How to read a shop
drawing in ten minutes

Start with the tolerance block

Most sheets have one, and most of them say something useless like "commercial tolerance". A tolerance you can reject a part against is a number with a unit: ±1 mm on aperture pitch, 1.2 mm per metre on flatness, ±0.5° on a fold angle.

If the block is missing or vague, the rest of the sheet is a drawing rather than a specification. You cannot hold anyone to it, and neither can the machinist.

Find the 1:5 details

An elevation at 1:20 shows intent. The joint at 1:5 shows what will actually be built. If the sheet has elevations but no 1:5 details, the joint has not been designed yet — it has been left to the workshop or to the site.

Count the junctions on the elevation, then count the details. The numbers should match. When they do not, the missing ones are where the argument will happen.

Check which face is visible

Every panel has two faces and at most one of them matters. A good sheet marks it, and that mark then follows the part through cutting, welding, packing and the crate.

When it is not marked, the part is handled on the finish side at least once, and the mark shows up as a scuff you cannot photograph usefully and cannot polish out.

Look for part numbers

A part number is what makes a drawing traceable. It should carry the zone, the elevation and the sequence, and it should appear on the part, in the packing list and on the trial-fit sheet.

If the sheet has no part numbers, the job will be packed by production order instead of install order — and the install order is what the crew actually needs.

A ten-minute check

1. Tolerance block: numbers with units, or nothing
2. 1:5 details: one per junction on the elevation
3. Visible face: marked on every view
4. Part numbers: zone, elevation, sequence
5. Install sequence: stated, not implied

If a sheet passes all five, it can be built and it can be argued about. If it fails two or more, ask for a revision before anyone orders sheet.