Every parameter you expose is a promise you have to keep. Forty named parameters where three ever move leaves thirty-seven states nobody rebuilt. One is waiting for whoever opens it next. Half of parametric work is deciding which dimensions are not allowed to move.

A polished bore reads deeper than it is, and more frames make it confident. Light bounces wall to wall before it reaches the sensor. The long path puts the surface behind where it sits, and every frame gets it wrong the same way. Noise averages out. This does not.

The useful half of a residual is where the signs flip, not how big it is. Random signs around a bore mean noise. Signs that group into three arcs mean a lobe, and the RMS is the same either way. An average hides the pattern it averaged. Plot residuals against angle.

The part worth measuring is usually the one that did not fail. The worn shaft tells you where it rubbed. The housing it ran in is unworn, in tolerance, and still holding the fit you have to hit. Ask for the assembly, not the casualty.

A parametric model is only editable in the directions its author thought about. The tree encodes a guess about which dimension moves next. Guess right and it solves. Guess wrong and you are rebuilding half the part. Parametric is not flexibility, it is a prediction.

The flattest patch on your scan is often where the scanner saw nothing. Targets get masked out, so each leaves a hole. The fill spans it with the smoothest surface that meets the edges, and smooth reads as clean. Those targets were sitting on your datum faces.

A cone fit has to find an apex that is nowhere near your part. A short band of gentle taper puts it off the bench. Nudge the half angle, slide the apex to suit, and the residual barely moves. Take the axis from a feature you trust, then fit circles square to it.

Reverse engineering that ends at a STEP file ended early. A solid says what the shape is. It does not say which diameter is a fit, which face is the datum, or which radius was a decision rather than whatever tool was in the holder. Whoever makes it next has to guess.

A tapped hole in a STEP file is a plain cylinder. The thread lived in a hole feature or a drawing note, and STEP carries neither. The hole keeps its diameter and forgets what it was for. Model the helix and you keep geometry, not the callout. Pick which half to lose.

A feature tree should follow how the part was made, not the order you found the features. Cast body first, machined faces as cuts, holes last. Change the wall thickness and the machining stays where it belongs. Discovery order is a record of your afternoon.

White plastic is translucent, and your scanner reads a little way inside it. Light enters the surface, scatters, and returns from under it. The depth lands low, the face sits inboard, and the part comes in small. Matte spray is not for the shine. It is for the depth.

An editable model you cannot read is not editable, it is unlocked. Changing one dimension means knowing which sketch owns it and what rebuilds downstream. That is reading, and nobody automated reading. A tree nobody can read gets rebuilt from scratch, quietly.

Section a bore off axis and it reads oval, and the fit will agree with you. Slice a cylinder at an angle and you get an ellipse. Minor axis is the diameter. Major axis is tilt. Fit the cylinder first, then section square to its axis. Wear is something you prove.

The part on your bench is a sample of one, and you are about to call it the nominal. Two castings off one pattern disagree before anyone fits them. Add wear and you measured one part's past, not the design. A second part costs a shipping label. The nominal is the job.

The same bore can arrive as a cylinder in one STEP file and a spline surface in the next. A cylinder carries a diameter and an axis. A spline carries control points that happen to be round. Hole recognition finds one of them. The other gets contoured, not drilled.

A tree that errors on the first edit is better built than one that quietly does not. The dangerous one absorbs it. A fillet solves to nothing, a sketch flips to the mirror solution, a hole lands on the face behind the one you meant. Quiet trees ship wrong parts.

The hard part of AI in CAD is not making the model, it is signing it off. Generated geometry still needs someone to say the bore is right. Checking a tree you did not build is slow work. The work did not disappear. It moved to the person with the most to lose.

A bolt pattern is one feature with two numbers, not six separate holes. Fit all six centres to one circle and the errors argue with each other. Pitch diameter and clocking come out tighter than any single hole. Model the holes from the pattern, not where they scanned.

The hardest call on a reverse engineering job belongs to whoever holds the scanner. Coverage, spray, which faces got a second pass: all settled before anyone asked what locates the part. A scan you did not plan is a guess about scope, in a file you now have to quote.

An STL triangle states its facing twice, and the two can disagree. One is the stored facet normal. The other is the vertex winding. Most readers ignore the normal and recompute from the winding. Some trust it, so a mesh flips in one tool. The file holds two opinions.