#Sketch: an overview

A sketch is the flat drawing every part stands on. You draw an outline, give it constraints and dimensions, then turn it into a body: extrude, revolve, sweep.

#The main rule: a sketch should be fully defined

The status line shows the degrees of freedom — how much can still be moved without breaking any constraint. While that number is above zero the sketch floats: it looks right, but the first change higher up the history can move it.

A fully defined sketch (zero degrees of freedom) behaves predictably: change a dimension and exactly what that dimension controls changes.

#What it is made of

• Entities — lines, circles, arcs, splines. What you see.
• Constraints — rules between them: horizontal, vertical, coincident, parallel, perpendicular, equal, tangent, symmetric, point on line, point on circle.
• Dimensions — constraints with a number: length, angle, radius. The number can be a formula.

#How drawing works

A tool is switched on by a button on the left or by its hotkey and stays on: you can draw several lines in a row. Esc leaves the tool.

Auto constraints (the wand in the top bar) add the obvious ones for you: draw a nearly horizontal line and you get a horizontal constraint. It saves time but sometimes guesses wrong; then delete the constraint from the list.

#Construction geometry

The X key toggles construction mode: such lines are dashed, never become part of a body profile and exist only as support for constraints — centre lines, diagonals, circles for arrays.