The CNC Machining Process for Investment Casting: How It Works, Benefits, and Applications

What is the CNC Machining Process?

CNC stands for Computer Numerical Control. CNC machining uses computer-controlled equipment to remove material from a metal component until specific dimensions, tolerances, and surface finishes are achieved.

Modern CNC equipment can repeatedly produce highly accurate features, making it ideal for components where certain dimensions must meet very tight tolerances.

Many manufacturers will use investment castings as the raw material rather than commercially available standard metal shapes.

What are Some CNC Machining Processes?

Common CNC machining operations include:

• Milling – metal removal using cutting tools that rotate.Considered one of the main metalworking processes for metal removal in mass for all geometries
• Turning – metal removal using a single-point cutting tool while the work revolves.  Generally used for producing round details such as diameters and bores, threads, etc
• Drilling – metal removal to form a hole in a piece of metal using a fluted cutting tool of a fixed diameter
• Boring – similar to drilling, except with a single-point cutting tool that can be adjusted for the desired hole size
• Reaming – similar to drilling, however, using a multi-fluted cutter of a fixed diameter to fine-tune the diameter of a drilled or bored hole 
• Tapping and threading – a fixed diameter cutting tool used to form a thread inside a drilled hole
• Grinding – a metal removal technique using an abrasive wheel to remove minute amounts of metal at a time.  Used for higher precision and where extremely smooth surfaces are required

CNC Machining of Investment Castings

Investment casting is well known for producing complex, near-net-shape metal components with excellent surface finishes and dimensional accuracy. But for many applications, the casting itself is only part of the manufacturing process. Critical surfaces, holes, threads, sealing areas, and tight-tolerance features may require additional finishing, which can most efficiently be achieved through CNC machining.

Combining investment casting with CNC machining can provide manufacturers with the best of both processes: the design flexibility and material efficiency of investment casting, along with the precision and repeatability of computer-controlled machining.

Examples of CNC Machining Applications with Investment Castings

One of the primary advantages of investment casting is its ability to produce complex shapes with close tolerances.

Unlike manufacturing a component entirely from a solid block of metal, investment casting places material where it is needed from the beginning. This can significantly reduce the amount of material that must be removed later.

However, some component features may require tolerances beyond what is practical with the casting process alone.

For example, a casting might require CNC machining for:

• Precision mounting surfaces
• Bearing locations
• Threads
• Sealing surfaces
• Mating surfaces
• Precision bores
• Critical hole locations
• Tight dimensional tolerances

The goal isn’t necessarily to machine the entire casting. Instead, investment casting produces the complex overall geometry, while CNC machining focuses on the areas requiring additional precision.

CNC Machining vs. Investment Casting

CNC machining

CNC machining is a subtractive process. A mill, lathe, or other computer-controlled machine removes material until the desired geometry remains.

Its main advantages are:

• Very tight dimensional tolerances 
• Excellent surface finishes 
• Fast production of prototypes and small quantities 
• Easy design revisions without replacing hard tooling 
• Good consistency and repeatability 
• Strong suitability for simple or moderately complex geometries 

Its main limitations are increased material waste, longer cycle times for complex parts, and restrictions due to tool access. Deep cavities, curved internal passages, and intricate undercuts may be difficult or impossible to machine economically.

Investment casting

Investment casting is a near-net-shape process. Wax patterns are assembled, coated with ceramic material, melted out, and replaced with molten metal. After solidification, the ceramic shell is removed, and the castings are finished.

Its main advantages are:

• Highly complex shapes and fine design details 
• Reduced machining and material waste
• Thin walls and curved surfaces 
• Undercuts and features that are difficult to machine 
• Ability to consolidate multi-piece assemblies into one component 
• Economical repeat production once tooling is established 
• Wide choice of ferrous and non-ferrous alloys 

Its main limitations are the initial tooling expense, longer process lead time, and less precise as-cast tolerances than CNC machining. Critical holes, sealing faces, bearing surfaces, and close-tolerance dimensions may still need machining.

Parts made from CNC Machining of Investment Casting

The differences between components produced strictly as investment castings and machined components that use investment castings as the base metal are almost non-existent due to the natural fit between casting and CNC machining.  This combination of investment casting and CNC machining is used across many industries where complex geometry and precision must coexist.

Industrial Equipment

Pumps, valves, fittings, housings, impellers, and other industrial components frequently benefit from investment casting followed by precision machining.

Food and Processing Equipment

Stainless steel investment castings can provide corrosion resistance and complex geometries for components used in food-processing, beverage, and material-handling equipment.

Transportation

Investment cast components can be used in automotive, commercial transportation, recreational vehicles, and other transportation applications where strength, weight, geometry, and repeatability are important.

Energy and Power Generation

Valves, flow-control components, brackets, fittings, and other specialized components can benefit from the combination of complex cast geometry and precision-machined surfaces.

General Manufacturing

Equipment manufacturers can use CNC-machined investment castings for everything from mounting components and levers to housings, brackets, hardware, and complex mechanical assemblies.

Investment Casting vs. Machining from Solid Material

CNC machining is an extremely capable manufacturing technology, but machining an entire complex component from solid material isn’t always the most economical solution.

Consider a component with a complicated exterior shape but only a few critical precision surfaces.

Machining the entire component could require extensive cutting time and remove a large percentage of the original material.

Investment casting takes a different approach.

The component can first be cast close to its final geometry. CNC machining then removes relatively small amounts of material from the areas requiring the greatest precision.

This hybrid manufacturing strategy can provide an excellent balance between complexity, accuracy, material utilization, production efficiency, and cost.

What is Investment Casting?

Investment casting typically begins by producing a wax pattern representing the desired component.

The patterns are assembled onto a gating system and repeatedly dipped into ceramic slurry and coated with refractory material. Once the ceramic shell reaches the required thickness, the wax is removed.

Molten metal is then poured into the ceramic mold.

After the metal solidifies, the ceramic shell is removed, and the individual castings are separated from the gating system.

Depending on the application, additional operations such as heat treatment, blasting, grinding, straightening, inspection, or other finishing processes may be performed.

Investment casting can produce geometries that may be difficult, expensive, or even impractical to manufacture entirely through machining.

These can include:

• Curved surfaces
• Thin sections
• Internal passages
• Complex contours
• Detailed external features
• Integrated component features

CNC machining can then be used only where tighter dimensional control is required.

Advantages of Using Investment Castings with CNC Machining

In many applications, investment casting and CNC machining are complementary rather than competing technologies. Investment casting produces the component close to its final shape, and CNC machining finishes only the critical features.

For example, an investment casting could form the component’s overall shape, mounting arms, ribs, bosses, lettering, and complex contours. CNC machining could then finish:

• Precision bores 
• Threads 
• Sealing surfaces 
• Bearing locations 
• Mating faces 
• Features requiring exceptionally tight tolerances 

This combined approach can provide investment casting’s design freedom and material efficiency while maintaining CNC-level accuracy where it matters.  However, there are many other benefits of combining investment casting with CNC machining, including:

Reduced Machining Time

If a component were manufactured entirely from bar stock, plate, or billet, substantial amounts of material might have to be removed.

Investment casting produces a near-net-shape component first, leaving CNC machining to concentrate primarily on critical features.

Less material removal can mean shorter machining cycles.

Reduced Material Waste

Machining a complex component from solid material can create significant amounts of chips and scrap.

Investment casting can greatly reduce this waste because the component begins much closer to its final shape.

This can be particularly valuable when working with expensive alloys.

Complex Design Capabilities

Investment casting can produce geometries that may be difficult, expensive, or even impractical to manufacture entirely through machining.

These can include:

• Curved surfaces
• Thin sections
• Internal passages
• Complex contours
• Detailed external features
• Integrated component features

CNC machining can then be used only where tighter dimensional control is required.

Excellent Dimensional Accuracy

Investment casting already provides good dimensional capabilities compared with many other casting methods.

Adding CNC machining allows manufacturers to achieve even tighter tolerances on selected critical features.

Excellent Surface Finishes

Investment casting is known for producing relatively smooth cast surfaces.

Where an even more precise surface finish is required. For example, on a sealing, bearing, or mating surface, CNC machining can provide additional finishing.

Potential Part Consolidation

Investment casting can sometimes combine several components or features into a single casting.

Instead of machining individual pieces and assembling them later, engineers may be able to design those features directly into the casting.

Optimize Your Metal Components with Single-Source Casting & CNC Machining

Combining near-net-shape investment casting with precision CNC machining delivers the ideal balance of complex geometry, tight tolerances, and material cost savings. At Niagara Investment Castings, we provide a complete turnkey solution—handling both your raw castings and secondary precision machining under one roof.

Why Choose Niagara Investment Castings for Turnkey Components?

• Streamlined Supply Chain: Eliminate multi-vendor coordination, shipping delays, and hand-off risks by sourcing your castings and finished CNC machining from a single accountable partner.
• Maximized Efficiency: Reduce machine cycle times and costly scrap by machining only critical tolerance areas on near-net cast shapes.
• Extensive Alloy Selection: Choose from hundreds of ferrous and non-ferrous alloys customized to meet your exact structural and environmental requirements.
• End-to-End Precision: Benefit from consistent quality control across the entire manufacturing sequence, from initial wax pattern to final machined surface.

Ready to streamline your supply chain and lower your component production costs?

Contact the Niagara Investment Castings Team Today to discuss your project specifications or request a custom quote.