Pros & Cons of CNC Machining

Pros & Cons of CNC Machining

CNC Machining for Prototyping and Production

CNC machining is commonly used in the production of tools, prototypes and end-use parts. With versatility as one of its strong points, the method is frequently applied in aerospace, medical, industrial, consumer products and electronics sectors. But what specific advantages are gained by using CNC machining instead of other manufacturing processes? There are pros and cons of both CNC milling and turning across various axes configurations.

Pros: Timely, reliable, accurate and robust

CNC milling and turning are highly accurate and repeatable processes. Tight tolerances between +/-0.001″ – 0.005″ can be achieved, depending on specifications. Machines can be programmed to reliably run for 24 hours, 7 days a week if necessary, so CNC milling is a good way of getting parts produced on demand.

Using standard tooling, CNC machining is particularly valuable for creating custom, one-off parts, i.e. for replacing legacy components or delivering a specialized upgrade to a customer. It is also conceivable to scale single-part production to runs exceeding 10,000 units. Depending on the unit number, size, and complexity, the turnaround for components can be as short as one day. With shipping and delivery, deadlines can be met within a week.

Another primary advantage of CNC technology is the achievable mechanical properties. By cutting away from a bulk material, rather than thermally transforming it as in injection-molding or additive manufacturing, all desirable mechanical properties of the metal or plastic of choice are retained. More than 50 industrial grade metals, alloys, and plastics can be machined using CNC milling and turning. This selection includes aluminum, brass, bronze, titanium, stainless steel, PEEK, ABS, and zinc. The only material requirement for CNC machining is that the part has an adequate hardness to be fixtured and cut.

cnc machined aluminum 6061
A rocket engine injector radial passage CNC machined from aluminum 6061 for engineers at UC Irvine. Source: https://www.xometry.com/blog/case-study-xometry-helps-uc-irvine-rocket-team-prepare-for-spaceshot

Cons: Cost of Geometric Complexity

One trade-off when taking advantage of the high performance of CNC machining is that geometric complexity comes at a cost. Simple, chunky parts are the best designs for CNC milling and turning. There will always be some design limitations due to tool access, although the degree of this effect is relative to the number of axes on the machine. In other words, the more axes used, the more complex the features can be achieved.

Tip: For more on how to design for CNC machining and understand tool access, see our quick guide here.

Another trade-off is that start-up costs for CNC machining can be expensive. It is necessary to have a trained professional perform the setup, tool loading, and programming on CNC mills and lathes. Luckily this cost is fixed, so by taking advantage of the same setup for multiple parts, it becomes more economical. Saving money is also achieved by keeping part repositioning to a minimum. Machining at 5-axis and above can sometimes be more economical on multi-faceted geometries because it eliminates the need to manually reposition the part.

Wire EDM method of machining can be slow and expensive relative to other processes, and the range of materials that can be used is shortened as they must be electrically conductive.

Pros: Costs that Work for You

To achieve the most cost-effective CNC machined parts, larger production runs are recommended to spread the cost of machine setup. Unit price decreases incrementally when CNC machining up to volumes in the thousands where it plateaus. Structured batch production and shipments of CNC parts can help mitigate wastage or inventory costs.

Of all the various machine configurations, 3-axis CNC milling, the simplest setup, is generally the cheapest method of making uncomplicated parts with high tolerances. CNC turning on a lathe is also a highly cost competitive process when cylindrical workpieces like threaded rods and shaft couplings are required. Typically, a lathe would cost 15% less than the 3-axis machine for a similar part.

With 5-axis CNC machining, the options are split into two configurations: indexed 5-axis CNC milling and continuous 5-axis CNC milling. In indexed 5-axis CNC milling, the workpiece is automatically rotated to give tools more access to mill features. The extra two directions of movement are done between milling steps without removing the part from its fixture. The difference with continuous 5-axis CNC milling is that the machine can simultaneously move in all directions as the workpiece is cut. Both processes eliminate the added cost and potential margin for human error that comes with manually repositioning a workpiece. Due to these benefits, 5-axis machining is the best solution for complex components.

In comparison to “basic” 3-axis CNC mills, 5-axis machining comes at an increased expense, with indexed 5-axis CNC milling being the cheaper of the two. Continuous 5-axis CNC milling typically costs over 20% higher than an indexed 5-axis machine, and about double that of a standard 3-axis mill.

5-Axis Milling Video

A demonstration of continuous (also known as simultaneous) 5-axis CNC milling on an MU-10000H, from Xometry manufacturing partner Okuma.

About Xometry

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