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Engineering Graphics and CAD · Lesson 31 of 35

Sheet-metal and fabricated parts (introduction)

Model bent sheet-metal parts at an introductory level and understand flat patterns and bends.

Readiness check

Learning objectives

By the end of this lesson you can:

Explain sheet metal as constant-thickness bent material.
Create a base flange and bends.
Interpret a flat pattern and bend allowance qualitatively.
Add relief and place holes away from bends.
Recognize basic sheet-metal design constraints.

Check your starting point

Five to ten minutes.

When you bend a metal bracket from a flat strip, does its thickness change?
If you needed to cut the flat shape before bending, what would you need to know?
What might happen to a hole placed right at a bend line?

Interpretation.

Q1: No; sheet metal keeps a constant thickness through bends. Skill 31.1.
Q2: The flat pattern (the unfolded shape) and its dimensions. Skill 31.3.
Q3: It would distort as the material bends through it. Skill 31.4.

You need L21 (features). Sheet metal is a distinct modelling mode.

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. Sheet-metal parts are made from flat stock of constant thickness, cut and bent into shape. CAD sheet-metal tools model the bent part and can unfold it into a flat pattern, the shape that is actually cut before bending.

Why an engineer needs it. Enclosures, brackets, panels, and chassis are commonly sheet metal, and they follow different rules from machined parts: constant thickness, bends with a radius, and features that must respect the bends. Modelling them with sheet-metal tools gives both the folded part and the flat pattern the shop needs.

What problem it solves. It models bent parts correctly and produces the flat pattern for manufacture, while respecting sheet-metal constraints.

What goes wrong when it is ignored. Modelling sheet metal like a solid ignores constant thickness and bend behavior; placing holes too close to bends distorts them; missing reliefs cause tearing at bends.

A simple mechanical example. An L-shaped sheet-metal bracket is a flat base flange with one bend. Unfolded, it is a flat strip (the flat pattern) that is laser-cut, then bent to the L. A hole must sit far enough from the bend that it does not distort.

Key ideas:

Constant thickness: sheet parts keep one thickness throughout.
Flange and bend: a base flange plus bends form the shape; each bend has a bend radius.
Flat pattern: the unfolded shape, which is what is cut. The bend allowance (how much material the bend consumes) and the K-factor (a material property describing where the neutral line sits) determine the flat length; these are introduced qualitatively here, as their exact values are material- and process-dependent and are flagged for verification.
Reliefs and hole placement: bends often need a relief cut to avoid tearing, and holes must sit a minimum distance from a bend to avoid distortion.

Part 6: Manufacturability and engineering judgement.

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

The lesson map. Sheet-metal and fabricated parts (introduction) becomes manageable when you move through the four checks in order and verify each result before continuing.

The skills, taught in order

Skill 31.1 - Model constant-thickness sheet parts

Concept. Sheet parts keep one thickness and are built from flanges and bends. Terminology. Sheet metal, thickness, flange, bend. Procedure. Use the sheet-metal tools: set the thickness, create a base flange, and add flanges/bends, keeping thickness constant. Reasoning. Constant thickness is the defining property; sheet-metal tools enforce it. Failure mode. Modelling a sheet part as a varying-thickness solid. Check. State what stays constant in a sheet-metal part.

Skill 31.2 - Create flanges and bends

Concept. Flanges add material; bends fold it with a radius. Terminology. Base flange, edge flange, bend radius. Procedure. Create the base flange, then add flanges and bends at the required angles with sensible bend radii. Reasoning. Flanges and bends build the folded shape the shop will produce. Failure mode. Zero-radius (sharp) bends, which sheet metal cannot form. Check. Add an edge flange with a bend radius.

Skill 31.3 - Read the flat pattern

Concept. The flat pattern is the unfolded shape that is cut before bending. Terminology. Flat pattern, bend allowance, K-factor. Procedure. Unfold the part to view the flat pattern; note that the flat length accounts for bend allowance (material consumed by bends), governed by the K-factor. Reasoning. The flat pattern is what is manufactured; its dimensions depend on the bends. Failure mode. Assuming the flat length is just the sum of the folded leg lengths. Check. State why the flat length differs from the sum of the folded legs.

Skill 31.4 - Place holes and reliefs correctly

Concept. Holes must sit away from bends, and bends often need relief. Terminology. Bend relief, minimum hole-to-bend distance. Procedure. Keep holes a minimum distance from bends (so they do not distort), and add bend reliefs where a bend meets an edge to prevent tearing. Reasoning. Material moves at a bend, so nearby holes distort and unrelieved bends can tear. Failure mode. Holes on or near a bend, or missing reliefs. Check. Relocate a hole that sits on a bend line.

Worked example 1: an L-flange sheet-metal bracket and its flat pattern

Problem. Model a simple L-shaped sheet-metal bracket (a base flange with one 90-degree bend to an upright flange) in a chosen thickness, and view its flat pattern.

Planning. Set the thickness, create the base flange, add an edge flange with a bend, and unfold.

Solution.

Thickness. Set the sheet thickness (say 2 mm); it stays constant throughout.
Base flange. Create the base flange (the horizontal leg) at the required size.
Edge flange and bend. Add an edge flange for the upright leg, with a 90-degree bend and a sensible bend radius (often close to the thickness). This forms the L.
Flat pattern. Unfold the part to view the flat pattern: a flat strip whose length accounts for the two legs plus the bend allowance (the bend consumes some material, so the flat length is not simply the sum of the leg lengths).
Result. A folded L-bracket and its flat pattern, ready to cut and bend.

Result. An L-shaped sheet-metal bracket of constant 2 mm thickness with one 90-degree bend, and its flat pattern showing the cut shape whose length includes the bend allowance.

Why the method works. Sheet-metal tools enforce constant thickness and compute the flat pattern from the bends, giving both the folded part and the manufacturable flat.

How to verify independently. The flat length should be slightly different from the simple sum of the two leg lengths, because the bend consumes material. That difference confirms the bend allowance is being accounted for.

Worked example 2: a hole too close to a bend

Problem. The L-bracket has a mounting hole placed close to the bend, so it would distort when bent. Show the problem, relocate the hole to a safe distance, and add a bend relief where needed. The complication is respecting sheet-metal constraints on hole placement and bends.

Planning. Identify the distortion risk, move the hole away from the bend, and add relief.

Solution.

The problem. A hole placed within the bend region (or too close to it) distorts when the material bends through that area, becoming oval or deformed. On the flat pattern, the hole sits inside the bend zone.
Minimum distance. Holes should sit at least a minimum distance from the bend (a common guideline is a few times the thickness plus the bend radius; the exact value is process-dependent and flagged for verification). Move the hole beyond that distance, onto the flat portion of the leg.
Bend relief. Where the bend meets a free edge, add a bend relief (a small notch) so the material does not tear at the bend corner.
Recheck the flat pattern. Unfold again: the hole now sits clear of the bend zone, and the relief appears at the bend-edge junction.
Result. The hole is relocated to a safe distance and a bend relief is added, so the part bends without distorting the hole or tearing.

Comparison. Leaving the hole near the bend distorts it; relocating it and adding relief produces a clean, manufacturable part. Sheet-metal features must respect the bend zones.

Result. The mounting hole is moved beyond the minimum distance from the bend and a bend relief is added, preventing hole distortion and edge tearing.

Independent check. On the flat pattern, confirm the hole lies outside the bend zone and a relief sits at the bend-edge junction. Clear placement confirms the constraints are respected.

Misconceptions and diagnostics

Misconception	Why it seems reasonable	Why it is wrong	Evidence that reveals it	Correction	Diagnostic question
"Model sheet parts like solids."	It is still a 3D part.	Sheet metal keeps constant thickness and unfolds; solid modelling ignores this.	A varying-thickness solid has no valid flat pattern.	Use sheet-metal tools with constant thickness.	"Is thickness constant and does it unfold?"
"Holes can go anywhere."	A hole is just a hole.	Holes near bends distort as the material bends.	The hole becomes oval after bending.	Keep holes a minimum distance from bends.	"Is this hole clear of the bend zone?"
"The flat length is the sum of the legs."	The legs add up.	Bends consume material (bend allowance), so the flat length differs.	The flat pattern length is not the simple sum.	Let the tool compute the flat pattern.	"Did I account for bend allowance?"

Practice ladder

Level A - Recognition

Task. Identify flange, bend, bend radius, and flat pattern on six sheet-metal examples. Deliverable. Six labelled examples. Success criteria. At least five correctly labelled. Answer guidance. The flat pattern is the unfolded shape. Common errors. Confusing a flange with a bend. Difficulty. Low.

Level B - Guided application

Task. Create a scaffolded two-bend sheet-metal part and view its flat pattern. Deliverable. The folded part and flat pattern. Success criteria. Constant thickness; sensible bend radii; valid flat pattern. Answer guidance. Base flange first, then edge flanges. Common errors. Zero-radius bends. Difficulty. Medium.

Level C - Independent application

Task. Model a sheet-metal bracket independently with correct hole placement and relief. Deliverable. The bracket and its flat pattern. Success criteria. Holes clear of bends; reliefs present; valid flat pattern. Answer guidance. Respect minimum hole-to-bend distance. Common errors. Hole in the bend zone. Difficulty. Medium.

Level D - Transfer and design

Task. Critique a supplied sheet-metal part for hole placement, reliefs, and bend radii, and revise it. Deliverable. The revised part plus a critique. Success criteria. All sheet-metal issues found and fixed. Answer guidance. Check bends, holes, and reliefs against the constraints. Common errors. Missing a needed relief. Difficulty. Medium to high.

Working with AI, and proving it yourself

Use AI as a tutor

Useful AI support:

Ask it to explain bend allowance and the flat pattern qualitatively.
Ask it to list sheet-metal design rules to check.
Ask it to suggest a minimum hole-to-bend distance (then verify against your shop's standard).

Limits:

A text assistant may quote K-factor or bend values that do not match your material and machine.
It cannot see your flat pattern.

Verify AI output against: the constant-thickness principle, the flat-pattern concept, and your shop's or supplier's sheet-metal guidelines for exact values.

Prove it yourself

A plausible but incorrect AI answer, and how to catch it. You ask, "What is the flat length of my bracket, just the two leg lengths added?" and the assistant replies: "Yes, add the two leg lengths to get the flat length."

This ignores the bend. Detect it with the flat-pattern principle: a bend consumes material (bend allowance), so the flat length is not the simple sum of the folded legs; it is computed from the geometry and K-factor. The evidence is the CAD flat pattern, whose length differs from the leg sum. Correct conclusion: let the sheet-metal tool compute the flat pattern; the flat length accounts for bend allowance.

Retrieval and spaced review

What stays constant in a sheet-metal part?
What is a flat pattern?
Why does the flat length differ from the sum of the legs?
Why keep holes away from bends?
What is a bend relief for?
What does a bend have that a sharp fold does not?
Cumulative (L30): How does sheet-metal manufacturing differ from machining in how material is handled?
Reconstruction task: From memory, describe modelling the L-bracket and getting its flat pattern.

Answers. 1: thickness. 2: the unfolded shape that is cut before bending. 3: bends consume material (bend allowance). 4: they distort as the material bends. 5: to prevent tearing where a bend meets an edge. 6: a bend radius (sheet metal cannot form a zero-radius sharp fold). 7: machining removes material from solid stock; sheet metal cuts a flat and bends it, keeping constant thickness.

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

Reference mapping and next step

Read further

Onshape docs (sheet metal)
Giesecke ch.13.

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

Finish the lesson

You can now: model constant-thickness sheet parts; create flanges and bends; read a flat pattern; and place holes and reliefs correctly.

Self-assessment checklist.

I model sheet parts at constant thickness.
I use sensible bend radii, never zero.
I read the flat pattern for the cut shape.
I keep holes clear of bends.
I add bend reliefs where needed.

Next lesson: L32 - Additive-manufacturing considerations. Why it follows: machining removes material and sheet metal bends it; additive manufacturing builds it up in layers, with its own opportunities and constraints. L32 completes the process trio before design-for-assembly.

Required files or submissions: submit your Level C sheet-metal bracket and flat pattern. Optional extension: in Onshape, model a simple sheet-metal bracket and flatten it.