DFM & Design
GD&T Basics: How to Read a Technical Drawing
GD&T tells the shop what actually matters on your part. Here is how to read the boxes, the letters and the symbols.
Key takeaways
- Size alone does not control shape, orientation or position.
- A feature control frame reads left to right: symbol, tolerance, datums.
- Datums are the reference planes the inspector measures from.
- Only call out what the function needs. Every frame adds inspection work.
Why size is not enough
A drawing that says "25.0 ±0.1" controls how big a hole is. It says nothing about whether the hole is round, straight or in the right place.
Two parts can both pass that callout and still not assemble. One hole is round and centred. The other is oval and 0.3 mm off position.
GD&T, or geometric dimensioning and tolerancing, fills that gap. It controls form, orientation, location and profile, not just size.
The feature control frame
A feature control frame is the rectangle on the drawing with symbols inside. Read it left to right.
| Position | Content | Meaning |
|---|---|---|
| 1st box | Geometric symbol | What is being controlled |
| 2nd box | Tolerance zone | How much variation is allowed |
| 3rd box onward | Datum references | What it is measured from |
Example: a flatness symbol, then 0.05, then nothing. That means the surface must lie between two parallel planes 0.05 mm apart. No datum is needed, because flatness is about the surface alone.
Example: a position symbol, then a diameter symbol with 0.2, then A, then B, then C. That means the centre of the feature must sit inside a 0.2 mm diameter cylinder located from datums A, B and C.
Datums
A datum is a reference. It is usually a surface, an axis or a plane that the part is located against during inspection.
Datums are lettered A, B, C in order of priority. A is the primary, which means the part sits on it first. B stops rotation. C stops the last degree of freedom.
Change the datum order and you change the measurement. That is why two shops can disagree about the same part.
Common symbols
| Control | Symbol name | What it does |
|---|---|---|
| Flatness | Flatness | Surface must lie between two parallel planes |
| Straightness | Straightness | A line element must stay within a band |
| Circularity | Roundness | Each cross-section must be round |
| Cylindricity | Cylindricity | The whole surface must be a true cylinder |
| Perpendicularity | Squareness | A surface must be 90° to a datum |
| Parallelism | Parallelism | A surface must stay parallel to a datum |
| Position | True position | A feature must sit inside a zone at its true location |
| Profile of a surface | Profile | A surface must follow its nominal shape within a band |
| Runout | Circular runout | A rotating part must not wobble beyond a limit |
Reading a callout step by step
Take a hole with this frame: position symbol, diameter 0.2, datum A, datum B, datum C.
- Step 1. The symbol says position, so location is being controlled.
- Step 2. The tolerance zone is a cylinder 0.2 mm in diameter.
- Step 3. The axis of the hole must lie inside that cylinder.
- Step 4. The cylinder sits at the true position defined by the basic dimensions.
- Step 5. The datums A, B and C set the coordinate system for that true position.
Maximum material condition, briefly
Sometimes you will see an M in a circle after the tolerance. That means maximum material condition, or MMC.
For a hole, MMC is the smallest allowed hole. If the hole comes out larger, you get bonus tolerance, because a bigger hole has more room to be off position and still assemble.
It is a useful tool for parts that must bolt together. It is also easy to misuse, so ask before adding it.
Common mistakes
- Using GD&T frames on every feature when a general tolerance would do.
- Forgetting to define datums, which makes the frame unmeasurable.
- Mixing ± dimensions and position tolerances on the same feature.
- Calling out profile on a surface that is not modelled accurately.
- Using a tolerance so tight that no CMM in the supply chain can verify it.
Common questions
Do I need GD&T on a simple part?
No. A plate with clearance holes and a general tolerance is fine. Add frames when fit, alignment or sealing depends on it.
What is a basic dimension?
A dimension in a box with no tolerance. It defines the true position, and the feature control frame supplies the allowable variation.
Can GD&T reduce cost?
Yes, when it replaces a blanket tight tolerance. A position callout often allows more variation than ±0.05 on X and Y, and it matches how the part works.
The bottom line
GD&T is a language for saying exactly what you need. Use it where function depends on geometry, define your datums, and keep the rest of the drawing simple.