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When designs are still fluid and product specifications are in flux, two common choices are 3D printing and CNC (computer numerical control) milling

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3D Printing VS CNC Machining Comparing two rapid prototyping alternatives

Dec 26, 2017

In the product development phase, there are many process options for creating models, prototypes, patterns or moulds.Selecting the best approach requires an appreciation for the project requirements, an understanding of the process considerations and an evaluation of the resulting time, cost and quality. Without this information, it is difficult to identify the optimal approach. The cover story elaborates on 3D printing to allow process selection based on an understanding of its capabilities and associated considerations. A read on…

When designs are still fluid and product specifications are in flux, two common choices are 3D printing and CNC (computer numerical control) milling. Both can meet the needs for early concept models through functional prototypes without the delays imposed by mould, tool, or die making. Both processes can make parts in a variety of materials.

However, the similarities end there. In many ways, 3D printing is the opposite of CNC milling. Most notably, the factors that influence time and cost are quite different. Additionally, the size, shape and configuration of suitable or ideal parts for 3D printing are a reversal from the ideal for CNC machining. To choose between the two, a thorough understand of both processes is necessary.

The 3D printing process

3D printing is a collection of additive processes that construct prototypes on a layer-by-layer basis. This eliminates the need for tooling and allows the fabrication of extremely complex geometry with no impact on time or cost. Fundamentally, this is the primary advantage of 3D printing, and it translates to rapid delivery of small batches of parts and the flexibility to quickly accommodate design changes.

The additive nature of 3D printing also supports parallel part processing — multiple parts are manufactured in a single job in less time than that needed to make each individually. This capability allows production of multiple parts from a single project, multiple revisions of a single part, or multiple parts from different projects.The only limiting factor is the XY area of the 3D printer’s working envelope. Another advantage is that there is never a need for multiple setups to make a part.After initiating a job, it runs until the parts are complete.This supports around-the-clock production without the need for operator oversight or intervention.

Workflows

The workflows of 3D printing and CNC, at the highest level, are the same. Following part design (in CAD), there is a programming step where the instructions for the machine’s operations are defined. Next, the machine is prepared for the job and the part is then manufactured. Upon completion, secondary processes are performed. The execution of these workflows, however, are quite different both in terms of time, labor and effort, which directly affects lead time and cost.

Design (CAD): Both 3D printing and CNC use a 3-dimensional design definition that is completed in CAD. The requirements for the CAD data are common between them so the time and labour cost are comparable.

Programme: For 3D printing, preparing files is commonly a 5- to 30-minute process that is highly automated. Upon importing an STL file (which is the output from CAD), a technician selects an orientation, specifies a material and selects from pre-defined build options. Beyond selecting an optimal orientation, there is little thought needed to prepare files. Note that for demanding jobs, there can be more operator interaction required, but this is uncommon for prototyping applications and does not increase processing time significantly.

For CNC, the CAD data is imported into a CAM program where all aspects of the machining process are defined, most of them manually. Among the decisions to be made are how to fixture the part; how many machining passes are needed; and what cutters, speed and feed rates to use.

Set-up: Preparing a 3D printer is a quick process. While each technology has its own procedures, many require only material replenishment and pre-heating. In just 5 to 15 minutes, the printer is ready to manufacture parts. For CNC, the machine operator will load the cutters needed for the job and fixture the work piece. When the CNC machine has a tool changer, the cutters are loaded only once. Lacking a tool changer, the machine operator will load the appropriate cutter for each machining operation.

Manufacture: With a 3-axis CNC, this is seldom the case since the workpiece must be repositioned to cut upon faces that are not up-facing (in the original orientation), and therefore, not accessible to the cutters. For each repositioning, an operator needs to reorient and re-fixture the workpiece.

The factors that influence the time to manufacture a part are distinctly different for 3D printing and CNC. Manufacturing times for 3D printing are dictated primarily by the volume of material in a part and the part’s height. Feature count and feature type have little impact on time. The opposite is true for CNC. Size and volume have marginal impact, but the number of features and their design characteristics dictate time.

Finish/assemble: Upon completion of the manufacturing process, 3D printed parts will require post-processing. Most, but not all, will need removal of a sacrificial support structure that attaches the part to the build plate of the printer and holds unrestrained features in place. Unlike the manufacturing process, the time for support removal is geometry-dependent. CNC parts on the other hand, have no mandatory secondary operations.

Time and cost illustrations

The following examples use a burdened labor rate of $37.00/ hour. For an hourly cost of machine time, the examples use $1.00 for both 3D Printing and CNC*. For material cost calculated based on the extent volume, 3D printing uses $1.00/ in3 while CNC uses $0.50/in3**.

Pocket tray

Although 3D printing excels at making complex parts, it can be competitive for even simple parts like the pocket tray shown in Figure 1. As per the project, 3D printing is 55% less expensive than CNC. However, it takes 77% longer to produce, assuming that both processes can start immediately and that there are no delays imposed by availability of labour, materials and machine.

Industrial robot adaptor

Although not as simple as the pocket tray, the adaptor for an industrial robot is not overly challenging for CNC. The result is that CNC is faster than 3D printing, but more expensive, as shown in Table 2. The adaptor once again shows that 3D printing can be the lower cost option with a 30% savings. This highlights the financial impact of the labour that CNC requires. With a total time that is 52% more than CNC, it also demonstrates the influence of a part’s design on the speed advantage that is possible with 3D printing. Even though a 3-axis CNC requires an operator to perform six set-ups, the milling is straightforward, consisting only of some face milling, bore milling and drilling.

Time drivers

Excluding set up and finishing for both 3D printing and CNC, the question of which process has higher throughput can only be answered after selecting a part design. The time factors for 3D printing are simple and straightforward. A part’s volume and height dictate most of the manufacturing time. The only design-related consideration that adds time results from support structures. When features need to be supported, a small amount of build time is added, usually in the 5% to 15% range. Unlike CNC, adding features that remove material, such as pockets and holes, will decrease the time needed to make a part.

CNC time is determined by the volume of material subtracted and the rate at which it is removed. The amount is dictated by the design and the size of the raw material stock. The rate is dictated by many factors. Workflow must also be considered when evaluating the total elapsed time to deliver a prototype. Here 3D printing has the advantage because it has fewer steps to coordinate, fewer labor demands, and fewer resource requirements.

Operations

The in-house machine shop has been displaced by outsourcing in many companies. There are numerous reasons for this shift, but many cite the expense of staffing and the challenge in finding skilled CAM programmers and CNC machinists.

3D printing is a bit different. While some companies will not add a full-blown 3D printing lab due to the overhead expense, many have installed 3D printers within their design and engineering teams. Effectively, 3D printing becomes a self-serve function. In doing so, there is no addition to labor costs, but there is an opportunity cost when an engineer spends a few minutes to launch a 3D printing job. Those that do justify a 3D printing lab build the justification partially on the labor efficiency; a single individual can support all functions of a lab with five or more 3D printers and still have time for other duties.

Having the process in-house makes it much more responsive and more cost effective, if utilization is reasonable. For example, one prototype shop noted that the average 3D printed prototype takes two to three days to deliver and costs $200 to $300. The same part, when CNC machined, would cost $400 to $500 and have a seven-day lead time. If the 3D printed prototype were done in-house, the difference would be much more dramatic with costs around $100 and the potential for same-day delivery.

In-house 3D printing also fosters a design and engineering culture of fast and frequent design iterations. When innovative ideas become a physical reality quickly and cost effectively, more concepts can be considered resulting in better products. The product development team can get more aggressive with its designs, knowing that tomorrow’s prototype will either validate the idea or illustrate that it will not work.

Time and cost considerations

Previously, it was shown that the pocket tray and robot adaptor could be made less expensively with 3D printing while taking longer to produce (ignoring workflow delays). However, neither example fully capitalizes on the advantages of 3D printing. Building on the concepts that influence time and cost, the following discussion illustrates the impact of leveraging 3D printing’s strengths.

Workflow

Assume that an order is released a half hour before the end of the business day. With 3D printing, the job can be processed and started before the shift ends. The parts are now available at the start of the next day. Considering the labor demands of CNC, machining may start the next morning.

  • Bottomline: 3D printed parts are available 1.3 to 2.5 hours earlier. Assume that the queue is already filled with orders and schedules cannot be juggled. For CNC, the projects have to wait until there is machine capacity. With 3D printing, the parts can be combined with other projects, so long as there is room in the machine.

  • Bottomline: 3D printed parts are available day(s) earlier. Assume that the requirement is for three pocket trays, which will fit in one run of the 3D printer. For both CNC and 3D printing, the set-up time remains constant. But for 3D printing the manufacturing time is four hours for three parts (not 3 X 2.3 hours). For CNC, assume that there are three machining operations that start without any delay.

  • Bottomline (time): 3D printed parts available in 4.3 hours and CNC parts available in 3.3 hours.

  • Bottomline (cost): $75.00 cost for 3D printing and $140.00 for CNC.

Building on the three pocket trays, now assume that each is a different version, reflecting alternative design concepts. For 3D printing, the total time is 4.5 hours since there is only a small addition of time to process three different files. For CNC, the total time would be 4.9 hours.

  • Bottomline (time): 3D printing is 0.4 hours faster (and the design team becomes more efficient)

  • Bottomline (cost): $128.00 cost for 3D printing and $218.40 for CNC.

Design

Assume that the pocket tray has cutouts on all four walls and several pockets on the bottom face. For 3D printing, there will be a slight decrease in manufacturing time. For CNC, there will be an increase in set-up time and a small increase in machining time.

  • Bottomline (time): The time for 3D printing and CNC become equal

  • Bottomline (cost): $20.00 cost for 3D printing and $80.00 for CNC.

Adding pockets and cutouts does not increase the difficulty to CNC the pocket tray. Just imagine the time and cost difference if this prototype’s design had thin ribs; deep, narrow channels; and undercuts. Also consider the significant time and cost advantage of 3D printing if all of the above scenarios came together at once—there were three different revisions of this challenging design, the order was placed at 5:00 PM, the schedule was full, and each action was preceded by an interruption in the workflow (refer table) 3D printing has the flexibility and efficiency to deliver when the conditions are less than perfect.

Conclusion

With an understanding of the difference between 3D printing and CNC, and a thoughtful examination of goals and requirements, companies, both small & large, have access to an alternative for the product development process. Also, add to that an open-mindedness to taking actions that were previously unthinkable or ignored, and the product development team can discover more about its designs faster and earlier in the cycle. There isn’t a single ‘best’ solution. 3D printing is an alternative to CNC that opens the door to more options and new capabilities. Step inside a well-equipped shop and you will see CNC mills sitting side-by-side with 3D printers. For them, and for those that have the insight to what makes these processes different, the combination creates a workflow where the best solution is always available. ..

Courtesy: Stratasys

Image Gallery

  • CNC (aluminium) on the left, 3D printed (plastic), right

    Image: Stratasys

  • CNC (aluminium) on the left, 3D printed (plastic), right

    Image: Stratasys

  • Time and cost estimates adjusted for typical product development conditions.

    Image: Stratasys

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