The Wonders of Chemistry in the Machine Shop

Kennametal’s proprietary Extrude Hone EVO technology introduces a new level of precision to electrochemical machining, achieving precision surface conditions without stray machining attack

Expectations of accuracy continue to become more stringent for machining high-precision components (like automotive fuel systems and powertrain parts), putting the suppliers of such products under constant pressure for quality and process efficiency. That is the opportunity that Kennametal aims to exploit with its new electrochemical machining (ECM) process.  Extrude Hone^™ EVO features a proprietary generator technology that delivers 3 kW to 100 kW of power, depending on machine configuration. “Additionally, EVO delivers another exclusive added value: electrochemical machining without stray machining attack,” said global marketing manager Bruno Boutantin. “Customers will enjoy the highest surface finish quality for improved component performance.”

ECM is used in contouring, radiusing, polishing, deburring, and flow tuning applications. It is a subtractive method that works on the principle of anodic metal dissolution. The primary advantages of the process are: a) targeted material removal in precise locations; b) no mechanical or thermal stress
; c) process stability
; and d) high productivity.

Each part to be machined requires a cathode (-) for selective material removal on the workpiece (+). The cutting speed is equal to the DC current applied to the part. The lack of contact between the tool (-) and the workpiece (+) is important.

An electrolyte solution (NaCl or NaNO₃ in water or glycol base) handles the charge transfer in the working gap. The resulting electron current releases metal ions from the workpiece.

The amount of material removed is defined by Faraday’s Law (equivalent to current x cycle time.) Using the electrolyte, the removed material is flushed out of the gap as a hydroxide. This must be captured by an adequate filter system to maintain constant electrolyte conditions.

There will be a constant gap, resulting in proper tooling and cathode life. The shape of the tool cathode determines the shape of the material removal.

Kennametal Precision Surface Solutions product manager Patrick Matt explained that in precision electrochemical machining (PECM) the workpiece dissolves anodically according to the geometry of the subsequent electrode. He added that this makes it possible to achieve complex geometrical shapes in practically all electrically conductive metals (e.g. highly tempered steel, rolling bearing steel, powder-metallurgy steel, and super-alloys.)

As such, PECM is effective for machining shapes that could not be manufactured feasibly (or at all) in the past.  Shaping, drilling, or microstructuring on external or internal areas are typical applications in which the process can be efficient. Regarding tolerances, it is able of achieving quality in the range of 2 to 5 μm. Roughing, finishing, and polishing are typically combined into one operation. Surface finishes of 0.05µm are achievable.

The feed rate of the cathode is between 0.1 mm/min and 2 mm/min (depending upon the application.) Typically, multiple parts are machined over one cycle to achieve efficiency with per-part costs that Matt described as “attractive.”