Machining vs. Casting Decision Depends on Volume
A resort manager stopped to watch a carpenter framing a new outbuilding on the resort’s property and was surprised to see the man driving screws with a large hammer. “You know what that little slot in the head is for?” the manager asked. “Sure,” replied the carpenter. “That’s to back it out if you drive it in the wrong place.”
Sometimes, the so-called “right tool for the job” can be a matter of opinion. The point is that a number of factors can go into choosing the right tool for any particular job, and sometimes the choice isn’t immediately obvious.
The Right Tool for Producing Metal Parts
In metal fabrication, available end-use production tools include die casting, sand casting, and machining:
- Die casting is similar to plastic injection molding. Molten aluminum, copper, zinc, or some alloy of those metals is forced into a steel die and allowed to cool. In some cases, additional machining may be required to achieve threading, better sealing surfaces, or certain details. Machining as a finishing process can allow multiple variations from a single casting.
- Traditional sand casting is a bit cruder than die casting. Surface finishes are granular and mismatches between mold halves and inserts can require post-processing due to loose tolerances in the casting. The process is used for non-cosmetic parts such as engine blocks for automobiles. Difficult high-volume applications such as creating engine coolant channels around cylinders often require post machining to finish the part. Some design solutions in sand casting can’t be replicated in die or CNC machining.
- CNC machining is typically used for lower volumes, think dozens to a few thousands. It is quick and responsive and doesn’t require preparatory tooling the way casting methods do. In addition to low-volume production, it can be used to create prototypes of die-cast or sand-cast parts until a final design is created and tooling can begin. It can also be used for bridge tooling to create production parts while tooling is produced for high-volume production.
Volume Should Determine Production Method
There are situations in which the choice of process is obvious. If you are producing a high volume of parts, think tens of thousands and beyond, casting is far more economical. Producing the die takes time, but each cast part can be produced quickly and at relatively low incremental cost.
CNC machining, on the other hand, produces the first parts much more quickly and economically, but there isn’t as much of an economy of scale after a certain point.
As a result, the first consideration in choosing a manufacturing method is volume. If you need dozens for prototyping or hundreds or a few thousand for low-volume, end-use production, then you should consider machining. For 10,000 or more, consider sand or die casting. Between those extremes, however, a variety of factors can influence your choice.
Moving from machining to casting can reduce machine time and the production of scrap. But casting also has its downsides including:
- Flash that needs to be cleaned off
- Porosity in thick cross sections that can be exposed by post-machining
- Flow lines and end-of-fill cold material that may be brittle
In some cases, 10,000 pieces may have to be machined due to the requirements of the part. This affects the part cost, but performance requirements sometimes take precedence over cost.
Machining has the advantage if you need parts quickly. Machined parts can be produced within hours of creating a CAD model, while producing a die and setting up for casting takes significantly more time. This gives machining an advantage even if production will ultimately be done by casting. A part can be modeled, machined, and tested at a fraction of the cost of casting a few prototypes. Using machining, and depending on the results of testing, the part can be redesigned—several times if necessary—to allow fine-tuning of the design before incurring the higher cost and longer delay of producing a die and associated fixturing. Production volume of prototypes can range from a single part for lab-bench testing to a larger number for a beta test or test marketing.
Ultimately, avoid the mistake of considering cost as the only, or even the primary, driver in choosing a manufacturing method. Start with system specifications and focus on total cost with your next project.
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Manufacturing Matchup is a closer look at various production methods.