Digital Alloys’ Guide to Metal Additive Manufacturing - Part 6
Comparison of Additive Manufacturing & CNC Machining
February 28, 2019
Boeing and other manufacturers use three primary criteria to measure the value of additive manufacturing (AM) against CNC Machining: part performance, cost and lead time (see our blog post on this). In the past, metal AM processes were expensive and slow. As a result, their use in production was primarily in applications where the value of improved part geometries outweighed the sacrifice in speed and cost. For this reason, the analysis of production cost and schedule efficiency has been neglected.
With new high speed, low cost AM technologies like Joule Printing™, AM can now compete with (or augment) machining in speed and cost. This elevates the importance of a more thorough comparison of these two manufacturing methods – the purpose of this blog.
Machining (Subtractive Manufacturing)
Machining is the most common choice for low volume manufacturing because it is flexible, and can produce strong, accurate parts. Machining removes material from a workpiece (typically a billet) by cutting it with rotating tools (milling) or rotating the workpiece itself against a tool (turning). These cutting processes are very accurate and have broad capabilities in part geometry and materials. In addition, machining has low tooling requirements – reducing fixed costs and lead time. Machining is usually the first choice for rapid, low volume manufacturing, and is used extensively for prototyping, tool production, and short-run production of end use parts.
While machining has advantages in low volume manufacturing, it is expensive and does not scale well for high volume production. Its shortcomings include: high material waste, expensive machinery, and a requirement for skilled programming time. In addition, for hard metals like titanium and tool steel, cutting times and tool wear are very high. Due to these limitations, manufacturers prefer more cost-effective processes like casting, stamping, and forging for high volume production.
Near-Net-Shape Manufacturing & The Buy-to-Fly Ratio
Many metal manufacturing processes - including casting, forging, and most additive manufacturing techniques - cannot directly produce the tolerances and surface finish demanded by the end application. As such, they are “near-net-shape” processes whose outputs require a secondary finish machining step to produce a finished part. Similarly, processes based primarily on machining usually have two steps, a “rough cut” step optimized for high material removal speed, and a “finishing” step that cuts more slowly but produces better accuracy and surface finish. Therefore, in analyzing the speed and cost of various production processes, we must consider both the near-net-shape and finish machining steps.
For simplicity, we will focus our analysis on the near-net-shape production step and compare machining with additive manufacturing. In practice, the speed and cost of the finish machining step do not vary as widely between processes.
A key cost driver in near-net-shape production is the “buy-to-fly” ratio, which is the ratio of the weight of purchased raw material (buy) to the weight of the final part (fly). Buy-to-fly determines the amount of material waste generated in the rough-cut step as well as the machine time needed to complete the rough cuts. When machining from raw bar stock or billet, the buy-to-fly ratio can be very high. “In aerospace the average is 11:1” - John Barnes, Barnes Group Advisors, and going up to 30:1 is not uncommon. To put this in perspective, a large aerospace manufacturer may purchase 50 tons of titanium in raw materials each day to produce only 4.5 tons of machined parts. In the below example of a 17:1 buy-to-fly ratio part, 94% of the workpiece ends up as waste material and the bulk of the machining time goes to rough cuts!
“Historic aerospace buy-to-fly ratios can range from 15:1 to 30:1 on the high end, depending on the shape of the part and the manufacturing process. In the next few years we expect additive manufacturing to significantly reduce these ratios, while also being cost-competitive”
Leo ChristodoulouAdditive Manufacturing of Near-Net-Shapes
Metal Additive manufacturing (AM) is a near-net-shape process that can cost-effectively replace conventional processes. AM builds up a part in layers, depositing metal only where it is needed. For some technologies, this reduces material waste as compared to CNC machining (powder bed technologies have difficulty reusing all un-printed powder). In addition, AM pre-processing can be much faster than the complex programming required for CNC toolpath generation. Finally, Metal AM requires no tooling, so can be cheaper than tooling-based processes (casting, forging, or stamping) for low-to-mid volume production.
The benefits of AM over CNC machining for near-net-shape production are particularly strong for high buy-to-fly ratio parts. In these applications, the reduction in material waste and rough cutting time is very valuable. If the near-net-shape can be printed quickly and cost-effectively, large overall benefits in time and cost result.
Machining vs AM - An Example of Time and Cost Savings
We will illustrate the cost and schedule benefits of AM using a manufacturing scenario of a low volume titanium component. We selected Titanium (Ti-64) because it is a popular material for which good data are available. We build our example around a 5 kg final part and a production run of 100 units, representative of relatively low volume industries like aerospace. The above example geometry (17:1 buy-to-fly) is used for the schedule analysis. Joule Printing™ is selected as the AM technology for this comparison since it has demonstrated the best combination of speed and cost.
We begin by comparing timelines for machining and Metal AM.
For CNC machining, a typical workflow and timeline for our example aerospace titanium machined part are as follows:
| Workflow Stage | Time (days) |
|---|---|
| Design fixtures & program toolpaths for their machining | 2 - 4 |
| Programming toolpaths for the cutter to remove material (*not critical path since it can be performed while waiting for billet) | 2 - 4* |
| Procure all the material and components needed to make the fixtures and parts | 10 - 20 |
| Manufacturer and QA fixtures | 2 - 4 |
| Machine setup and test run of first parts (program try out) + QA | 2 - 4 |
| Toolpath, tool, and machining parameter optimizations to improve speed and reduce costs. May include simulation as well as physical iteration. | 2 - 4 |
| Rough machining is run in serial production with quick change overs between parts | 10 - 30 |
From final design to manufacturing the near-net-shape part can take anywhere from 30-70 business days for a machining program like this.
A typical Joule Printing™ based AM workflow and timeline for our example titanium near-net-shape part program are:
| Workflow Stage | Time (days) |
|---|---|
| Programming toolpaths for Joule Printing | 1 - 3 |
| Joule Printing in serial production with quick changeovers between parts | 10 - 20 |
| QA | 1 |
The Joule Printing workflow is much simpler with fewer steps. The timeline is less than half that of Rough Machining (17 vs 40 business days in our example). The primary areas of time savings are: programming, material ordering, manufacturing fixtures, and the machining time. Metal AM programming is much faster than CNC. Also, eliminating the need for custom fixtures saves time and reduces material requirements. Now let’s take a look at the economics.
Comparison of Costs
The major cost components in near-net-shape machining are equipment (CNC machine time, cutting tools, maintenance); materials (billet for part and fixtures, fixture components), and labor (programming, setup, changeover, QA). When working with high cost, difficult to cut materials like titanium, the largest cost drivers are usually the cost of billet and machining time. The numbers we use in our analysis are:
| Aerospace grade titanium billet (*Ranges from $40/kg to $70/kg dependent on billet size, quantity purchased, market dynamics, etc. For this model we assume the midpoint.) | $55/kg* |
| Recouped on the recycled titanium chips | -$5/kg |
| Average cost for removing titanium in a rough machining operation, including all associated components and overhead ($35-65/kg range provided by aerospace manufacturer) | $50/kg |
| Total average programming cost for a 5 kg titanium aerospace part program (15 hours @ $100 per hour) | $1500 |
| Total average fixturing cost for a 5 kg titanium aerospace part program ($500 materials, $500 manufacturing) | $1000 |
| Total average machine setup cost (2.5 hours @ $100 per hour) | $250 |
The major cost components in near-net-shape metal AM are printing (printer time, maintenance), materials (wire, consumables), and labor (programming, setup, changeover, QA). For this analysis, the data are:
| Aerospace grade titanium wire ranges from $150/kg to $180/kg dependent on quantity purchased, market dynamics, etc. For this model we assume the midpoint. | $165/kg |
| Average cost of Joule Printing process | $78/kg |
| Fixed cost per build (assuming 1 part per build in this scenario) | $75/build |
| Total average programming cost for Joule Printing | $100 |
Using the above data, the near-net-shape unit cost ($/kg) is calculated as a function of the buy-to-fly ratio:
As the chart shows, Joule Printing unit cost is independent of geometry, whereas CNC machining cost is directly proportional to the buy-to-fly ratio. For both processes, the fixed costs (programming, fixturing, setup) are mostly negligible when amortized into quantities of 100+ units; the dominant unit costs are the material and processing. For titanium parts, Joule Printing is more cost effective than CNC machining for buy-to-fly ratios greater than 3:1. At 7:1 buy-to-fly ratio, the cost of Joule printing the near-net-shape is less than half that of machining. There are many aerospace and industrial parts with buy-to-fly ratios between 15:1 and 30:1 – offering huge opportunities for cost savings.
Other Considerations
“AM can reduce the “cradle-to-gate” environmental footprints of component manufacturing through avoidance of the tools, dies, and materials scrap associated with CM processes” (From the article, Insights Into Additive Manufacturing At Boeing)
Leo ChristodoulouOther benefits of using Metal AM to augment CNC machining include insulation from titanium price fluctuations and environment sustainability. Titanium and other high-performance metals are not only expensive but their price is also highly variable. This is a big issue for large manufacturers that purchase thousands of tons of these materials annually. Reducing their use decreases these company’s sensitivity to the dynamic material pricing. Metal AM shrinks the environmental footprint by reducing use of energy and metal resources. Machining away 1 kg of titanium requires over 40 kwh of energy whereas Joule Printing™ requires less than 1 kwh per kg added. This huge difference in energy efficiency is multiplied further with a high buy-to-fly part. We will be releasing a future post going into more depth on this topic.
Conclusion
Metal AM for near-net-shape vs machining represents a significant opportunity to reduce time and costs. It is not surprising that leading manufacturers are now considering metal AM when looking for more efficient methods to manufacture high buy-to-fly ratio parts.
To take advantage of Metal AM to compress schedules requires that the process is qualified and quickly deployable. This raises important questions around implementation. We encourage you to read our post on Metal AM Application Criteria and stay tuned for more information on these topics.
A big thank you to the following individuals that contributed to this article:
Leo Christodoulou - Boeing
John Barnes Kevin Slattery - The Barnes Group Advisors
Joris Peels - 3DPrint.com
Peter Rogers - Autodesk
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Alex Huckstepp
VP Business Development
Digital Alloys is committed to providing the technology and expertise manufacturers need to use metal additive manufacturing in production — enabling them to save time, shrink costs, and produce valuable new product.