The art of casting, which involves pouring molten metal into a mold, has a history spanning more than 6,000 years and continues to be extensively employed today. With the advancements in casting technology, it is now possible to produce high-quality components with precise tolerances, similar to CNC machining, a more recent manufacturing technique that is also widely utilized.
However, have you ever pondered over the idea of utilizing CNC machining for a part that is typically cast? Or are you currently contemplating which method would be most suitable for manufacturing your design? This article aims to explore both manufacturing processes and provide an analysis of the advantages and disadvantages associated with each.
What is Casting?
Metal casting involves pouring molten metal into a pre-made mold with cores or inserts for intricate internal geometries and cavities. Once the part cools and solidifies, it undergoes cleaning and processing to achieve the desired surface finish. Occasionally, CNC machining is utilized in a secondary operation to create tighter tolerances or features that are unsuitable for casting.
Various components such as train wheels, bus pedals, turbine blades, and engine blocks are commonly produced through casting. These cast parts find applications across a wide range of industries, including automotive, aerospace, consumer products, and medical devices.
What is CNC machining?
CNC machining is a manufacturing technique that involves the removal of material using a cutting tool to achieve the desired shape. The machine is guided by a computer program that utilizes G-code to determine the tool’s movement and cutting instructions.
CNC machining is utilized in a wide range of applications, from metal laptop enclosures to small components in cell phones, aerospace and automotive parts, as well as fixtures, jigs, and casting molds.
Choosing Your Manufacturing Method
Designing a part involves considering two main approaches for determining the manufacturing method (CNC vs casting): Design engineers typically begin with a specific manufacturing method in mind to ensure that the design is practical, feasible, and incorporates elements that align well with that particular process. Conversely, the requirements of a part’s function may necessitate specific dimensions, shapes, or geometries that influence the choice of manufacturing process. CNC machining and casting are each most suitable for particular types of geometries.
Casting vs CNC Machining: Choosing Based On Part Geometry
Choosing the appropriate manufacturing method begins with analyzing the geometry of the part. For instance, certain designs necessitate a significant amount of material removal, leading to time-consuming processes and excessive waste generation. Such designs are typically well-suited for casting, such as a hollow part with thin walls and large cavities. Moreover, intricate geometries are also prime candidates for casting due to the prolonged machining time they require.
Internal cavities are frequently best suited for casting, as shaping them through CNC machining can prove to be highly intricate. This is particularly evident for sealed internal cavities that demand assembly or metal joining procedures in addition to CNC machining. On the other hand, casting enables you to streamline the design of a part or assembly. Instead of fabricating numerous small components and assembling them, you can cast the entire part and eliminate the need for assembly steps.
Certain characteristics may be challenging to attain through casting. Internal features and undercuts, for instance, necessitate intricate molds with cores and inserts. Sharp corners and edges are not recommended for casting, and draft angles are essential for all features of cast parts. Moreover, large or irregularly shaped parts with thick walls are more suitable for CNC machining due to the complexities involved in filling and cooling such geometries uniformly in the casting process.
Version 1: Some attributes can be challenging to achieve through casting. Internal features and undercuts, for example, require complex molds with cores and inserts. Sharp edges and corners are not ideal for casting, and all features of cast parts need a draft angle. Additionally, large or oddly shaped parts with thick walls are better suited for CNC machining because filling and cooling such geometries evenly during the casting process can be difficult.
Casting vs CNC Machining: Choosing Based On Materials
Casting minimizes material wastage more effectively than CNC machining. The casting process efficiently utilizes the necessary material for the end product, with only a small portion of metal being lost through the gate, runner, and sprue. The surplus material can be easily eliminated, melted down, and repurposed for another component. However, the range of material options for casting is significantly narrower than that for CNC machining. While virtually all alloys are compatible with CNC machining, the selection of alloys suitable for casting is more limited.
Casting vs CNC Machining: Choosing Based on Production Speed
The lead time for the casting process is typically a few weeks. Once the mold is ready, the casting process itself is quite efficient, making it ideal for producing high volumes of parts. However, the design and machining of the mold require some time. Quality checks and maintenance are also necessary for the mold, although some casting methods involve expendable molds. On the other hand, die casting uses a permanent mold, resulting in decreased time and cost per part as the volume of parts increases.
CNC machining offers a faster production speed, leading to a quicker turnaround for low volumes of parts. With the CAD model, CAM program, and a CNC machine in place, there is no need to wait for mold manufacturing to begin making parts.
Casting vs CNC Machining: Choosing Based on Repeatability and Precision
1. Repetition plays a crucial role in manufacturing, and the casting procedure yields consistent parts. Once the mold and process are fine-tuned, each part can be replicated accurately. In die-casting aluminum, tolerances can be maintained within +/- 0.25mm up to 25.4mm, and for precision casting, it is 0.01” up to 1” for standard and .002” per 1”. The tolerances are influenced by the material used. Aluminum, for example, has a higher melting point than zinc, necessitating higher injection pressure and a more aggressive process. This leads to looser tolerances in cast aluminum compared to machined aluminum. On the other hand, tighter tolerances can be achieved when casting zinc. CNC machining metal is another highly repeatable and precise process that yields much tighter tolerances than casting. For standard metal machining, a tolerance of 0.025mm (0.001”) is considered reasonable. Fictiv precision machining is capable of producing parts with tolerances as tight as +/- 0.0002”.
Casting vs CNC Machining: Choosing for Prototyping
CNC machining is a more suitable option for prototyping as it allows for easier modifications in the design process. Unlike casting, where expensive molds need to be recreated or adjusted with every design change, CNC machining offers flexibility by simply altering the model and updating the CAM program. This eliminates the need for costly and time-consuming mold adjustments, making it a more efficient choice for prototype manufacturing.
Casting vs CNC Machining: Choosing based on Quantity and Demand
The relevance of the quantity of parts needed cannot be overlooked. During the initial stages of prototyping, a smaller number of parts may suffice, making CNC machining a more logical option. Another factor to consider is the variability in demand. If the need for producing more parts fluctuates, CNC machining would be a preferable choice.
CNC machining offers the advantage of obtaining parts faster and on demand, especially with the advancements in digital transformation within the manufacturing industry. This makes CNC machining a financially sound decision, as you can order parts as and when needed, rather than trying to predict demand and ending up with excess inventory.
However, if you anticipate high demand and are transitioning into production, casting is a suitable choice. It is better equipped to handle the production of thousands of parts or more in a shorter period.
Summary
When deciding between casting and machining, it is crucial to assess your position in the development process, the quantity of parts needed, demand fluctuations, and the complexity of the part. The optimal manufacturing method will depend on your specific circumstances. You might encounter a unique set of requirements that necessitate creativity and an innovative approach.
Despite the increasing popularity of CNC machining, casting still has its place in manufacturing. As discussed in the aforementioned article, each method has its own set of pros and cons. Armed with this information, you can make an informed decision for your project based on factors such as part complexity, quantity, quality standards, and project timeline. Your project may benefit from a combination of casting, CNC machining, and finishing processes.
