The machining blank is the piece of material you start from before any material is removed: round, hexagonal or square bar, flat, plate, sawn block, tube, section, or a part pre-formed by forging, casting or cutting. The difference between the blank and the finished part, the machining allowance, is the material that will be turned into chips.
The workshop chooses the blank from the sizes offered by suppliers, for example bars several metres long or plates in standard formats, to limit offcuts. It saws the blank to the required length, then machines it. The delivery condition matters: hot-rolled is economical, but its surface is scaled and its dimensions vary; cold-drawn or ground stock has precise dimensions and a clean surface. The cost of a machined part depends heavily on the ratio between the mass of the blank and that of the finished part. In aerospace this is sometimes called the “buy-to-fly” ratio.
Choosing the blank well reduces cost: for a large run, a blank close to the final shape (forged, cast or cut part) avoids machining large volumes. For a one-off part, a block or plate in standard dimensions is generally the simplest. The material purchased is also what carries the material certificate, and so the traceability of the part. The guides on materials for machined parts and the price of CNC machining expand on this; the CNC machining page presents the capability.
In a quote request. State the exact material grade (for example EN AW-6082 T6, S355J2 or 1.4301) and its temper or condition, rather than a generic name. Say whether a material certificate (EN 10204 type 3.1) is required. Avoid imposing the blank shape without a reason: by giving the overall dimensions of the part, you let the workshop choose the most economical blank.