Deciding which manufacturing process to choose can be difficult; there are many different factors to consider. You may start out with a die casting process because it can deliver the quantity you need and meet your required tolerances. However, down the line you may need to change to a different manufacturing process. This may happen if the demand for the part changes or if your lead time or quality needs change. If you find yourself needing to redesign your part from a casting process to a CNC machining process, this article is for you. 

cylindrical metal part made using die casting
An example of a die cast metal part

When to Choose CNC Machining Over Casting            

If you began with die casting, why choose to redesign your part and switch to CNC machining? While casting is more cost effective for high quantities of parts, for low to medium quantities CNC machining is your best bet. 

CNC machining is more capable of meeting tight lead times, because there’s no time or cost required up front to manufacture a die or dial in the process. Plus, die casting often requires machining as a secondary operation, anyway. Post-machining is used to achieve certain surface finishes, to drill and tap holes and meet the tight tolerances of cast parts that mate with other parts in an assembly. And that post-machining requires a custom fixture, which can itself be complex to make.

CNC machining is also more capable of producing higher quality parts. You can be more confident that every part will be consistently made within your tolerance requirements. CNC machining is naturally a more precise manufacturing process, and doesn’t risk defects — like porosity, sink, and failure to fill properly — that occur with casting. 

Additionally, casting complex geometries requires more complex dies, with extra components like cores, slides, or inserts. These all add up to large investments in cost and time before even beginning production. And not only complex parts make more sense for CNC machining. For example, a CNC machine can easily fabricate a flat plate by machining stock material to the size and thickness you need. But casting that same plate could easily result in filling, warping, or sink issues. Lastly, some parts are just designed without good casting principles in mind — which makes the decision to switch from casting to machining easy!

metal CNC machined part
This CNC machined part would require an extremely complex mold to die cast it.

How to Turn a Casting Design Into a CNC Machining Design

If you do decide to redesign your part to make it more suitable for CNC machining, there are multiple critical adjustments to make. You’ve got to consider the draft angle, pockets and cavities, wall thickness, critical dimensions and tolerances, and material choice. The good news is that switching to a CNC machining process opens up many possibilities!

Ditch the Draft Angles

If you originally designed the part with casting in mind,  it ought to have incorporated draft angles. As with injection molding, draft angles are essential so that the part can be removed from mold or die once it’s cooled. With machining, draft angles are unnecessary, and should be removed. Designs incorporating draft angles require a ball end mill to machine and increase your overall machine time. Extra machine time, extra tooling and extra tool change operations means extra cost — so save some money and ditch the draft angle design! 

Avoid large, deep pockets and cored out cavities

Pockets and cored out cavities are usually avoided in casting, because thick areas don’t tend to fill well and can cause defects like sink marks. Those same features take a long time to machine and doing so generates a lot of wasted material. And because all the force is on one side, the stress of machining out a deep cavity can cause warping once the part is released from its fixture. If pockets aren’t critical design features, consider filling them in if you can afford the added weight, or add ribs or gussets to prevent warping or deflection.

a rectangular machined part
The complex details and the thickness of the material make this a better candidate for CNC machining

Thicker Walls Are Better

Similarly, you’ll want to consider the wall thickness. The recommended wall thickness for cast parts depends on the structure, function, and material, but it’s usually relatively thin, and ranges from 0.0787 in. (2.0 mm) to 0.138 inches (3.5mm). For very small parts, the wall thickness can be even smaller, but requires fine-tuning the casting process. CNC machining, on the other hand, has no upper limit for wall thickness. In fact, thicker is generally better because that means less machining and less material waste. Additionally, you avoid any risk of warping or deflection during machining that may occur with thin-walled parts.

Tighten Up Your Tolerances

Casting is generally not as capable of holding tight tolerances as CNC machining, so you may have made allowances or compromises in your casting design. With CNC machining, you can fully realize your design intent and make more precise parts by eliminating those compromises and implementing tighter tolerances 

Consider Using a Wider Range of Materials

Last but not least, CNC machining provides a much wider range of material options than casting. Aluminum is a very common material for die casting, and it converts well to aluminum 6061. Zinc and magnesium are also commonly used for die casting. Other metals, such as brass, copper, and lead, require more specialty treatment in order to make good parts. Carbon steel, alloy steel, and stainless steel are more rarely die cast because of their tendency to rust. 

With CNC machining on the other hand, there are many more metals that lend themselves to the process. You can even try making your parts out of plastic, as there are many plastics that also machine well and have useful material properties. 

Conclusion

While casting is a great process in certain circumstances (many parts that we use every day were made with die casting), CNC machining is sometimes better suited to the function of your part or your manufacturing needs. If this is the case, make sure to redesign your part for the most efficient and cost effective CNC machining process. 
Looking to CNC machine a previously cast part? Check out Fictiv’s CNC machining capabilities.