Why Learn to Read Sheet Metal Drawings?
Technical drawings are the universal language of manufacturing. Whether you are an engineer sending parts out for fabrication, a procurement officer requesting quotes, or a workshop technician setting up a machine, understanding drawings is essential.
A misread dimension means scrapped parts. A missed tolerance means rejected assemblies. A wrong material callout means wasted time and money. This guide teaches you the fundamentals of reading sheet metal fabrication drawings so you can communicate clearly with your fabricator and catch errors before they reach the shop floor.
The Three Standard Views
Sheet metal drawings follow third-angle projection (used in the US, Vietnam, and most of Asia) or first-angle projection (used in Europe). The projection method is indicated by a small symbol in the title block.
Third-Angle Projection (Most Common in Vietnam)
In third-angle projection, the views are arranged as if you are looking at the object from outside:
- Front view — The most informative face of the part, placed in the center
- Top view — Placed directly above the front view
- Right side view — Placed to the right of the front view
Think of it as “what you see is what you get” — the top view shows what the part looks like from above, placed above the front view.
Isometric / 3D Views
Many modern drawings include a 3D isometric view for visual clarity. This is not dimensioned but helps you understand the overall shape of the part, especially for complex bent sheet metal where flat views can be confusing.
Tip: Always refer to the dimensioned 2D views for fabrication. The 3D view is for reference only.
Line Types and Their Meanings
Lines on a technical drawing are not all the same. Each line type conveys specific information:
| Line Type | Appearance | Meaning |
|---|---|---|
| Continuous thick | _ | Visible edges and outlines |
| Continuous thin | _ | Dimension lines, leader lines, hatching |
| Dashed | _ _ _ _ _ | Hidden edges (features behind the visible surface) |
| Chain (long-short) | .... | Center lines, axes of symmetry |
| Chain (long-short-short) | ........ | Cutting plane lines (section views) |
| Phantom (long-double-short) | ...._.. | Bend lines, alternate positions |
Bend Lines
In sheet metal drawings, bend lines are critically important. They are typically shown as:
- A chain line (center line style) on the flat pattern view
- A phantom line on the 3D or folded view
- Often annotated with the bend direction: UP (toward you) or DOWN (away from you)
The bend line shows exactly where the material will be folded. It is not an edge — it is the fold axis.
Reading Dimensions
Dimensions tell you the exact size of every feature. Understanding dimension conventions prevents the most common fabrication errors.
Dimension Components
Every dimension has three parts:
- Dimension line — A thin line with arrows at both ends indicating what is being measured
- Extension lines — Thin lines extending from the feature to the dimension line
- Dimension value — The number (in millimeters for metric drawings)
Types of Dimensions
| Dimension Type | Format | Example | Measures |
|---|---|---|---|
| Linear | Number only | 150 | Distance between two points |
| Diameter | Prefix with symbol | dia. 10 or 10 with circle symbol | Circular hole diameter |
| Radius | Prefix R | R5 | Arc or fillet radius |
| Angle | Degrees symbol | 90 deg | Angle between two surfaces |
| Counterbore | Symbol + dims | Counterbore 12 x 5 deep | Stepped hole: bore diameter x depth |
| Countersink | Symbol + angle | Countersink 10 x 90 deg | Conical recess: diameter x angle |
Flat Pattern vs Folded Dimensions
Sheet metal drawings often include two sets of dimensions:
- Folded dimensions — The dimensions of the finished (bent) part. These are what your assembly drawing references.
- Flat pattern dimensions — The dimensions of the flat blank before bending. These are what the laser cutter actually cuts.
Important: The flat pattern is always slightly smaller than the sum of the folded dimensions because material stretches at the bend. This is accounted for by the bend allowance or K-factor. Your fabricator handles this calculation, but you should be aware that flat and folded dimensions will not add up exactly.
Understanding Tolerances
No manufacturing process is perfectly precise. Tolerances define the acceptable range of variation for each dimension.
Tolerance Notation
| Notation | Meaning | Example |
|---|---|---|
| Bilateral equal | Plus and minus same amount | 50 +/- 0.3 (acceptable: 49.7 to 50.3) |
| Bilateral unequal | Different plus and minus | 50 +0.5 / -0.2 (acceptable: 49.8 to 50.5) |
| Unilateral | Variation in one direction only | 50 +0.5 / -0 (acceptable: 50.0 to 50.5) |
| Limit dimensions | Min and max stated directly | 49.8 / 50.3 |
General Tolerances (ISO 2768)
Many drawings do not tolerance every single dimension. Instead, they reference a general tolerance standard in the title block:
| ISO 2768 Class | Linear 0.5-6mm | Linear 6-30mm | Linear 30-120mm | Angles |
|---|---|---|---|---|
| f (fine) | +/- 0.05 | +/- 0.1 | +/- 0.15 | +/- 0.5 deg |
| m (medium) | +/- 0.1 | +/- 0.2 | +/- 0.3 | +/- 0.5 deg |
| c (coarse) | +/- 0.2 | +/- 0.3 | +/- 0.5 | +/- 1.0 deg |
| v (very coarse) | +/- 0.5 | +/- 1.0 | +/- 1.5 | +/- 1.5 deg |
For sheet metal: Class “m” (medium) is the standard default. Laser cutting achieves +/- 0.1mm, CNC bending achieves +/- 0.3mm, so medium tolerance is readily achievable without special measures.
Geometric Dimensioning and Tolerancing (GD&T)
Advanced drawings use GD&T symbols in feature control frames to specify flatness, perpendicularity, parallelism, and position tolerances. While a full GD&T explanation is beyond this beginner’s guide, here are the most common symbols you will encounter in sheet metal:
| Symbol | Name | Controls | Typical Value (Sheet Metal) |
|---|---|---|---|
| Flatness (parallelogram) | Flatness | Surface deviation from a perfect plane | 0.5-1.0mm per 1000mm |
| Perpendicularity (inverted T) | Perpendicularity | Angle deviation from 90 degrees | 0.3-0.5mm per 100mm |
| Position (crosshair circle) | True Position | Hole location from theoretical position | 0.2-0.5mm |
Bend Notes and Callouts
Sheet metal drawings contain specific bend information that is unique to this type of fabrication.
Bend Annotation Format
A typical bend callout looks like this:
BEND: R1.5 x 90 deg UP
This means:
- R1.5 — Inside bend radius is 1.5mm
- 90 deg — Bend angle is 90 degrees
- UP — Bend direction is toward the viewer (the flange goes up from the flat view)
Bend Radius Rules
| Material | Standard Inside Bend Radius |
|---|---|
| Mild steel | 1x material thickness |
| Galvanized steel | 1x material thickness |
| Stainless 304 | 1x material thickness (minimum) |
| Aluminum 5052 | 1x material thickness |
| Aluminum 6061-T6 | 2-3x material thickness (prone to cracking) |
| Copper | 0.5-1x material thickness |
Tip: If no bend radius is specified on the drawing, most fabricators default to 1x the material thickness. If your design requires a specific radius, always call it out explicitly.
Bend Direction Conventions
| Callout | Meaning | On Flat Pattern |
|---|---|---|
| UP / Inside | Flange bends toward the viewer | Bend line with UP arrow |
| DOWN / Outside | Flange bends away from the viewer | Bend line with DOWN arrow |
Getting bend direction wrong results in a mirrored part — the most common sheet metal fabrication error. Always double-check bend directions, especially on asymmetric parts.
Material Callouts
The title block or notes section of a drawing specifies the material. Common formats:
| Callout | Material | Thickness | Standard |
|---|---|---|---|
| SS400 t2.0 | Mild steel | 2.0mm | JIS |
| SGCC t1.5 | Hot-dip galvanized | 1.5mm | JIS |
| SECC t1.2 | Electro-galvanized | 1.2mm | JIS |
| SUS304 2B t1.5 | Stainless 304, 2B finish | 1.5mm | JIS |
| SUS304 #4 t1.5 | Stainless 304, No.4 brushed | 1.5mm | JIS |
| A5052 H32 t2.0 | Aluminum 5052, quarter-hard | 2.0mm | JIS |
| SPCC t1.0 | Cold-rolled carbon steel | 1.0mm | JIS |
In Vietnam, JIS (Japanese Industrial Standard) material designations are the most common. If your drawing uses ASTM, DIN, or EN designations, your fabricator can cross-reference them.
Surface Finish Specifications
Surface finish requirements are usually noted in the title block or as specific callouts:
| Specification | Meaning |
|---|---|
| As-cut | No surface treatment after laser cutting |
| Deburr all edges | Remove laser cutting burrs on all edges |
| Powder coat RAL 7035 | Apply powder coating in light gray |
| Brushed No.4 | Stainless steel with directional brush finish |
| 2B | Standard stainless mill finish (smooth, reflective) |
| Anodize clear | Aluminum with clear anodized coating |
| Passivate | Stainless with chemical passivation treatment |
| Zinc plating | Electrolytic zinc coating after fabrication |
Reading a Complete Drawing: Checklist
When you receive or review a sheet metal drawing, go through this checklist:
- Title block — Part name, material, thickness, finish, quantity, revision
- Projection method — First angle or third angle (check the symbol)
- General tolerances — ISO 2768 class or custom tolerance notes
- Overall dimensions — Verify the part fits in the expected envelope
- Hole sizes and positions — Check all diameters, counterbores, countersinks
- Bend information — Radius, angle, direction for every bend
- Flat pattern — If included, verify it matches the folded dimensions
- Surface finish — Coating, plating, or special finish requirements
- Notes — Any special instructions, inspection requirements, or standards
- Revision history — Make sure you have the latest revision
Common Drawing Mistakes to Watch For
- Missing bend radius — Forces the fabricator to guess, which may not match your design intent
- Bend relief not designed — Internal corners where two bends meet need a small relief cut or slot to prevent tearing
- Hole too close to a bend — Holes should be at least 2x material thickness away from a bend line, otherwise they distort
- Inconsistent dimensions — Over-constrained or conflicting dimensions cause confusion
- Missing material callout — “Steel” is not enough. Specify the grade (SS400, SUS304, etc.)
- No flat pattern — For complex parts with many bends, always include a flat pattern view
Need Help With Your Drawings?
If you are new to technical drawings or unsure about how to communicate your design to a fabricator, we are here to help. Our engineering team provides free DFM (Design for Manufacturability) reviews. We will check your drawings for common issues and suggest improvements before production begins.
Visit our FAQ page for answers to common technical questions, or contact us directly to discuss your project. We are happy to work from sketches, 3D models, or even reference photos — we will help you produce a proper fabrication drawing.