Every feature on a part drawing should have a tolerance. This is one of the fundamental rules of ASME Y14.5, which states that each dimension shall have a tolerance, except for dimensions specifically identified as reference, maximum, minimum, or stock.
This article explains what the rule requires, how a feature gets its tolerance, and why complete tolerancing matters when a part goes through first article inspection (FAI) under AS9102.
What the rule requires
A tolerance defines the limits within which a feature is acceptable. Requiring a tolerance on every feature means every feature on the drawing has a defined answer to the question: what makes this feature acceptable or rejectable?
The rule does not require that every tolerance be tight. A feature with no functional role can carry a wide tolerance. Oftentimes, a non-critical feature may have a looser tolerance to reduce machining and inspection costs.
Additionally, not every feature needs to be measured on every part. How often a feature is inspected, and with what method, is decided by manufacturing and quality. The drawing only has to define the acceptance limits so that, when the feature is inspected, there is a requirement to compare the result against.
How a feature gets its tolerance
A tolerance does not have to be written next to every dimension. A drawing can assign tolerances in several ways, and most drawings use more than one.
| Method | How it appears on the drawing | Example |
|---|---|---|
| Direct tolerance | Plus/minus or limit values written with the dimension | 1.250 ±0.005 |
| General (title block) tolerance | A table in the title block that applies to any dimension without a direct tolerance, usually by number of decimal places | .XX ±0.01, .XXX ±0.005, angles ±0.5° |
| Geometric tolerance | A feature control frame, with the feature located or defined by basic dimensions | Position Ø0.010 at MMC to datums A, B, C |
| General note | A note that applies a requirement to all features of a type, or to features defined only by the CAD model | "Unless otherwise specified, all surfaces within profile 0.030 to datums A, B, C" |
ASME Y14.5 exempts a small set of dimensions from the rule. For example, a reference dimension, shown in parentheses, is for information only and is not inspected; a dimension marked MIN or MAX is limited in one direction only; and a stock dimension applies to commercial stock material, whose tolerance is set by the material specification.
A basic dimension, shown in a box, is not an exception. It has no tolerance of its own because its tolerance comes from the feature control frame that references it.
Tolerances should be based on function
The size of each tolerance should come from what the feature does. In order to set the tolerance, the engineer needs to know how the part functions, which parts it connects to, and which features affect fit, form, or performance.
Features that locate, seal, bear load or mate with another part generally need tighter control, often with geometric tolerances tied to the datums that represent how the part is assembled. Features with no functional interface, such as clearance pockets or external edges, can usually be covered by the general tolerance.
What a drawing does not define
A drawing defines the requirements for the finished part. It does not specify the manufacturing process, the machine used, the inspection equipment, or the sampling plan. Those decisions belong to manufacturing and quality.
Engineers should still consider them when choosing tolerances. A tolerance that the supplier's process cannot hold, or that available inspection equipment cannot reliably measure, can lead to rejected parts or deviation requests.
Why complete tolerancing matters for first article inspection
AS9102 requires a first article inspection report to account for every design characteristic on the drawing. On Form 3, each characteristic is listed with its drawing location, its requirement, and the actual measured result. If a characteristic has no defined tolerance, there is no requirement to record, and the supplier cannot show whether the feature conforms.
When this happens, the supplier has to request clarification from the customer before the FAI can be closed. This delays the first article and, if the supplier assumes a tolerance instead, can lead to the report being rejected during customer review.
The drawing issues that most often cause this are:
- No general tolerance block, or a block that does not cover every decimal precision used on the drawing
- A basic dimension that is not referenced by any feature control frame
- A general note, such as "break all sharp edges," with no measurable limit
- Features defined only in the CAD model, with no note stating that the model governs and no general profile tolerance
- A dimension marked as reference that the customer still expects to be verified
Reviewing a drawing for these items before it is released to a supplier reduces clarification requests and helps the first article close on the first submission.
How GroundControl supports this
GroundControl's FAI Generation Tool builds the AS9102 report from the drawing, listing each characteristic with its requirement on Form 3. Characteristics without a defined requirement are visible in the report before it is submitted. For customers receiving FAIRs from their suppliers, FAIR Review lets the quality team review each report on the platform and return questions to the supplier.
