Parametric design

In a parametric model, dimensions are linked by rules: outside = part + 2 × (clearance + wall). Change the part, and everything that depends on it updates. Name the numbers that drive the design, and work the rest out from them.

The board your case was designed for has been replaced by a bigger one. Do you start again, or change one number?

In Onshape (and Fusion, and Tinkercad's newer parameters) a design can have variables: #part = 68.6 mm, #clearance = 1 mm, #wall = 2 mm. Every other size is an expression using them, such as #part + 2 * (#clearance + #wall). That is a parametric model: the numbers that matter drive it, and the rest is worked out.

Predict: Read the problem and decide what you think before you try anything. A wrong guess is fine: it shows you what to look for.
Try: Try it on the board and compare the result with your prediction.
Investigate: Work out why it happens, one step at a time.
Modify: Change the problem on purpose and see what happens.
Practise: A few on your own, to check you have it.
Make: Make your own problem using what you have learned. Expect a few tries; that is normal.

WidthMore contexts, same idea.

PredictRead the problem and decide what you think before you try anything. A wrong guess is fine: it shows you what to look for.

A bigger part

In a parametric case, the part gets 10 mm wider. What happens to the outside width?Nothing until you redraw itThe walls get thinner to make roomIt grows by 20 mmIt grows by 10 mm by itself

TryTry it on the board and compare the result with your prediction.

Try: change the part

Try it yourselfYour edits are saved in this browser.

Make the part 10 mm wider (78.6 mm). What is the outside now?

Use the board. This ticks when you get there.

Now halve the clearance (0.5 mm). How much does the outside change?

Use the board. This ticks when you get there.

InvestigateWork out why it happens, one step at a time.

Worked example: write the rules

Worked exampleBefore each step, say what you think comes next.

$\#part = 68.6, \; \#clearance = 1, \; \#wall = 2$ The driving dimensions: the only numbers typed in.
$\text{inside} = \#part + 2 \times \#clearance = 70.6$ Worked out, never typed.
$\text{outside} = \text{inside} + 2 \times \#wall = 74.6$ Worked out from the inside.
$\#part = 75 \Rightarrow \text{outside} = 81$ Change one number; every size that depends on it follows.

Now a 50 mm part.

Part 50 mm, clearance 1 mm, wall 2 mm. Outside width (mm)?

Sam has to redraw the whole case for the new board. Click the line where Sam's design first goes wrong.

Modify TypicalChange the problem on purpose and see what happens.

Do it in Onshape

In Onshape: I created variables #part, #clearance and #wall, sketched the case with the outside dimension typed as #part + 2 * (#clearance + #wall), extruded it, then changed #part and watched the case update.

PractiseA few on your own, to check you have it.

Rules

Which size should be a driving variable rather than worked out?The case's outside widthThe width of the part that goes insideThe lid's outside widthThe case's inside width

A lid sits over the case with 0.5 mm clearance each side and its own 2 mm wall. Outside = 74.6. Lid outside (mm)?

MakeMake your own problem using what you have learned. Expect a few tries; that is normal.

The biggest part

Try it yourselfYour edits are saved in this browser.

Need a hint?

outside = part + 2 × (1 + 2) = part + 6.

Another hint

The outside can be at most 80.

Another hint

part = 80 − 6 = 74.

The case must be no more than 80 mm wide outside (clearance 1 mm, wall 2 mm). Set the biggest part that fits.

Use the board. This ticks when you get there.

For teachers

Students change the driving dimensions of a parametric case and watch the outside follow, write each size as an expression of the part, clearance and wall, see why Sam's typed-in number forces a redraw, build the same model with variables in Onshape, and work back from a size limit to the largest part. About 40 minutes.

Open the teacher guide: answers, solutions, slides and a worksheet.

Students will be able to

I can tell driving dimensions from driven ones.
I can write a size as an expression of variables.
I can change a parametric model by changing one number.

Before class

Onshape: free education plan, runs in the browser; variables are made with the Variable feature (#name) and used in any dimension box. Fusion works the same way with Parameters. This links to Maths (expressions and substitution) and to Code (variables).

Running the lesson

Predict: ask what the case's outside depends on.
Worked example: underline the driving numbers.

The hard part (on purpose)

Students type the number they worked out. Insist: if you worked it out, it's an expression.

When students get stuck

A student's model doesn't update. Ask: Is that dimension a number or an expression? Fix: Type the expression with the variables.

Stretch and support

Stretch: Add a #fillet variable for rounded corners and a #lid_gap for the lid. Which sizes depend on each?

Support: A three-column table: variable, value, which sizes use it.

Where it leads

Next (optional): Laser cutting and kerf.

Sources

Onshape Learning Center: variables (link) (checked 12 October 2026)

What each step is for

a bigger part: Committing to what changes when the part grows tests the new move: in a parametric model, dependent sizes update themselves.
Try: change the part: Changing the driving numbers on the board settles the prediction and shows which numbers each size depends on.
Worked example: write the rules: Writing each size as an expression of the driving variables models the move. Sam's typed-in number is planted after the method.
Do it in Onshape: Creating variables in Onshape and using them in sketch dimensions is the real skill this lesson points at.
rules: Reading and writing dependent sizes.
the biggest part: Working back from a limit on the outside to the largest part that fits reverses the rule.

Now you can

Tell driving dimensions from driven ones.
Write a size as an expression of variables.
Change a parametric model by changing one number.

Next lesson2. Laser cutting and kerfA laser beam burns away a thin strip as it cuts: the kerf. So a cut slot comes out wider than drawn (by the kerf) and a cut-out part a little smaller. Draw slots narrower than the board for a snug press fit.

Source: content/engineering/parametric/parametric/lesson.md