Aluminum Machining and Casting Processes
A machined part is cut out of solid metal. A cast part is poured into a mold. Most aluminum products use one, or cast first and machine second. Here is what each route does well, and what it costs you.
CNC machining
CNC machining is subtractive. A programmed machine removes material from a bar, plate or block until the part is left. No tooling is made for the part, only fixtures to hold it, so one piece costs the same setup as fifty and a design change is a new program, not a new die.
The blank is wrought aluminum (6061, 7075, 6082 and the rest of the alloy list), which is denser and stronger than cast metal and has no cast porosity. Typical shop capability is ±0.05 mm on general features and ±0.01 mm on reamed or bored holes and critical faces when it is called out, with a machined surface of about Ra 1.6 µm. See the tolerance guide before tightening a drawing.
| Operation | What it does |
|---|---|
| Milling (3-axis) | A rotating cutter removes material from a fixed block. Covers pockets, faces, slots, holes and threads on one or two sides of the part. The workhorse for plates, housings and brackets. |
| 4-axis and 5-axis milling | A rotary axis or two lets the cutter reach several faces in one clamping. Fewer setups means tighter relation between features and less handling. Used for manifolds, impellers, angled ports and sculpted surfaces. |
| Turning and mill-turn | The bar rotates against a fixed tool. Fastest route to round parts: shafts, bushings, fittings, knobs. Mill-turn centres add live tooling so flats, cross holes and keyways come off the same machine. |
| Wire and sinker EDM | Spark erosion cuts shapes a cutter cannot reach, such as sharp inside corners. Slower and priced by time, so it is a finishing option for specific features rather than the main route in aluminum. |
The cost of machining follows machine time and the amount of metal removed, so a part cut from a thick block is expensive for the chips it turns into. That is the main reason high-volume parts move to casting. The cost guide breaks down what drives a quote.
Casting
Casting is formative. Molten aluminum goes into a mold cavity and solidifies in the shape of it. The mold costs money, then every further part is cheap. The method names below differ in how the mold is made and how the metal gets in, and that decides surface, tolerance, strength and volume.
Cast parts use casting alloys, not the wrought grades. A356 is the common choice for structural castings because it can be heat treated to T6, A380 for high-pressure die casting because it fills thin sections, and 319 where a part needs to be a little tougher at temperature. Typical yield is roughly 100 to 200 MPa in the as-cast or T6 condition, below 276 MPa for 6061-T6.
| Method | Typical alloys | Tooling | Volume | Tolerance | Surface | Wall |
|---|---|---|---|---|---|---|
| High-pressure die casting | A380, A383, A360 | Steel die, high cost | Thousands to millions | ±0.1 to ±0.3 mm on small features | Ra 1.6 to 3.2 µm | From about 1.5 mm |
| Low-pressure die casting | A356, A357 | Steel die, high cost | Hundreds to tens of thousands | ±0.2 to ±0.5 mm on small features | Ra 3.2 to 6.3 µm | From about 3 mm |
| Gravity permanent mold | A356, 319, A357 | Metal mold, medium cost | Hundreds to tens of thousands | ±0.3 to ±0.8 mm | Ra 3.2 to 6.3 µm | From about 3 mm |
| Sand casting | A356, 319, A206 | Pattern only, low cost | 1 to a few thousand | ±1 mm class, looser on large parts | Ra 6.3 to 25 µm | From about 4 mm |
| Investment casting | A356, A357 | Wax tool, medium cost | Tens to thousands | ±0.1 to ±0.3 mm on small features | Ra 1.6 to 3.2 µm | From about 1.5 mm |
| Plaster mold casting | A356, 319 | Pattern, medium cost | Tens to hundreds | ±0.2 to ±0.5 mm | Ra 1.6 to 3.2 µm | From about 1.5 mm |
| Lost foam | A356, 319 | Foam pattern tooling, medium cost | Thousands and up | ±0.5 to ±1 mm | Ra 6.3 to 12.5 µm | From about 3 mm |
| Squeeze casting and semi-solid | A356, A357, A380 | Steel die, high cost | Thousands and up | ±0.1 to ±0.3 mm on small features | Ra 1.6 to 3.2 µm | From about 2 mm |
Typical published ranges for aluminum, not a guarantee for your part. The foundry confirms what it can hold on your geometry and the tolerance applies to small features, not across the whole casting.
High-pressure die casting
Molten aluminum is injected into a steel die at high pressure and solidifies in seconds. Lowest cost per part at volume and the thinnest walls of any casting route. The fast fill traps gas, so die castings carry some porosity and are normally not heat treated to T6. Vacuum-assisted dies cut the porosity but add cost.
Low-pressure die casting
Gas pressure pushes metal up into the die from a furnace below, so the fill is calm and the casting is denser than a high-pressure one. Parts can be heat treated to T6. Slower cycle, so it suits wheels, housings and structural parts where integrity matters more than speed.
Gravity permanent mold
Metal is poured by gravity into a reusable steel or iron mold. Sand cores form internal cavities. The chilled mold gives a finer grain than sand casting, so strength and elongation are better, and the tooling is cheaper than a high-pressure die.
Sand casting
Metal is poured into a one-use sand mold made around a pattern. Any size, from grams to several tonnes, and a pattern can be wood, plastic or 3D printed, so it is the cheapest way to get a first casting. Rough surface and loose tolerances mean most faces get machined.
Investment casting
A wax pattern is coated in ceramic, the wax is melted out and metal is poured into the shell. Reproduces fine detail, thin walls and shapes with undercuts that a die cannot release. Cost per part is higher, so it fits low to medium volume complex parts.
Plaster mold casting
A plaster slurry is cast around a pattern and the mold is broken after pouring. Smooth surface and thin sections for small runs of prototype or low-volume parts. The mold insulates, so metal cools slowly and the cast structure is coarser.
Lost foam
A foam pattern stays in loose sand and vaporizes as metal enters. No parting line and no cores, so complex internal passages can be cast in one piece. Mostly seen on engine and pump housings at volume.
Squeeze casting and semi-solid
The metal is filled slowly and held under high pressure while it solidifies (squeeze casting), or injected as a part-solid slurry (semi-solid). Both give very low porosity and can be heat treated, which is why they are used for safety-related parts. Fewer shops run them.
Cast first, machine second
Casting rarely finishes a part. A foundry delivers the shape; a machine shop brings the sealing faces, bores, threads and mounting surfaces to tolerance. Leave a machining allowance on every face that will be cut, usually 0.5 to 1 mm on a die casting and 2 to 3 mm on a sand casting.
Cast aluminum machines differently from bar. Silicon in the alloy wears tools faster, and gas pockets just under the skin can open up when a face is cut, so a face that looked sound can show porosity. Deep cuts into a die casting are where this happens. Read the guide to machining cast aluminum before drawing the datum scheme.
For a part that could be made either way, the machining against die casting comparison works through the volume break-even in detail.
Which route fits
| Your situation | Route |
|---|---|
| One to a few hundred parts, any shape you can cut | CNC machining from bar or plate |
| A prototype or first article of a part that will be cast later | CNC machining, or sand casting from a printed pattern |
| Thousands of the same housing with thin walls and ribs | High-pressure die casting, then machine the critical faces |
| A structural part that must be heat treated and pressure tight | Low-pressure, gravity permanent mold or squeeze casting, then machine |
| A large part, a few pieces, tooling budget near zero | Sand casting, then machine |
| Complex shape with undercuts, tens to hundreds of parts | Investment casting, or 5-axis machining from billet |
| Tight tolerance on bores, threads and sealing faces | Machining, whatever the blank was made by |
Rule of thumb only. Part size, wall thickness, alloy and how much of the part is a precision feature move the answer, and the break-even moves with them.
Sources
General references for the process definitions. Figures on this page are typical ranges, not values from a single document.
Questions
Is die casting cheaper than CNC machining?
Per part at volume, yes. The die costs money before the first part exists, so below a few thousand pieces machining is usually cheaper overall and above that casting wins. The exact break-even depends on the part size, wall thickness and how much of the cast part still has to be machined.
Can I machine a cast aluminum part?
Yes, and most castings are. The casting supplies the shape and the machining supplies the tolerance on sealing faces, bores and threads. Leave machining allowance on those faces and expect to find porosity just under the cast skin on some parts.
Why is cast aluminum weaker than 6061 or 7075?
Cast alloys are chosen for how they flow and fill a mold, not for strength, and the cast structure holds some porosity. Typical yield for common cast alloys is roughly 100 to 200 MPa, against 276 MPa for 6061-T6 and about 500 MPa for 7075-T6. Heat-treated low-pressure and squeeze castings sit at the top of the cast range.
Which casting method gives the best surface and tolerance?
High-pressure die casting and investment casting hold the finest detail and the tightest as-cast tolerance. Sand casting is the roughest. None of them replaces machining on a fit or a seal.
Does this site cast parts?
No. We match machining requests with shops that cut aluminum. If your part is a casting that needs finish machining, send the drawing and the blank details and we match the machining step. The casting itself comes from a foundry.
Machined, or a casting that needs machining?
Send the drawing and we match it with up to 3 shops that cut aluminum. Free for buyers, reply within 24 business hours.