What a good RFQ drawing includes

When a supplier cannot see which dimensions matter, they have two options: ask, which costs time, or assume, which costs money. A defensive assumption usually means tighter tolerances than you need, a more expensive material condition, or a contingency in the price.

The irony is that the most expensive drawings to quote are often the ones marked with tight tolerances everywhere, because nothing distinguishes the features that control function from the ones that do not.

• Drawing number and revision, so everyone is quoting the same part. Revisions arriving mid-quote are a common source of dispute.
• Material specification and condition — not just 'steel', but the grade and whether it arrives annealed, normalised or hardened.
• The dimensions that control function, with tolerances. Mark them. Everything else can sit at a general tolerance note.
• Surface finish where it matters, stated as a value rather than 'good finish'.
• Heat treatment, plating or coating, including whether the supplier coordinates it or you do.
• Quantity now and the expected repeat. This changes the process route more than most buyers expect.
• The date you actually need it, separated from the date you would prefer it.

A dimension without a datum is an instruction without a reference point. Where a feature must be concentric, square or parallel to something else, saying so explicitly changes how the part is fixtured and in what order it is machined.

This is the single most common gap on drawings that otherwise look complete. It tends to surface late, when a part measures within every stated tolerance and still will not assemble.

A sentence describing the part's function is worth more than it looks. It lets a manufacturing engineer distinguish a bore that locates a bearing from a bore that merely clears a bolt, and to flag a feature that will be difficult to hold before it becomes a problem.

It also makes it possible to suggest alternatives. A feature that is expensive to machine can sometimes be redesigned at no functional cost, but only if the supplier understands what it is for.

This is common with obsolete and imported spares, and it is workable. What helps most is the sample itself, or clear photographs with a scale reference alongside.

Add the machine it belongs to, what the part does, any mating components, and the failure history — what wore, cracked or seized. Wear patterns carry information about loading that a measurement of a worn part cannot.

• The failed part, or photographs with a scale reference
• Machine make and model, and the part's function
• Mating parts or an assembly drawing if one exists
• Failure history and how urgent the replacement is

Before sending a drawing out to quote, read it as though you had never seen the part. If you cannot tell which three dimensions matter most, neither can the supplier — and the quote you get back will reflect that uncertainty rather than your requirement.