# Gear hobbing

Generating external gear teeth with a synchronized hob: access, tool selection, finishing allowances, and inspection.

Reviewed: 2026-11-01

Tags: processes, shaping, gears, machining

In the worked spur-gear setup, a **two-start hob at 600 rev/min** generates a **40-tooth gear at 30 rev/min**. At **1.2 mm per workpiece revolution**, axial feed is **36 mm/min**. A **24 mm face + 8 mm approach + 4 mm overrun** takes **one minute** at constant feed. Add loading, extra passes and other cycle activities separately.

NIDEC describes hobbing as synchronized rack-and-pinion generation with feed across the face. Verify rigidity, tool access and strength for external spur or helical work. [NIDEC, Hobs](https://www.nidec.com/en/products/machine-tools/precision-cutting-tools/hobs/NMTJ-hob/)

## Process choice and access

Continuous rotary cutting makes hobbing attractive for repeated external gear production, but cutter access is a design constraint. Include the hob's approach, exit, diameter, and mounting envelope when assessing a gear beside a shoulder or another gear. An apparently accessible tooth face does not establish that the complete cutter can traverse it.

YANMAR's technical review, dated January 27, 2016, contrasts hobbing with reciprocating pinion-cutter shaping. It identifies internal teeth and nearby obstructions as reasons to choose shaping, illustrating a stepped gear that cannot be hobbed with the shown arrangement. Treat that example as a geometry-dependent comparison, not a claim that every stepped shaft needs shaping. [YANMAR technical review, section 3.1](https://www.yanmar.com/global/about/technology/technical_review/2016/0127_4.html)

| Route | Distinguishing motion and selection question |
| --- | --- |
| Hobbing | Rotating hob and blank generate teeth continuously; can the hob and holder clear the complete stroke? |
| Gear shaping | Reciprocating pinion cutter; is its stroke and return clearance available near the obstruction? |
| Power skiving | Synchronized cutter and workpiece with crossed axes; is the machine/tool combination qualified for this internal or external geometry? |

FHUSA-TSA explicitly distinguishes power skiving from shaping and from hard hobbing, which is also sometimes called “skiving.” Specify the actual process in a quotation or route rather than relying on that ambiguous word. Its power-skiving page describes crossed-axis kinematics and internal/external applications; these do not make a conventional hob an internal-gear cutter. [FHUSA-TSA, Skiving Tools, undated](https://www.fhusa-tsa.com/en/skiving/)

## Define the gear and the hob together

A usable enquiry should include tooth count, normal or transverse module as applicable, pressure-angle convention, helix angle and hand, profile modification, face width, root requirements, blank dimensions, and gear datum surfaces. “Module 2 hob” alone is insufficient: normal and transverse quantities must be interpreted consistently, and the required root or finishing allowance can change the tool form.

NIDEC's tool range distinguishes module, diametral-pitch, and circular-pitch hobs; topping/non-topping, roughing/finishing, pre-shaving/pre-grinding, and protuberance variants; and separate tools for worm wheels, sprockets, and other toothed profiles. These are choices to resolve with the tool designer, not interchangeable labels. A pre-grinding hob must support the specified subsequent operation, including its root transition and stock distribution. [NIDEC-IPT, Gear Hobs, undated](https://www.nidec.com/en/nidec-machinetool/corporate/network/factory/nidec-machinetool-india-fa/products/gear-hobs/)

Also record hob starts, hand, diameter, usable cutting length, mounting bore or shank, cutting material/coating, and resharpening condition. More starts can raise productivity, but NIDEC cautions that they can reduce the available blades and introduce polygonal error. Improper resharpening can impair tool precision. Neither a start-count increase nor a freshly sharpened cutter automatically preserves gear quality. [NIDEC, generating and sharpening guidance](https://www.nidec.com/en/products/machine-tools/precision-cutting-tools/hobs/NMTJ-hob/)

## Establish the manufacturing route

State material grade and incoming hardness/condition before selecting cutting conditions. Identify whether hobbing produces the final flank or leaves stock for shaving, grinding, or another qualified finishing operation. Define that allowance on the relevant flank geometry; do not substitute a generic radial allowance for a specified normal flank allowance.

Heat treatment can change gear geometry after the soft-cut inspection. YANMAR describes post-heat-treatment grinding or honing as a route for addressing demanding gear accuracy and noise requirements, with chamfering/deburring and sometimes shaving preceding heat treatment. Its account concerns automotive-oriented production and does not promise that every hobbed gear requires or benefits from the same sequence. [YANMAR technical review, section 2 and process diagram](https://www.yanmar.com/global/about/technology/technical_review/2016/0127_4.html)

For process planning, document workholding datums, blank runout checks, cutting direction, lubrication or approved dry-cut arrangement, chip evacuation, burr control, and tool-life limits. Cutting speed, axial feed, passes, and coolant practice require the selected material/tool/machine combination; a coating supplier's demonstration is not a universal recipe.

Hob shifting distributes use along the cutter, but physical tooth length is not all available for shifting. NIDEC's one-page *Effective hob shift length* note relates usable shift to the engaged tooth length and end allowances. Its worked helical-gear example has 130 mm of tooth length but only 87 mm of effective shift. That result depends on its stated geometry and cutting arrangement. The retrieved note has no explicit issue date, so no revision year is inferred from its web address. [NIDEC Cutting Tool News](https://www.nidec.com/files/user/www-nidec-com/machine-tool/knowledge/column/cutting-tool/hob-02/en_hob-02.pdf)

## Worked example: synchronize rotation before estimating time

This original planning example assumes a **spur gear**, 40 teeth, a two-start hob, hob speed 600 rev/min, and axial feed **1.2 mm per workpiece revolution**. These are hypothetical inputs, not recommended cutting settings. For this simple generating relationship, each hob revolution advances two tooth pitches:

- Workpiece speed = 600 × 2 / 40 = **30 rev/min**.
- Axial travel rate = 1.2 × 30 = **36 mm/min**.
- Assume a 24 mm face plus separately established 8 mm approach and 4 mm overrun: total travel = **36 mm**.
- Constant-feed traverse time = 36 / 36 = **1 minute**.

Multiplying feed by hob speed would incorrectly give 720 mm/min because it uses the wrong revolution basis. The one-minute result excludes loading, acceleration, radial positioning, extra passes, shifting, and inspection. Approach and overrun here are assumed values, not formulas for cutter clearance. Helical-gear synchronization additionally depends on helix and feed kinematics; do not transfer this spur-gear calculation unchanged. The example illustrates the synchronized generating principle described by NIDEC, not the machine controller's complete cycle calculation. [NIDEC, Processing by hobbing](https://www.nidec.com/en/products/machine-tools/precision-cutting-tools/hobs/NMTJ-hob/)

## Inspection and release

Specify the acceptance standard and edition, measured characteristics, datum setup, and inspection stage. ISO 1328-1:2013, edition 2, September 2013, establishes flank tolerance classification for individual cylindrical involute gears; ISO records its confirmation in 2024. Its scope does not replace drawing requirements for material, hardness, size, burrs, or the assembled transmission. Do not quote a quality class without its standard and applicable deviations. [ISO 1328-1 official scope and edition record](https://www.iso.org/standard/45309.html)

Check profile, lead/helix, and pitch with suitable gear metrology, alongside specified tooth thickness and datum-related checks. Distinguish analytical measurements from a composite rolling result: a single rolling number cannot identify every individual flank deviation. As a bounded equipment example, Gleason's GRSL combines optical pitch, lead, and profile inspection with double-flank rolling tests; its listed maximum workpiece diameter is 250 mm and maximum module 7.2 mm. Those product limits do not establish accuracy for an arbitrary part or fixturing arrangement. [Gleason GRSL, undated product specification](https://www.gleason.com/en/products/metrology/metrology-systems/laser-and-hybrid-metrology/grsl-laser-inspection-with-double-flank-rolling)

A practical handoff includes the controlled gear definition, blank/material state, hob identification and sharpening record, verified clearance envelope, finishing allowances, heat-treatment sequence, and inspection results at the agreed stages. Release against those requirements rather than the fact that teeth were successfully cut.

## Related reference

[Manufacturing processes](../index.md)