8 | Fastest |
Relative speed comparison between methods; actual cycle time depends on module, number of teeth, and material.
Gear Hobbing
Hobbing is the most widely used CNC gear cutting method for external spur and helical gears. A multi-tooth hob rotates in sync with the gear blank and cuts all teeth in a continuous motion. For example, it is fast and repeatable. Cost-effective at volume. A module-3, 50-tooth helical gear runs in 18–22 seconds on a modern machine.
A blank can pass its material cert and still vary 5–8 HRC across the cross-section. At piece 80 in a long run, that variation causes unpredictable hob wear and profile errors that only show up in final inspection.
Helical Gear Milling
5-axis CNC milling uses a form cutter or solid carbide end mill to cut helical gears. Most guides say milling is only for prototypes. That rule breaks down on medium-volume jobs like 200 to 2,000 pieces. Here the helix angle is above 30° or the material is a tough alloy like 4340 or 9310.
On these tasks, setting up a hobbing centre takes multiple trial cuts to dial in the helix and lead. A 5-axis milling centre skips these variables. The result: 25–40% lower total cost on medium-volume helical runs in difficult materials, when you count setup, trial-cut scrap, and tooling together.
Gear Shaping
Gear shaping uses a reciprocating cutter that looks like a small gear. It is slow but it reaches places hobbing cannot. For example, internal gears, gears with a shoulder next to the teeth, and cluster gear arrangements. If your gear is internal and the volume is not suitable to justify skiving, shaping will sort it out.
Gear Skiving
Skiving is a continuous cutting process where the cutter runs at a crossed angle to the gear blank. It produces internal ring gears about eight times faster than shaping. But the thing is your machine needs a rigid spindle with EGB synchronisation. Not every shop has this equipment.
Gear Grinding
Gear grinding is a finishing step, not a primary cutting method. You use it after hobbing or shaping, usually after heat treatment, to correct distortion and reach a tighter accuracy grade. It is not optional when the gear runs above 5,000 RPM. The interesting thing is that grinding brings runout down to ±0.005 mm and surface finish to Ra 0.4–0.8 µm.
Gear Broaching
Broaching pulls a multi-tooth broach through the bore in one pass. Internal splines cut in seconds. It is the fastest method at high volume. But the tooling cost is high — it only makes sense above around 10,000 pieces per run.
Types of Gears Produced by CNC Machining
| Gear Type | CNC Method | Common Use | Typical Module |
| Spur gear | Hobbing / milling | Gearboxes, conveyors | 1–8 |
| Helical gear | Hobbing / 5-axis milling | Automotive transmissions | 1–6 |
| Bevel gear | 5-axis / Gleason | Differentials, right-angle drives | 1–10 |
| Worm gear | Hobbing, forming | High-ratio reducers | 1–5 |
Custom Spur Gears
Spur gears have teeth that go parallel to the shaft axis. You will find no axial thrust. Because the geometry is simple and the cost is very low. So it is a good choice for parallel shafts, and moderate loads.
Helical Gears
Helical gear teeth sit at an angle to the shaft, usually 15–30°. Because multiple teeth engage at once, the load is spread out. That means less noise and higher load capacity than spur gears of the same size.
Custom Bevel Gears
Bevel gears transfer power between shafts that meet at an angle. It is most often 90°. Straight bevel gears are simpler and cheaper to machine. Similarly, spiral bevel gears carry higher loads more smoothly. Both are used in differentials, right-angle gearboxes, and power tools.
Worm Gears
A worm gear set gives you a high reduction ratio in a compact space. This set includes:
- The worm
- A screw-shaped shaft
- Meshes with a bronze wheel.
The worm is hobbed or milled on CNC. Bronze for the wheel and it runs against steel without seizing.
Gear Cutting Tools
Types of Tools
The main gear cutting tools are:
Hobs
Used for hobbing spur, helical, and worm gears. Single-start for accuracy, multi-start for speed.
Gear shaper cutters
Disk, shank, or cup-shaped cutters for gear shaping, especially internal gears.
Skiving cutters
These are for internal ring gear skiving on EGB-sync machines.
Form end mills and disc cutters
These are for 5-axis milling of helical and bevel gears, as well as tools for thread milling.
CBN and aluminium-oxide grinding wheels
Manufacturers use these tools for gear grinding and finishing.
Materials and Coatings
HSS-E handles most wet hobbing jobs on standard alloy steels. Cemented carbide runs at three times the surface speed — 180–250 m/min dry — and produces less heat in the workpiece.
Coating choice is important more than most engineers realize:
- TiN (for general purpose)
- TiAlN (for dry hobbing)
- AlCrN (for high-temperature alloys)
Gear Machining Materials
| Material | Typical Use | Heat Treatment |
| AISI 4140 | Gearboxes, machinery | Q&T or nitriding |
| AISI 8620 | Automotive, aerospace | Carburize to 58–62 HRC |
| 17-4PH stainless | Marine, medical, food | H900 |
| Bronze C93200 | Worm wheels | None |
| Acetal / Delrin | Light-duty, consumer | None |
Steel Gears
AISI 8620 is a good choice for carburized gears in automotive and aerospace. Carburizing to 58–62 HRC gives a hard, wear-resistant surface over a tough core that handles shock loads. AISI 4140 is good for nitrided gears. For high-strength applications, titanium alloys are also an option.
Note: For forged blanks, the certification test bar does not always reflect the hardness across the full blank.
Non-ferrous and Polymers
Bronze is standard for worm wheels because it runs against steel worms without galling. Brass is good for small parts. By the way, engineering plastics give you low noise and there is no lubrication requirement.
Custom Gear Manufacturing Process
Gear Blank Preparation
The first step is that you start with a forging blank because forged blanks are better for high stress gears. Then you turn the blank to its datum diameter, bore, and face. This step is more important than anyone thinks.
For example, an unintentional runout in the blank at this stage carries straight through to the finished gear’s pitch circle runout. So hold the blank to 0.005 mm TIR on the turning arbor, not 0.015 mm.
CNC Gear Cutting
You pick the cutting method based on the gear type and production volume. One setup check that is almost never written into a process sheet: verify hob axial alignment to 0.002–0.003 mm TIR on the actual cutting arbor partway through the run, not just at the start.
Thermal growth during a two-hour run can shift the hob by 0.005 mm. That shift causes heavier loading on one tooth flank, and the gear will pass dimensional inspection but fail in the field. Senior machinists do this check without being told. It takes 30 seconds.
Heat Treatment
For 8620 carburized gears, the process is: gas carburized at 900–950°C, quench, temper to 58–62 HRC. The important number to plan around: heat treatment generally distorts the gear enough to shift pitch circle runout by 0.02–0.05 mm.
Gear Grinding and Finishing
Profile grinding corrects heat-treat distortion and brings the gear to its final accuracy grade. After grinding, gear honing or lapping takes off the last bit of roughness. For example, Ra below 0.4 µm and produces the most uniform tooth contact pattern.
Inspection and Quality Control
Full inspection means a CNC gear measuring centre — Klingelnberg, Mahr, or Wenzel. It measures:
- Profile deviation
- Lead deviation
- Pitch error
- Total radial composite error
A CMM is not a substitute. Not for this at least.
On 8620 carburized production gears with controlled hob shift and coolant temperature, total radial composite error below 0.015 mm is routine. Most supplier datasheets quote 0.025–0.035 mm as a practical limit. It shows the difference is process discipline, not machine capability.
Tolerances and Quality Standards
AGMA and ISO both define gear quality grades, but they run in opposite directions. AGMA goes from Q3 to Q15. Higher numbers means better quality.
ISO/DIN goes from Grade 3 to Grade 12. Lower number means better quality.
Accuracy Parameters
You need to understand three parameters to write a complete gear drawing:
- Profile deviation
- Lead deviation
- Total radial composite error
The tolerance table below shows you the complete stats:
| Process | ISO Grade | AGMA Equiv. | Runout (mm) | Surface Ra (µm) |
| CNC hobbing | DIN 6–8 | Q8–Q10 | ±0.020 | 1.6–3.2 |
| 5-axis milling | DIN 6–7 | Q9–Q10 | ±0.015 | 1.6–3.2 |
| Gear shaping | DIN 7–9 | Q7–Q9 | ±0.025 | 1.6–3.2 |
| Gear skiving | DIN 6–7 | Q9–Q10 | ±0.012 | 0.8–1.6 |
| Gear grinding | DIN 3–5 | Q11–Q13 | ±0.005 | 0.4–0.8 |
| Honing/lapping | DIN 3–4 | Q12–Q13 | ±0.003 | 0.2–0.4 |
Case Study: Compound Spur Gear for Heavy-Duty Transmission
This was a compound spur gear to a heavy-duty transmission gearbox. These were two gears with varying modules cut on the same shaft and which were operating in a single gearbox case. It was 20CrMnTi alloy steel, a Chinese standard grade, which was comparable to AISI 8620, which carburizes and quenches well under high-load, high-wear conditions.
The machining order was: turn to datum, hobby the individual gear parts, cut the inside spline and heat treat.
First: both gear sections share the same shaft, so they have to stay concentric with each other. Any runout shift between the two sections causes mesh noise on both gears at the same time.
Second: heat treatment distorted the part. Both the tooth profiles and the overall runout moved outside tolerance after carburizing and quenching.
Solution: So the fix addressed both problems together. FastPreci left extra stock on the bore and datum faces before heat treatment, then finish-turned those surfaces after quenching to re-establish the datum. The hobbing offset for each section was adjusted to account for the expected distortion direction. After heat treatment, we lightly ground the important surfaces that the measuring equipment locates on.
In result, the total runout held within 0.015 mm across both gear sections. Both gears mesh smoothly without adjustment. Noise in assembly testing came in below the customer’s limit.
How to Choose a Custom Gear Manufacturer
Equipment Checklist
Before you send a drawing, check that the supplier has the right machines for your job:
- CNC hobbing centres with EGB synchronisation
- 5-axis machining centres
- CNC gear grinding machines
- CNC gear measuring centre
Certifications
ISO 9001:2015 is the minimum quality system baseline.
- For automotive supply chain, you need IATF 16949.
- For aerospace, AS9100 Rev D.
You will confirm that the supplier’s inspection procedures reference ISO 1328-1:2013. Also, ask to see a completed inspection report from a recent comparable production run, with real measurement numbers.
Capability: Prototyping vs. Production
A good custom gear manufacturer can handle both early-stage prototypes and full production runs on the same equipment. Ask whether they offer DFM review before finalizing drawings.
Conclusion
CNC gear cutting is not one process. Hobbing for most external gears at volume, 5-axis milling for medium-run helical jobs in tough materials, shaping and skiving for internal gears, and grinding.
For design engineers: put lead deviation on your drawing, ask your manufacturer what profile modifications they recommend before you finalise the design.
For procurement managers: the gear price is not the cost. Evaluate suppliers on their process control documentation, their gear measuring equipment, and how they handle problems mid-run.
Both decisions come down to the same thing: working with a manufacturer who has the equipment, the process discipline, and the experience to tell you what the drawing does not say.
Send your drawing and speed data to FastPreci. We will review it, flag any process or profile modification recommendations, and send you a quote with a completed process control plan.
Frequently Asked Questions
What are straight-cut gears?
Straight-cut gears are spur gears. Their teeth run parallel to the shaft axis. No axial thrust load, simple geometry, lowest production cost.
What is the most common CNC gear cutting method?
Gear hobbing. It handles external spur and helical gears at medium to high volumes faster and more cost-effectively than anything else.
Can hobbing cut internal gears?
No. The hob needs clearance to over-run the blank at the end of each cutting pass. Internal gear geometry does not give that clearance.
What tolerance can CNC gear cutting achieve?
Total radial composite error below 0.015 mm is achievable with proper hob shift and coolant temperature control. Industry datasheets often quote 0.025–0.035 mm as the practical limit. Gear grinding brings runout to ±0.005 mm. Honing or lapping gets to ±0.003 mm and Ra below 0.4 µm.
When is gear grinding required?
Two main situations. First: the gear has been case-hardened and heat-treat distortion needs correcting. Second: the application runs above roughly 5,000 RPM. At that speed, noise, vibration, and fatigue life all require ISO Grade 5 or better.
