Casting - is over 6000 years old and is still widely used. Advances in the technology of this process have enabled the casting of high-quality parts with narrow tolerances. CNC machining is also ubiquitous, but much newer. Have you ever thought about machining your regular castings? Or maybe you're wondering which method to choose to implement your project? In this article, we will present two manufacturing processes and discuss the advantages and disadvantages of each. Stainless steel, ductile iron, and other materials have specific properties in the workpiece. How to implement the advantages of plate geometry? Is casting better, or stainless steel machining? What are the disadvantages and advantages associated with this? We write about this below!
What is casting?
Casting is an ancient craft that is still used in many forms. There are many types of castings, but all use molds. The molten metal is then poured or placed in such a mold, where it cools and hardens to its final form. Cores or inserts can create complex internal geometries. After hardening, the part is cleaned and subjected to a heat treatment process to the desired surface finish. Sometimes this additional operation includes CNC machining to achieve tighter tolerances. Examples of such castings are train wheels, bus pedals, turbine blades, and engine blocks. They are used in almost every industry, including automotive, aviation, consumer products, and medical devices. Casting is applied to spheroidal cast iron isothermally hardened.
What is CNC machining and turning?
CNC machining, or CNC turning, belong to a production technology known as the subtractive method, which means that the cutting tool removes material to achieve the desired geometry. A computer program using code tells the machine where to move and how to cut. Such a process can produce very precise parts, which are used practically everywhere. Metal laptop casings, small elements inside cell phones, aviation and automotive components, as well as equipment and tools are produced using CNC machining.
Choosing a production method based on geometry – CNC or casting
When designing parts, the process usually starts with the production method, so that realistic, achievable features that are well suited to the way of work can be planned. On the other hand, the function of the part may require a specific size, shape, or geometry, which in turn may dictate a specific production process. CNC machining and casting are tailored to a specific specification.
For example, some projects require significant removal of material from the initial metal block. This would take a lot of time and lead to excessive waste of material, so such a case might be a good candidate for casting. Good examples are hollow parts with thin walls and large cavities. Complex figures are also good candidates for casting, as they require complicated solutions and long processing times.
Casting is a great process for simplifying the design of a single part or an entire assembly. Instead of machining many small elements and then assembling them, you can cast the whole thing at once and avoid many assembly stages.
However, some features are more difficult to achieve by choosing this method. Undercuts or internal elements will be complex and require complicated molds with cores and inserts. Castings also require all parts to have a spiral angle. Large or unusually thick-walled parts are better suited for CNC machining. During casting, these geometries are difficult to fill and cool evenly. CNC machining is ideal for thick parts that require high precision. However, remember that cutting can create cutting resistance, which makes cutting difficult. CNC uses carbide inserts that operate in the cutting zone.
Pros and Cons
Apart from differences in geometry, casting and machining processes have their own pros and cons.
Casting - pros and cons
- Casting reduces material losses. Only the materials needed to make the final part are used in the process, with a small amount of metal flowing through the gates, runners, and guides. Excess material can be easily removed and remelted for use again in another section.
- It is also a fast process when the mold is ready. This makes it suitable for the production of large volume parts. Single-use molds are used for some types of solutions. However, in pressure casting using a permanent mold, the time and cost per part decrease with its volume increase.
- It can also be a fast, repeatable process. Repetition is important in production, and casting provides very similar parts. After choosing the mold and processing, each detail can be made exactly the same way.
- During pressure casting of aluminum, tolerances can be maintained up to +/- 0.25 mm for up to 25.4 mm and 0.05 for precision casting (0.01″ for up to 1″ for standard and 0.002″ for precision). This will vary depending on the material. Aluminum has a higher melting temperature than, for example, zinc, so it requires higher injection pressure and generally a more aggressive process. Therefore, during zinc casting, even tighter tolerances can sometimes be maintained.
- On the other hand, CNC machining provides a faster return with fewer parts. There's no need to wait for mold production. All you need is a CAD model, CAM program, and CNC machine.
- Changes in the project are easier thanks to CNC milling. When the concept is constantly changing, all you have to do is remodel the solution and redo the CAM program instead of rebuilding it or starting from scratch.
- CNC machining is also a very repeatable process. For standard metal cutting, 0.025 mm (0.001 inch) is a reasonable expected tolerance.
- The development stage is related to the degree to which the project elements undergo continuous evolution. At the beginning of the design process, this can be repeated many times. In this case, CNC machining would be a better choice due to the relative efficiency of changes in this process.
- The number of required parts is also important. Earlier in prototyping, many elements may not be needed, so CNC machining might make more sense.
- Another thing to consider is the variability of your needs. How often do you actually need these parts? In case of demand fluctuations, CNC machining might be a better solution. CNC machined parts are available faster, especially as the digital transformation progresses. This could be a good option from an economic standpoint, as you can order them as needed, instead of trying to predict what you will need and end up with excessive inventory.
- You may be compelled to have a larger number of parts after the project completion and in production. At this point, casting is a good choice as it is more suitable for producing thousands or more elements.
- Finally, quality matters. CNC machining is able to maintain tighter tolerances on parts, making it a more precise process. Precision CNC machining can produce them within a tolerance of +/- 0.0002 inch, while precision aluminum castings are more like 0.002 inch. If this level of accuracy is extremely important for your application, CNC machining may be a better choice for you.
- The production process always depends on specific demand. You may find yourself with a unique combination of requirements that forces you to be creative and find innovative solutions.
- No mold costs: unlike casting, CNC machining does not require molds. CNC machines for shaping products use various cutting tools controlled by digital computer code. Therefore, it allows saving on tooling costs.
- Production in one place: CNC machining can be applied to small and complex parts production. However, casting has limitations that are not suitable for manufacturing complicated parts. Using castings for complex products can lead to dimensional errors and rough surfaces.
CNC machining takes place at constant feeds, with appropriate tooling and low tolerance dimensional coding, making it efficient to use stainless steel products for complex but precise production. - Faster production cycle: CNC machining of stainless steel is a faster production process compared to casting this material. Aligning the mold is a time-consuming operation, followed by melting and cooling the metal and ejecting the final product. On the other hand, CNC machining involves generating computer code, setting up the workpiece, and post-production ejection of stainless steel parts.
- Dimensional accuracy and stability: CNC operations are managed by a computer, which reduces the risk of human error in dimensional accuracy. A stainless steel casting may require extreme heating and cooling, which can alter the dimensional accuracy of the final product.
On the other hand, CNC machining can be performed with low dimensional tolerances. Additionally, it does not require extreme heating or cooling of the stainless steel sheet. Therefore, dimensional changes caused by temperature fluctuations are excluded in CNC processes.
CNC Machining Pros and Cons
Casting or machining?
Knowing the above facts, would you choose casting over machining or vice versa? A few key things to consider are the stage of development, the number of required parts, variability of requirements, and the previously discussed part geometry.
CNC machining has become a very common craft, but there is still room for casting! Each production method has its advantages. Instead of conducting business as usual, try to reassess your project to see if it is better suited for casting or for CNC machining based on attributes such as: part geometry, quantity, quality needs, and schedule.
Advantage of stainless steel machining using CNC
Factors that make CNC machining better than stainless steel casting.
Here are some examples of CNC machining of stainless steel that make it a better alternative to stainless steel castings.
Due to the aforementioned advantages of CNC machining of stainless steel, stainless steel castings were quickly replaced by it and proved useful in many applications.
Summary of CNC and casting topics
Generally speaking, CNC milling is an alternative to casting stainless steel, as it provides quick product creation, maintains a shiny surface finish, and ensures high strength and durability for products made from this material. However, for different parts, various technologies such as CNC, turning, drilling, etc. can also be applied.