Gear inspection is the point where an OEM order moves from "drawing approved" to "parts that actually perform on the line". For purchasing and quality engineers, the real question is not whether a supplier inspects, but how that inspection is planned, measured, recorded and made traceable. A repeatable quality-control system covers incoming material, every critical process step and final acceptance, and it produces documents you can archive against your own inspection record, first-article approval or PPAP package. When inspection expectations are agreed at the quotation stage, there are no surprises at goods-in and no last-minute redesign of the acceptance criteria.
Incoming-material inspection happens before the first chip is cut. The steel grade on the mill certificate — typically C45, 42CrMo4, 20CrMnTi, 16MnCr5, SCM440 or a stainless — is checked against the order, and an EN 10204 3.1 mill certificate is retained for traceability. Where the buyer demands it, optical-emission spectrometry confirms carbon and alloy content on the delivered batch, while Brinell or Rockwell hardness checks and a visual inspection for seams, laps, cracks or decarburization complete the release. Any bar stock that fails is quarantined and returned, never sent to the hobbing line. This step is cheap insurance: most in-gear heat-treat failures can be traced back to a material that was never the grade ordered.
In-process inspection begins with a first-article check right after gear cutting, before heat treatment. We measure over-pin or span between wires, chordal tooth thickness, radial runout, bore diameter and face-runout on the datum surfaces. These dimensions are deliberately held with a small allowance so that stock remains for finish grinding after the predictable distortion of carburizing or quenching. Between articles, tool-wear and machine-drift checks on tooth depth and center thickness keep the hobbing or shaping process inside its control band, and the cutting tools are changed on a defined life rather than on feel.
After heat treatment the gears are re-measured, because distortion is unavoidable and must be controlled, not ignored. Runout, lead and profile are checked again; the grinding stage then corrects the bore, the locating faces and the tooth flanks to drawing. Surface hardness is verified as HRC or HV, and case-hardening depth is confirmed either by a microhardness traverse or by a nital-etched mount — commonly 0.3 to 0.8 millimetres of effective case for through-hardened or case-carburized gears, chosen to match the module and the loading. Gears that move beyond the allowable distortion are flagged before grinding, not after.
The equipment behind these measurements is part of the quality story. A CNC gear measuring center evaluates tooth profile, lead, single- and double-flank composite error, cumulative and single pitch error, and runout against the selected accuracy class. A gear-rolling or single-flank tester checks contact pattern, backlash and transmission noise before assembly. A coordinate measuring machine verifies bore size, perpendicularity, datum flatness and any geometric callout, while a contour and roughness instrument confirms tooth-flank finish. The inspection room is temperature controlled and the instruments are calibrated on a schedule traceable to national standards, because a measurement taken on a hot shop floor is only a guess.
Accuracy grades are where drawing, cost and inspection meet. ISO 1328-1 and the older DIN 3962 define tolerances for profile, lead, pitch and runout. Most commercial, automotive and agricultural gears ship comfortably at ISO grade 7 or 8; precision reducers, machine tools, servo drives and robotics commonly ask for grade 6 down to grade 5. A tighter grade does not just lower the numbers — it raises grinding time, scrap risk and inspection effort, which is why the grade must be agreed on the drawing before production begins. You cannot inspect a grade that was never specified, and a buyer who leaves the class blank usually receives whatever was convenient to manufacture.
Backlash and datum selection deserve their own attention, because they are where a correct gear still fails in assembly. Backlash is controlled through tooth thickness and center distance and is usually specified as a range rather than a single value; too little and the gears bind when warm, too much and the drive hammers on reversal. The inspection plan must also agree which surfaces are the datums — typically the bore and one locating face — so that runout, profile and lead are all measured relative to the same references the customer will use in the gearbox. Measuring tooth geometry against a different datum than the assembly uses is a common source of "good on the report, wrong on the bench" complaints.
Surface condition is inspected as carefully as dimension. Ground tooth flanks are typically finished to Ra 0.8 to 1.6 micrometres, and a smoother flank directly reduces pitting risk and mesh noise. Deburring, edge break and root condition are checked so that stress raisers do not start in the fillet, and high-precision ground gears are inspected by nital etch for grinding burns and tempering that would shorten rolling-contact life. Hardness, case depth and surface finish together determine whether a gear will survive its rated torque, and they are all recorded as measured values, not as a generic "passed".
The inspection plan itself balances 100% inspection against sampling. First-article and final dimensional checks are standard on every order, and safety-critical or high-load gears receive 100% inspection on gear geometry and runout. For standard parts, attribute and dimensional sampling follows ISO 2859-1, commonly general inspection level II with an agreed AQL, on appearance and non-critical dimensions. The plan — what is 100%, what is sampled, and to which acceptance quality limit — is set during the RFQ so that neither side re-opens it at delivery.
The value of inspection is delivered as documentation. A typical shipment is accompanied by the mill material certificate, the heat-treatment and hardness report, and a dimensional or gear inspection report listing the actual measured values for each controlled characteristic rather than a simple pass stamp. For new tooling or regulated customers we can extend this into a full PPAP package: part-submission warrant, first-article inspection, control plan and process-flow documentation. Each finished gear carries a lot number that links it back to its heat batch and mill lot, so a single suspect part can be traced forward and backward through the production history.
Inspection should be agreed at quotation, not discovered at goods-in. Send us your drawing, the required accuracy grade, hardness and case depth, the backlash range, the datums to inspect against, and the reports your quality team archives, and we will quote the inspection plan, sampling level and documentation as a defined part of the price. That brief list avoids the most common disputes: an unspecified accuracy class, a missing hardness requirement, a requested full report that was never priced, or a material certificate the buyer assumed came free. Our quality system and the inspection reports behind it are described in detail on our quality pages, and our engineers are glad to review an existing inspection plan and tell you honestly which tests add value and which add cost without benefit.