How to draw a spur gear in CAD
A spur gear in CAD is a closed 2D profile, extruded or cut. The blank is a disk. The tooth is four pieces: involute flank, tip arc, the mirrored flank, and the root fillet plus root arc. Pattern that tooth z times about the center.
A reliable construction
- Draw the reference pitch circle, d = m z, or d = z / Pd. Draw the base circle, db = d cos α. Draw the outside and root circles from the addendum and dedendum you actually intend to cut, including profile shift.
- Generate one flank. Evaluate the involute at a few dozen radii from the base circle to the tip and spline the points, or use the CAD system’s involute if it has one and you trust the parameterization. Do not approximate a meshing flank with a single circular arc. An arc is close over a short height and wrong at the ends.
- Mirror the flank about a radial centerline. The half-angle at the pitch circle has to match the tooth thickness you want, including backlash: half the thickness, divided by the pitch radius, in radians.
- Close the tip with an arc of the outside circle. If the flanks meet before that circle, the tooth is pointed. Shorten the blank. Do not let the sketch cross itself.
- Close the root with the fillet radius you will actually hob or grind, tangent to the root diameter and to the flank. A hobbed fillet is a trochoid of the hob tip. A drawing that calls out a circular fillet of that radius is what this tool draws.
- Pattern the tooth about the center z times. Cut it from a blank of the outside diameter, or extrude the tooth space as a cut. Put the bore, key, and set screw on the drawing, not in the tooth generator.
What to dimension
Dimension the things a shop checks: tooth count, module or diametral pitch, pressure angle, profile shift or tooth thickness (span, or over pins), outside diameter, root diameter, face width, bore, and the AGMA or ISO quality grade you are willing to pay for. A quality grade is a tolerance band on the finished tooth. Pick it from the standard you work to. Do not invent one, and do not copy a grade table you do not have a license for.
What the SVG is
The SVG from the designer is a true-scale outline. In module mode one drawing unit is one millimetre. In diametral-pitch mode one unit is one inch. Pitch, base, outside, and root circles are separate groups, and the tooth outline is its own path. Use it as a check plot or a CAM underlay.
It is not a manufacturing drawing. It has no face width, no bore, no hardness, and no tolerance. DXF of the same outline is planned as a Pro download and is not sold here yet. For strength, life, and a tolerance stack, use a calculator that carries those calculations. For a cut gear, use a shop that cuts gears.
If the pinion is small, read the undercut note before you pattern the tooth. A pretty involute that a hob will cut away is not the part you will get.