Internal Gear Machining: Shaping, Broaching and Grinding

Introduction to Internal Gear Manufacturing

Internal gears and ring gears present unique manufacturing challenges compared to external gears. The cutting tools must operate within the gear bore, creating constraints on tool diameter, overhang, and chip evacuation. Internal gears are used extensively in planetary gear systems, automatic transmissions, differential assemblies, and industrial gearboxes where space-efficient power transmission with concentric input/output shafts is required. The manufacturing processes for internal gears—shaping, broaching, and grinding—each offer distinct capabilities in terms of precision, productivity, and geometric flexibility. This article examines each method in detail, providing engineers with the technical information needed to select the optimal process for their internal gear applications, covering module ranges from m0.2 to m20 and gear diameters from 10 mm to 600 mm.

Gear Shaping for Internal Gears

Gear shaping with a pinion-type cutter is the most widely used method for internal gear cutting. The shaping cutter—a hardened and ground pinion with cutting edges—reciprocates axially through the workpiece while both cutter and workpiece rotate in a timed relationship that generates the internal involute profile. Shaping can produce internal spur and helical gears, with helix angles up to 35° achievable using helical cutters and guide systems. The stroke length must exceed the gear face width by 5–10 mm at each end for full profile generation. Cutting speeds of 15–40 m/min for HSS cutters and 50–100 m/min for carbide-tipped cutters determine cycle time. Shaping achieves IT7–IT9 precision (DIN 7–9) in the cut condition, sufficient for most medium-precision applications. Shaping cutter design is critical: the cutter outside diameter must be smaller than the internal gear root diameter with adequate clearance for chip flow.

Parameter	Gear Shaping	Rotary Broaching	Internal Grinding
Achievable precision	DIN 7–9 (IT7–IT9)	DIN 7–8 (IT7–IT8)	DIN 4–6 (IT4–IT6)
Module range	m0.5–m12	m0.3–m6	m0.5–m10
Maximum bore diameter	600 mm	150 mm	400 mm
Min bore diameter	20 mm	10 mm	30 mm
Face width limit	Up to 200 mm	Up to 80 mm	Up to 200 mm
Cycle time (typical, m2, 50mm bore)	45–120 sec	10–30 sec	120–300 sec
Tooth surface finish Ra	1.6–3.2 µm	0.8–1.6 µm	0.2–0.8 µm

Rotary Broaching for Internal Gears and Splines

Rotary broaching (also called internal broaching or push broaching) is the most productive method for cutting internal tooth profiles in medium-to-high volume production. A broaching tool—a multi-tooth cutting tool with progressively increasing tooth height—is pushed or pulled through a pre-drilled or pre-bored hole, removing material in a single pass. For internal gears and splines, broaching achieves IT7–IT8 precision with excellent repeatability across the production run. The broach tool is custom-manufactured for each gear specification, with tool costs ranging from $500 to $5,000 depending on the number and complexity of teeth. Broaching is most economical for bore diameters of 10–150 mm with gear face widths up to 80 mm. Production rates are very high—10–30 seconds per part—making broaching the preferred process for high-volume automotive internal gears and transmission splines. Broach tool life ranges from 5,000 to 50,000 parts between resharpenings, depending on material and hardness.

Broaching Parameter	Typical Range	Effect on Quality	Optimal Setting
Cutting speed	3–10 m/min	Surface finish degrades above 8 m/min	5–7 m/min for steel gears
Tooth rise per tooth	0.02–0.08 mm	Larger rise increases cutting force	0.03–0.05 mm for HSS broaches
Chip load (av)	0.02–0.06 mm/tooth	Affects tool wear and finish	0.03–0.04 mm for 45# steel
Coolant pressure	10–30 bar	Insufficient coolant causes chip jamming	20–25 bar with water-soluble oil
Broach pull force	50–300 kN	Excessive force causes tensile breakage	Below 80% of broach shank capacity
Stock removal	0.5–3.0 mm (total)	Excessive stock shortens tool life	1.0–2.0 mm total per pass

Internal Gear Grinding

When internal gear precision requirements exceed DIN 7, internal grinding is necessary. Internal gear grinding uses a small-diameter grinding wheel (typically 30–100 mm) mounted on a high-speed spindle (10,000–60,000 RPM) that reciprocates along the tooth flank to correct heat treatment distortion and achieve the final profile and lead accuracy. Two main approaches exist: form grinding with a profiled wheel and generating grinding with a threaded wheel. Form grinding uses a diamond-dressed wheel matching the tooth space profile and produces accurate tooth forms for small-to-medium batch production. Generating grinding uses a worm-type threaded wheel and is more productive for larger batches, achieving similar precision. Internal grinding achieves DIN 4–6 precision with tooth surface finish of Ra 0.2–0.6 µm. The grinding stock is typically 0.10–0.25 mm per flank for case-hardened gears. Grinding burn is a critical concern—coolant flow of 40–80 L/min directed precisely at the grinding zone prevents thermal damage.

Heat Treatment Considerations for Internal Gears

Heat treatment of internal gears presents specific challenges due to the enclosed tooth geometry. Case hardening of internal gears made from 20CrMnTi or 20CrMo achieves surface hardness of HRC 58–62 but causes bore shrinkage of 0.02–0.08 mm per 50 mm diameter, which must be accounted for in the pre-heat-treatment bore dimension.

Heat Treatment Type	Typical Materials	Surface Hardness	Distortion (bore diameter change)	Post-HT Correction Needed
Carburizing + quench	20CrMnTi, 20CrMo, SCM415	HRC 58–62	−0.02 to −0.08 mm per 50 mm	Internal grinding required
Gas nitriding	42CrMo, 40Cr, 38CrMoAl	HV 800–1100	+0.005 to +0.020 mm	Minimal or none
Induction hardening (internal)	45#, 40Cr	HRC 50–55	±0.01 to ±0.03 mm	Light bore honing if needed
Through-hardening	45#, 42CrMo	HRC 28–38	±0.02 to ±0.05 mm	Bore reaming sufficient
Carbonitriding	Low-carbon steels	HRC 58–64	−0.01 to −0.04 mm per 50 mm	Grinding or honing

Distortion is asymmetric: internal gear teeth tend to open up (increase in tooth space width) after carburizing and quenching, requiring compensation in the pre-heat-cut geometry. Nitriding of 42CrMo internal gears produces HV 800–1100 hardness with minimal distortion (typically 0.005–0.020 mm bore diameter change), making it preferred for precision internal gears that cannot tolerate post-heat-treatment grinding. Induction hardening of large ring gears (above 200 mm diameter) can be applied selectively to the tooth flanks, but the coil design for internal tooth spaces is complex and requires specialized inductor geometry.

Small Module Internal Gears: MIM and Powder Metallurgy

For small module internal gears (m0.2–m1.5), metal injection molding (MIM) and powder metallurgy (PM) offer cost-effective net-shape alternatives to cutting. MIM produces internal gears with complex tooth profiles, undercuts, and integrated mounting features in a single sintering operation. Precision of IT8–IT10 is typical, with surface finish Ra 1.6–3.2 µm. Materials include 17-4PH stainless steel, 316L, and Fe-2Ni alloys with sintered density above 96%. PM pressing is suitable for module m0.5–m2.0 internal gears with straight tooth profiles, achieving IT8–IT11. PM gears are often used in oil pump applications and low-load actuator mechanisms. Both MIM and PM eliminate the tool clearance constraints of shaping and broaching, allowing internal gears with very small bores (down to 3 mm), narrow face widths, and high tooth counts (up to 120 teeth). The trade-off is lower mechanical strength compared to wrought steel, making these processes suitable for torque ratings below 50 Nm.

Process Selection Guide for Internal Gears

The optimal manufacturing process for internal gears depends on geometry, precision, batch size, and material. For prototype and low-volume production (1–500 pieces), gear shaping is the most flexible and cost-effective option. For medium volumes (500–5,000 pieces), shaping with optimized tooling or broaching for standard profiles provides good economics. For high-volume production (5,000+ pieces), broaching for bores under 150 mm and shaping with carbide cutters for larger bores offer the lowest per-piece cost. When precision requirements demand DIN 5–6, internal grinding is required regardless of batch size, with pre-grinding by shaping or broaching. brm-metal offers a complete range of internal gear manufacturing capabilities from shaping (up to m12, 600 mm diameter) through broaching (m0.3–m6, 10–150 mm bore) to precision internal grinding (DIN 4–6), supporting materials from 45# steel through case-hardening grades and nitriding steels.

Summary: Manufacturing High-Quality Internal Gears

Internal gear manufacturing requires careful process selection balancing precision requirements, production volume, and cost constraints. Shaping offers flexibility for medium-to-large internal gears across a wide module range. Broaching delivers unmatched productivity for smaller bores in high-volume production. Internal grinding provides the precision needed for demanding applications. Combined with appropriate heat treatment—carburizing, nitriding, or induction hardening—these processes enable internal gears that meet the most stringent quality standards. brm-metal's internal gear manufacturing facility is equipped with modern gear shapers, internal broaching machines, and CNC internal grinders, supporting custom and standard gear production from prototype to mass production. Contact our engineering team with your internal gear specifications for a process recommendation and competitive quotation.