CNC Milling vs. ECM
CNC milling is one of the most common and popular manufacturing tools, used for machining both simple and complex parts. However, a lesser-known option with an array of advantages is pulsed electrochemical machining (PECM). While neither process is the “right answer” in all cases, we’re here to help you figure out which one works best for you.
In CNC milling, materials are shaped using a rotating cutting tool according to a 3D model that has been programmed into the machine. In pulsed electrochemical machining (PECM) on the other hand, materials are shaped into the inverse geometry of an electrode tool by dissolving the metal into a charged electrolyte solution.
While CNC milling can be used across a variety of material classes – e.g. metal, plastic, ceramic, and composite – PECM only works on conductive metals, albeit a wide variety of them. A more detailed description of PECM-compatible materials can be found here.
Despite this essential difference, these subtractive manufacturing processes share other similarities, e.g., CNC milling and PECM both can be used as finishing processes and have high precision. However, there’s a reason we call PECM “pixel-perfect machining” at Voxel. Because CNC milling and PECM use entirely different processes to complete the same job, they can yield very different results.
High Force Milling vs. Low Force PECM
CNC milling uses a contact-based process to remove material, which can place forces on the cutting tool, workholding, and milling spindle. When CNC milling can use short or large-diameter tooling and a rigid setup, the process can be very fast and accurate. But some part features require the use of long tools to shape the part, which can lead to impaired accuracy, reduced removal rates, and longer machining times.
Figure 1: Relationship between deflection and tool length, indicating opportunities for ECM usage.
On the other hand, PECM has low forces which are primarily dictated by the fluid flow of electrolyte between the tool and workpiece. Due to these low forces, its speed and accuracy are also less sensitive to tool aspect ratio. Instances when a long tool but low deflection would be required are an excellent potential opportunity for PECM.
The low process forces can have additional benefits as well. For example, CNC milling can achieve thin walls, especially when the aspect ratio is small or low tool loads (slower speeds) are used. However, when even thinner walls are required and especially when faster speeds are required, an unconventional machining process like PECM becomes the clear winner. As a non-contact process, PECM applies no pressure (and thereby deflection) on the thin wall, and causes no vibration. This means thin walls stay within tolerance and maintain finishes with very low roughness, all without damaging the tools used to create them.
Surface Finish, Speed, Precision, and Accuracy: All Interconnected
Both CNC milling and PECM can be used as either a roughing or a finishing process, but many simple roughing operations are much better suited to CNC milling. At Voxel, we refer to the specific removal rate to designate the metal removal rate per size of the machining tool (cutter, electrode, grinding wheel, etc). The specific removal rate for CNC milling can be much higher than electrochemical processes – after all, it is removing material in chunks (or chips) while PECM is dissolving the metal atom by atom. This means that applications where a larger roughing end mill or face mill are used will almost always be faster with CNC milling than PECM.
However, the unique strength of PECM is that we can make our electrode much larger, adding tool surface area in a way that cannot be matched by a CNC milling operation. This advantage is especially apparent when the feature sizes are small. For example, a plate with an array of 5000x 1mm / .040” holes would be processed one at a time via CNC milling, while with PECM, we would process thousands simultaneously, dramatically improving the process speed.
Figure 2: Tool marks on surface when machined with CNC milling (left) vs. mirror-like surface when machined with PECM (right).
Finishing of contoured surfaces, such as are used for fluid flow surfaces (i.e. turbine/compressor blades), also can be an incredibly slow with CNC milling given the need for small tools and step-overs during the machining process to create a smooth surface. PECM, in contrast, can machine the entire surface simultaneously, resulting in a speed advantage that climbs linearly with the size of the surface and exponentially with its complexity.
Accuracy and repeatability are also considered differently in CNC milling vs PECM. On a contoured surface or with delicate or high aspect ratio features, accuracy in CNC milling requires slowing down the process to manage cutter deflection. Conversely, accuracy in PECM requires a tool modification iteration process, which makes the up-front cost of PECM higher than competing technologies, such as CNC milling.
Alternatively, once both processes are dialed into the right accuracy, PECM can often be more repeatable due to its low forces and an unchanging tool geometry. This means that if your part geometry is relatively simple, likely to change (i.e. prototyping phase), or destined for low volumes, CNC milling is a better process. On the other hand, if your features are very complex (e.g. thin walls, contoured surfaces), have high part quantities or high quantities of repeating features, PECM may be a better process.
Sensitivity to Hardness/Toughness
When using CNC milling, the physical material properties have a dramatic effect on machining rates. Conversely, PECM is unaffected by physical properties and can machine hard or soft materials with similar speeds.
Figure 3: Relative removal rates between materials for a given manufacturing process. In ECM, physical material properties have little to no impact on machining speeds.
For example, the elasticity of nitinol makes it challenging to CNC mill, because either the part moves on contact or absorbs the cutter force. Work-hardening materials such as stainless steels and super alloys can also be difficult to CNC mill as it requires optimal parameters to create the right size metal chip to remove heat without adversely affecting tool wear or cutter deflection.
Alternatively, PECM is sensitive to the chemical properties of the material and although our removal rates may be relatively unaffected, different material chemistries can affect the surface finish.
Tool Wear and Burrs
CNC milling’s contact-based process means it can have a tool life as short as 14 minutes, with no indication when tool failure will occur and a potential for catastrophic failures (1). PECM, as a non-contact process, creates virtually no wear on the tool, making PECM tool life near-infinite.
Additionally, PECM’s non-contact process creates no burrs, naturally smoothing corners instead. This equates to benefits in many applications. For example, no burrs on a surgical tool means no metal particles left in the body. No burrs in a precision environment means no burrs contaminating the environment or leading to wear in moving parts. In these and other cases, machining – or at least finishing – with PECM is a wise choice.
PECM is advantageous as a non-contact, non-thermal process that is compatible with any conductive metal or alloy. With PECM, thin walls, speed, repeatability, and low surface roughness are all possible.
That said, either PECM or CNC milling can be the better choice, depending on the situation, and we’re happy to help you figure that out. Contact us to discuss if PECM is a good fit for your geometric – and budgetary – parameters.
1. Pennington, Tony. “WNT applications team cuts tooling costs for Velden Engineering.” Engineering Subcontractor, October 2015, pg. 38, http://www.rbpublishing.co.uk/engsubcon/archives/escoct2015.pdf
This article is part of our ongoing “PECM vs. Competing Processes” series that compares PECM to other, popular machining processes. Find other articles in the series below:
Electropolishing vs PECM - coming soon