Parametric habits that survive the third revision

The first version of a part always works. The trouble starts at revision three, when someone asks for a bigger bore and half the features turn red. These are the habits that made my models stop breaking — none of them depend on a particular CAD package.

Name the parameters that matter

A model with d12, d47 and d103 is a model nobody can change. I give a real name to every dimension someone might ask about and derive the rest from them:

bore        = 25
pcd         = 125
hole_count  = 4
hole_dia    = 18
thickness   = 18
outer_dia   = pcd + 2 * hole_dia + 4

Everything else in the model refers to these names. If a number appears in a sketch without a name, it is either cosmetic or a bug waiting to happen.

Sketch from the origin

Anchor the first sketch to the origin planes and put the part's symmetry on them. A shaft lies along an axis; a flange is centred on the origin. Features that reference the origin planes survive almost any edit. Features that reference a face created three steps earlier break as soon as that face moves.

Fillets and chamfers go last

Rounds and chamfers pick edges, and edges are the most fragile thing in a model: they get renumbered whenever the geometry before them changes. If the fillet is the last feature, an edit higher up can only break the fillet, not everything after it. On a flange that means: disc, bore, holes, pattern, and only then the chamfer on the bore.

Pattern the feature, not the sketch

Draw one bolt hole and pattern it around the axis with hole_count as the instance count. Drawing all eight holes in one sketch looks quicker, but changing to four holes then means editing the sketch by hand.

Keep a table for standard sizes

For catalogue parts — nuts, washers, flanges — I keep one table per standard and let a single "size" parameter pick the row. The model never contains a hard-coded M16; it contains size, and the table says what M16 means.

A good parametric model is boring to edit. You change one number and nothing interesting happens.

Related: the flange checklist and the note on involute gears.