MYP Design 3D CAD Modelling Notes

CAD Is A Model That Remembers How You Built It

1. CAD stands for computer aided design, and a 3D CAD model is a solid object described in software rather than a picture of one.
2. Fusion 360, SolidWorks, Onshape and Tinkercad all work the same way underneath, building solids from flat sketches and keeping a record of every step.
3. Parametric modelling means the model is driven by numbers you can go back and change, so a 40 mm hole becomes a 50 mm hole with one edit.
4. Those numbers are called parameters, and they can be linked so that setting a wall thickness once updates every wall that refers to it.
5. The software knows the model is solid, so it can report volume, mass in a chosen material and centre of gravity without you measuring anything.
6. One hand drawing shows one view of an idea, while one CAD model can produce every view, a render and a cut file from the same data.

Sketch First, Then Give It Thickness

1. Every solid starts as a 2D sketch on a flat plane, usually the front, top or right plane, or a flat face of something you have already made.
2. Extrude pushes a closed sketch a set distance to make a solid, so a 60 mm by 40 mm rectangle extruded 12 mm gives a block.
3. Extrude also cuts, so a 6 mm circle sketched on a face and extruded as a cut through all leaves a clean hole.
4. The sketch has to be a closed profile, because an open loop has no inside for the software to fill with material.
5. Sketch on whichever plane makes the shape easiest to describe, since a badly chosen plane turns a two step part into a ten step one.
6. Type real dimensions as you draw rather than dragging things until they look right, or every later edit will fight you.

Revolve And Loft Make Shapes Extrude Cannot

1. Revolve spins a sketch around an axis, which is how you get a bottle, a bowl, a wheel or any round part in a single step.
2. Sketch half the outline of a drinking glass, revolve it 360 degrees about the centre line, and you have the whole glass with its inside already hollow.
3. Loft blends between two or more sketches sitting on different planes, so a square base and a round top join in one smooth transition.
4. A loft is how you model a shampoo bottle or a helmet shell whose cross section changes all the way up.
5. Sweep drags a profile along a path instead, which suits handles, tubing and a kettle spout.

Fillets And Chamfers Are Added Last For A Reason

1. A fillet rounds an edge to a chosen radius, and a 3 mm fillet on a phone stand stops the corners digging into a hand.
2. A chamfer cuts the edge off at an angle, usually 45 degrees, which helps a part start into a hole and removes a sharp burr.
3. A rounded internal corner also spreads load, so a filleted corner survives force that would crack a sharp one.
4. Add both near the end of the model, because filleting early forces every later sketch to work around a curved edge.
5. A radius bigger than the material around it makes the feature fail, so a 4 mm fillet on a 3 mm wall will not build.

The Feature Tree Is Your Build History With Names

1. The feature tree, called the timeline in Fusion 360, lists every sketch, extrude, revolve and fillet in the order you made them.
2. Any entry can be reopened and edited, and everything after it rebuilds on top of the change.
3. Features depend on what came before, so a hole sketched on a face disappears if you delete the extrude that created that face.
4. Dragging a feature earlier or later changes the finished shape, which is how you move fillets to the end of the build.
5. Rolling the tree back shows the part half built, which gives you a screenshot sequence of how the model grew.
6. Constraints inside a sketch, such as horizontal, parallel, equal or tangent, lock relationships that dimensions alone cannot hold.
   1. A sketch with every line constrained and dimensioned is fully defined, and most software turns it a different colour to tell you.

Assemblies: Parts That Know How They Fit

1. An assembly brings separate part files together so you can check they fit before any material is touched.
2. Mates, called joints in some software, tell the model how two parts relate, such as this face sitting flat on that face.
3. A coincident mate sticks two faces together, a concentric mate lines up two round features, and a revolute joint lets one part spin on a single axis.
4. Each mate removes degrees of freedom, and a part with all six removed cannot move at all.
5. Leave the right freedom in place and the assembly moves on screen, so a hinged lid swings and a drawer slides.
6. Edit one part file and the assembly updates, so widening a bracket by 5 mm shows straight away whether the screw holes still line up.

Change The Number, Not The Drawing

1. On paper, making a box 20 mm taller means redrawing every view by hand and re-measuring the lot.
2. In CAD you reopen the extrude, type 70 instead of 50, and the holes, fillets and assembly rebuild around the new height.
3. Set key sizes up as named parameters at the start, such as a wall thickness of 3 mm, then call that name inside your sketches.
4. Trying three heights then costs three numbers and three screenshots, which is exactly the evidence developing ideas asks for.
5. A model built from vague sketches with no dimensions cannot be edited this way and usually ends up rebuilt from nothing.

Where A CAD Model Goes Next

1. The same model can be rendered for a client, sliced for a 3D printer, flattened to a cut file or run through a stress test.
2. Model in millimetres at full size from the start, because rescaling later breaks dimensions that were set to real numbers.
3. Model what you can actually make, so a 0.4 mm wall is pointless if the school printer lays 0.4 mm lines.
4. Keep the native CAD file as your master and treat every export as a one way copy.