Form Factors
Reading a connector drawing for evidence
A connector drawing controls the dimensions that set fit and interface: mating face geometry, contact pitch, keying, mounting holes, and the datums those features are measured from.
A connector drawing controls the dimensions that set fit and interface: mating face geometry, contact pitch, keying, mounting holes, and the datums those features are measured from. Everything else, surface finish on non-critical faces, radii, chamfers, plating thickness unless called out, is left to the supplier's process. The revision block records when any of the controlled items changed, so a part number alone does not tell you which drawing it was built to.
Which dimensions on a connector drawing are controlled?
Controlled dimensions are the ones tied to a datum and given a tolerance. On a connector, that usually means the mating interface: the position and diameter of each contact cavity, the pitch between them, the keying slot, the shell outline at the mating face, and the mounting hole pattern. These are the features that decide whether two halves mate and whether the assembly fits the panel.
A dimension is controlled when it carries a tolerance, either directly next to the number or through a general tolerance note in the title block. A dimension without a tolerance and without a general note is a reference dimension, shown in parentheses on many drawings. Reference dimensions are informational. They are derived from other controlled dimensions and are not inspected.
The datum structure is what makes the control meaningful. If the mating face is datum A and the contact pattern is positioned from A and B, then the position tolerance applies to that frame. Move the datum and the same numbers mean something different. This is why a drawing that looks unchanged can produce a part that does not fit: the geometry is the same, the reference frame is not.
For readers who spend time with technical records of this kind, the same discipline of separating what a document fixes from what it merely reports appears in other fields, for example in the incident and integrity records collected at pipeline safety data, where the wording of a cause code matters as much as the number beside it.
What does a tolerance note leave open?
A tolerance note fixes a band and leaves the distribution inside that band open. A note reading ±0.1 mm on a contact position does not say the supplier must aim at nominal. It says any value inside the band is acceptable. The supplier may run at one edge of the band if the process is stable there.
That has consequences. Two suppliers can both meet the drawing and still produce parts that behave differently in a housing, because one runs near the low limit and the other near the high. The drawing permits both. If the assembly needs a tighter distribution than the tolerance band allows, the drawing has to say so, through a tighter tolerance, a statistical note, or a capability requirement.
A general tolerance note in the title block, often something like ±0.2 mm unless otherwise stated, leaves even more open. It applies to every dimension not individually toleranced, which means features the designer may not have thought about carefully are governed by a blanket rule. Angles, radii, and edge breaks frequently fall under it.
Tolerance notes also leave open the measurement method unless the drawing names one. A position tolerance measured with a functional gauge and the same tolerance measured on a coordinate measuring machine can accept different parts at the boundary. The drawing controls the requirement, not the inspection strategy, unless it explicitly does.
Why does a drawing carry a revision block?
The revision block exists because a part is not a fixed object. When a controlled dimension, a datum, a tolerance, or a note changes, the drawing revision changes with it, and the revision block records what changed and when. Without it, a part number would point at a moving target.
In practice the revision block is the only way to answer a simple question: was this part built to the drawing I am holding? A supplier who built to revision C and a supplier who built to revision D may both ship a part that carries the same part number, if the change was not flagged as interchangeable. The revision letter is the evidence.
Revision blocks also carry the reason for the change, sometimes in a short description column. That description is often the most useful line on the sheet. A change from ±0.15 to ±0.10 on a contact position tells you the assembly had a fit problem. A change to a datum tells you the inspection frame moved. Neither fact is visible from the current geometry alone.
Table: what a drawing note fixes and what it leaves open
| Note | What it fixes | What it leaves open |
|---|---|---|
| Individual dimension with tolerance | The nominal value and the acceptable band | Where inside the band the supplier runs |
| General tolerance in the title block | A default band for untoleranced dimensions | Which features the designer considered critical |
| Datum callout | The reference frame for position tolerances | How the datum feature is fixtured during inspection |
| Surface finish callout | A maximum roughness on the named face | The process used to reach it |
| Material and plating note | The specified material and coating | Thickness distribution across the part unless stated |
| Revision block entry | What changed and at which revision | Whether earlier parts remain interchangeable |
Checklist for reading a drawing against a part
- Confirm the revision letter on the drawing matches the revision on the part's paperwork or marking.
- Identify the datums and check that the features you are measuring are positioned from the same frame.
- Separate toleranced dimensions from reference dimensions; do not inspect the reference ones.
- Read the general tolerance note and apply it to every dimension without its own tolerance.
- Check whether any note names an inspection method, and follow it if it does.
- Look at the revision block description for the last change and ask whether it affects the feature you care about.
- Compare the part number and revision together, never the part number alone.
Common mistakes
- Treating a reference dimension as a controlled one. Parentheses or a missing tolerance mean the number is informational, and holding a supplier to it is a misreading of the drawing.
- Assuming the part number identifies the geometry. It identifies the part, not the revision. Two parts with the same number can differ if the change was not made interchangeable.
- Ignoring the general tolerance note. It governs more dimensions than most readers expect, and it is usually the reason a feature that looks uncontrolled still has a limit.
- Reading the revision block as a formality. The description column is often the only record of why a dimension moved, and it is frequently the fastest route to understanding a fit problem.
- Assuming a tolerance note guarantees a distribution. It guarantees a band. The distribution inside the band is a process question, not a drawing question.
The page behind these facts
The figures on this page are read from the source the text names, nist.gov, consulted on September 6, 2026. How this desk reads a document, and what it does with a figure it cannot source, is set out in sources and method.