CNC Machining and Casted Materials(cold rolled steel vs cast iron Archibald)

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CNC (Computer Numerical Control) machining is a manufacturing process that uses computer-controlled tools to shape raw material into finished parts and products. CNC machining is commonly used with metals, plastics, wood, foam, and wax blocks. However, casted materials are also frequently machined with CNC equipment.
What is a Casted Material?
A casted material refers to any object that has been formed by pouring molten metal, plastic, or other material into a mold and allowing it to solidify. The casting process allows for the economical production of complex shapes that would be difficult or uneconomical to produce by other methods. Common casted materials used in CNC machining include aluminum, magnesium, zinc, bronze, iron, steel, and thermoplastic polymers.
Benefits of CNC Machining Casted Parts
There are several benefits to using CNC machining on casted parts:
- Improved dimensional accuracy: The casting process can produce inconsistencies and imperfections on the surface and dimensions of the part. CNC machining can improve tolerance and achieve more precise finished dimensions.
- Better surface finish: CNC machining can smooth out rough surfaces and remove defects left over from the casting process, resulting in more visually appealing and consistent surface finishes.
- Machining complex geometries: Intricate shapes and features like threads, holes, slots, and grooves can be added through CNC machining. These details may be difficult or impossible to produce directly in the casting.
- Reduced secondary processing: CNC machining casted blanks reduces the need for additional manual grinding, sanding, drilling, and tapping. It consolidates much of the secondary processing into one automated CNC operation.
- Flexible, efficient production: CNC automation allows casted parts to be machined rapidly and consistently in medium to high volumes, with the flexibility to easily change the programming to create design variations.
Preparing Casted Parts for CNC Machining
There are some important considerations when preparing casted parts for CNC machining:
- Casting tolerances: The casting process should produce parts within the required dimensional tolerances for the follow-on CNC operations. Excessively out-of-tolerance castings may not machine accurately or efficiently.
- Allowances for machining: The casting design should provide enough excess stock material to allow for removal by the CNC tooling to achieve the net shape and dimensions.
- Fixturing: Castings must be securely fixtured in the machine to avoid vibration, deflection, and inaccuracies during high-speed metal removal. Custom fixtures matched to the part geometry may be required.
- Machinability: Cast alloys and metals have varying machinability. Materials like aluminum and brass machine faster than steels. The proper tooling, speeds, feeds, and depths must be selected.
- Surface condition: The casting surface may need to be cleaned, smoothed, or skimmed prior to finer finishing operations. This removes any oxidization or residue from the casting process.
- Internal stresses: Internal stresses from the casting process can cause casted parts to distort during machining. Proper fixturing, sequencing, and allowing for stress relieving pauses in the CNC program can mitigate these issues.
CNC Operations for Casted Parts
Common CNC machining operations performed on casted parts include:
- Facing: A facing operation skims a flat surface on the top and/or bottom of the workpiece, providing a clean reference surface for other operations.
- Roughing: Roughing uses large tools to efficiently hog away the bulk of the excess casting material to get close to the final dimensions.
- Semi-finishing: Semi-finishing employs smaller tools and lighter cuts to begin reducing surface roughness and fine-tuning the dimensions.
- Contouring: Contouring operations use ball end mills and other shaped cutters to machine complex 3D geometric features.
- Finishing: The finishing pass uses very light cuts, small tools, and fast feeds to achieve the required surface finish, geometry, and dimensional accuracy.
- Drilling and tapping: Holes are drilled and tapped as needed to fabricate threaded holes or produce bolt hole patterns.
- Engraving and text: Engraving tools mark text, logos, and identification details onto the surface of casted parts.
Maximizing CNC Machining of Casted Parts
To get the most out of CNC machining for casted components, engineers should:
- Design parts with machining in mind: Avoid thin, long, unsupported sections that can vibrate or deflect. Include enough stock allowance for machining operations.
- Select casted materials suited to machining: Softer, gummier alloys like aluminum and brass machine faster than steels.
- Choose castings with closer tolerances: Tighter casting tolerances reduce unpredictable inaccuracies during machining.
- Consider heat treating: Heat treating casted parts can relieve stresses and improve machinability.
- Allow parts to normalize: Letting parts sit for a few days after casting allows stresses to normalize before machining.
- Review toolpaths for smart machining: Optimize toolpaths to avoid abrupt direction changes, minimize rapid movements, and properly sequence operations.
- Adjust speeds and feeds: Start with conservative cutting parameters on first articles and adjust as needed based on tool wear and finish.
With careful planning and execution, CNC machining is an efficient secondary processing method for adding precision, function, and surface finish to casted parts and materials. CNC Milling