--- marp: true theme: default html: true --- # Parametric design Parametric design · Linked dimensions --- ## Today **We are learning to** 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. - 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. --- ## Why it matters > The board your case was designed for has been replaced by a bigger one. Do you start again, or change one number? --- 💬 Discussion ## Predict **In a parametric case, the part gets 10 mm wider. What happens to the outside width?** - Nothing until you redraw it - The walls get thinner to make room - It grows by 20 mm - It grows by 10 mm by itself --- ✍️ Activity ## Try it {{embed: https://studio.learningwaypoints.com/tutorials/_playground-board/index.html?id=deck%2Fengineering%2Fparametric%2Fparametric%2Fstep-2&code=widget%3A+fab%0Amode%3A+param%0Apart%3A+68.6%0Aclearance%3A+1%0Awall%3A+2%0A&fresh=1}} [Open this step in the tutorial](https://studio.learningwaypoints.com/tutorials/engineering/parametric/parametric/?full=step-2#step-2) --- ✅ Check ## What happened? * It grows by 10 mm by itself: outside = part + 2 × (clearance + wall), and only the part changed. --- 💬 Discussion ## Driving and driven Now a 50 mm part. $$\#part = 68.6, \; \#clearance = 1, \; \#wall = 2$$ 1) $\text{inside} = \#part + 2 \times \#clearance = 70.6$ — Worked out, never typed. 2) $\text{outside} = \text{inside} + 2 \times \#wall = 74.6$ — Worked out from the inside. 3) $\#part = 75 \Rightarrow \text{outside} = 81$ — Change one number; every size that depends on it follows. --- ✅ Check ## Driving and driven **Part 50 mm, clearance 1 mm, wall 2 mm. Outside width (mm)?** * Answer: 56 --- ✅ Check ## Find the mistake **Sam has to redraw the whole case for the new board. Click the line where Sam's design first goes wrong.** ``` 1 Sam measures the Uno: 68.6 mm wide. 2 Sam types the case's outside width as 74.6 mm. 3 The project switches to a board 75 mm wide. 4 Sam redraws the whole case from scratch. ``` * Answer: line 2 --- ✍️ Activity ## Do it in Onshape --- ✅ Check ## Which drives? **Which size should be a driving variable rather than worked out?** - The case's outside width - The width of the part that goes inside - The lid's outside width - The case's inside width * Answer: The width of the part that goes inside --- ✅ Check ## The lid **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)?** * Answer: 79.6 --- ✍️ Activity ## Make: the biggest part [Open this step](https://studio.learningwaypoints.com/tutorials/engineering/parametric/parametric/?full=step-6#step-6) --- ✅ Check ## Exit ticket **Why make a case's outside width an expression instead of a number?** * So it updates by itself when the part, clearance or wall changes, instead of being redrawn.