A gear drawing looks like an ordinary engineering drawing until you look closely — it carries a set of parameters that exist only for gears, and getting any one of them wrong means the gear will not mesh, will be noisy, or will fail prematurely. Whether you are sourcing gears for the first time or reviewing a supplier's quote, being able to read the drawing yourself is the fastest way to catch errors before they become expensive parts.
The first block of gear-specific data is usually in the title block or a gear data table. It lists the module (or diametral pitch), number of teeth, pressure angle, helix angle (for helical gears), face width and material. Module is the ratio of pitch diameter to number of teeth, expressed in millimetres — a module 3 gear has a pitch diameter of 3 mm per tooth. Pressure angle is almost always 20° for modern gears, though 14.5° appears on older designs, and the two are not interchangeable: a 20° pinion will not run quietly with a 14.5° gear. If any of these values is missing or ambiguous, ask before cutting metal — they are not optional.
The second block covers dimensions and tolerances. The pitch diameter is the reference circle on which all tooth geometry is based, and it is usually given as a theoretical dimension rather than a machined one. The outside diameter (tip diameter) and root diameter are machined dimensions with tolerances. Bore diameter, keyway width and depth, and hub dimensions all carry standard tolerances unless otherwise specified. The most important tolerance on a gear drawing is often the tooth tolerance grade — AGMA Q8 to Q12, or DIN 4 to 9 — which controls profile error, pitch error and runout, and directly determines how quietly and smoothly the gear will run. A higher grade means more finishing operations (grinding, skiving, honing) and a higher price, so it is worth confirming that the specified grade matches the actual application.
The third block covers surface finish, heat treatment and Backlash — the amount of play between mating teeth — is usually specified as a range on the centre distance or as a direct backlash value, and it is critical: too little and the gear will bind when it heats up; too much and it will be noisy and imprecise. Surface finish on the tooth flank is typically Ra 1.6 to Ra 3.2 for ground gears and Ra 3.2 to Ra 6.3 for hobbed gears. Heat treatment is stated as a hardness range — for example, "carburised and hardened, 58–62 HRC on tooth flank, core 250–300 HB" — and it tells the manufacturer exactly what furnace operations are needed.
The fourth block is the notes and general tolerances. This is where the drawing specifies which standard applies (ISO, AGMA, DIN, JIS), whether the gear must be balanced, whether magnetic particle inspection is required, and any special requirements such as deburring, phosphate coating or anti-rust treatment. It is also where missing information often hides — if the drawing says "all dimensions in mm" but does not specify a tolerance standard, the manufacturer will apply its own default, which may not match what the designer intended. Reading the notes carefully is often the difference between a gear that fits and one that does not.
At our factory we review every drawing before quoting, and we will flag missing or ambiguous parameters before we cut metal rather than after. Send us your gear drawing in PDF, DWG or STEP format, and we will return a clear quote with the manufacturing route, material, heat treatment and tolerance class — plus a list of any parameters that need confirmation before production begins.