Machining Aluminum Castings 101

Aluminum Casting Process

Aluminum castings are made by pouring molten metal into a custom mold. When the casting comes out, sometimes we just trim the gates off and the part is ready to go. But sometimes, the casting goes through secondary operations to meet the final specs. These operations might include machining, heat treat, sand core removal, polishing, powder coating, or anodizing.

Let’s talk secondary machining.

Why Might we Machine Castings?

Sometimes castings go through secondary machining operation to…

Add Features

Some features can be cast-in, such as sand core channels, pull core holes, or threaded steel inserts.

Some features may need to be added in secondary machining such as drilling, tapping, threading, and other key features for assembly.

Remove Features

Casting designs require draft, radii, and a certain wall thickness. But if your final assembly requires a 90-degree corner, zero draft, or drastic wall thickness changes, these features can be achieved during the machining process.

Tighter Tolerances

Sometimes castings go though a machining operation to achieve tighter tolerances than the casting process is capable of. Be sure to call out critical specs on your print that could be achieved by casting or machining.

Permanent mold casting tolerances are twice as tight as sand casting.

linear tolerance permanent mold casting

Machined castings can hold 0.0005 inch tolerances.

Considerations when Machining Aluminum Castings

If you machine a casting, consider…

Sourcing

Will you use a one-stop supplier like Batesville Products for casting and machining operations, or will you source separate foundries and machine shops?

Fixturing & Inspection

What’s the best way to fixture this part in the CNC machine? Fixtures are often included in the tooling cost.

During the machining process, aluminum castings may release tension or warp if not fixtured properly. For the best results, you want to load flat castings with minimal flash into the CNC machine.

Consider using flatness gauges, profile gauges, go-no-go gauges, and other inspection tools to verify critical specs before/after machining.

Raw vs Heat Treated Castings

Raw aluminum castings are flexible, which can work to your advantage or disadvantage during machining.

Machining ease depends on the aluminum alloy casted. Some aluminum alloys are almost always heat treated before machining, otherwise they are too soft for precise machining. For permanent mold casting, we only use A356.

A356 castings can be machined with or without heat treat. A356 castings without a heat treat are more flexible than those T6 heat treated. T6 heat treat ages the part and makes it stronger and more rigid.

Machine Stock Allowance

If machining, you will need to add a small machine stock allowance to your casting design.

casting machine stock allowance

Machining from Billet vs. Casting

What’s the best manufacturing method for your design? Casting with secondary machining, or machining from billet?

Casting from Ingot

Efficient and cost effective production for higher quantities and complex shapes
Have a broader size range
Can cast-in features to that quicken machining (cores, fins, bosses, windows, inserts, etc.)
Limit material waste by producing a near-net shape
Have design requirements such as draft, radii, and wall thickness (which can be removed in secondary machining)

Machining from Billet

Cost efficient for low quantities and prototyping or simple shapes
Size limited by size of CNC machine
Each feature must be carved out, resulting in high material waste, long cycles, and sometimes multiple cycles
Great design flexibility

Learn more about the pros, cons, and when to choose each manufacturing method here.

Part Machined from Billet Switches to Casting

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Questions About Casting & Machining?

Call or email us today! We’ve been aluminum casting and machining custom components for over 75 years!