Two letters on a hob drawing decide more of your finished gear quality than almost anything else on the purchase order, and most enquiries get those two letters wrong in one of two directions.
In one direction, a buyer asks for Class AA on a gear that will be profile ground afterwards, and pays a premium for flank accuracy that the grinding wheel removes anyway. In the other, a buyer asks for Class B on a gear that is hobbed, hardened and shipped with nothing in between, then cannot work out why the profile chart fails at final inspection.
This assumes hobbing is already your chosen process but if you’re still deciding between hobbing or shaping for this part, that decision should come before any of the below.
What the class actually controls
A hob class is not one number. It is a pass or fail against a table of separately toleranced features, and the tool only carries the class if every one of them sits inside the limit. A single feature outside tolerance drops the whole hob a grade, which is why two hobs marked with the same letter can still behave differently in the cut.
The toleranced features fall into three groups.
The mounting references
Bore diameter and bore form, keyway, radial runout of the hubs, axial runout of the hub faces. Nobody quotes these in a sales conversation, and they matter enormously. They are the surfaces the tool was manufactured on, the surfaces it is trued in on every time it goes into the hobber or the sharpener, and the datum that every other measurement is taken from.
If you’re new to the process, our recent article “how a gear hob is manufactured” walks through why these reference surfaces are machined first and held to such tight tolerance before any cutting edge is ground.
The cutting edge positions
Spacing between adjacent gashes, cumulative gash spacing around the tool, gash lead over the hob length, form and position of the cutting faces, hob lead between cutting edges on the same thread, individual and cumulative base pitch, radial runout on the tooth tips, tooth thickness at the pitch cylinder, and deviation of the flank from the design profile. This group is what generates the involute, so it drives what the gear chart looks like.
For a quick reference on matching cutter number to tooth count, see our involute cutter number chart.
The overall geometry
Diameter over the hob length, outside diameter, active length of the toothed portion.
Two structural details are worth knowing before you read any tolerance table.
First, the numbers are banded by module. A tolerance quoted for module 2 is not the tolerance for module 12, and the limits widen substantially as module increases. Any hob tolerance presented as a single figure per class, with no module band attached, is a simplification rather than a specification.
Second, under DIN 3968 the tolerance on the straight flank is one sided and negative. The flank is allowed to fall below the design line but not to sit above it. A hob slightly under profile cuts a slightly thick tooth, which downstream operations can correct. A hob over profile has already taken metal that cannot be put back.
Three class systems, and why quotations disagree
Here is where buyers get caught, and it costs real money. The letter grades are not portable between standards.
| Standard | Status | Grades, highest accuracy first |
| DIN 3968 (Germany, 1960) | Still catalogued and still the common commercial language | AA, A, B, C, D |
| ISO 4468:2020 | Current edition, reviewed and confirmed in 2026 | 4A, 3A, 2A, A, B, C, D |
| ISO 4468:1982 | Withdrawn | AA, A, B, C |
| ANSI/AGMA 1102-C19 (USA) | Current, reaffirmed January 2025 | Trade usage runs AAA, AA, A, B |
| IS 8733:1978 (India) | Adoption of DIN 3968:1960 | AA, A, B, C, D |
| JIS B 4355 (Japan) | Current | Equivalent grade structure |
Read the ISO rows again, because that is the trap. When ISO 4468 was revised, the old AA, A, B, C ladder was scrapped and replaced by 4A, 3A, 2A, A, B, C, D, with 4A at the top.
The consequence: Class A sits one step below the top under DIN 3968, and three steps below the top under the current ISO standard. A drawing note reading only “hob accuracy Class A” is therefore not a specification. It is an ambiguity, and the two readings are separated by several tolerance grades.
Always write the standard next to the letter: Class A to DIN 3968, or Grade 2A to ISO 4468:2020. If you are buying across borders, settle the standard in writing before the tool is cut, not after it is inspected.
The AAA problem
There is no AAA class in DIN 3968. There never has been.
Manufacturers who offer AAA are working to their own tightened fraction of the Class AA table. Published practice sits somewhere between 67 and 75 percent of the AA values, and which fraction gets used is the maker’s decision, not the standard’s. Samputensili has published 67 percent. Others work to 75 percent.
So AAA from two suppliers is not the same tool, and if a dispute arises there is no normative table to arbitrate it. If you genuinely need better than AA, stop relying on the third letter. Name the micron limits on the features that matter for your gear, which in most cases means flank profile, gash lead, adjacent and cumulative gash spacing, and tooth tip runout, and require the measured values on the inspection report rather than a pass mark.
What each class buys you in finished gear quality
Tool manufacturer guidance puts the working expectation roughly as follows, on the understanding that a rigid machine, a true arbor and a careful setup can lift the result by about one grade.
| Hob class (DIN 3968) | Typical achievable gear quality | Rough AGMA equivalent |
| Class AA | DIN 3962 grade 8, and grade 7 or better on a modern CNC hobber with a two cut cycle | AGMA 8 |
| Class A | DIN 3962 grade 9 | AGMA 7 |
| Class B | DIN 3962 grade 10 | AGMA 6 |
Treat those as indicative, not contractual. The published sources do not fully agree, and the disagreement is instructive. Samputensili guidance maps Class AA to DIN grade 8. Gear Technology India recommends Class AA specifically for finish hobbing of parts calling for DIN grade 7 or 6. Both positions are defensible, because the difference is not in the tool. It is in the hobbing machine, the arbor, the fixture, the blank, the number of starts, and whether the cycle roughs and finishes or cuts once.
Two further points come from the shop floor rather than the catalogue.
The grades do not move together. On a good NC machine, helix and pitch on the finished gear often land a grade better than profile, because profile inherits hob rake and sharpening error more directly than the other elements do.
And the tool stops being the limiting factor somewhere around grade 7. Practitioners report that holding DIN grade 7 consistently in production is hard even with the best available tool class on a modern machine. If the drawing calls for grade 6, budget for grinding or honing. No hob class delivers it on its own.
Choose the class by what happens after hobbing
This is the decision rule that saves the most money and prevents the most rejections. Work backwards from the finishing route, not forwards from the tool catalogue.
Route 1: hobbed, hardened, shipped
No finishing operation, so the hob error is the gear error. Nothing downstream corrects anything. Buy Class AA if the drawing calls for DIN grade 8 or better. A lot of agricultural, general industrial and low speed transmission work falls here, going straight from the hobber to heat treatment.
Route 2: pre-grind hobbing
The gear will be profile ground, typically with around 0.10 mm of stock. Grinding removes the flank profile error, so paying an AA premium for profile accuracy you are about to grind away is money spent twice. Class A is normally sufficient.
Do not read that as accuracy being irrelevant here. Gash spacing, tooth tip runout and lead still matter, because they drive stock variation around the gear, and uneven stock is what makes the grinding cycle long and the wheel wear badly.
Route 3: pre-shave hobbing
Use Class AA. This is the case most often specified wrongly, usually on cost grounds. Shaving is a light finishing operation and it does not fully remove errors carried over from the roughing pass, so a proportion of whatever the hob puts in will still be there on the finished gear. Pre-shave hobs belong in Class AA.
Number of starts changes the answer
Multi start hobs raise output substantially, and each extra start adds another lead and pitch error path into the generated gear. Single start gives the better accuracy and the better finish, which is why it stays the default on tight quality work.
DIN 3968 is written for single start hobs. Multi start tools are generally manufactured against the same tolerance tables, but the achievable gear quality usually drops by about a grade, so a multi start Class AA hob does not deliver what a single start Class AA hob delivers. If the drawing calls for a tight grade and the process plan calls for multiple starts, one of the two has to move.
Check divisibility as well. The number of starts against the workpiece tooth count decides which teeth get cut by which start, and that lands directly in the pitch variation on the finished gear.
Class C and D are effectively obsolete
Class C and D hobs carry unground finished profiles. Their geometric accuracy is poor, and there is a metallurgical problem sitting behind the geometric one: with no grinding pass after heat treatment, decarburisation is left in place at the surface, so hardness and wear resistance at the cutting edge suffer. Coating an unground profile does not repair that.
The economics have moved as well. When profile grinding was a manual operation, the price gap between AA, A and B was significant. Today, when standard manufacture runs on CNC relief grinders working to AA geometry as a matter of course, that gap has largely closed. The remaining premium for Class AA over Class A is reported at around 5 percent, and what it buys is greater care on bore, face and collar grinding plus stricter inspection of every manufacturing parameter.
The commercial implication is blunt: dropping from AA to A to save cost is not the saving it used to be. Specify the class the gear needs and spend the argument somewhere it matters.
The class you bought is not the class you are cutting with
A hob accuracy class describes a shipped condition. It is a claim about the tool on the day it left inspection, and your shop can undo it in an afternoon.
Mounting is the big one. Class is a property of the tool and the spindle it turns on, taken together. Put an AA hob on a worn or bent arbor and the gear cannot tell the difference between that and a far cheaper tool. Arbors that were once high precision lose it through ordinary wear, so they need their own inspection interval. Where spacers are used, their faces have to be parallel within about 2.5 microns, and on threaded nut arbors the clamping face of the nut has to be perpendicular to the threads to the same order.
The check itself takes two minutes. Mount hand tight and indicate for runout. If it does not run true, look at the centres for chips before blaming the tool, then indicate the arbor along its full length in case it is bent. Tighten and re-check. If spacers are in the stack and runout persists, rotate the spacer and check again, and if it survives two or three attempts, check the spacer for parallelism.
Hub diameters and hub faces deserve the same protection. They are the reference surfaces used to make the tool and to true it in every time it is mounted, so a nick on a hub face is damage to the datum, not cosmetic damage. Hob data should be etched or laser marked for the same reason. Engraving raises material around the mark, and raised material stops a tool running true.
Resharpening is the third route by which class is lost, and it is covered in detail in our guide to resharpening the hob. The point that belongs here is narrower: rake error changes the pressure angle the tool generates, so a hob returned from a regrind at the wrong rake is no longer working to the class printed on its face. Ask your grinding service for a documented inspection against the original class after every regrind, and keep the original hob drawing on file so there is something to inspect against.
What to put on your enquiry
Most hob enquiries arrive missing half of the following, which is why quotations come back with assumptions built into them and why comparing two quotes is often comparing two different tools. Send this and you will get a like for like answer from any serious manufacturer.
- Module or diametral pitch, and pressure angle
- Gear tooth count, helix angle and hand, face width
Haven’t nailed these numbers down yet? Our spur gear calculator can generate them from your center distance and ratio
- Gear material and hardness at the time of cutting
- Required gear quality grade and the standard it refers to, for example DIN 3962 grade 8 or ISO 1328 grade 8
- Downstream operation: none, shave, grind or hone
- Hob accuracy class and the standard it is quoted against, for example Class A to DIN 3968 or Grade 2A to ISO 4468:2020
- Bore and keyway, hub dimensions, and the maximum outside diameter and length your machine will take
- Number of starts and hand of cut
- Substrate: HSS M2, M35, M42, or powder metallurgy HSS such as ASP 2030
- Coating: TiN, TiCN, TiAlN, AlCrN, or uncoated
- Profile features: topping, semi topping, protuberance, tip relief. Note that DIN 3968 does not cover tolerances for protuberance or semi topping profiles, so those have to be agreed separately with the manufacturer
- Inspection documentation required, with measured values on the features you care about rather than a pass mark
Where Maxwell sits
Maxwell Tools Company has manufactured gear cutting tools in Rajpura, India since 1976 and exports to more than 50 countries. Our gear hobs are supplied in DIN 3968 Class AA, A and B, in modules 0.5 to 25, up to 250 mm diameter, in HSS M2, M35, M42 and powder metallurgy HSS ASP 2030, with TiN, TiCN, TiAlN and AlCrN coating options.
Every hob is vacuum heat treated for dimensional stability, bore ground on a Doimak grinder so the mounting datum runs true, and dimensionally verified on an Accurate Spectra coordinate measuring machine before dispatch, with inspection documentation on every order.
Send your gear drawing or a sample. Our engineers will recommend the class that matches your quality grade and your finishing route, which is not always the class that produces the larger invoice.
Frequently asked questions
Publish these on the page and mirror them in the FAQPage schema at the end of this document.
1. What is a gear hob accuracy class?
A gear hob accuracy class is a graded set of manufacturing tolerances covering the bore, hub runout, gash spacing, gash lead, cutting face position, tooth tip runout, flank profile and lead. A hob carries a class only if every toleranced feature meets it, so the weakest feature sets the grade.
2. What are the DIN 3968 hob classes?
DIN 3968 defines five grades: AA for high precision ground hobs, A for precision ground, B for commercial ground, C for precision unground and D for commercial unground. AA is the most accurate. C and D carry unground profiles and are rarely supplied today.
3. Is Class A under ISO 4468 the same as Class A under DIN 3968?
No. ISO 4468:2020 grades hobs 4A, 3A, 2A, A, B, C and D, with 4A the highest precision, so Class A sits three steps below the top. Under DIN 3968, Class A sits one step below the top grade AA. Always state the standard alongside the letter when specifying a hob.
4. What is a AAA hob?
AAA is not defined in DIN 3968. It is a manufacturer designation for a hob held tighter than Class AA, typically to between 67 and 75 percent of the AA tolerance values depending on the maker. Because no normative table exists, AAA from two suppliers is not necessarily the same tool, so it is better to specify micron limits on the features that matter.
5. What gear quality can a Class AA hob produce?
As a working expectation, a Class AA hob produces around DIN 3962 grade 8, and grade 7 or better on a modern CNC hobbing machine with a true arbor and a careful setup. Class A is generally associated with grade 9 and Class B with grade 10. Machine condition, arbor accuracy, fixture rigidity and the number of starts influence the result as much as the tool class does.
6. Which hob class should I use for pre-grind hobbing?
Class A is normally sufficient when the gear will be profile ground afterwards, because grinding removes the flank profile error. Gash spacing, lead and tooth tip runout still matter, since they cause uneven grinding stock around the gear.
7. Which hob class should I use for pre-shave hobbing?
Class AA. Shaving does not fully remove errors carried over from the roughing pass, so a pre-shave hob has to be accurate in its own right.
8. Does a multi start hob give the same accuracy as a single start hob?
No. Multi start hobs raise productivity but add extra lead and pitch error paths, and the achievable gear quality typically drops by around one grade. DIN 3968 is written for single start hobs.
9. Can mounting affect the accuracy class of my hob?
Yes, decisively. A Class AA hob mounted on a worn or bent arbor behaves like a much lower class tool. Spacer faces should be parallel within about 2.5 microns, hubs and hub faces must be free of nicks, and the mounted hob should be indicated for runout before cutting.