Casted Materials in CNC Machining(riveting process Michelle)

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Computer numerical control (CNC) machining utilizes programmed commands to control machine tools that cut and shape stock material into custom parts and products. A wide variety of materials can be machined with CNC including metals, plastics, wood, composites, and casted materials. Casted materials refer to parts that have been formed by pouring molten liquid into a mold which is then allowed to cool and solidify. Common casted materials used in CNC machining include cast iron, cast aluminum, and cast zinc.
Casted parts are valued in machining for their cost effectiveness, dimensional accuracy, smooth surfaces, and material properties. However, casted materials often require additional CNC machining before they are ready for end-use. This article will provide an overview of casted materials in CNC machining, including:
- Benefits of Casted Materials
- Common Casted Materials Used in CNC
- Casting Methods
- Casting Tolerances
- CNC Machining Operations on Casted Parts
- Fixturing Considerations for Casted Parts
- Achieving Precision from Casted Raw Material
- Secondary Finishing Processes
- Examples of CNC Machined Casted Parts
Benefits of Casted Materials
There are several benefits that make casted materials advantageous choices for CNC machining:
- Cost effective - Casting molds can produce many identical parts quickly and economically. Machining time is reduced since the basic part shape is formed through casting.
- Dimensional accuracy - The mold cavity precisely forms the general shape, requiring less CNC machining. Tolerances of ±0.005 in (±0.13 mm) are typical.
- Smooth surfaces - Casting produces smooth surfaces that reduce machining time versus raw stock material.
- Material properties - Casting alloys parts with improved mechanical properties that cannot be achieved with other fabrication methods.
- Complex shapes - Intricate shapes can be casted that would be impossible or uneconomical to machine from solid material.
Common Casted Materials Used in CNC
Here are some of the common casted materials that are used in CNC machining:
- Cast Iron - A hard, durable material commonly used for engine blocks, gearboxes, cylinders, brake components and machine tools and beds. Ductile cast iron and gray cast iron are most frequently machined.
- Cast Aluminum - Lightweight aluminum alloys primarily used for housings, fixtures, mounts and motor components. Popular alloys are 319, 380, A356 and A357.
- Cast Zinc - Zinc alloys provide good hardness, wear resistance and dimensional stability. Common applications include hardware components, electronics housings and fixtures.
- Cast Steel - Alloy steel is applied to parts needing high strength such as automotive, aerospace and industrial components. Common alloys are 4140, 4340 and 8620.
- Cast Tool Steel - Leaded tool steel alloys offer very high hardness for long-wearing tooling applications. Common alloys are P20, H13 and 420 stainless.
Casting Methods
There are various casting methods used to produce parts later machined through CNC:
- Sand Casting - The most common and cost effective method. Sand molds and cores shape the part cavity.
- Investment Casting - Produces highly accurate and complex shapes. A primary metal part is formed first and used to shape the ceramic mold.
- Die Casting - Forced high pressure injection of molten metal into reusable steel dies. Commonly used with zinc, magnesium and aluminum alloys.
- Permanent Mold Casting - Reusable metal molds provide closer tolerances and better surface finishes than sand casting. Used for medium to high production volumes.
Each process has its own advantages and disadvantages regarding accuracy, surface finish, part complexity, and production quantities. The optimal casting technique depends on the application requirements.
Casting Tolerances
Castings are formed in molds that never produce 100% identical parts. Dimensional variances will exist within a certain tolerance range. Common casting tolerance grades include:
- Grade 22: ±0.025 in (±0.64mm) general tolerance
- Grade 24: ±0.010 in (±0.254mm) precision sand casting
- Grade 25: ±0.005 in (±0.127mm) precision sand casting
- Grade 27: ±0.003 in (±0.076mm) investment casting
Higher precision grades cost more but reduce additional CNC machining. Allowance must also be made for shrinkage as the molten metal cools. Typical casting shrink rates are approximately 1% although each alloy is unique.
CNC Machining Operations on Casted Parts
Raw casted parts will require various CNC machining operations including:
- Facing - Squaring the sides of a surface and making it flat. This is often the first op to create a datum plane.
- Drilling - Holes are drilled with twist drills or through coolant drills. Achieving hole location and alignment can be difficult on casted soft surfaces.
- Boring - Enlarging and smoothing holes to precise diameters. Boring is preferred over drilling for tight tolerance holes.
- Turning - External turning on a lathe removes excess material and brings outer diameters to spec. Allows finishing of spherical and contoured surfaces.
- Milling - Removing material using rotating end mills and cutters. Performing 2D or 3D contours, slots, cavities, and specialty features.
- Tapping - Forming internal screw threads in holes through tapping. May be done on a CNC mill/lathe or manually.
- Deburring - Removing sharp edges, chips, and burrs from machining. Done either through manual grinding/filing or special CNC deburring tools.

Fixturing Considerations for Casted Parts
Proper workholding is critical for machining casted parts. Irregular surfaces and dimensional variations in the raw casting make setup more difficult than machining uniform raw stock. Here are some fixturing challenges and solutions:
- Soft Surfaces - Avoid using vice jaws directly on porous cast surfaces which can damage the part. Use soft jaw inserts or aluminum riser blocks to protect the part.
- Locating Off Cast Surfaces - Fixed locators like end stops and dowel pins should locate off flatter machined surfaces rather than directly from the raw casting.
- Clamping Forces - Care should be taken not to deform or crush thin cast walls when clamping. Distribute clamping pressures over larger areas.
- Alignment Issues - Inspect for potential alignment problems between features in different casting planes. This may require indicating parts in multiple orientations during setup.
- Frequent Measurement - Continuously check critical dimensions to account for workpiece movement and identify errors early.
Achieving Precision from Casted Raw Material
Casted parts can make precision CNC machined components but require some additional planning:
- Prototype Sample - Have a casting sample made first to locate the part origin and machine datums. Use this to test and optimize the CNC program.
- Casting Tolerancing - Specify tighter casting tolerances for critical features to reduce machining required. Allow wider tolerances on non-critical areas to reduce cost.
- Semi-Finishing First - Leave an extra 0.010-0.020” of stock for the first passes to clear away the casting skin and machine a stable datum surface.
- Roughing Near Net Shape - Remove the bulk of the material during roughing leaving approximately 0.010” for the finish pass. This reduces tool pressure and deflection.
- Finishing with Finer Tools - Use lower-flute count and finer pitch finishing tools to minimize vibration and achieve a smoother surface finish.
Secondary Finishing Processes
Post-machining secondary finishing is often required to achieve the final part appearance, function, and specifications:
- Shot Blasting - A stream of steel abrasive directed at the part to provide a uniform surface finish and improve paint adhesion. Removes casting skin and residual sand.
- Vibratory Finishing - Corn cob or ceramic media tumble, polish, and deburr parts in a vibratory tub. A very gentle finishing method.
- Pressure Washing - Powerful water jets remove debris while leaving critical dimensions unchanged. Ideal for deep intricate geometries.
- Passivation - An acid bath treatment that removes free iron particles from stainless steel castings to restore corrosion resistance.
- Heat Treating - Hardening and tempering through precise heating and cooling to refine grain structure and enhance mechanical properties.
- Plating/Coating - Electroplated coatings like zinc, nickel, and chromium provide corrosion protection, improve wear, and enhance appearance.
Examples of CNC Machined Casted Parts
Many recognizable products start as raw castings before being CNC machined into precision finished components. Some examples include:
- Engine Blocks - Cast iron block castings machined for cylinders, water jackets, bolt holes, and mating surfaces.
- Turbochargers - Cast steel turbine and compressor housings aerodynamically profiled by 5-axis machining.
- Leg Vises - Ductile iron bodies machined with precision guideways and threaded components.
- Payphone Housings - Rugged cast aluminum alloy enclosures CNC milled for keypads, coind slots, and other features.
- Gearbox Housings - Die cast aluminum alloy forms make engine gearbox cases that are accurately machined for bearing bores and bolt patterns.
- Medical Scanners - Cast zinc or aluminum parts become precision finished MRI, CT, or PET scanner components.
In summary, casted materials are a versatile option for CNC machining a wide range of products and components. When properly applied, castings offer cost savings, dimensional accuracy, surface finish, and material advantages over raw CNC machined stock. With considerations for workholding, measurement, tooling strategies, and secondary processing, precision casted parts help bring innovative products to life through CNC manufacturing. CNC Milling CNC Machining