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You Are Buying the Block, Not the Part

September 21, 2026 by The Alu CNC Parts editorial desk / how this is checked

The price of a machined part tracks the block it started as, not the object that arrives in the box. A 200 gram bracket cut from a 2 kilogram billet is priced as a 2 kilogram job, because somebody paid for that metal and somebody spent machine hours turning most of it into chips.

That makes stock form one of the few genuine cost levers a buyer controls from the drawing.

The forms and where they fit

Plate. Rolled, supplied in standard thicknesses, the default for flat parts and anything larger than it is thick. Carries rolling stress, so specify T651.

Bar (round, square, rectangular). Extruded or rolled, good for shafts, spacers, small housings and anything roughly prismatic. Generally lower stress than plate.

Cast tooling plate. MIC-6 and equivalents, machined flat both faces and stress relieved. More expensive per kilo, cheaper per part where flatness matters.

Extrusion. A profile pushed through a die. If a standard profile is close to your cross section, you start much nearer the finished shape and machine only the ends and the features.

Forging and casting. Only relevant at real volume, where tooling cost spreads over enough parts to beat machining from solid.

Design to the sizes that exist

Aluminum plate comes in standard thicknesses. In metric markets that usually means 3, 5, 6, 8, 10, 12, 15, 20, 25, 30, 40, 50 mm and upward. In imperial markets it follows fractional inches. Bar follows its own standard steps.

A part designed 52 mm thick forces the shop to buy 60 mm plate and machine 8 mm away across the whole face. Designing it at 50 mm means the plate arrives close to final and one face may not need touching at all.

Two habits worth building:

  1. Ask what the shop stocks before finalising a thickness. It is a one line question on an RFQ.
  2. Leave the non-critical dimension flexible. 50 mm, MAY INCREASE TO NEXT STANDARD STOCK SIZE tells the shop it is free to pick the cheapest start.

Where a part is oriented in the stock also matters

A long thin part cut from plate can often be nested from bar instead, using far less material. A part that is nearly round is cheaper from round bar than milled from square. These are decisions the shop makes, and a shop quoting quickly will pick the safe option rather than the clever one.

Saying on the RFQ that you are open to suggestions on stock form invites the shop to propose the cheaper start, and most will.

When an extrusion changes the economics

If your part has a constant cross section over most of its length, an extrusion is worth checking. Heat sinks, rails, enclosures, frame members and channel parts all fit this pattern.

Two routes:

  • Standard profile. Free to use, available from stock, and if one is close to your section it is dramatically cheaper than machining from solid.
  • Custom die. Costs real money up front and several weeks of lead time, then produces material very cheaply. It starts to make sense in the thousands of metres, not the tens.

Extruded stock is usually 6063 or 6061 rather than the plate alloys, and 6063 is softer, so check that it still meets your strength requirement before committing.

What this means for the RFQ

Say the part’s function, give the critical dimensions, and mark the ones that are negotiable. A drawing that fixes every dimension to a decimal place tells a shop it has no room, so it quotes the safe, expensive start.

THICKNESS 50 mm NOMINAL. MAY INCREASE TO NEXT
STANDARD STOCK SIZE IF IT REDUCES COST.
OVERALL PROFILE NOT CRITICAL, FEATURES 'A' TO 'D' ARE.

That is a cheaper part for the same function, and it costs you three lines on the print.

Sources

Figures in this guide are checked against the following. Published values are typical ranges for test material, not a guarantee for your part.

Frequently asked questions

How much difference does stock size really make?

On a part that starts as a block and ends as a bracket, material and the time spent removing it can be a third of the quote or more. Dropping from 80 mm plate to 60 mm because the part only needed 55 mm is a real and immediate saving.

Should I supply my own material?

Usually not. Shops buy at trade prices, handle certification and carry the risk if a bar turns out to be the wrong temper. Supplying your own stock moves that risk to you, and a scrapped part becomes your problem rather than theirs.

Is an extrusion always cheaper?

Only at volume, and only if a standard profile fits. A custom extrusion die costs money and takes weeks. A standard profile that happens to suit your part is one of the cheapest starting points there is.

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