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Engineering Graphics and CAD · Lesson 22 of 35

Patterns, mirrors, and repeated geometry

Create repeated features efficiently and parametrically instead of modelling each by hand.

Readiness check

Learning objectives

By the end of this lesson you can:

Create linear and circular patterns.
Mirror features or bodies across a plane.
Drive pattern count and spacing with parameters.
Choose pattern versus mirror versus sketch-driven repetition.
Explain how patterns encode design intent and ease change.

Check your starting point

Five to ten minutes.

To make six identical bolt holes evenly around a circle, would you model each one by hand?
If you copy a hole four times and then need eight, how much rework is there?
If a part is symmetric, is there a way to model one side and get the other automatically?

Interpretation.

Q1: No; a circular pattern makes all six from one seed. Skill 22.1.
Q2: A lot; hand copies must each be edited or new ones added. A pattern would update by changing one number. Skill 22.3.
Q3: Yes; mirror across a symmetry plane. Skill 22.2.

You need L21 (features to repeat).

0 or 1 weak itemsContinue with this lesson.

2 weak itemsReview Lesson 21, then return.

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

The core idea

What it is. Patterns and mirrors create repeated geometry parametrically. A pattern repeats a feature in a line (linear) or around an axis (circular); a mirror reflects a feature across a plane. The repeats are associative: they update when the original (seed) feature or the pattern parameters change.

Why an engineer needs it. Real parts repeat features constantly: bolt circles, rows of holes, symmetric ribs. Modelling each by hand is slow and, worse, fragile: a design change (four holes to eight) means editing every copy. A pattern captures the repetition as one parametric feature, so a single parameter change updates them all.

What problem it solves. It creates repeated geometry once, parametrically, so changes propagate automatically and the intent (a bolt circle, a symmetric pair) is captured.

What goes wrong when it is ignored. Hand-copied features do not update together; changing the count or spacing means manual rework, and copies can drift out of alignment.

A simple mechanical example. A flange with six bolt holes on a bolt circle is one hole (the seed) plus a circular pattern of six. Change the count to eight and all eight appear, evenly spaced, from one edit. A symmetric bracket rib is modelled once and mirrored, so the two ribs always match.

Pattern types:

Linear pattern: repeats along one or two directions, by count and spacing.
Circular pattern: repeats around an axis, by count and angle (or even spacing over 360 degrees).
Mirror: reflects across a plane, enforcing symmetry.

Parameters drive repeats. Count, spacing, and angle are parameters; changing them updates every instance. This is the whole advantage over copies.

Neutral terminology: linear/circular pattern and mirror are named similarly across Onshape, SolidWorks, Fusion, NX, and Creo; some tools call a pattern an "array."

Part 4: Parametric CAD foundations.

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

The lesson map. Patterns, mirrors, and repeated geometry becomes manageable when you move through the four checks in order and verify each result before continuing.

The skills, taught in order

Skill 22.1 - Create linear and circular patterns

Concept. Patterns repeat a seed feature by count and spacing (linear) or count and angle (circular). Terminology. Linear pattern, circular pattern, seed feature, instance, count, spacing, angle. Procedure. Select the seed feature, choose linear or circular, set the direction or axis, and enter count and spacing/angle. Reasoning. One pattern feature replaces many hand copies and stays associative. Failure mode. Copying the feature by hand instead of patterning. Check. Create a bolt circle of six holes from one seed with a circular pattern.

Skill 22.2 - Mirror for symmetry

Concept. Mirror reflects a feature across a plane, enforcing symmetry. Terminology. Mirror, symmetry plane. Procedure. Select the feature (or body) and a symmetry plane; the mirror creates the reflected instance, kept associative to the original. Reasoning. Mirroring guarantees the two sides match and update together. Failure mode. Modelling the second side independently, so the two can drift apart. Check. Mirror a rib across the part's symmetry plane.

Skill 22.3 - Drive repeats with parameters

Concept. Count, spacing, and angle are parameters that update all instances. Terminology. Pattern parameter, associativity. Procedure. Set count and spacing/angle as the pattern's parameters; to change the repetition, edit the parameter, not the instances. Reasoning. Parametric repeats make design changes a one-number edit. Failure mode. Editing individual instances instead of the parameter. Check. Change a hole pattern from four to eight by editing the count.

Skill 22.4 - Choose pattern, mirror, or sketch repetition

Concept. Match the tool to the repetition: linear/circular pattern for arrays, mirror for symmetry, sketch repetition for simple in-sketch cases. Terminology. Pattern, mirror, sketch-driven repetition. Procedure. Use a circular pattern for radial arrays, a linear pattern for rows/grids, a mirror for symmetry, and in-sketch repetition only for simple cases. Reasoning. The right tool captures the intent and keeps the model clean. Failure mode. Forcing a mirror where a pattern belongs, or vice versa. Check. Choose the tool for a symmetric pair of brackets (mirror) versus a row of holes (linear pattern).

Worked example 1: a bolt circle by circular pattern

Problem. A flange needs six identical holes of diameter 8 evenly spaced on a bolt circle of diameter 60. Model them from one seed hole using a circular pattern driven by a count parameter.

Planning. Make one seed hole on the bolt circle, then circular-pattern it six times about the flange axis.

Solution.

Seed hole. With the hole tool, place one diameter-8 hole on the bolt circle (30 from the axis). Fully locate it. This is the seed.
Circular pattern. Select the seed hole, choose a circular pattern about the flange axis, set the count to 6 and even spacing over 360 degrees. Six evenly spaced holes appear.
Parameter. The count (6) is a parameter; the spacing is 360 divided by the count, so the holes stay even whatever the count.
Check. All six holes are identical and evenly spaced, from one seed and one pattern feature.
Edit readiness. Changing the count to 8 would produce eight evenly spaced holes with no other work.

Result. Six evenly spaced diameter-8 holes on the 60 bolt circle, made from one seed hole and a circular pattern driven by a count parameter.

Why the method works. The pattern captures the bolt-circle intent (evenly spaced holes) as one associative feature, so the holes are consistent and change-ready.

How to verify independently. Change the count to 8: eight evenly spaced holes should appear with no manual placement. That single-edit update confirms the pattern is parametric.

Worked example 2: mirror a rib and re-parameter a linear pattern

Problem. A bracket has a reinforcing rib on one side that must be matched by an identical rib on the other side (symmetry), and a row of four cooling slots that later must become eight. Mirror the rib and build the slots as a linear pattern, then change the count. Contrast with hand-copying. The complication is combining a mirror (symmetry) with a re-parametered pattern (change).

Planning. Mirror the rib across the symmetry plane; make the slots a linear pattern; then edit the count from four to eight.

Solution.

Mirror the rib. Model one rib, then mirror it across the bracket's symmetry plane. The second rib is associative: if the first rib changes, the mirror updates, so the two always match.
Linear-pattern the slots. Model one slot (the seed), then linear-pattern it four times along the bracket with a set spacing. Four evenly spaced slots appear.
Change the count. Edit the linear pattern's count from 4 to 8 (and adjust spacing if the length is fixed). Eight slots appear from one edit.
Contrast with hand copies. Had the ribs and slots been hand-copied, matching the mirrored rib would require manual duplication (and re-doing it after any rib change), and going from four to eight slots would mean adding and placing four more by hand. The pattern and mirror do both automatically.
Intent captured. The mirror encodes "these ribs are symmetric"; the pattern encodes "these slots are a uniform row." Both intents survive edits.

Comparison. Mirror and pattern update automatically and keep intent; hand copies must be edited one by one and can drift. For symmetric and repeated features, mirror and pattern are both faster and more robust.

Result. The rib is mirrored (symmetry, associative) and the slots are a linear pattern re-parametered from four to eight in one edit; hand-copying would require manual rework for each.

Independent check. Change the seed rib's height: the mirrored rib should change too. Change the pattern count: all slots update. Both automatic updates confirm associativity.

Misconceptions and diagnostics

Misconception	Why it seems reasonable	Why it is wrong	Evidence that reveals it	Correction	Diagnostic question
"Copy and paste each repeated feature."	It is direct.	Copies do not update together; changes mean manual rework.	Changing the count means adding copies by hand.	Use a pattern so instances update from a parameter.	"Will these repeats need to change together?"
"Mirror just duplicates static geometry."	It looks like a copy.	A mirror stays associative; the reflection updates with the original.	Editing the original updates the mirror.	Use mirror for symmetry so both sides match.	"Must these two sides always match?"
"Edit each instance to change spacing."	Instances are what you see.	Spacing and count are parameters; edit the parameter, not the instances.	Editing one instance leaves the pattern inconsistent.	Change the pattern's count/spacing parameter.	"Am I editing the parameter or an instance?"

Practice ladder

Level A - Recognition

Task. For eight repeated-feature cases, choose linear pattern, circular pattern, or mirror. Deliverable. Eight choices. Success criteria. At least six correct. Answer guidance. Radial equals circular; row/grid equals linear; symmetry equals mirror. Common errors. Using a pattern where a mirror (symmetry) is meant. Difficulty. Low.

Level B - Guided application

Task. Build a scaffolded bolt-circle pattern (seed given), then change the count. Deliverable. The pattern before and after the count change. Success criteria. Even spacing; count driven by a parameter; correct update. Answer guidance. Even spacing equals 360 divided by count. Common errors. Placing holes individually. Difficulty. Medium.

Level C - Independent application

Task. Pattern and mirror features on a supplied part (a bolt circle and a symmetric pair), no prompts. Deliverable. The patterned and mirrored part. Success criteria. Correct pattern and mirror; associativity confirmed. Answer guidance. Seed once, then pattern or mirror. Common errors. Independent second side instead of a mirror. Difficulty. Medium.

Level D - Transfer and design

Task. Given a design change (four holes to eight, or add a symmetric feature), implement it by editing a pattern parameter or adding a mirror, and reflect on the time saved versus hand copies. Deliverable. The updated model plus a short reflection. Success criteria. Change implemented parametrically; reflection compares with manual rework. Answer guidance. Change the parameter, not the instances. Common errors. Re-modelling instead of editing the parameter. Difficulty. Medium to high. (This is a design-change micro-test.)

Working with AI, and proving it yourself

Use AI as a tutor

Useful AI support:

Ask it to explain when to pattern versus mirror.
Ask it to describe circular-pattern spacing and confirm with your bolt circle.
Ask it to suggest how to make a repeated feature change-ready.

Limits:

A text assistant cannot see your pattern's associativity.
It may suggest copying features.

Verify AI output against: the pattern-the-feature-not-copies principle, associativity (edit the seed, the instances update), and parameter-driven counts.

Prove it yourself

A plausible but incorrect AI answer, and how to catch it. You ask, "I need four holes now but maybe eight later. Should I just copy the hole four times for now?" and the assistant replies: "Yes, copy it four times; you can copy four more later if needed."

This throws away parametric change. Detect it with the design-intent principle: a pattern makes four-to-eight a one-number edit, while copies require manual additions and can drift. The evidence is the change test: editing a pattern count updates all instances; copies do not. Correct conclusion: build the holes as a linear or circular pattern so the count is a parameter, not hand copies.

Retrieval and spaced review

What is the difference between a pattern and a mirror?
What parameters drive a circular pattern?
Why pattern the feature instead of copying it?
When do you use a mirror?
What does associativity mean for a pattern or mirror?
How do you change a four-hole pattern to eight?
Cumulative (L18): How do patterns and mirrors express design intent, like the stable-reference idea from L18?
Reconstruction task: From memory, describe building the six-hole bolt circle.

Answers. 1: a pattern repeats a feature in a line or around an axis; a mirror reflects it across a plane. 2: axis, count, and angle/spacing. 3: patterns update from a parameter and stay associative; copies do not. 4: for symmetry, so two sides always match. 5: instances update when the seed or parameters change. 6: edit the pattern's count parameter from four to eight. 7: they capture the intent (a bolt circle, a symmetric pair) as one parametric feature that updates together.

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

Reference mapping and next step

Read further

Onshape docs (patterns, mirror).

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

Finish the lesson

You can now: create linear and circular patterns; mirror for symmetry; drive repeats with parameters; and choose the right repetition tool.

Self-assessment checklist.

I pattern features instead of copying them.
I mirror for symmetry so both sides match.
I drive count and spacing with parameters.
I can change a pattern count in one edit.
I choose pattern versus mirror by the intent.

Next lesson: L23 - Feature order, design intent, and robust models. Why it follows: you now have the full feature toolkit; the capstone CAD lesson pulls it together into models whose feature order and references express design intent and survive change, the difference between a model that looks right and one that is robust.

Required files or submissions: submit your Level C patterned and mirrored part and your Level D design-change micro-test. Optional extension: in Onshape, build a six-hole bolt circle and change it to eight; confirm all update.