Stage 4: Assemblies and Mechanisms

Goal#

After this stage you can split a design into real parts, connect them so they move the way the product should, prove that they do not collide, list them in a bill of materials, and reuse one part across several designs.

This matters for product design because nobody ships a single solid. A consumer product is a base, a lid, a hinge, fasteners, a board, maybe a gear train. Clients ask whether the lid opens 110 degrees, whether the battery door clears the hinge, and how many screws to order. An assembly answers all three before a prototype is printed, and it gives you the part list that turns a model into a product.

Before you start#

Stage 3 (parametric design): you can drive a model from a parameter table, read the timeline, and repair a broken feature. The enclosure you built there is reused in Challenge 4.
You can sketch, extrude, revolve, fillet and pattern, and use Measure.
You can save a design to a project so it appears in the Data Panel (external references need this).
Licence note: everything required here works on a standard Fusion licence, with two caveats from Autodesk's help. The Bill of Materials is not available with a Fusion for personal use licence, so the BOM tasks give you a manual route. External components need cloud storage in a project. Nothing in this stage requires a paid extension.

Learn#

Practice#

Do the practice exercise on Components vs Bodies
Do the practice exercise on Top-Down vs Bottom-Up Design
Do the practice exercise on Joints and Joint Types
Do the practice exercise on Joint Limits and Motion Links
Do the practice exercise on Contact Sets
Do the practice exercise on Motion Study
Do the practice exercise on BOM and Parts Lists (use the manual route if the BOM is unavailable on your licence)
Do the practice exercise on External References

Drills (10 to 20 minutes each):

Drill 1, bodies to components. Model two 40 mm cubes as two bodies in the default component. Try to create a joint between them and note what Fusion allows. Then right-click the Bodies folder and use Create Components from Bodies. Rename the components Cube_A and Cube_B and join them. Write down the difference you saw before and after.
Drill 2, wrong component. Create a component Plate and activate it. Sketch and extrude a 60 x 40 x 5 mm plate. Without activating the Default Component, create a second extrusion for a different part. Look in the Browser, see where the body landed, and fix it by activating the correct component first. Record the rule in one sentence.
Drill 3, degrees of freedom. On paper, list the joint type and degrees of freedom you would use for: a laptop hinge, a drawer, a ceiling fan on its shaft, a ball-end camera mount, a bolted bracket. Then build the first two in Fusion and drag each part to confirm you got the motion you listed.
Drill 4, limits and contact. Build a lid on a base with a Revolute joint. Set the joint limits to 0 to 110 degrees using Edit Motion Limits, then Drive Joints to 110 and confirm the lid stops there. Turn on a selective contact set between lid and base and check that driving the lid to a negative angle is also blocked by collision.

Checkpoint project#

Project: print-in-place hinge or spur gear pair, then a variation. Time estimate: 3 to 5 hours (2 to 3 hours for the tutorial, 1 to 2 hours for the variation).

Pick one tutorial and complete it exactly first:

Tutorial 4: Print-in-Place Hinge, a three-knuckle hinge with a captive pin and 0.3 mm clearances.
Tutorial 3: A Meshing Spur Gear Pair, a 20-tooth and 30-tooth module-2 pair at the correct centre distance.

When the tutorial works, make a variation without following new steps.

Variation if you chose the hinge: a five-knuckle hinge with a larger pin.

Parameter	Tutorial	Variation
`kn_len` (knuckle length)	10 mm	8 mm
Number of knuckles	3	5
`pin_d`	5 mm	6 mm
`gap` (radial clearance)	0.3 mm	0.25 mm
`ax_gap` (axial clearance)	0.4 mm	0.4 mm

Expected values: total length = 5 x 8 + 4 x 0.4 = 41.6 mm; bore diameter = 6 + 2 x 0.25 = 6.5 mm.

Variation if you chose the gears: a 16-tooth and 24-tooth module-2 pair (still a 3:2 ratio).

Expected values: pitch diameters 32 mm and 48 mm; centre distance = (32 + 48) / 2 = 40 mm; rotating the 16-tooth gear one full turn turns the 24-tooth gear 240 degrees.

Deliverables

The tutorial result working: the hinge swings its full range, or the gears turn together with the Motion Link
The variation built by editing parameters or re-generating, with the expected values above read off with Measure
Every moving part in its own component, named, with joints that behave when you drag them
A selective contact set (not Enable All Contact) between the moving pairs, and a Motion Study play-through with no interpenetration
An Interference check or a section view showing the clearance you chose (0.25 mm radial for the hinge variation, centre distance 40 mm for the gears)
A one-paragraph note on what changed between tutorial and variation and which parameter you had to touch

Challenges#

Challenge 1: Sliding-lid pencil box#

Brief: A stationery client wants a pencil box with a lid that slides open along the length of the box and stops before it falls out.

Constraints: Box 170 mm long, 50 mm wide, 25 mm tall, wall 3 mm, FDM print. The lid is a separate part 170 mm long and 3 mm thick. Grooves 1.2 mm deep are cut into the inside of both long walls, so the groove-to-groove width is 44 + 2 x 1.2 = 46.4 mm and the lid is 46.2 mm wide (0.1 mm clearance per side, typical). Travel 120 mm.

Deliverables

Box and lid as separate components, with the box pinned
A Slider joint with limits 0 to 120 mm
A contact set that stops the lid against the box when it is pushed the wrong way

Self-check

Drive Joints to 120 mm: the lid stops, and Measure shows 50 mm of lid still inside the box (170 minus 120)
The lid cannot be dragged past either limit
Measure the groove against the runner: the gap reads 0.1 mm per side
Renaming the lid component changes its name in the Joints folder, not the joint behaviour

Stretch: Add a detent bump and make the clearance a parameter from Stage 3.

Hint: see Joints and Joint Types and Joint Limits and Motion Links.

Challenge 2: Two-link desk lamp arm#

Brief: A lighting client wants a desk lamp arm with a shoulder and an elbow and wants to know how close the lamp head can get to the base.

Constraints: Link 1 is 120 mm between pivot centres; link 2 is 100 mm. Both pivots use 6 mm pins. The shoulder rotates 0 to 90 degrees from vertical. The elbow rotates 0 to 150 degrees from the straight position. Base fixed.

Deliverables

Base, link 1, link 2 and head as separate components
Two Revolute joints with limits
A Motion Study of the elbow through its full range

Self-check

With the elbow at 0 degrees, the distance from shoulder pivot to head pivot is 220 mm
With the elbow at 150 degrees, the pivot-to-pivot distance is about 60 mm (the square root of 120 squared plus 100 squared plus 2 x 120 x 100 x cosine 150 degrees)
No joint can be dragged beyond its limit
The head stays attached to link 2 at every angle

Stretch: Add a contact set so link 2 cannot swing through the base at shoulder angle 0.

Hint: see Joint Origins for the pivot points.

Challenge 3: Rack-and-pinion drawer opener#

Brief: A kitchen-gadget client wants a rack that slides out when a small knob is turned.

Constraints: Module 2, 20-tooth pinion (pitch diameter 40 mm). Rack travel 100 mm. The pinion is a Revolute joint, the rack a Slider.

Deliverables

Pinion and rack as separate components, meshing at the correct centre-line height
A Motion Link relating pinion rotation to rack travel
Limits on the rack so it cannot leave its guide

Self-check

One full turn of the pinion moves the rack pi x 40 = 125.7 mm
A 90 degree turn moves the rack about 31.4 mm
The rack hits its 100 mm limit after about 286 degrees of pinion rotation (100 / 125.7 x 360)
Contact between teeth is blocked in the Motion Study (no tooth overlap)

Stretch: Replace the pinion with a 16-tooth one and update the Motion Link; the rack should now move 100.5 mm per turn.

Hint: see the rack-and-pinion worked example on Joint Limits and Motion Links.

Challenge 4: The enclosure, assembled and counted#

Brief: Turn your Stage 3 enclosure into a shippable assembly with a part list.

Constraints: Base, lid, a board placeholder (60 x 40 x 1.6 mm), four M3 screws (10 mm long, typical) and four standoffs already in the base. Lid fixed to the base with the screws.

Deliverables

Every real part converted to a component, with names such as Base, Lid, Board, Screw_M3x10
The four screws as copies (instances) of one component
Joints: base pinned, board and lid located with joints, screws rigid
A parts list, from the Bill of Materials if available, otherwise a table you write from the Browser

Self-check

The part list has 4 distinct rows (base, lid, board, screw) and 7 items total (1 + 1 + 1 + 4)
The screw row shows quantity 4 (or you can show 4 instances in the Browser)
Changing pcb_l to 80 mm, as in Stage 3, rebuilds the assembly with the lid still fitting
Each component has a name and a part number

Stretch: Add a hinge at one short edge and open the lid to 110 degrees with a contact set that stops it on the base.

Hint: see BOM and Parts Lists, "Instances vs copies".

Challenge 5: One bracket, two products#

Brief: A client sells two products that use the same mounting bracket. A design change to the bracket must reach both.

Constraints: Bracket 40 x 20 x 4 mm with two holes of diameter 4 mm. Products are two separate designs, each inserting the bracket as an external component.

Deliverables

The bracket saved as its own design in a project
Two product designs that each insert it as an external component and joint it in place
A change to the bracket hole diameter from 4 mm to 5 mm, propagated

Self-check

After the change, both product designs offer an update and both show 5 mm holes once you update
You can read the bracket's version state in the Browser before and after
Break Link on one product leaves that product's bracket unchanged when the source is edited again
No component is shown as unresolved

Stretch: Create a circular reference on purpose, read the message, then remove it.

Hint: see External References, "Update behavior worth knowing".

You have finished this stage when#

I can tell a body from a component and convert one into the other.
I can choose the right joint type for a motion and explain its degrees of freedom.
I can limit a joint's travel and link two joints with a Motion Link.
I can use a selective contact set so moving parts stop on collision.
I can run a Motion Study and read it as kinematics only.
I can produce a parts list with correct quantities, or build one by hand from the Browser.
I can share a part between designs and update it when the source changes.
I have completed a mechanism tutorial and a variation of it.

Next stage#

Continue with Stage 5: Design for Manufacture.

Stage 4: Assemblies and Mechanisms

Goal#

Before you start#

Learn#

Practice#

Checkpoint project#

Challenges#

Challenge 1: Sliding-lid pencil box#

Challenge 2: Two-link desk lamp arm#

Challenge 3: Rack-and-pinion drawer opener#

Challenge 4: The enclosure, assembled and counted#

Challenge 5: One bracket, two products#

You have finished this stage when#

Next stage#

Sources

Related pages