When it comes to CNC machining, understanding the material you’re working with is just as important as knowing how to operate the machine itself. The properties of the material can significantly affect the machining process, from the selection of tools to the settings used for cutting. Let’s explore how material properties impact CNC programming and machining, and why getting this right can make all the difference.
The Basics of Material Properties
Materials used in CNC machining come in various forms such as bars, billets, metal sheets, forgings, and castings. Some materials might have already been processed on another machine, and they can be either solid or hollow. The size and shape of the material determine how it will be set up and secured in the machine. Additionally, the type of material (e.g., steel, cast iron, brass) influences the choice of cutting tools and the conditions under which they operate.
You can’t write an effective CNC program without knowing the material type, size, shape, and condition.
Material Uniformity
One critical aspect that is often overlooked by programmers and managers alike is the uniformity of the material batch. Imagine ordering material from two different suppliers and finding slight differences in dimensions, hardness, or even shape. Another example is when materials are cut into smaller pieces, each piece might vary slightly in length within an acceptable range.
Such inconsistencies can make programming more challenging and time-consuming. The best programming approach in these cases is to prioritize machining safety over machining time. This might mean accepting some air cuts or slightly slower feed rates, but it ensures that the cutting load doesn’t exceed the tool’s capacity.
Another method is to categorize irregular materials into different groups and write separate programs for each group. Ideally, the program should accommodate all known and predictable inconsistencies, for instance, by using skip cycle functions.
Cutting Performance Index
Another important aspect of material description is its cutting performance. Large machining companies often maintain charts that recommend cutting speeds and feed rates suitable for common materials. These charts are invaluable for programming, especially when dealing with unfamiliar materials. These recommended values serve as a good starting point and can be optimized as more is known about the material’s properties.
In the imperial system, cutting performance index units are feet per minute (ft/min), and they can be represented as surface feet per minute (SFM), constant surface speed (CSS), cutting speed (CS), peripheral speed, or surface speed. For metric calculations, the unit is meters per minute (m/min).
Given a tool diameter (for milling) or workpiece diameter (for turning), spindle speed (r/min) can be calculated using these formulas:
For the imperial system:
r/min = 12× ft/min/(pi×D)
For the metric system:
r/min = 1000 ×m/min/(pi× D)
Where:
- r/min = spindle speed in revolutions per minute,
- D = tool diameter (for milling) or workpiece diameter (for turning) in inches or millimeters,
- ft/min = cutting speed in feet per minute,
- m/min = cutting speed in meters per minute,
- pi = constant, approximately 3.14159.
Practical Considerations in CNC Programming
Let’s delve into some practical considerations when dealing with different materials in CNC programming.
Tool Selection
The type of material greatly influences tool selection. For instance, machining steel requires tools that can withstand high temperatures and have good wear resistance, such as carbide tools. On the other hand, softer materials like aluminum can be machined with high-speed steel (HSS) tools.
Cutting Conditions
Material properties such as hardness and toughness will dictate the cutting conditions. Harder materials require slower cutting speeds and higher feed rates to avoid excessive tool wear. Conversely, softer materials can be machined at higher speeds but may require careful attention to avoid chatter and poor surface finish.
Coolant Use
The choice of coolant is also influenced by the material. For example, water-based coolants are suitable for aluminum to prevent the material from sticking to the tool, while oil-based coolants are better for harder materials like steel to improve lubrication and cooling.
Example: Machining Different Materials
Let’s consider a practical example where different materials are machined using the same CNC machine.
Material: Steel (AISI 1045)
- Tool: Carbide
- Cutting Speed: 250 ft/min (76 m/min)
- Feed Rate: 0.015 in/rev (0.38 mm/rev)
- Coolant: Oil-based
Material: Aluminum (6061-T6)
- Tool: High-Speed Steel (HSS)
- Cutting Speed: 600 ft/min (183 m/min)
- Feed Rate: 0.025 in/rev (0.64 mm/rev)
- Coolant: Water-based
These parameters illustrate how different materials require different machining strategies to achieve optimal results.
Understanding the impact of material properties on CNC machining is crucial for successful programming and efficient production. From tool selection to cutting conditions and coolant use, every aspect of the machining process is influenced by the material being used. By taking into account material uniformity and cutting performance, CNC programmers can optimize their strategies to ensure high-quality, consistent results.
