Checking Drawings by the Numbers

About this book

A drawing checker is the last person to read a drawing before strangers act on it. Once the drawing is issued, a supplier quotes from it, a machinist cuts metal to it, an inspector accepts or rejects parts against it, and a buyer orders the bought-out items its parts list names. Every fault the checker misses is found later by one of them, at a higher price. The signature in the checked box says that someone looked, deliberately, for the faults that would cost the most.

This book is a method for that look. It works through a drawing in the order a checker should: the sheet and its title block, the views, the dimensions, the limits and fits, the stacks across an assembly, the notes on material, surface texture and treatment, and the assembly drawing and parts list that tie the set together. Each chapter gives a procedure, the numbers that test it, a fault a real drawing office could plausibly miss and how it was caught. The last chapter turns to the checking process itself: how to record findings, measure what checking catches and decide where effort pays.

It is written for drafters who check their own and others’ work, design engineers who sign drawings, and leads who run a small drawing office. It assumes you can read a mechanical drawing to AS 1100 and do the arithmetic of limits and fits. What the conventions on a drawing mean is the subject of the Library’s Drawings That Make Parts; how to set tolerances and budget a stack is covered in Fits That Work. This book is about finding where a drawing falls short of them.

The running case

Every chapter checks one fictional drawing set. This is an illustration. A small machinery builder has designed a belt tensioner for a conveyor drive, drawing TA-200. Its arm, TA-201, is a ductile iron casting with a machined pivot boss 30 mm long, a bore Ø16.000 to 16.018 mm for a bush, and a pulley spindle hole 120 mm from the pivot. A sintered bronze bush, TA-203, is pressed into the boss, and the arm turns on a zinc-plated steel pivot pin, TA-202, of Ø11.973 to 11.984 mm, held axially between two washers and a retaining ring. The assembly drawing lists seven items, three made and four bought out.

The drafter has finished the set and passed it to the checker. Each chapter checks part of it, with real numbers, and most find a fault that would otherwise have reached the workshop. The business, the drawings and the faults are invented; none describes KEVOS® work or a real supplier, and supplier data in the case is illustrative.

Source limits and professional responsibility

The calculations in this book are arithmetic on the limits shown on the running case’s drawings, and every one was checked by running it. Where the case needs supplier behaviour, such as how much a pressed bush closes up, the value is labelled as an illustration; real work needs the supplier’s data or a trial.

Checking reduces risk; it does not transfer responsibility. The designer remains responsible for the design, and the person who approves a drawing for issue answers for its fitness. Where a drawing covers safety-critical or regulated equipment, such as guarding, lifting gear or pressure equipment, the checks in this book are part of, not a substitute for, the engineering review and approvals that Australian law and the relevant standards require.