Revision conflict on an aluminium bracket drawing
A Lagos consultancy has issued a drawing for an aluminium mounting bracket for a roof-mounted ventilation unit, built by a fabricator in Ikeja. Revision B, marked for construction, gives hole centres of 250 mm and overall length 260 mm, with a general tolerance of plus or minus 1 mm. The fabricator's site sketch shows the same holes at 248 mm, and a handwritten note on the drawing says 'check on site'. The title block says revision B, but the consultant's drawing register lists revision C as current and no revision C drawing can be found. Six of twenty brackets have already been drilled at 248 mm. The installation subcontractor says the unit will not line up and refuses to accept the brackets. The project manager wants the fabricator to continue and settle the difference later so the programme is not delayed. The unit manufacturer's catalogue shows a slotted hole of 22 x 14 mm for 12 mm bolts.
1. Identify which dimensional information governs the bracket and explain how you would resolve the revision conflict before production continues.
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The controlled issued-for-construction drawing governs, not a site sketch and not a handwritten note. But the title block and the drawing register disagree, and revision C cannot be produced. That is a document-control nonconformance: until revision C is retrieved or the consultant confirms in writing that revision B is the latest issued version, no one can say which document is contractually current. The fabricator's sketch is an uncontrolled record and cannot authorise drilling.
My action: instruct the fabricator in writing to stop drilling; place the six drilled brackets on hold; raise a formal request for information to the consultant asking for revision C or written confirmation of the governing revision and the correct hole centres; check the transmittal and distribution records to see whether revision C was ever issued to the fabricator. The project manager's proposal to continue and settle later is unacceptable because every additional bracket drilled to the wrong setting increases scrap and delay, and aluminium brackets are not easily reworked by welding. Resolution must precede production, and the answer must be recorded in the drawing register and reissued under a controlled transmittal.
2. Assess the 250 mm and 248 mm values against the tolerance and the slotted-hole geometry, and state whether the six drilled brackets can be accepted.
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The drawing value is 250 mm with a general tolerance of plus or minus 1 mm, so the permitted range is 249 to 251 mm. A 248 mm setting is 2 mm below nominal, which is outside that range by 1 mm. If the bracket is drilled symmetrically about its centre, each hole moves about 1 mm inward, so the error is small but it is still a nonconformance.
The 22 x 14 mm slot for a 12 mm bolt gives adjustment of about 5 mm each way along the 22 mm dimension and about 1 mm each way across the 14 mm dimension. That means the 1 mm per-hole shift can be absorbed only if the slots are oriented in the direction of adjustment and the bolt head and washer still have full bearing. Edge distance, bolt bearing and the unit's fixing centres must also be checked. My conclusion: the six brackets are not automatically acceptable. They may be accepted conditionally only after the designer confirms that the slot take-up covers the error and records a written concession; otherwise they should be re-drilled or replaced. The slot is a design allowance, not a licence to ignore tolerance.
3. Draft the communication you would issue and describe how you would handle the already-drilled brackets.
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I would issue a formal request for information and a site instruction, not a phone call. The request would state the drawing number and revision, quote the 250 mm and 248 mm values, attach the register extract showing revision C, and ask for the governing revision and confirmed hole centres. The site instruction would tell the fabricator to stop drilling, quarantine the six brackets, and take no action based on the sketch. I would mark the sketch 'for information only, not for construction' and record it in the document register.
For the six brackets: raise a nonconformance report, inspect and record actual hole positions, edge distances and slot condition, and tag them as held. Proposed dispositions are re-drill or slot to the approved detail if the designer allows, or replace. I would send the approved dimension to the installation subcontractor in writing, with a transmittal record, and require the fabricator to acknowledge it. All instructions go through the site instruction book or formal correspondence so that the eventual decision is traceable and no one can later claim they built to a verbal instruction.