Designing a spur gear profile no longer requires expensive CAD software. This guide walks you through every parameter — from module and pressure angle to profile shift — and shows you how to export production-ready SVG and DXF files using a free online generator.
Almost every gear made today — from automotive transmissions to watch movements — uses the involute tooth profile. The involute curve is the path traced by the end of a taut string as it unwinds from a cylinder. This geometry has two key properties that make it ideal for power transmission:
The involute profile is mathematically generated from a base circle (radius rb = rpitch × cos α). Everything above the base circle follows the pure involute curve; below it — in the root fillet zone — the profile transitions to a trochoid curve generated by the tip of the mating rack.
Before generating a profile, you need to specify six parameters. Here is what each one controls:
| Parameter | Symbol | Typical range | What it controls |
|---|---|---|---|
| Module | m | 0.5 – 10 mm | Overall tooth size. Pitch diameter = m × z. All meshing gears must share the same module. |
| Number of teeth | z | 5 – 200 | Determines pitch circle diameter and gear ratio. Below ~17 teeth (at α = 20°) undercutting occurs without profile shift. |
| Pressure angle | α | 14.5°, 20°, 25° | Angle of the contact force. 20° is the global standard (ISO 53). Higher angles increase tooth strength but raise bearing loads. |
| Addendum coefficient | ha* | 0.8 – 1.2 | Tooth tip height above the pitch circle, as a multiple of module. Standard value is 1.0. |
| Dedendum coefficient | hf* | 1.0 – 1.4 | Tooth root depth below the pitch circle. Standard is 1.25, providing 0.25 m clearance between tip and root of the mating gear. |
| Profile shift | x | −1.5 – +1.5 | Radial displacement of the rack cutter, in multiples of module. Eliminates undercutting, adjusts tooth thickness, or fine-tunes centre distance. |
The module is the single most important parameter for gear interchangeability. Two gears can only mesh if they share the same module and pressure angle. The pitch circle diameter is simply d = m × z, making it trivial to size a gear train: a 20-tooth and a 40-tooth gear at m = 2 have pitch diameters of 40 mm and 80 mm, with a theoretical centre distance of 60 mm.
The 14.5° standard is a legacy from early American practice and is now largely obsolete. The dominant global standard (ISO 53, DIN 867) uses 20°, which balances tooth strength, undercutting resistance, and bearing loads. Use 25° when maximum tooth strength is required and noise is not a concern — common in heavy machinery. For fine-pitch gears (m < 1) in instruments, 20° remains the standard.
Profile shift (x) moves the generating rack outward (positive x) or inward (negative x) before the involute is cut. This is one of the most powerful — and least understood — tools in gear design.
Undercutting occurs when the rack tip penetrates below the base circle, cutting away part of the involute flank. The minimum tooth count to avoid undercutting at α = 20° with no profile shift is zmin = 17. For z < 17, apply a positive profile shift of at least:
xmin = 1 − z × sin²(α) / 2
For example, a 12-tooth gear at 20° needs x ≥ 0.22 to avoid undercutting.
When a pair of gears has a fixed (non-standard) centre distance, opposite profile shifts (+x on one gear, −x on the other) allow the gears to mesh correctly without altering the tooth count or module.
In a gear pair where the pinion (smaller gear) carries higher load cycles, applying a positive shift to the pinion and a negative shift to the gear balances the bending stress at the roots of both gears.
GearProfile.app generates the exact tooth geometry using the same rack-generation simulation used in professional gear software. Here is how to get from parameters to file:
The live gear preview in the left panel updates as you change parameters, using a canvas-rendered simulation of the full gear with all teeth. This lets you immediately see the effect of profile shift or an unusual pressure angle before committing to an export.
GearProfile.app supports two export formats, each suited to a different workflow:
SVG (Scalable Vector Graphics) is an XML-based format that can be opened in Inkscape, Adobe Illustrator, Affinity Designer, or any browser. The exported SVG includes the tooth profile polygon plus reference circles (pitch, tip, root) as dashed overlays. SVG works well for:
DXF (Drawing eXchange Format) is the format of choice for CAD, CNC, and laser cutting workflows. The exported DXF uses AC1015 (AutoCAD 2000) compatibility, which is supported by virtually every CAD application. With High Quality Export, spline curves are written as true SPLINE entities, preserving the smooth curve data. DXF works well for:
GearProfile.app offers two distinct calculation methods, each producing different output quality:
Raw export computes the gear profile through boolean subtraction: a polygon of the involute tooth is subtracted from the gear blank, and the result is exported as a dense polyline. This method faithfully captures the exact geometry but produces a high point count (typically 2,000 – 8,000 vertices for a full gear). The DXF output uses LWPOLYLINE entities — straightforward and compatible with every CAD package, but consisting of straight line segments approximating the curves.
High Quality export computes each section of the tooth analytically:
The resulting DXF file contains smooth SPLINE and ARC entities — not polylines. When you import this DXF into FreeCAD or SolidWorks and create a 3D extrusion, the resulting solid has smooth, analytically defined faces rather than faceted approximations. This matters for FEA (finite element analysis) and for high-precision CNC paths.
Exporting to SVG and importing as a canvas sketch in Fusion 360 (or as an SVG extrusion in FreeCAD) is the fastest path to a printable gear STL. Recommended parameters: m = 2–3, z = 15–30, α = 20°, x = 0. Keep the hub diameter and keyway in mind — the profile export covers only the tooth geometry; you will need to add the hub in CAD.
For acrylic, plywood, or MDF gears, export to DXF and import directly into your laser software. Tooth thickness at the pitch line = π × m / 2 for a standard gear. For 3 mm acrylic, use m ≥ 2 to avoid brittle tips. LightBurn accepts DXF natively; Inkscape can open both SVG and DXF.
A CNC milling toolpath for a gear profile requires the DXF contour to be offset inward by the cutter radius. Most CAM packages (Fusion 360 CAM, HSMWorks, FreeCAD Path) will apply this offset automatically. Use High Quality Export for CNC work to get smooth SPLINE entities that the CAM solver can interpolate accurately.
In FreeCAD: File → Import DXF → select the exported file → it appears as a Draft drawing. Use Part → Extrude to create a 3D solid. In Fusion 360: Insert → Insert DXF → choose a sketch plane → extrude. In SolidWorks: open the DXF via File → Open, confirm the import wizard, and the sketch is ready to extrude.
Generate and preview involute gear profiles instantly. No installation, no account required. Export SVG and DXF with a Pro plan.
Open Gear Generator →Module (m) is the metric system's tooth size parameter: pitch diameter = m × z. Diametral pitch (DP) is the imperial equivalent: pitch diameter (in inches) = z / DP. The conversion is DP = 25.4 / m. A module 1 gear is roughly equivalent to a 25.4 DP gear. GearProfile.app uses module exclusively.
The current version generates external spur gears only. Internal gear profiles require a different generation algorithm (the rack is inverted) and will be added in a future update.
Two external spur gears mesh correctly when: (1) they share the same module, (2) they share the same pressure angle, and (3) the actual centre distance matches the theoretical centre distance (d₁/2 + d₂/2 for a standard pair, or the corrected value when profile shifts are used). The tip circle of each gear must not overlap the root circle of the other.
The export produces the complete gear outline — all teeth in one closed profile. If you see only one tooth in your CAD software, check that the import units are set correctly (the DXF is in millimetres). In FreeCAD, set the document unit to mm before importing.
The minimum tooth count to avoid undercut: zmin = 2 / sin²(α). At 14.5°: zmin ≈ 32. At 20°: zmin ≈ 17. At 25°: zmin ≈ 11. For tooth counts below these limits, apply a positive profile shift x ≥ 1 − z·sin²(α)/2.
Both SVG (Insert → Canvas, then trace) and DXF (Insert → DXF into Sketch) work in Fusion 360. DXF is cleaner for parametric modelling because it imports as a sketch you can dimension and constrain. Use High Quality Export for smooth SPLINE entities.