Deciding which manufacturing process to choose may be difficult; There are many different factors to consider. You can start with the die-casting process, because it can provide the quantity you need and meet the tolerance you need. However, next you may need to change a different manufacturing process. This can happen if the requirements for parts change, or your lead time or quality needs change.
When to choose CNC machining instead of casting
If you start from die casting, why do you choose to redesign your parts and use CNC machining instead? Although casting is more cost-effective for high quantities of parts, CNC machining is the best choice for low to medium quantities of parts.
CNC processing can better meet the tight delivery cycle, because there is no need to manufacture the mold, time or cost in advance during the processing. In addition, in any case, die-casting usually requires machining as an auxiliary operation. Post machining is used to achieve certain surface finishes, drill and tap holes, and to meet strict tolerances for cast parts that mate with other parts in the assembly. And the post-processing needs customized fixture, which is very complex in itself.
CNC processing can also produce higher quality parts. You can be more confident that each part will be manufactured consistently within your tolerance requirements. CNC processing is naturally a more accurate manufacturing process, and there is no risk of defects occurring in the casting process, such as porosity, depression and improper filling.
In addition, casting complex geometry requires more complex molds, as well as additional components such as cores, sliders, or inserts. All of this adds up to a large amount of investment in cost and time even before production starts. Not only the complex parts are more meaningful for NC machining. For example, CNC machine tools can easily manufacture flat plates by processing stock materials to the required size and thickness. But casting the same metal plate is easy to cause filling, warping or sinking problems.
How to transform casting design into CNC machining design
If you decide to redesign the part to make it more suitable for CNC machining, several key adjustments are required. You must consider draft angle, groove and cavity, wall thickness, key dimensions and tolerances, and material selection.
Remove draft angle
If you initially considered casting when designing a part, it should include a draft angle. As with injection molding, the draft angle is very important so that the part can be taken out of the mold after cooling. During machining, the draft angle is unnecessary and should be removed. The design including draft angle requires a ball end milling cutter to process and increase your overall processing time. Additional machine time, additional tools and additional tool change operations mean additional costs - so save some money and give up the draft angle design!
Avoid large and deep grooves and hollow cavities
Shrinkage cavities and hollow cavities are usually avoided in casting because thicker areas are often poorly filled and may lead to defects such as dents. These same functions take a long time to process, which will produce a lot of waste materials. Moreover, since all the forces are on one side, once the part is released from the fixture, the stress of processing the deep cavity will lead to warping. If grooves are not a key design feature, if you can afford the extra weight, consider filling them, or adding ribs or gussets to prevent warping or deformation.
The thicker the wall, the better
Again, you need to consider wall thickness. The recommended wall thickness of castings depends on structure, function and material, but is generally relatively thin, ranging from 0.0787 to 0.138 inch (2.0 to 3.5 mm). For very small parts, the wall thickness can even be smaller, but the casting process needs to be fine tuned. On the other hand, CNC machining has no upper limit on wall thickness. In fact, thicker is usually better, because it means less processing and less material waste. In addition, you can avoid any risk of warping or deflection of thin-walled parts during machining.
Strict tolerance
Casting usually cannot maintain strict tolerances like CNC machining, so you may make concessions or compromises in the casting design. With CNC machining, you can fully realize your design intent and manufacture more accurate parts by eliminating these compromises and implementing stricter tolerances.
Consider a wider range of materials
Last but not least, CNC machining offers a wider choice of materials than casting. Aluminum is a very common die-casting material. Zinc and magnesium are also commonly used in die casting. Other metals, such as brass, copper and lead, require more special treatment to produce high-quality parts. Carbon steel, alloy steel and stainless steel are seldom die cast because they are easy to rust.
On the other hand, in CNC processing, there are more metals suitable for processing. You can even try to make your parts with plastic, because there are many plastics that can be processed well and have useful material properties.
