CAD Modeling Techniques

Feature-based modelling builds a part as a sequence of engineering features, such as base extrusion, hole, rib, fillet, pocket and pattern. Each feature stores its parameters (dimensions, depth, position) and its parent relations. Features carry design intent, so the part can be edited by changing a parameter, and the model rebuilds. Features may be sketch-based (extrude, revolve, sweep, loft) or applied (hole, fillet, chamfer, shell, draft).

Sketch-based operations

1. Extrusion: a closed 2D profile is pushed through a distance (blind, mid-plane, up to a face or through all). Result: prismatic solid. Cut-extrude removes material.

  [ profile ]  ->  |=======|
                   |       |   depth d

2. Revolution: a profile on one side of an axis is rotated by an angle (up to 360 deg). Result: shafts, pulleys, bottles. Revolved cut makes grooves.

   |  profile       axis
   |  +--+           |
   |  |  |  ->    (solid of revolution)

3. Sweep: a profile moves along a path (guide curve); used for pipes, springs, handrails. The profile may stay perpendicular to the path.
4. Loft (lofting): a solid or surface is fitted through two or more profiles at different planes; guide curves control the shape. Used for aerofoils, bottle necks, ducts.
5. Blend: a smooth transition between two or more sections, often of different shapes (circle to square duct), with straight or smooth (rotational) transition; unlike loft, usually the sections are parallel and joined point by point.

Feature tree and design changes

The feature (history) tree lists features in order of creation. Editing a feature or its sketch dimension triggers a regeneration of all child features. Order can be rearranged, features can be suppressed, and patterns and mirrors reuse features. Parent-child relations must be kept clean: deleting a parent affects its children.

Advantages

• Fast design changes by parameter edit.
• Design intent preserved through constraints and equations.
• Family of parts (configurations) from one model.
• Features map directly to manufacturing operations (hole, pocket), supporting CAM.

(a) Assembly constraints (mates)

An assembly is made by placing parts and removing their degrees of freedom (a free part has six: three translations and three rotations) using constraints.

A fully constrained assembly may be built bottom-up or top-down. Interference check and clearance check confirm correct assembly.

(b) Limits and fit

Hole:

• Upper limit $=40+0.025=40.025$ mm; lower limit $=40.000$ mm.
• Tolerance $=40.025-40.000=0.025$ mm.

Shaft:

• Upper limit $=40-0.009=39.991$ mm; lower limit $=40-0.025=39.975$ mm.
• Tolerance $=39.991-39.975=0.016$ mm.

Clearances:

$$\begin{aligned} C_{max}&=\text{hole}_{max}-\text{shaft}_{min}=40.025-39.975=0.050\ \text{mm} \\ C_{min}&=\text{hole}_{min}-\text{shaft}_{max}=40.000-39.991=0.009\ \text{mm} \end{aligned}$$

Fit tolerance $=C_{max}-C_{min}=0.041$ mm $=0.025+0.016$.

Since $C_{min}>0$, the shaft is always smaller than the hole, so it is a clearance fit (running/sliding fit, similar to H7/g6).

Answer: hole 40.000 to 40.025, shaft 39.975 to 39.991; C_max = 0.050 mm, C_min = 0.009 mm; clearance fit.

Before an entity (line, arc, face, feature, part) can be edited, it must be selected. After selection, commands manipulate it.

Selection methods

• Pick (click): direct selection of one entity; with a pick box (aperture) around the cursor.
• Window: a rectangle from left to right selects entities completely inside.
• Crossing window: a rectangle from right to left selects entities inside or touching it.
• Fence / lasso: a line or free-hand shape selects whatever it crosses or encloses.
• Multiple (Ctrl/Shift) and select-all: add or remove entities from a set.
• Filter / by property: select by layer, colour, type, name or feature.
• Drag: hold and move an entity or handle (grip) to change position or size.
• Previous / last: reuse the last selection.

Manipulation (transformation) commands

• Translate / move, rotate, scale, mirror (reflect), copy, pattern (linear, circular), stretch.
• These use a $4\times4$ homogeneous matrix; for a 2D scaling by $S_x, S_y$ about the origin: $x'=S_xx,\ y'=S_yy$.

Editing commands

• Trim / extend a curve to a boundary, break, join, offset, fillet, chamfer, explode (split a group).
• Delete, undo/redo, modify a dimension, edit sketch, suppress or roll back a feature.
• Grip editing: move control points of a curve or a surface to change its shape.

Good selection tools reduce errors and speed up modelling: working on layers, groups and filters avoids selecting the wrong entity.