- A basic dimension (value in a box) is theoretically exact. It has no tolerance of its own, title-block tolerances don't apply to it, and it locates or shapes a tolerance zone. You inspect the feature control frame that uses it.
- A reference dimension (value in parentheses, or marked REF on older drawings) is information only. It is not inspected, and a part is never rejected on it.
- Whether to balloon basic dimensions on an FAI varies by customer. Many shops don't; some customers require it. Check the PO and the customer's quality requirements, then apply one rule consistently.
- Every drawing item that states a requirement gets a balloon: toleranced dimensions, title-block-toleranced dimensions, GD&T feature control frames, and requirement notes.
What is the difference between a basic dimension and a reference dimension?
A basic dimension is a theoretically exact value that defines where a tolerance zone sits; the tolerance comes from a feature control frame. A reference dimension is information only and carries no requirement at all.
| Basic dimension | Reference dimension | |
|---|---|---|
| Looks like | Value in a rectangle: [ 42.00 ] | Value in parentheses: (42.00). Older drawings may write 42.00 REF. |
| Meaning | Theoretically exact. Defines where a tolerance zone sits, or its true shape. | Information only. Usually derived from other dimensions, or repeated for convenience. |
| Tolerance | None of its own. Title block tolerances don't apply. The allowed variation comes from a feature control frame (position, profile, angularity...). | None. It doesn't govern manufacturing or inspection. |
| Do you inspect it? | Indirectly, through the frame that uses it. | No. |
| Can it fail a part? | Not on its own. The frame passes or fails. | Never. |
| Balloon it? | Depends on the customer. See below. | Usually not. Some customers ask for it, marked as reference. |
A quick example. A bolt hole sits at [ 30.00 ] from datum B and [ 18.00 ] from datum C, with a position frame Ø0.20 | A | B | C. The two boxed numbers say where true position is. The frame says the hole axis may sit anywhere inside a Ø0.20 cylinder centered there. There's no "30.00 ±something" to check. You check the position, which you calculate from the measured X and Y offsets. (The true position calculator does that sum.)
Do you balloon basic dimensions?
It depends on the customer: many shops don't balloon basic dimensions, and some customers require them. Practice varies, so the customer's FAI requirements decide. You'll see three practices:
| Practice | What Form 3 shows | Why shops choose it |
|---|---|---|
| 1. Don't balloon basics | Only the feature control frame gets a row, with the measured position or profile as the result. | Basics have no tolerance, so there's nothing to accept or reject. Fewer rows, no "N/A" noise. |
| 2. Balloon, mark as basic | A row with requirement 30.00 BASIC and a result like "Verified by bln 12" pointing to the frame. | Every number on the sheet has a balloon, so the reviewer can see nothing was skipped. Some customers require this. |
| 3. Balloon and report the measured value | 30.00 BASIC, result 30.04, no pass/fail, with the frame result on its own row. | Gives the engineer and machinist the raw location data. Useful on a new program or a difficult part. |
Whichever practice you use, three rules hold:
- Never invent a tolerance for a basic dimension, from the title block or anywhere else.
- Never reject on a basic dimension alone. Disposition the frame.
- Pick one practice per customer and write it down, in your inspection procedure or a customer requirements matrix, so two inspectors don't produce two kinds of report.
Read the notes before you read the boxes. ASME Y14.5 allows basic dimensions to be identified by a general note, such as "UNTOLERANCED DIMENSIONS ARE BASIC", instead of a box around each one. On those drawings, a dimension with no box and no tolerance is basic, not "title block ±0.1". Drawings that use this note should not also carry a general ± tolerance block; if yours has both, ask the customer which one governs. Model-based definitions often work this way, with profile of a surface doing the real work.
Do you inspect reference dimensions?
No. A reference dimension carries no requirement, so it is not inspected and it cannot fail a part. It is a convenience for whoever reads the print: an overall length that is already defined by two other dimensions, or a stock size.
Two situations still need care:
- The measured part disagrees badly with a reference dimension. Not a reject, but a flag. Either the controlling dimensions are wrong on the part, or the drawing has an error. Check the controlling dimensions, and if they pass, raise it with the customer's engineer rather than letting it go silently.
- A feature is defined only by a reference dimension. That's a drawing problem, not an inspection decision. Ask the customer how they want it controlled, get the answer in writing, and reference it on the FAI.
Some customers want reference dimensions ballooned so the balloon count matches every number on the sheet. If so, put the requirement as (42.00) REF and the result as "Reference, not inspected". Don't record a measured value as if it were a result.
Want to see how the free ballooner treats boxed and parenthesized dimensions on your own drawing?
Try the free balloonerShould I balloon and inspect it? The decision table
Balloon it if the drawing states a requirement the part must meet; skip it if the item only identifies or informs. That is the working rule from our ballooning guide, applied here to everything you'll meet on a print.
| On the drawing | Balloon? | Inspect? | Notes |
|---|---|---|---|
Dimension with tolerance or limits (25.00 ±0.05, 25.00/25.10) | Yes | Yes, record the value | The bread and butter of Form 3. |
| Plain dimension covered by title-block tolerance | Yes | Yes | Write the resulting tolerance in the requirement, so the row stands on its own. |
| Basic dimension (boxed) | Per customer | Via the frame | See the three practices above. |
| Reference dimension (parentheses or REF) | Usually no | No | If ballooned, mark "Reference, not inspected". |
Single-limit dimension (R2 MAX, 10.0 MIN DEPTH) | Yes | Yes | The open side is limited only by other features and design elements. |
| GD&T feature control frame | Yes, its own balloon | Yes | Separate from the size dimension it sits under. A frame with two segments (composite) usually gets two results. |
| Datum feature symbol (the triangle with a letter) | Usually no | No | It identifies a feature. The feature's own size or form controls get the balloons. |
4X Ø6.60 THRU | Yes | Every instance | One balloon with four results, or sub-balloons 4.1 to 4.4. Follow the customer. |
R3 TYP | Yes | Every like feature | TYP doesn't say how many. Count the instances, state the count on Form 3, and ask if it's unclear. Y14.5 prefers an explicit count such as 4X; TYP is still common on older drawings. |
| Hole callout with counterbore and depth | Yes, each element | Yes | Hole Ø, C'bore Ø, C'bore depth are three characteristics. Sub-balloon them. |
| General note with a requirement ("Break sharp edges 0.2 max") | Yes | Yes, often attribute | Easy to miss, because notes sit away from the views. |
| Flag note (note number in a flag, pointing at features) | Yes | At each flagged location | Balloon the note and account for every place it's flagged. |
| Surface finish symbol | Yes | Yes | Include the default finish note if there is one. |
| Material, heat treat, plating spec | Per customer | Via certs | Usually accounted for on Form 2. See C of C vs material cert. |
| Key or critical characteristic symbol | Yes | Yes, variable data | Record the designator in Form 3 field 7 (Characteristic Designator), using the drawing's or customer's symbol. |
| Dimension in an oval or rounded frame | Yes | Yes | Some drawing and customer standards use this to mark an inspection dimension with a set check frequency. Find the defining note or spec. |
| Title block notes (scale, sheet size, drawn by) | No | No | Not product requirements. |
These are common practices, not rules from the standard. Where the customer's FAI requirements or purchase order say otherwise, they win.
What does the customer's reviewer check on basic and reference dimensions?
The customer's reviewer checks that every requirement has a balloon and a result, and that nothing was accepted or rejected on a dimension that carries no requirement. When a supplier quality engineer (SQE) opens your FAI report, the questions about these items are predictable:
- Does every requirement have a balloon? They'll scan the notes first, because that's where gaps hide.
- Does the balloon count match the Form 3 row count? Basics and references you chose not to balloon shouldn't leave unexplained gaps in the numbering.
- Was anything rejected, or accepted, on a dimension that carries no requirement? A reference dimension on an NCR is a red flag about the inspector, not the part.
- Is the basic-dimension practice consistent across the report and with what they asked for?
- Are TYP and nX features fully accounted for, with a result for every instance?
For the rest of what gets reports bounced, see why FAI reports get rejected and the first article inspection checklist.
FAQ
What is the difference between a basic dimension and a reference dimension?
A basic dimension, shown in a box, is a theoretically exact value that locates or defines a tolerance zone; the allowed variation comes from a feature control frame such as position or profile. A reference dimension, shown in parentheses or marked REF on older drawings, is for information only. It does not govern manufacturing or inspection, and a part cannot be rejected on it.
Do you balloon basic dimensions for a first article inspection?
Practice varies by customer. Many shops do not balloon basic dimensions separately, because they are verified through the feature control frames that use them. Some customers require every basic dimension ballooned, marked BASIC, and referenced to the frame that verifies it. Check the customer's FAI requirements and purchase order, then apply one rule consistently across the report.
Can a part be rejected for a reference dimension?
No. A reference dimension carries no requirement, so it cannot be the basis for rejecting a part. If the measured part disagrees badly with a reference dimension, check the dimensions that actually control the feature. If those pass, the disagreement may point to a drawing error, which is worth raising with the customer's engineer in writing.
What tolerance applies to a basic dimension?
None of its own. Title block general tolerances do not apply to basic dimensions. The variation allowed comes from the geometric tolerance, such as position or profile, in the feature control frame that uses the basic dimension. Never invent a tolerance for a basic dimension, and never reject a part on a basic dimension alone; disposition the frame instead.
Where software helps
Deciding what to balloon is judgment; numbering it all and typing it into Form 3 is just work. Deburr's ballooner reads dimensions, GD&T frames and notes, and flags anything it isn't sure about with a reason, so you make the calls on basics, references and TYP rather than hunting for them. A person reviews every balloon before anything is signed. Try it on your own drawing with the free AI ballooner.
Related: How to balloon a drawing · AS9102 Form 3 explained · GD&T cheat sheet · MMC bonus tolerance · Free ballooning software, compared
Sources
- Reference dimension, Wikipedia
- "Untoleranced dimensions are basic" note in drawing, Eng-Tips forum
- ANSI Y14.5M-1982 and use of TYP, Eng-Tips forum
- AS9102C Form 3 details (field 7, Characteristic Designator), Paradygm
- How to fill out AS9102 forms, GroundControl