Drawing numbering and revision blocks that survive 20 years

The wrong part on the bench is rarely a wrong design. It is a right design carrying a number that changed meaning, or two revisions of the same number sitting in one bin. The drawing was correct when it was issued; the numbering let it down two years later.

A part-number scheme has to survive more than the drawing it was invented for. In 20 years it will pass through three CAD systems, two ERPs, a supplier change, a material substitution and at least one merger of product families. drawing numbering that encodes today's material or today's plant becomes wrong at the first change and forces a renumber that breaks every record behind it.

The rules below cover the number, the revision block and the change note, and they are written so that an operator at a machine can act on all three without calling engineering.

What does drawing numbering have to survive?

A part number is an identifier, not a description. Its only job is to point at one part, uniquely and permanently, for as long as the part exists and after it is discontinued.

That produces five constraints, and most schemes break at least one:

• Stability. The number cannot change when the design, material or supplier changes.
• Uniqueness. No two parts share a number, and a retired number is never reissued.
• Occam. It has to fit the ERP field, the barcode and the human voice on a telephone.
• Sortability. Related parts should sort together when the family is what matters.
• Neutrality. It should not encode anything that can change.

The last one is the hardest to accept, because an intelligent number is convenient on the day it is written and a liability ever after.

Should a part number be intelligent or dumb?

Use a dumb sequence with a short family prefix, and keep the attributes in the ERP. That is the decision rule, and the arithmetic behind it is simple.

An intelligent number such as GM-EL-0142-CR15-A encodes material CR and thickness 15. At revision C the design moves to 2.0 mm. The options are to keep a number that now lies about the thickness, to renumber the part and lose its purchase history, or to add a second suffix that nobody reads. All three happen in real factories, and all three end with a wrong part in a bin.

A hybrid works better than either extreme. Encode only what will never change: the product family, a sequence and a variant for mirror parts. Two letters, four digits, one variant letter and one check digit give a 10-character number:

• EL — elevator family, 2 characters.
• 0142 — sequence, 4 digits, never reused within the family.
• A — variant, 1 letter, for left-hand, right-hand or mirror parts.
• 7 — weighted check digit over the first nine characters.

Ten characters fit an 18-character ERP material field with room for a six-character suffix where the ERP needs one, and they print as a Code 128 label about 41 mm wide at a 0.25 mm module, which is readable at arm's length on a bin.

The sequence capacity is the part people underestimate. Four digits give 9,999 numbers per family. At 300 new parts a year that is 33 years, and at 60 a year it is longer than the product line will last. If a family ever runs out, add a letter to the family prefix rather than widening the sequence.

A scheme that holds for 20 years

The check digit is not bureaucracy. Part numbers are typed into ERPs, spoken over the phone and read off a screen by a person holding a drawing. A single transposed pair of digits is the most common entry error, and a check digit catches every single-digit error and every adjacent transposition except 0 and 9 swapped. On a laser program library, that is the difference between cutting the right part and cutting 200 of the wrong one.

Three habits keep the scheme intact through a system change:

Map, do not renumber. When the ERP changes, migrate the old number into a legacy field and keep it searchable. A customer who ordered EL-0142-A for 15 years must still find it.

Tombstone, do not delete. A discontinued part keeps its master record, its drawing and its revision history, marked obsolete. Deleting it frees the number for reuse, which is the one failure the scheme cannot detect.

Put the number in the file name. Name the DXF, the bend program and the STEP file as NUMBER_REV.ext. A folder of files called final, final2 and final_revised is how a revision reaches the machine without its letter.

What belongs in the revision block?

A revision block is a contract between the person who changed the drawing and the person who has to make the part. ISO 7200 sets out the data fields a title block and document header should carry, and ASME Y14.35 sets out how revisions are recorded and lettered.

The fields that earn their space:

• Revision letter. Advance one letter per release. Skip I, O, Q, S, X and Z, which leaves 20 of the 26 letters and 20 years of revisions at one change a year before the sequence continues to AA.
• Date. The date the revision was released, not the date it was drawn.
• Change description. What changed, where, and why, in language an operator can act on.
• Zone. The sheet reference from ISO 5457's grid, so the reader can find the change without comparing two prints.
• Change document number. The ECN or change request that authorised it.
• Drawn and approved. Two names, because one person cannot both propose and release.
• Effectivity. The serial, date or quantity from which the new revision applies.
• Interchangeability. One flag: can this revision and the previous one be mixed in a bin?

The change description is where most revision blocks fail. "Hole changed" tells an operator nothing. "Hole Ø6.5 to Ø8.0 at zone B3 for M8 fixing; not interchangeable with rev B" tells them what to check, where to look and whether the parts in the bin are still usable.

The zone reference is worth the extra 20 seconds. On an A1 sheet with a full grid, a zone note narrows the search from a whole drawing to a 100 mm square, and it is the difference between a first-piece check that finds the change and one that confirms what was already there.

Why do two revisions end up in one bin?

Because a revision letter implies interchangeability, and nothing on the drawing says otherwise.

A revision that moves a hole 1.5 mm is a new part. It will not fit the mating bracket, it will not pass the checking fixture, and it cannot be mixed with the previous batch. But it carries the same number as the parts already on the shelf, so the store issues whichever comes to hand, and the assembly line discovers the problem at the fixture.

The cost is measurable. Sort a bin of 400 parts by revision, at 30 seconds each with a gauge in hand, and the job takes 3.3 hours and still relies on the parts being visibly different. If they are not, the sort fails. A part number that changes with the revision costs nothing at that moment and saves the bin.

The rule is worth writing into the drawing standard:

Geometry changes are the common case. A bend radius change from R2 to R3 on 2 mm CRCA moves the bend allowance by about 1.6 mm per bend, so a four-bend box grows 6.3 mm in the flat while every 2D dimension on the drawing stays the same. That is a new part number, not a revision, and the drawing revision traps guide sets out the arithmetic.

What does the shop need, and what does Ganesh check?

The number and the revision have to arrive together, on the same document, at the same time. In practice that means the PDF, the DXF or STEP and the revision field all agree, and the file name carries both.

At Ganesh the part number and revision are keyed on the router, the cutting program, the bend program and the first-article record. A revision that changes the flat pattern needs a new DXF as well as a new PDF, because the flat pattern cannot be regenerated from a 2D view. Where only a STEP file is supplied, the flat is generated from the model and the bend allowance is applied from the material and tooling data, which is why a model that disagrees with the PDF by a bend is a defect the DFM review will stop.

The in-house tool room keeps the checking fixtures, and each fixture is tagged with the part number and the revision it was built for. A fixture built to rev A and used on rev C is a pass on a part that does not fit, which is worse than a fail.

The design and CAD hub covers the rest of the drawing standard. Revision control for sheet metal covers how a released change reaches the floor, and the drawing for quotation guide covers what a supplier needs in the first issue. The tools and dies capability page shows where fixtures and checking gauges are built.

When the scheme has to be replaced, and when it must not be

A scheme is replaced when it cannot express a part, not when it is inconvenient. Two cases force it: two companies merge with overlapping families, or a product line is renumbered because the old system encoded a plant that no longer exists.

In both cases the rule is the same. Issue new numbers for new parts, keep the old numbers alive for existing parts, and put a cross-reference in the ERP. A big-bang renumber across a live product line breaks purchase orders, service spares, warranty history and every drawing already in a customer's filing cabinet, and it buys nothing that a cross-reference field does not already give you.

What must not change is the revision discipline. A new scheme with no interchangeability flag and no effectivity is the old problem in a new format.

What to put on the drawing

• The part number in the title block, with the revision letter in its own field, per ISO 7200.
• The sheet size, zone grid and revision table position per ISO 5457.
• The change description with the zone, the feature and the reason.
• The effectivity as a serial, date or quantity.
• The interchangeability flag, Y or N, with a one-line note when it is N.
• The superseded revision number, so the reader knows what this replaces.
• The material, thickness and finish as attributes, not inside the number.
• The associated file names: number, revision, and the DXF or STEP that matches the PDF.

The number that outlives the drawing

A drawing archive outlives its authors, its CAD system and often the product. The number is the only part of it that a customer, a store and a machine all read the same way. Ten characters, a check digit, a revision field and a flag that says whether the bin can be mixed will still work when the third CAD system arrives.

Send the drawing with the revision and the effectivity to request a quote. The review starts with the number, because a part built to the right drawing with the wrong revision is still the wrong part.