SOLIDWORKS Design Approach · Part 09
Six degrees of freedom, and how to remove them
An assembly is a collection of parts positioned and oriented correctly relative to one another. Everything difficult about assemblies follows from that word: relative.
Foundations
What makes assembly modelling different
Parts are manufactured first and assembled second; in CAD the same order applies. Two things distinguish assembly work from part work: mates, and the assembly tree.
A mate is a geometric condition between faces or other entities of two components that positions and orients them relative to each other. Its purpose is to prevent unwanted relative motion — to mimic reality. An assembled component should be anchored in space unless it is genuinely free to move for functional reasons, such as a motor shaft that must rotate.
The degrees-of-freedom arithmetic
A rigid body in space has six degrees of freedom: three translations along the X, Y and Z axes, and three rotations about them. In general a component needs about three well-chosen mates to lock all six, because constraining one degree of freedom frequently constrains another as a side effect. That is why mate count is a poor measure of whether a component is properly located — what matters is which freedoms remain.
Standard mates
Coincident, parallel, perpendicular, tangent, concentric, lock, distance and angle. These handle the large majority of ordinary assembly work.
Mechanical mates
Cam, gear, rack and pinion, screw, hinge, slot, universal joint. These encode kinematic relationships rather than static position, and are what make functional motion possible.
Advanced mates
Profile centre, symmetric, width, path, linear coupler, and limit mates that constrain motion between defined bounds.
Where a shaft runs in a bearing, mate the shaft's cylindrical face concentric with the bearing bore — not an edge to an edge that happens to give the right picture. Mates that mirror the real constraint survive design change; mates chosen for convenience do not. This is design intent expressed at assembly level.
Two strategies
Bottom-up and top-down
Parts first, then relationships
- Step 1 Create each part independently.
- Step 2 Insert them into an assembly document.
- Step 3 Apply mates to position them.
More intuitive and therefore more common. Parts are fully independent, which makes them easy to reuse, easy to check out separately in a data management system, and immune to accidental cross-part references.
Weakness: nothing guarantees that independently created parts will fit. Dimensional coordination has to be managed by the designer.
Relationships first, then parts
Also called in-context design. A layout sketch — a skeleton or napkin sketch — establishes how components sit relative to one another, and parts are then built referencing it. Some designers view the layout as a way of claiming space for each component; others as a block diagram that establishes the parameters of the assembly.
Strength: preferred for conceptual design, where a team is still conceiving the product and its layout. Also more efficient for large, complicated assemblies, because coordination is centralised and there are fewer opportunities for parts to disagree.
Weakness: external references create dependencies. A part built in context of another part will change when that other part changes, which is the intention — and is also the risk.
In-context references are powerful and quietly dangerous. Decide as a matter of policy which parts are permitted to carry them, where the driving geometry lives, and how references are broken or locked when a design is released. An assembly nobody can open without rebuilding half of it usually got that way one convenient reference at a time.
Structure
Reading the assembly tree
The assembly tree differs from a part feature tree in two important ways.
- Caster Assembly
- (f) Mount Plate Fixed. Exactly one component is normally anchored — the base that everything else is located from.
- (−) Fork Subassembly Floating, then constrained by mates. The minus symbol means degrees of freedom remain.
- (−) Axle
- (−) Wheel × 1 Instances of the same part appear as separate nodes but share one definition.
- Mates A single folder holding every mate in the assembly — unlike features, mates are not interleaved with components.
First, the nodes are components and mates rather than features, and there is only one mates folder no matter how many mates exist. Second, an assembly tree cannot be rolled back the way a part tree can; components can be suppressed, but there is no equivalent of stepping backwards through construction history. Each component node carries a symbol showing whether it is fixed or floating, and that state can be toggled.
Verification
Interference and collision detection
Two related checks that answer different questions.
| Interference detection | Collision detection | |
|---|---|---|
| Analyses | Stationary components | Moving components |
| Question answered | Do parts occupy the same space in the assembled position? | Do parts strike one another anywhere through their range of motion? |
| Typical cause of a hit | Wrong dimension, wrong mate, wrong component revision | Insufficient clearance, wrong stroke, missing limit |
| Output | Paired components with the overlapping volume shaded | The position at which contact occurs during motion |
| Handling intended contact | Press fits and threads can be excluded from the report | Contact may be the design intent — a cam and follower must touch |
Both checks produce false positives if run naively. Interference detection will flag every threaded fastener modelled with real thread geometry, and every deliberate press fit. Distinguishing coincidence — contact arising from a coincident mate — from true material overlap is usually a setting worth turning on. The discipline is to configure the check once for the assembly and then run it on every revision, rather than to run it once and conclude that it is noisy.
Behaviour
Exploded views, animation and motion study
An exploded view displaces components along the axes of the assembly space, normally from the isometric orientation. The system holds this as a display state, so the assembly can be shown collapsed or exploded from the same document, and the explode steps can be played back as an animation that communicates build order.
Motion study is a different capability and is worth distinguishing carefully:
Animation
Motion driven by key points on a timeline and constrained by the assembly mates. It shows how something moves; it does not tell you whether it can. Fast, and adequate for communication.
Basic motion
A more realistic simulation that adds physical elements — springs, gravity, motors and contact — on top of the mates. Slower, and much closer to a behavioural answer.
Full motion analysis, which solves for forces, torques and reactions rather than positions alone, belongs with the analysis tools covered in Part 17. The practical rule: use animation to explain, basic motion to sanity-check, and motion analysis to size a component.
Gear, rack-and-pinion, screw and cam mates propagate motion at the correct ratio, so a driven wheel turns because the driving gear turned, not because someone key-framed it. Where an assembly exists to demonstrate a mechanism, building it on mechanical mates rather than positional ones is the difference between a model and a picture.
Variants
Assembly design tables
Design tables work at assembly level much as they do at part level. A spreadsheet drives multiple configurations, controlling which configuration each component uses, the state of individual mates, and the distance and angle relationships between components. The result is a single assembly document that represents a family of product variants — different lengths, different optional equipment, different handing — without duplicating files.
The same naming discipline applies. Rename components and mates to meaningful terms before building the table, because the table addresses them by name and an unreadable table is worse than no table.
Key takeaways
- Mates exist to remove degrees of freedom; count remaining freedoms, not mates.
- Mate to the entities that carry the real physical constraint, and prefer mechanical mates where motion is the point.
- Bottom-up gives independence and reuse; top-down gives coordination and suits conceptual and large-assembly work.
- Govern in-context external references explicitly — they are the main source of fragile assemblies.
- Interference is static, collision is dynamic; configure both once and run them every revision.
- Animation communicates, basic motion sanity-checks, motion analysis sizes.
Series
Continue the pathway
The SOLIDWORKS Design Approach series works through computer aided design as an engineering discipline, from first principles to manufacture.