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A 3D model on screen is a promise; the part in your hand is the proof. Between the two lie checks, drawings, file exports, machine programming, manufacturing and inspection. Knowing each step helps you design parts that are made right the first time, and helps you talk clearly with the people who make them.
1. Requirements
Every part starts with what it must do: the loads it carries, what it connects to, the space it must fit, the environment it works in, how many are needed and what it may cost. Write these down before modelling. They are the test every later decision is checked against.
2. The 3D CAD model
A good model captures design intent, not just shape. Sketches are fully defined, key dimensions drive the geometry, and features are built in the order the part would be made: the main body first, then holes, then fillets and chamfers. When a dimension changes, everything that depends on it should update correctly.
The part is then placed in its assembly with mates that match the real joints, so fit, clearance and motion can be checked against the parts around it. Material and appearance are assigned, which gives the mass and centre of gravity.
3. Check and analyse
- Interference and clearance checks in the assembly.
- Finite element analysis (FEA) for stress, deflection, vibration or heat, where hand calculations aren’t enough.
- Tolerance stack-ups for critical gaps and fits.
- A design review against the requirements, and a manufacturability review with the people who will make it.
Every change found here is cheap. The same change after tooling or machining is expensive.
4. Drawings and files
The model is turned into what each supplier needs:
| File | What it is | Used for |
|---|---|---|
| Native CAD file | The full parametric model | Further design work |
| STEP (.step, .stp) | Neutral, exact 3D geometry (ISO 10303) | Sending models to suppliers, CAM, other CAD systems |
| Parasolid (.x_t), IGES | Other neutral 3D formats | Older systems, specific CAM packages |
| PDF drawing | The 2D drawing with dimensions, tolerances and notes | Quoting, manufacturing and inspection |
| DXF / DWG | 2D outlines | Laser, plasma and waterjet cutting of flat parts and sheet-metal flat patterns |
| STL / 3MF | A triangle mesh of the surface | 3D printing |
A STEP file carries the exact shape but not the tolerances, so the drawing is still the controlling document for anything that must be accurate.
5. Make
How the part is made depends on the process chosen during design:
- CNC machining: a CAM program uses the 3D model to generate toolpaths for each operation, and a post-processor turns them into G-code for the specific machine. The machinist sets up the stock, fixtures and tools, then runs and checks the first part.
- Sheet metal: the flat pattern from CAD (with the right bend allowance) is cut by laser, punch or waterjet, then bent on a press brake, and welded or fastened if needed.
- 3D printing: the STL or 3MF file is sliced into layers with chosen orientation, walls, infill and supports, then printed and post-processed.
- Casting and moulding: the model of the raw casting or moulding is used to make a pattern or mould, and the functional faces are machined afterwards.
6. Inspect
The first part is measured against the drawing in a first article inspection: every dimension, tolerance and note. Hand gauges (callipers, micrometers, bore and thread gauges) cover simple parts; a coordinate measuring machine (CMM) checks complex shapes and GD&T, often comparing directly with the CAD model. Results are recorded in an inspection report.
7. Assemble and test
The parts are assembled with the specified fasteners and torques and tested for function: does it fit, move, seal and carry load as intended? A prototype may also go through endurance or environmental tests.
8. Release and feedback
Problems found in manufacturing, inspection or testing go back into the CAD model and the drawing as a new revision, never as a hand-written change on one copy of a drawing. Once approved, the design is released for production, and the model, drawings and BOM are kept under revision control.
Worked example: from the model to a first estimate
A machined aluminium bracket (6082-T6, 2.70 g/cm³) has a CAD volume of 48.5 cm³ and a bounding box of 80 × 50 × 25 mm. It will be machined from a block 85 × 55 × 30 mm. Estimate the part mass, the stock mass and the roughing time at a removal rate of 36 cm³/min.
Step 1: part mass from the CAD volume.
Step 2: stock volume and mass.
Step 3: material to remove and roughing time.
Estimates like this, taken straight from the model, show where the cost is. Here most of the metal is cut away, so a part designed to be bent from sheet or cut from an extruded profile might be much cheaper. The removal rate comes from the manufacturing calculations guide.
Key points
- Start from written requirements; check every later decision against them.
- Model with design intent so changes update cleanly, and check fit in the assembly.
- Send STEP for geometry, a PDF drawing for tolerances, DXF for flat cutting and STL or 3MF for printing.
- Inspect the first part against the drawing, and feed every change back into the model as a new revision.