- 01Material, alloy and temper, with a no substitution noteWrite 6063-T5, not aluminium and not 6063. Add that the alloy may not be substituted without written approval. Substitution is rarely anyone cheating, it is a purchasing clerk finding the right thickness in the wrong alloy on a Tuesday and solving a problem nobody told them was a colour problem.
- 02A general tolerance in the title blockPlus or minus 0.1 mm suits a machined keyboard case. Tighter than plus or minus 0.05 mm everywhere forces slower passes, more inspection and more scrap on features where half a millimetre would have been fine.
- 03Specific callouts on the five dimensions that matterGasket shelf depth, plate pocket fit, screw and boss pattern position, port cutout position, and flatness on any plate. Everything else runs at the block tolerance.
- 04Position tolerance to a datum, not a chain of sizesA hole pattern dimensioned as a chain from one edge accumulates tolerance and tells the shop nothing about what has to agree with what. Give the pattern a datum and a position tolerance instead.
- 05Whether dimensions apply before or after finishingCoatings build thickness and a pocket gets it on both walls. Type II adds little, hardcoat and e-coating add enough to close a precise fit. This one line prevents the classic prototype that measures perfectly and will not assemble.
- 06Cosmetic faces identifiedMark which surfaces a buyer sees constantly, which they see occasionally and which are hidden. This changes the price more than most designers expect, because finishing effort follows the marking.
- 07Blast media and grit, as a numberMatte is not a specification. Glass bead at 120 grit is. The grit sets the texture you feel and it shifts the apparent colour, so a repeat run blasted differently will not match.
- 08Coating, colour reference and batching instructionProcess, then a Pantone or RAL number as a starting point, then a line requiring that parts meeting in a visible seam share one finishing load.
- 09Where racking marks are allowed to sitAnodising requires the part to be held and the contact point does not take colour. It is unavoidable, so decide where it goes rather than being surprised by wherever the anodiser chose.
- 10Threads with depth, and whether inserts are wantedTapped holes in aluminium that get unscrewed repeatedly will strip eventually. If the board is meant to be opened often, say so and ask about threaded inserts.
The tolerance stack, once
Take the one assembly that matters most, usually plate to case, and add every term in the chain.
- Case shelf or pocketYour drawn dimension, plus its tolerance.
- Coating on both wallsTwice the build thickness, and it is not zero on anything except PVD.
- Gasket thicknessSupplied with its own tolerance, and it compresses.
- Plate thickness and flatnessTwo separate terms. Flatness is the one people forget.
- Boss or standoff heightPlus the coating on that too.
What to do with the total
You are not trying to design a perfect stack. You are finding out which single term dominates, so you can tighten that one and leave the rest alone. On a gasket board the gasket then absorbs what remains, which is a genuine manufacturing advantage of the mount and one of the reasons it forgives small variation that a rigid mount turns into a rattle.
The full reasoning is in which tolerances actually matter, and the drawings themselves belong in the package described in the RFQ template.