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Inside the CNC Machine Shop: Additive vs. Subtractive Prototype Fabrication

[April 28, 2014] On the CNC machine shop floor, the manufacturing method of prototyping is an integral step in the production process of milling and turning parts. Given the nature of the job, and before commencing with full-scale CNC machining, it may be in the best interest of the customer to begin with a model or prototype of the part to be machined. At APM's Colorado machine shop, our CNC machinists often construct a prototype that acts to serve as both the customer's, and our own model for a check on a part's dimensions, material strength, tolerances, etc., before full-scale production is initiated. A prototype is also a great platform for testing materials and verifying CNC programs and operations. In the design and precision manufacturing industry, there are two very distinctive prototyping methods, additive and subtractive, each with different applications, and each with their own advantages and disadvantages. Here, we will provide the reader with a brief overview, and some pros and cons of both technologies.

Additive manufacturing (AM), is a technology that first gained traction in the mid-1990s, and now goes by several different names such as 3D printing, stereo-lithography, and fused deposition. It is a great platform for prototype development. Instead of removing material from a part via the subtractive processes used in traditional CNC machining, additive methods involve depositing and joining materials, such as liquid resin, papers, or powders layer upon layer to produce the desired prototype from 3D model data. Some pros and cons of AM in prototype development include:

[Pros]

  • CAD driven technology allows for limitless design options and revisions. Complex features can be previewed first, revised if necessary, and then manufactured into a prototype.

  • AM results in shorter job turnarounds. For small-batch jobs inherent in prototype production, design to final build is a quicker process.

  • Significant costs savings are realized because less raw materials are used.

[Cons]

  • AM may reduce the numbers of qualified CNC machinists needed. 3D printing technology is available to anyone with a printer, raw materials, and the right software.

  • The use of intellectual property rights and piracy is an issue with AM. Blueprints may be easily obtained online for 3D printed prototypes.

  • AM materials can be more expensive than those used by traditional CNC mills, such as bar or billet stock.

Subtractive manufacturing (SM) has been in existence since humans began working with metal. Simply put, it involves the removal of raw material from a blank. Today, this is most efficiently done in a precision machine shop where materials are removed via a CNC machining process involving an array of specialty tools including CNC mills and CNC routers for example. Over the last ten years, subtractive technologies have dramatically improved, with enhanced design and development capabilities. These improvements have allowed for more precise, and more creative prototype designs. Until the advent of AM, SM had been the only tool in use on the CNC machine shop floor to manufacture prototypes, and is utilized by a majority of shops today. Prior to full-scale production runs, SM has some advantage over AM, but also some disadvantages.

[Pros]

  • SM can be faster than AM; even for small batch runs associated with prototyping because of advances in design software.

  • SM techniques do not require outsourcing or investment in new 3D printing equipment.

  • SM requires less finishing work to be done once a prototype has been machined compared to AM.

[Cons]

  • On the startup, or entry level, SM requires more capital investment in precision machining technology than does the cost associated with AM investment.
  • More raw materials are utilized and may be discarded as scrap. Operating costs go up.
  • SM is inherently a longer process, both in design and production, when compared with AM. The result is longer turnaround times.

The process of producing a prototype for a customer will always be a necessary step in the precision machining process. Both prototyping technologies will always require the right combination of machine, manpower, design, setup time and materials costs. There is a case to be made for techniques. The bottom line is that additive manufacturing should not be thought of as a replacement or alternative to the traditional method of subtractive prototyping. Depending on the part to be machined or printed, the material, the schedule, and a host of other factors, both technologies have a spot on the floor of the future CNC machine shop.

For more information on AM and SM techniques, or to fulfill your prototyping needs, contact APM's Colorado CNC machine shop today.

 

About the Author

 

Gerry Dillon is a co-founder, current owner and certified CNC machinist at Advanced Precision Machining (APM), a full-service machine shop located in Longmont, Colorado. Before making his home in the United States in 2000, Gerry was born and raised on the emerald isle of Ireland and took an interest in milling and machining from an early age, ranking #1 in the Irish National Apprenticeship Program. In 2005, he partnered with his friend and colleague, Kirk Tuesburg, currently APM’s machine shop manager, together launching what’s grown into a leading Colorado machine shop. Gerry brings over 30 years of machining experience to the shop floor, and is certified in all aspects of geometric dimensioning and tolerancing.

 

 

 

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