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Engineering Graphics and CAD · Lesson 25 of 35

Assemblies: structure and mates

Combine parts into an assembly with correct mates/constraints that reflect how parts actually connect.

Readiness check

Learning objectives

By the end of this lesson you can:

Build an assembly structure from parts and subassemblies.
Apply mates (fastened, revolute, slider, cylindrical, planar, and others).
Place mate references on functional geometry.
Choose the mate that matches the real joint.
Explain how assembly modelling differs from part modelling.

Check your starting point

Five to ten minutes.

In an assembly, do you move parts into place by dragging, or by defining relationships?
Why might the first part in an assembly be fixed in place?
If a hinge lets a part rotate about one axis, what kind of joint is that?

Interpretation.

Q1: By defining relationships (mates), so positions are parametric and update. Dragging alone is not enough.
Q2: To ground the assembly, giving everything else a fixed reference. Skill 25.1.
Q3: A revolute (rotational) joint, allowing one rotation. Skill 25.2.

You need L24 (parts with deliberate interfaces to mate).

0 or 1 weak itemsContinue with this lesson.

2 weak itemsReview Lesson 24, then return.

3 or more weak itemsWork through the prerequisite examples before continuing.

The core idea

What it is. An assembly positions multiple parts (and subassemblies) relative to each other using mates (also called joints or constraints), each of which encodes how two parts connect and how they may move. In Onshape, mates are placed using mate connectors (small coordinate frames) on functional geometry.

Why an engineer needs it. Products are assemblies. Modelling an assembly correctly lets you check that parts fit, see how a mechanism moves, and produce assembly drawings and a bill of materials. Mates make the assembly parametric: change a part and the assembly updates.

What problem it solves. It defines the spatial and motion relationships between parts so the assembly is positioned, movable where intended, and updatable.

What goes wrong when it is ignored. Dragging parts roughly into place gives no real relationships, so nothing updates and motion cannot be studied. Wrong mate types give wrong motion (a hinge that slides, a slide that rotates).

A simple mechanical example. The clamp assembly has a base (fixed as ground), a pivoting jaw connected by a revolute mate (it rotates about the pivot pin), and a screw that drives the jaw. Each mate encodes a real joint, so the assembly moves like the real clamp.

Assembly structure. An assembly is built from parts and, for larger products, subassemblies (assemblies used as components). The first component is usually fixed (grounded) to anchor everything else.

Mates and their motion (Onshape names):

Fastened: no relative motion (0 degrees of freedom).
Revolute: one rotation (a hinge, a pin joint).
Slider: one translation (a linear slide).
Cylindrical: rotation plus translation along one axis.
Planar: motion in a plane.
Pin-slot, ball, parallel: other specific relationships.

Mate connectors on functional geometry. In Onshape, a mate is defined between two mate connectors placed on the functional features (a hole axis, a face). Placing them on functional geometry (not arbitrary points) makes the mate meaningful and robust. (This differs from tools that mate face-to-face directly; the principle of mating functional geometry is the same.)

Part 5: Parts, assemblies, and drawings.

Check: explain the decision in your own words before using a CAD command.

The lesson map. Assemblies: structure and mates becomes manageable when you move through the four checks in order and verify each result before continuing.

The skills, taught in order

Skill 25.1 - Build the assembly structure and ground it

Concept. Assemble parts and subassemblies, fixing the first as ground. Terminology. Assembly, subassembly, fixed/grounded component, bottom-up assembly. Procedure. Insert the parts, fix the base component, and group related parts into subassemblies for larger products. Reasoning. A grounded base gives every other part a fixed reference. Failure mode. No fixed component, so the assembly floats. Check. State why the first component is fixed.

Skill 25.2 - Choose the mate that matches the joint

Concept. Each real joint maps to a mate type with the right motion. Terminology. Mate/joint, fastened, revolute, slider, cylindrical, planar. Procedure. Identify the real joint's motion, then pick the mate that allows exactly that (hinge equals revolute, slide equals slider, rigid equals fastened). Reasoning. The mate type determines the motion; the wrong one gives wrong behavior. Failure mode. Using fastened where motion is needed, or the wrong motion type. Check. Choose the mate for a hinge (revolute) and for a rigid bracket (fastened).

Skill 25.3 - Place mate connectors on functional geometry

Concept. Define mates between connectors placed on functional features. Terminology. Mate connector, functional geometry. Procedure. Put mate connectors on the hole axes, faces, or edges that actually locate the joint, then mate them. Reasoning. Functional placement makes the mate meaningful and robust to edits. Failure mode. Mating arbitrary points, giving a fragile or wrong joint. Check. Place a revolute mate on a pin-hole axis.

Skill 25.4 - Understand assembly versus part modelling

Concept. Part modelling builds geometry; assembly modelling positions parts and defines motion. Terminology. Part Studio (Onshape) versus Assembly. Procedure. Model geometry in the part environment; bring finished parts into the assembly and relate them with mates. Reasoning. Separating geometry from positioning keeps both clean. Failure mode. Trying to model geometry in the assembly or mate in the part environment. Check. State where geometry is built and where mates are applied.

Worked example 1: assemble the clamp with a revolute jaw

Problem. Assemble the mechanical clamp: fix the base, then connect the pivoting jaw with a revolute mate about the pivot pin so it rotates as the real jaw does.

Planning. Ground the base, place mate connectors on the pivot axis, and apply a revolute mate.

Solution.

Insert and ground. Insert the base and the pivoting jaw. Fix the base as ground, so the jaw moves relative to it.
Mate connectors. Place a mate connector on the pivot-hole axis of the base and one on the pivot-hole axis of the jaw (functional geometry).
Revolute mate. Apply a revolute mate between the two connectors. This aligns the pivot axes and allows the jaw one rotation about the pivot, matching the real hinge.
Check motion. Drag the jaw: it rotates about the pivot only, as intended. It has one rotational degree of freedom.
Result. A clamp assembly with a grounded base and a jaw that pivots correctly.

Result. The clamp is assembled with the base grounded and the jaw on a revolute mate, giving exactly the pivoting motion of the real clamp.

Why the method works. Grounding the base and mating on the pivot axis with a revolute mate encodes the real hinge, so the assembly moves correctly.

How to verify independently. Try to move the jaw in any other way (slide it, lift it): it should not move except to rotate about the pivot. Only rotation is allowed, confirming the revolute mate.

Worked example 2: choosing the mate for the clamp's travel

Problem. The clamp closes by a screw that advances the jaw. Should the moving jaw's travel be modelled with a revolute mate (rotation) or a slider mate (translation)? Add the correct motion and compare the two schemes. The complication is matching the mate to the actual mechanism.

Planning. Determine how the jaw actually moves as the screw turns, then choose the mate.

Solution.

Two candidate schemes. (a) Revolute: the jaw pivots about a pin. (b) Slider: the jaw translates along a guide as the screw advances.
What the mechanism does. In a pivoting toggle clamp, the jaw rotates about a pin, so a revolute mate is correct, and the screw drives the rotation. In a sliding vise-style clamp, the jaw translates along a guide, so a slider mate is correct, and the screw drives the translation.
Model the correct one. For the pivoting design, keep the revolute mate from Worked Example 1 and drive its angle. For the sliding design, use a slider mate along the guide axis and drive its distance with the screw.
Compare. Choosing revolute for a sliding mechanism (or slider for a pivoting one) would give the wrong motion: the jaw would move in a way the real clamp does not. The mate must match the physical joint.
Screw drive. In either case, the screw's advance can drive the chosen degree of freedom (angle for revolute, distance for slider), so turning the screw closes the clamp in the model as in reality.

Comparison. Revolute suits a pivoting jaw; slider suits a sliding jaw. The correct choice depends on the actual mechanism, and using the wrong mate produces motion the real clamp does not have.

Result. Model the clamp's travel with the mate that matches its real joint (revolute for a pivoting jaw, slider for a sliding jaw), driven by the screw; the wrong mate gives wrong motion.

Independent check. Actuate the model (change the driving angle or distance) and compare with how the real clamp closes. If the modelled motion matches, the mate is correct.

Misconceptions and diagnostics

Misconception	Why it seems reasonable	Why it is wrong	Evidence that reveals it	Correction	Diagnostic question
"Drag parts into place to assemble."	It positions them visually.	Dragging gives no relationships; nothing updates or moves correctly.	Editing a part leaves others behind.	Define mates on functional geometry.	"Are the parts related by mates, or just placed?"
"One mate type fits every joint."	Mates all connect parts.	Each mate allows different motion; the type must match the joint.	A hinge modelled as fastened will not rotate.	Match the mate to the real joint's motion.	"What motion does the real joint allow?"
"The first part does not need fixing."	Parts can float freely.	An ungrounded assembly has no fixed reference and drifts.	The whole assembly moves when you drag one part.	Fix the base component as ground.	"Is a base component grounded?"

Practice ladder

Level A - Recognition

Task. Match eight real joints (hinge, slide, ball joint, rigid bolt) to their mate types. Deliverable. Eight matches. Success criteria. At least six correct. Answer guidance. Hinge equals revolute; slide equals slider; rigid equals fastened; ball equals ball. Common errors. Confusing cylindrical with revolute. Difficulty. Low.

Level B - Guided application

Task. Assemble two parts with a guided mate (connectors indicated), grounding the base. Deliverable. The mated assembly. Success criteria. Base grounded; correct mate; expected motion. Answer guidance. Place connectors on functional geometry. Common errors. Mating arbitrary points. Difficulty. Medium.

Level C - Independent application

Task. Assemble the clamp (Project P5) independently, grounding the base and mating the jaw. Deliverable. The clamp assembly. Success criteria. Correct structure, grounding, and mate; the jaw moves as intended. Answer guidance. Identify the joint, choose the mate, place connectors on the pivot. Common errors. Wrong mate type for the jaw. Difficulty. Medium to high.

Level D - Transfer and design

Task. For a given mechanism, choose and justify the mate for each joint, then assemble it. Deliverable. The assembly plus a joint-to-mate justification. Success criteria. Each mate matches its joint's motion; the mechanism moves correctly. Answer guidance. Analyze each joint's real motion first. Common errors. A mate that allows the wrong motion. Difficulty. High.

Working with AI, and proving it yourself

Use AI as a tutor

Useful AI support:

Ask it to map real joints to mate types.
Ask it to explain mate connectors in Onshape.
Ask it to suggest an assembly structure for a product.

Limits:

A text assistant cannot see your assembly or its motion.
It may suggest a mate that gives the wrong motion.

Verify AI output against: the joint-motion match (does the mate allow exactly the real motion?), the grounded-base rule, and functional mate-connector placement.

Prove it yourself

A plausible but incorrect AI answer, and how to catch it. You ask, "How should I connect a sliding jaw that moves in a straight line along a guide?" and the assistant replies: "Use a revolute mate; it is the standard way to connect moving parts."

This gives the wrong motion. Detect it with the joint-match principle: a sliding jaw translates, so it needs a slider mate; a revolute mate would rotate it instead. The evidence is the motion test: actuate the model and the jaw would swing rather than slide. Correct conclusion: use a slider mate for straight-line motion; match the mate to the real joint.

Retrieval and spaced review

How are parts positioned in an assembly?
Why is the first component fixed?
What motion does a revolute mate allow? A slider?
What is a mate connector, and where should it be placed?
How does assembly modelling differ from part modelling?
Why must the mate type match the joint?
Cumulative (L24): Why does mating on functional interface geometry (from L24) make an assembly robust?
Reconstruction task: From memory, describe grounding the clamp base and adding the revolute jaw.

Answers. 1: by mates that define relationships, not by dragging alone. 2: to ground the assembly so everything else has a fixed reference. 3: revolute allows one rotation; slider allows one translation. 4: a small coordinate frame placed on functional geometry (a hole axis, a face) to define a mate. 5: part modelling builds geometry; assembly modelling positions parts and defines motion. 6: the mate type determines the allowed motion, so it must match the real joint. 7: functional geometry is stable and meaningful, so mates placed there stay correct through edits.

Suggested review intervals. 1 day, 3 days, 7 days.

Reference mapping and next step

Read further

Onshape docs (Assemblies, mates).

Standards details must be checked against the current official edition used by your institution or employer.

Finish the lesson

You can now: build and ground an assembly; choose mates that match real joints; place mate connectors on functional geometry; and distinguish assembly from part modelling.

Self-assessment checklist.

I ground a base component.
I choose mates by the real joint's motion.
I place mate connectors on functional geometry.
My assemblies move as the real mechanism does.
I keep geometry in parts and relationships in the assembly.

Next lesson: L26 - Degrees of freedom, interference, and clearance. Why it follows: with the clamp assembled and moving, you next verify it is correctly constrained (the right degrees of freedom) and that parts fit without unintended interference, the checks that confirm an assembly is sound.

Required files or submissions: submit your Level C clamp assembly (Project P5). Optional extension: in Onshape, assemble two parts with a revolute mate and confirm the motion is a single rotation.