Top-Down Design
How master sketches drive every part — one source of truth for the whole robot
Bottom-Up Vs Top-Down
Bottom-up: model each part alone, fit them later.
- Parts have no shared reference — gaps appear.
- Top-down: define the whole layout first.
- Every part pulls dimensions from one master.
- Change the master, every part updates.
Open with the pain students already know: they CADded a gearbox plate and a bearing block separately, then nothing lined up at assembly. That is bottom-up. Top-down flips it — you sketch the skeleton of the mechanism FIRST, then build parts onto it. Stress that real FRC subsystems (intakes, elevators, swerve drive) are too interconnected to model part-by-part in isolation.
What Is A Master Sketch
A single sketch holding the key geometry.
- Wheelbase, pivot points, gear centers, tube lines.
- Lives at the top of the component tree.
- Parts reference it — never the reverse.
- Think blueprint the whole team agrees on.
Call it the 'skeleton' or 'layout sketch' — frcdesign calls it a master sketch. Demo: create a New Component, then a sketch inside it that ONLY contains driving geometry, no part detail. Emphasize construction lines (the X key in sketch toolbar) for reference geometry that won't become real edges. The master sketch is the contract — everyone references it, it references nothing.
Build The Layout Sketch
- Start a Component, add a sketch on a datum plane.
- Draw the mechanism's centerlines and pivots.
- Fully constrain it — no blue lines left.
- Use construction geometry for reference points.
- Name it 'Master Sketch' in the browser.
Live demo. Hammer the fully-constrained point: a blue (under-defined) master sketch will shift unpredictably and break every downstream part. Show the constraint badges. Tip: dimension off the origin so the sketch is locked to the world. Rename via double-click in the browser — sloppy names kill team collaboration.
Drive Parts From The Master
Project master geometry into each part's sketch.
- Use Project / Include (shortcut P).
- Projected edges turn purple — they're linked.
- Build the body on those projected lines.
- Move the master → the part follows.
This is the mechanical heart of top-down in Fusion. Inside a child component's sketch, hit P (Project) and click the master geometry — projected curves are purple and stay associative. Common mistake: students re-draw the geometry instead of projecting it, which silently breaks the link. Test it live: edit a master dimension and watch the projected part move.
Parameters As Single Source
Modify > Change Parameters for key numbers.
- Make wheelbase, tube size, bore into User Parameters.
- Reference parameters in master sketch dimensions.
- Type the parameter name into any dim box.
- One edit cascades to the whole robot.
This is Fusion's version of Onshape variables. Modify > Change Parameters > add User Parameter. Then when you place a dimension, type 'wheelbase' instead of a number. Now the master sketch AND parameters together are the single source of truth. FRC example: define hex_bore = 0.5in once; every bearing pocket and shaft references it, so switching to 3/8 hex is one number.
ONE EDIT, WHOLE ROBOT MOVES
The master sketch and parameters are the single source of truth — every part listens to them, never the other way around.
Top-Down Vs Bottom-Up
- Parts modeled in isolation
- Manual re-fitting at assembly
- Change breaks unrelated parts
- No shared design intent
- Layout drives all parts
- Parts auto-update together
- One number changes the robot
- Whole team reads one master
Use this to contrast directly. For robots specifically: subsystems share mounting holes, belt spans, and gear centers — that interconnection is exactly where bottom-up collapses. Caveat to mention: top-down has upfront cost (you must plan the skeleton), and over-linking can make edits brittle. Keep masters lean — only true driving geometry.
Where We Use It
- Drivetrain: wheelbase + module centers in master.
- Elevator: stage heights and belt path driven.
- Intake: pivot point and roller spacing first.
- Gearboxes: shaft centers set 20DP gear mesh.
- Swerve mounts project from the chassis master.
Make it concrete to our season. Gear mesh distance is a perfect example: for 20DP gears, center distance = (T1+T2)/(2*20). Put gear tooth counts as parameters, drive the shaft-center dimension off them, and the gearbox plate updates automatically. Same logic for #25 chain center distance or 5mm HTD belt spans. This is why mentors insist on master sketches.
Common Mistakes
Leaving the master under-defined (blue lines).
- Re-drawing geometry instead of projecting it.
- Putting part detail in the master sketch.
- Hard-coding numbers that should be parameters.
- Referencing parts back into the master (loops).
Walk through each failure they will actually hit. Blue lines = unstable master. Re-drawing = broken associativity, the link looks fine but isn't. Bloated master = slow and confusing; keep it to centerlines and key points only. Circular references (master depends on a part that depends on the master) cause Fusion to throw dependency errors — keep the flow strictly one direction, downhill.
Your Task
- New component 'Drivetrain', add Master Sketch.
- Parameter wheelbase = 24in, drive a 2x1 frame.
- Project rails into a tube component.
- Change wheelbase to 27in — confirm it updates.
- File > Share > Public Link in Fusion.
- Copy the generated share URL.
- Paste the link on AltHub.
- Note which parameter you changed.
Give them 20-25 minutes. Success criteria: when they edit the wheelbase parameter, the projected tube frame visibly resizes with zero manual edits. Walk the room checking for blue master sketches and re-drawn (non-projected) geometry — those are the two failures that mean they didn't really do top-down. Remind them to fully constrain before projecting.
Top-Down Design One Source Of Truth
- Master sketch + parameters drive every part.
- Project geometry to keep parts associative.
- Design intent flows one direction — downhill.
Your Task
- Model what this lesson covers in Fusion 360.
- Use the AltSkripts tools where they apply.
- Save it with a clear name.
- In Fusion: Share → Public Link → Copy.
- Paste the link below.
- A coach reviews it in AltHub.