When an OEM engineer opens a gear drawing, the accuracy grade is the single callout that most often decides whether the parts perform on the line, fit the budget, and arrive on the quoted lead time. It sets how tightly pitch, profile, lead and runout are held; it determines which cutting and finishing processes the shop can use; and it directly drives inspection effort, scrap risk and cost. Buyers who leave the grade blank, or who copy the grade from a similar drawing without checking it against the actual speed, noise and load of their gearbox, usually discover the mismatch at the worst possible moment: during first-article approval, or after the gears have already been built into a transmission that hums, whines or hammers on reversal. This guide walks through what grades IT4 through IT8 actually mean, how they line up across ISO, DIN and AGMA systems, where each grade is appropriate, and how to write the grade into an RFQ so that the supplier quotes the process you really need.
Three grade families dominate commercial gear drawings, and they line up closely enough that a buyer can read one and translate it. Under ISO 1328-1, accuracy is expressed as a numerical grade from 0 (finest) to 12 (coarsest); grades 4 through 8 cover almost every commercial transmission you will ever buy. The older German DIN 3962 uses a nearly identical scale, so a DIN grade 4 gear is effectively an ISO grade 4 gear, and DIN grade 9 matches ISO grade 8. The American AGMA 2000 system numbers quality in the opposite direction: a higher AGMA quality number is tighter, and Q8 through Q12 sit across the same window as ISO grades 8 down to 4. In practice this means an AGMA Q10 gear is roughly an ISO grade 6 gear, an AGMA Q9 is close to ISO grade 7, and AGMA Q12 is in the neighborhood of ISO grade 4. Most international shops now read ISO grades natively, and a drawing marked "ISO 1328 grade 6" is understood the same way in Luoyang, Stuttgart and Detroit. The differences between the three systems live mostly in the detailed tolerance tables and in how individual deviations are grouped; for a purchasing decision, the grade number is the common language.
Within the IT4-to-IT8 window, each grade has a natural home. Grades 4 and 5 are the precision end of the market: high-speed turbine and compressor gears, servo and robotics reducers, machine-tool spindle drives, instrumentation and any transmission where noise, positional accuracy or dynamic balance cannot tolerate tooth-to-tooth error. These gears are almost always finish-ground after heat treatment, and they are inspected on a CNC gear-measuring center with full profile, lead and pitch reports. Grades 6 and 7 are the workhorse classes of industrial gearing. Helical and spur gears for conveyors, elevators, general reducer boxes, small agricultural and construction drivetrains, and most automotive accessory drives ship comfortably at grade 6 or 7. They may be hobbed, shaved and then run through a finish grind or honing pass, or produced by a high-quality hobbing line with controlled heat-treatment distortion. Grade 8 is the economic baseline for slow, heavily loaded, low-noise-critical service: open gearing on mills, kilns, large crushers, agitator drives and construction equipment where torque matters more than smoothness. These gears are usually hobbed and, when case-hardened, ground only where the mating fit demands it.
The grade you specify is, in effect, the process you buy. A hobbed-only gear, even with a carefully chosen tool and a stable heat-treat cycle, rarely lands better than grade 8 on a controlled measurement report; hobbing sets the tooth form, but it cannot correct the distortion that follows carburizing or through-hardening. Hob plus shave takes the tooth from a cut grade 8 to a shaved grade 7, and in very stable shops a shaved-and-honed grade 6 is achievable on through-hardened parts. Once you cross into grades 5 and 4, finish grinding of the tooth flanks is almost mandatory: grinding removes the heat-treat distortion and brings profile, lead and runout back to the drawing, but it adds a setup, a wheel-dressing cycle, machine time and a second inspection. Roughly speaking, moving from grade 8 to grade 6 on the same geometry and material adds a shaving or light-honing step and may extend lead time by a week; moving from grade 6 to grade 4 adds tooth grinding, pushes cost up by a further 30 to 60 percent, and can extend the cycle by two to four weeks once grinding-fixture design and first-article reports are included. Scrap rate also rises sharply as the number drops, because a single heat-treat warp beyond the grinding allowance turns a good gear blank into a scrapped part. Batch size compounds the effect: on a 50-piece order the fixed cost of setting up the grinding fixture and programming the measuring center is spread thinly, but on a ten-piece prototype run it dominates the unit price, and many buyers are surprised to learn that a small-batch grade 4 gear can cost more per piece than a large-batch grade 8 gear. Specifying a grade tighter than the application needs is one of the most expensive habits in gear buying, and it is rarely recoverable at the negotiation stage because the process choice is already baked into the quote.
Choosing between grade 6 and grade 8 is the most common buyer decision, and it is driven by three physical inputs: pitch-line speed, noise expectation and load spectrum. If the pinion runs above roughly 10 metres per second at the pitch circle, or if the gearbox sits in a space where transmitted noise matters, such as an office environment, a residential ventilation unit or a passenger vehicle accessory drive, specify grade 6, because pitch error and tooth-to-tooth transmission error are what generate high-frequency whine. If the gear runs slowly, say below 3 metres per second, and the application is not noise-sensitive, grade 8 is almost always sufficient and a grade 6 will not buy you anything perceptible. Load spectrum is the third lever: a gear that carries steady torque in a predictable duty cycle tolerates more profile error, while a gear that starts and stops repeatedly, reverses under load, or sits in a fatigue-critical drivetrain benefits from the better contact pattern that a tighter grade produces. When in doubt, ask the supplier to compare a grade 7 option against the drawing grade; on many drivetrains, a grade 7 is the sweet spot that keeps noise acceptable without pushing the part into the grinding cycle. The mistake to avoid is specifying grade 6 "just to be safe" on a slow, heavy-duty pinion that will never leave the floor of a factory. A second, related mistake is the reverse: dropping to grade 8 on a high-speed reducer because the price looks attractive, and then discovering that the transmission whines in its installed location because pitch error was never held tightly enough. The decision is not a preference; it follows the speed, the noise target and the duty cycle, and it should be written into the RFQ with those numbers attached rather than left as a blanket quality claim.
Writing the grade correctly on the RFQ prevents the supplier from guessing. The cleanest callout names the system, the number and the scope, for example "ISO 1328-1 grade 6, with profile and lead within tolerance on both flanks, single-pitch error controlled, runout measured on the bore datum." Pair the grade with the required tooth-flank finish, usually Ra 0.8 to 1.6 micrometres for a ground gear and Ra 1.6 to 3.2 for a shaved or hobbed finish, because the buyer who asks for grade 6 but leaves roughness unspecified will receive whatever flank the grinding cycle happened to leave. State whether the gear will be run as a matched set, and if so, whether the pinion and wheel are measured together on a single-flank or double-flank tester; matched sets benefit from selective assembly, which the supplier can plan only if the RFQ asks for it. Finally, name the inspection you expect: an actual measured-values report on a gear-measuring center, or a simpler attribute check. The more specific the grade, the flank finish and the inspection are on the RFQ, the closer the quotation comes to the part you actually want.
The accuracy grade is where engineering intent, manufacturing process and purchasing budget meet on a single line of the drawing. Read it in the system your supplier uses, match it to the speed, noise and load of the application, write it down with flank finish and inspection requirements, and you will avoid the two most common gear-buying failures: an underspecified part that whines in service, and an over-specified part that costs twice what it needed to. If you are unsure which grade fits your next drivetrain, send us the drawing or a sketch with the speed, torque and noise target; we will map the options against the real process steps and quote the grade you actually need, not the grade that looked safest on the page.