Home / Courses / Engineering Graphics and CAD / Lesson 20

Engineering Graphics and CAD · Lesson 20 of 35

Constraints and fully constrained sketches

The single most important CAD skill, constraining a sketch so its geometry is fully and intentionally defined.

Readiness check

Learning objectives

By the end of this lesson you can:

Apply geometric constraints (coincident, horizontal, vertical, parallel, perpendicular, tangent, equal, concentric, midpoint, symmetric).
Apply dimensional constraints.
Achieve and recognize a fully constrained sketch.
Diagnose under-constrained and over-constrained sketches.
Use symmetry to constrain efficiently and encode intent.

Check your starting point

Five to ten minutes.

If you drag a corner of a rectangle and it changes shape, is the sketch fully defined?
What is the difference between telling two lines to be "equal" and giving them each a dimension?
What might happen if you add a dimension that conflicts with an existing constraint?

Interpretation.

Q1: No; if it moves, it still has freedom (it is under-constrained). A fully constrained sketch cannot move.
Q2: "Equal" is a geometric relation (they stay equal, whatever the value); a dimension fixes the value. Both remove freedom, in different ways.
Q3: A conflict (over-constraint); the CAD tool warns you. Skill 20.4 covers this.

You need L19 (a clean sketch to constrain).

0 or 1 weak itemsContinue with this lesson.

2 weak itemsReview Lesson 19, then return.

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

The core idea

What it is. Constraining a sketch means removing its degrees of freedom (the ways its geometry can still move) with geometric constraints (relations like parallel, equal, coincident) and dimensional constraints (values), until the sketch is fully constrained: every point is pinned and nothing can move unintentionally.

Why an engineer needs it. A fully constrained sketch behaves predictably: it holds its shape, updates cleanly when a driving dimension changes, and encodes design intent. An under-constrained sketch drifts when edited or dragged, producing unpredictable models. This is the single most important CAD skill.

What problem it solves. It makes geometry deterministic and intentional, so the model does exactly what the designer meant and edits behave predictably.

What goes wrong when it is ignored. Under-constrained sketches move unexpectedly on edits, giving wrong geometry. Over-constrained sketches conflict and error. Both undermine the model.

A simple mechanical example. A rectangle needs its corners square (perpendicular/horizontal-vertical relations) and its size fixed (two dimensions). With those, it is fully constrained: it cannot skew or resize on its own. Without them, dragging a corner distorts it.

Two kinds of constraint:

Geometric constraints state relationships: coincident (points meet), horizontal/vertical, parallel, perpendicular, tangent, equal, concentric (share a center), midpoint, symmetric (about a line).
Dimensional constraints fix values (lengths, diameters, angles).

Order of work: apply geometric relations first, then dimensions. Relations capture intent (this stays square, these stay equal); dimensions then set the sizes with the fewest values.

Recognizing the state. A fully constrained sketch has zero degrees of freedom; CAD tools indicate this (in Onshape and SolidWorks the geometry turns black; blue means it still has freedom; Fusion shows a lock). Under-constrained geometry can still be dragged; over-constrained geometry conflicts and the tool warns.

Symmetry is a powerful constraint: making geometry symmetric about a centerline halves the dimensions needed and encodes the intent that the two sides stay equal.

Neutral terminology: "fully constrained" (Onshape, Fusion, NX, Creo) equals SolidWorks "fully defined." Geometric constraints are called "constraints" or "relations."

Part 4: Parametric CAD foundations.

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

The lesson map. Constraints and fully constrained sketches becomes manageable when you move through the four checks in order and verify each result before continuing.

The skills, taught in order

Skill 20.1 - Apply geometric constraints

Concept. Geometric constraints state relationships that remove freedom and capture intent. Terminology. Coincident, horizontal, vertical, parallel, perpendicular, tangent, equal, concentric, midpoint, symmetric. Procedure. Add relations that reflect the design (square corners, equal sides, concentric circles) before dimensioning. Reasoning. Relations encode intent that persists as sizes change. Failure mode. Skipping relations and forcing everything with dimensions. Check. Add relations to make a four-line shape a proper rectangle.

Skill 20.2 - Apply dimensional constraints

Concept. Dimensions fix the remaining sizes with the fewest values. Terminology. Dimensional constraint, driving dimension. Procedure. After relations, add only the dimensions needed to fix the size; symmetry and equal relations reduce how many you need. Reasoning. Fewer, intentional dimensions are easier to edit and less likely to conflict. Failure mode. Adding redundant dimensions that cause over-constraint. Check. State the minimum dimensions to fix a rectangle whose corners are already square (answer: two, width and height).

Skill 20.3 - Reach and recognize fully constrained

Concept. A fully constrained sketch has zero degrees of freedom and is indicated by the tool. Terminology. Degrees of freedom, fully constrained/defined. Procedure. Add relations then dimensions until the geometry indicates fully constrained (turns black, or the DOF reach zero); test by trying to drag it (it should not move). Reasoning. Zero freedom means the geometry is deterministic. Failure mode. Leaving a sketch under-constrained because it "looks right." Check. Try to drag a fully constrained sketch; it should not move.

Skill 20.4 - Diagnose under- and over-constraint

Concept. Under-constraint leaves freedom; over-constraint creates conflict. Terminology. Under-constrained, over-constrained/conflict, redundant constraint. Procedure. If geometry drags, add the missing constraint. If the tool reports a conflict, find and remove the redundant constraint or dimension. Reasoning. Both states must be resolved for a reliable sketch. Failure mode. Adding more dimensions to a conflicting sketch, worsening the conflict. Check. Given a conflict message, identify and remove the redundant dimension.

Worked example 1: fully constrain the L-bracket profile

Problem. Take the L-bracket profile from L19 (an under-constrained set of lines) to fully constrained, adding relations then dimensions, and confirm zero degrees of freedom.

Planning. Apply horizontal/vertical and coincident relations first, anchor to the origin, then add the minimum dimensions.

Solution.

Anchor. Make one corner coincident with the origin, so the sketch cannot float. This removes translation freedom.
Geometric relations. Constrain the outline segments horizontal and vertical (the L has only right angles), and ensure the endpoints are coincident so the loop is closed. These relations make every corner square.
Dimensions. Add the sizes: vertical arm height 50 and width 10; foot length 40 and thickness 10. Because the corners are already square (relations), only these lengths are needed; no angle dimensions.
Check the state. The geometry indicates fully constrained (turns black); dragging any point does nothing.
Count. With the anchor, the horizontal/vertical relations, coincident endpoints, and the four size dimensions, every degree of freedom is removed.

Result. A fully constrained L-bracket profile: anchored at the origin, corners squared by relations, sized by four dimensions, with zero degrees of freedom.

Why the method works. Relations capture the right-angle intent so that dimensions only need to set lengths, giving a minimal, conflict-free, fully constrained sketch.

How to verify independently. Drag any point: nothing moves. Change the arm height dimension to 60: the arm lengthens cleanly and the sketch stays fully constrained. Both behaviors confirm it is correctly constrained.

Worked example 2: an over-constrained sketch and its fix

Problem. A student fully constrains a rectangle (corners square, width 40, height 20), then adds a diagonal dimension "to be safe." The tool reports a conflict. Explain the over-constraint, fix it, and contrast with a correct symmetric scheme. The complication is a redundant dimension on an already-defined sketch.

Planning. Recognize that a square rectangle of fixed width and height already has a determined diagonal; adding a diagonal dimension over-defines it.

Solution.

Why the conflict. A rectangle with square corners and fixed width (40) and height (20) is already fully constrained; its diagonal is determined (it must be the value from the width and height). Adding an independent diagonal dimension tries to fix a value that is already fixed, creating a conflict (over-constraint).
The tool's report. The CAD tool flags the redundant dimension as conflicting or redundant, refusing to accept two definitions of the same thing.
The fix. Remove the diagonal dimension (or mark it as a reference/driven dimension, which reports the value without driving it). The sketch returns to fully constrained.
Correct symmetric alternative. If the rectangle should be centred on the origin, a cleaner scheme uses a symmetric constraint about the vertical and horizontal axes plus width and height dimensions. Symmetry centres it with no extra dimensions and no conflict, and encodes the intent that it stays centred.
Lesson. Do not add dimensions "to be safe"; a fully constrained sketch needs no more, and extra dimensions conflict.

Comparison. The diagonal dimension over-defines and conflicts; the symmetric scheme constrains cleanly with minimal dimensions and captures centering intent. Constrain with intent, not with redundancy.

Result. The over-constraint is the redundant diagonal dimension; remove it (or make it reference). A symmetric constraint plus width and height is the clean, conflict-free way to centre the rectangle.

Independent check. After removing the diagonal, the sketch is fully constrained and drag-proof; the diagonal, if shown as a reference, simply reports the computed value. No conflict remains, confirming the fix.

Misconceptions and diagnostics

Misconception	Why it seems reasonable	Why it is wrong	Evidence that reveals it	Correction	Diagnostic question
"If it looks right, it is defined."	The shape looks correct.	Under-constrained geometry drifts on edits even if it looks right now.	Dragging a point changes the shape.	Add relations and dimensions until fully constrained.	"Can I drag any point and change the shape?"
"Add every dimension to be safe."	More seems safer.	Redundant dimensions over-constrain and conflict.	The tool reports a conflict.	Remove redundant dimensions; use relations.	"Is this value already determined by other constraints?"
"Relations and dimensions are interchangeable."	Both remove freedom.	Relations capture intent that persists; dimensions set values. Use relations first.	A shape forced by dimensions loses its intent (squareness) when edited.	Apply geometric relations first, then dimensions.	"Have I captured the intent with relations before dimensioning?"

Practice ladder

Level A - Recognition

Task. For eight sketches, state whether each is under-constrained, fully constrained, or over-constrained, using the indicators. Deliverable. Eight judgements. Success criteria. At least six correct. Answer guidance. Draggable equals under; conflict equals over; black/locked equals fully constrained. Common errors. Calling a black (fully constrained) sketch under-constrained. Difficulty. Low.

Level B - Guided application

Task. Fully constrain a scaffolded sketch, adding relations then dimensions, with prompts. Deliverable. A fully constrained sketch. Success criteria. Relations first; minimal dimensions; geometry indicates fully constrained. Answer guidance. Anchor to the origin; square corners with relations. Common errors. Dimensioning before adding relations. Difficulty. Medium.

Level C - Independent application

Task. Fully constrain a supplied profile from scratch, anchored and sized with the fewest dimensions. Deliverable. A fully constrained sketch. Success criteria. Zero degrees of freedom; no redundancy; symmetry used where sensible. Answer guidance. Relations capture intent; dimensions set sizes. Common errors. Leaving one point free. Difficulty. Medium.

Level D - Transfer and design

Task. Repair a supplied broken sketch that is both under-constrained in one place and over-constrained in another (a model-repair task), and explain each constraint you added or removed. Deliverable. The repaired, fully constrained sketch plus an explanation. Success criteria. Both problems resolved; the sketch fully constrained with intent; each change justified. Answer guidance. Add the missing constraint where it drags; remove the redundant one where it conflicts. Common errors. Adding dimensions to the conflicting area instead of removing the redundant one. Difficulty. High. (This is model-repair assessment evidence.)

Working with AI, and proving it yourself

Use AI as a tutor

Useful AI support:

Ask it to list the geometric constraints and their meanings, then confirm in your tool.
Ask it to explain why a sketch is over-constrained from your description.
Ask it to suggest a symmetric scheme for a centred profile.

Limits:

A text assistant cannot see your sketch's degrees of freedom.
It may suggest adding dimensions where a relation is better.

Verify AI output against: the drag test (fully constrained does not move), the relations-first principle, and the tool's conflict report.

Prove it yourself

A plausible but incorrect AI answer, and how to catch it. You ask, "My rectangle is fully constrained but I want to be safe, should I also dimension the diagonal?" and the assistant replies: "Yes, add the diagonal dimension for extra certainty."

This causes over-constraint. Detect it with the definition: a square rectangle with fixed width and height already determines the diagonal, so dimensioning it duplicates a fixed value and conflicts. The evidence is the tool's conflict message. Correct conclusion: a fully constrained sketch needs no more constraints; if you want the diagonal shown, add it as a reference (driven) dimension, not a driving one.

Retrieval and spaced review

What does "fully constrained" mean in terms of degrees of freedom?
Should you add geometric relations or dimensions first, and why?
How does the tool indicate a fully constrained sketch?
What causes over-constraint, and how do you fix it?
How does symmetry help constrain a sketch?
What is the drag test?
Cumulative (L12): How is "constrain with the minimum, intentional set" like "dimension without redundancy"?
Reconstruction task: From memory, list the relations and dimensions that fully constrain the L-bracket profile.

Answers. 1: zero degrees of freedom; nothing can move. 2: relations first, because they capture intent that persists as sizes change; dimensions then set values. 3: the geometry turns black (or shows a lock); blue means freedom remains. 4: a redundant constraint or dimension; remove it or make it a reference. 5: it centres or mirrors geometry, halving the dimensions and encoding equal-sides intent. 6: trying to drag the geometry; a fully constrained sketch will not move. 7: both use the fewest, intentional constraints and avoid redundancy that causes conflict.

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

Reference mapping and next step

Read further

Onshape docs (Constraints/dimensions).

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

Finish the lesson

You can now: apply geometric and dimensional constraints; reach and recognize a fully constrained sketch; diagnose under- and over-constraint; and use symmetry to constrain with intent.

Self-assessment checklist.

I apply geometric relations before dimensions.
I can reach a fully constrained (black) sketch.
I test with the drag test.
I can find and remove a redundant constraint.
I use symmetry to reduce dimensions and encode intent.

Next lesson: L21 - Solid features: extrude, revolve, cut, hole, fillet, chamfer. Why it follows: with fully constrained sketches, you can now turn them into solids reliably. L21 introduces the core feature set and the end conditions that make features robust.

Required files or submissions: submit your Level C fully constrained sketch and your Level D sketch repair. Optional extension: in Onshape, take one of your sketches to fully constrained and confirm it turns black and will not drag.

Part IV continues in 15-part4-lessons-cont.md with L21 (Solid features), L22 (Patterns, mirrors, and repeated geometry), and L23 (Feature order, design intent, and robust models).

# Engineering Graphics and CAD - Phase 4: Full Lesson Content, Part IV (continued), L21-L23

Continues 14-part4-lessons.md. Holds L21 (solid features), L22 (patterns, mirrors, and repeated geometry), and L23 (feature order, design intent, and robust models). CAD actions given for Onshape with the software-neutral principle first. No em dashes.