Design around standard stock sizes

The shop buys standard plate and bar. A part that is just over a standard size needs the next size up, so you pay for material that is then turned into chips.

Plate

Milled plate parts are usually skimmed on both faces to get a clean, flat surface. A part that is 20.0 mm thick therefore often needs 25 mm plate. Designing it at 19.5 mm lets the shop use 20 mm plate. See the list of common stock sizes.

Bar

For turned parts, the largest diameter decides the bar. A Ø40.5 mm flange needs Ø45 bar; at Ø39 it can be made from Ø40 bar, with less material and less cutting.

Cast plate

Cast aluminium tooling plate (for example 5083 cast plate) is flat and stress relieved and often used at its supplied thickness without skimming. If your part is a flat plate with holes, ask the shop whether they can use cast plate as supplied.