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Two different approaches to building assemblies. Top-down: design the whole, then split into parts. Bottom-up: design each part separately, then combine. Each has strengths.
Design approaches.
Use these three in order. Each builds on the one before.
In one paragraph, explain top-down vs bottom-up.
Walk me through splitting a body into parts.
I have a 15-part design. Which approach?
BOTTOM-UP DESIGN:
Workflow:
1. Design each part individually.
2. Combine into an assembly.
3. Test fit.
Each part: its own CAD file. Parameters defined per part.
Pros:
- Each part is fully encapsulated.
- Easy to modify one part without affecting others.
- Good for: existing standardized parts (off-the-shelf hardware).
Cons:
- Hard to enforce inter-part relationships.
- Parts might not fit until you assemble.
- Manual coordination of dimensions.
When to use:
- Building from a parts library.
- When parts will be sourced/printed/manufactured separately.
TOP-DOWN DESIGN:
Workflow:
1. Design the whole product as one body or one big assembly.
2. Identify split lines.
3. Split into individual parts.
Often: one master file with multi-bodies that share parameters.
Pros:
- Parts inherit parameters from the whole.
- Fit is guaranteed (parts are split from one mesh).
- Easier to maintain relationships.
Cons:
- File can become large.
- One person works on the whole; less parallelism.
When to use:
- Custom product where all parts must fit.
- Iterating quickly on a complete design.
HYBRID APPROACH (most common):
- Top-down for the main structure.
- Bottom-up for standardized parts.
E.g.: enclosure (top-down) + screw hardware (bottom-up).
EXAMPLE: CUSTOM ENCLOSURE.
TOP-DOWN:
Step 1: design the overall enclosure as one body.
- Solid block 80×50×30mm.
Step 2: identify split: top + bottom + side flaps.
Step 3: split body using sketches.
Step 4: each piece is now a separate body.
Step 5: refine each piece as needed.
Result: an enclosure where all parts mate by construction.
BOTTOM-UP:
Step 1: define each part separately.
- top: 80×50×3mm.
- bottom: 80×50×3mm.
- side panels: 80×30×3mm.
Step 2: assemble: place each in 3D space.
Step 3: check fits with measurements.
Result: an enclosure where each part is standardized; fits derive from numbers.
WHEN TO MIX:
For a 50-part assembly:
- Use top-down for: the main housing (designed in one file as multi-body).
- Use bottom-up for: standardized hardware (each in its own file or library).
PARAMETRIC LINKING:
Across files:
- In Fusion 360: 'Derive' from another file.
Bring parameters from File A into File B.
Changing File A propagates to File B.
- In OnShape: 'Link' between documents.
Similar functionality.
This lets you have a 'master parameters' document that all other files derive from.