When to choose CNC machining instead of casting
If you start with die casting, why do you choose to redesign your parts and use CNC machining instead? Although casting is more cost-effective for high volume parts, CNC machining is the best choice for low to medium volume 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 process. 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 the strict tolerances of cast parts that fit with other parts in the assembly. And the post-processing needs to customize the fixture, which is very complicated.
CNC machining can also produce higher quality parts. You can be more confident that every part will be consistently manufactured within your tolerance requirements. CNC machining is naturally a more accurate manufacturing process, and there is no risk of defects in the casting process, such as pores, dents 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 these add up to a huge investment in cost and time even before production begins. Not only complex parts are more meaningful to CNC machining. For example, CNC machines can easily manufacture flat plates by machining stock materials to the required size and thickness. But casting the same metal plate can easily lead to 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 the draft angle, groove and cavity, wall thickness, key dimensions and tolerances, and material selection.
Remove draft angle
If you initially considered casting when designing the part, it should include the draft angle. As with injection molding, the draft angle is very important so that the parts can be removed from the mold after cooling. During machining, the draft angle is unnecessary and should be removed. The design including draft angle needs a ball end milling cutter to process and increase your overall processing time. Extra machine time, extra tools and extra tool changing operations mean extra costs - so save some money and give up the draft angle design!
Avoid large and deep grooves and hollow cavities
In casting, shrinkage cavities and hollow cavities are usually avoided, because thicker areas are often poorly filled and may lead to defects such as depressions. These same functions require a long time to process, which will generate a lot of waste materials. Moreover, since all the forces are on one side, once the part is released from the fixture, the stress in the deep cavity will cause warpage. If grooves are not a critical design feature, and if you can afford 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 the wall thickness. The recommended wall thickness for castings depends on structure, function and material, but is usually relatively thin, ranging from 0.0787 to 0.138 inches (2.0 to 3.5 mm). For very small parts, the wall thickness can be even 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 that may occur to thin-walled parts during processing.
Strict tolerance
Casting usually cannot maintain strict tolerances like CNC machining, so you may have made concessions or compromises in 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 using 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 rarely die cast because they are easy to rust.
On the other hand, in CNC machining, more metals are suitable for machining. 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.
