February 2010 Archives

(4) Current efficiency and process speed of electrolytic machining

Processing speed of the electrolytic machining can theoretically be obtained using Faraday's law of electrolysis. However, it is necessary that the following conditions are met.

In general, the actual metal removal speeds may not match the calculated theoretical values. This may often be so because one or more of the conditions show above may not be fulfilled.

The electrical current efficiency can be calculated by the following equation.

This represents a percentage of current actually used for intended purpose out of the total current flowing through the electrolyte.

In actual practices the amount of actual metal removal amount may be larger than the theoretical value, and the current efficiency may seem to exceed 100%. The reason for this may be...

On the other hand, if the actual metal removal amount may be smaller than the theoretical amount, and the current efficiency may seem lower than 100%, the following may be the cause.

One of the most problematic sub reactionary processes is anode gas generation. The overvoltage level of the anode increases as the current density increases. When this overvoltage increases enough and the anode's electrical potential reaches a point to allow oxygen generation, a part of the electrical current will be consumed for this gas generation.
As can be seen so far, increasing the anode current density to hasten the machining speed past a certain point will result not only in metal elusion but also gas generation, and the etching efficiency degrades and machining speed decreases.

(2) Configuration of a Electro-machining system

A typical electro-machining system configuration is shown in [Fig.1]. As seen in [Fig.1], the work and the tool (electrode) is held very closely (0.02~0.7mm typical) to each other. A DC current is applied to the work as an anode (positive) and the tool as a cathode (negative) through the electrolyte.

Normally, the voltage used is 5~20V, and current density is 30~200A/cm2. This voltage/current level is quite high in comparison to Electrolytic-polishing and plating. The higher voltage and current results in faster process speed, improved accuracy and better surface roughness.

In order to constantly maintain a fixed positive/negative electrode distance (process gap), the cathode is fed into the work piece. The feed speed is 0.5~10mm/min based on the current density specified. As the result, the work piece is milled in a shape inverse of the tool (cathode) shape.

When the current is applied, hydrogen gas is formed from the cathode surface, and dissolved product from the anode surface. Also, the electrolyte temperature rises due to Joule heating by the electrical current application. If the electrolyte temperature reaches a boiling point, controlling of the electrical current becomes impossible. It is important to remove these negative factors immediately. To do this, the electrolyte is forced to flow between the electrodes at high speeds by the recirculation pump at 6~60m/S, typically.

The used electrolyte falls into the storage tank, then the impurities are let sedimented, centrifuged, filtered, and cooled down. The clean electrolyte is reused.

(3) Electrolyte

Desired properties for the electrolyte used are: (1) does not promote generation of passive solubles that may passivate the metal surfaces; (2) positive ions in the solution do not electro-deposit on the electrodes; (3) high conductivity and low viscosity; (4) produces good accuracy and finished surfaces; (5) low corrosiveness and toxicity; (6) stable solution composition and low cost, easy to obtain; etc.

Neutral salt solution has a lower conductivity in comparison with acids and alkali, but is often used. For instance, sodium chloride (NaCl) solution is used for steel alloys, nickel-chrome alloys, and most other metals. This solution contains chloride ions and prevents anode passivation. Also, the positive Na ions prevents electrodeposition on cathodes.

(1) Outline

As opposed to Chemical Etching which chemically dissolves the work material, the electrolytic etching dissolves the work material electro-chemically by applying DC current to anodic work pieces. Processes that utilize electrolytic etching are: Electrolytic machining; Electrolytic-grinding; Electrolytic-honing; and Electrolytic-lapping. These processes all use pre-formed cathode electrodes shaped into required configurations. Work pieces as anodes, they are placed very close to each other and let electrochemical dissolution take place. The electrochemical etching reactions are concentrated on to desired portions and controlled.

Both electrolytic and chemical etching are performed by concentrating and controlling the etching effects on certain parts of the work piece, but the electrolytic etching has more controlling options available as compared to the chemical counterpart.

With the chemical method, the only control option available is to use masks where etching effect is to be avoided. The same can also be used with the electrolytic method, but most applications utilize electrodes pre-formed into desired shapes for electrolysis to efficiently produce intended products.

1) Electrolytic machining

Electrolytic machining is also called electrochemical machining. The process uses electrolytes that do not generate any passivation film and utilizes only the electrolytic etching effects to perform complex milling work. Electro-machining has established its own position as an alternative among the other conventional methods.

2) Electrolytic-grinding

Electro-grinding is a combination of mechanical grinding and electrolytic etching. This grinding method uses grinding stones made of abrasives bound with conductive adhesive. The grinding stone mechanically grinds off passivation film to expose fresh metal surface, and the conductive adhesive facilitates the electrolytic dissolution of the metal. The benefits are: increased grinding efficiency, reduction of: grinding stone consumption; heat; and grinding resistance.

3) Electrolytic-honing, Electrolytic-lapping

Electro-honing and electro-lapping share the same principle with electro-grinding. They are the counterparts to mechanical honing and mechanical lapping. The benefits are the same as that of electro-grinding. Since honing and lapping are finishing processes and the amounts of metal removal are very small, they should perhaps be considered as part of surface etching classification.

(4)Chemical milling applications

1）Outer skin of aircraft fuselage

The aircraft fuselage skin in [Fig.1] is first tension formed into curvature, and chemically milled.

The web portion which constitutes an 80% of the surface area on one side is chemically milled down to the thickness of 0.625mm from 6.25mm, and the weight is reduced from 24kg to 6kg. There was no method to produce such a part in a single piece, previously. They were previously produced by welding or riveting the stiffeners on 0.625mm plates. Dramatic reduction in part count and assembly time were realized and production cost savings were attained.

2) Example of entire perimeter milling

It is difficult to make a certain portion of forged part extremely thin. In the example of the forged part [Fig.2] (a), it is easy to form the thick portion by forging but difficult to make the thin central portion. For such case, the part can be made by forging with the central portion as thin as possible with some margin material left on the entire part, then the overall dimension can be reduced down afterwards to that of the desired by chemical milling.
This technique is also applicable for mechanically milled parts and extruded parts, as shown in [Fig.2] (b).

3) Thickness reduction of cast parts

It is very difficult to produce cast parts with thin features. Here, it is possible to produce cast parts with enough safety margin added to prevent cast failures, then remove the excess material by chemical milling.

4) Tapered milling applications

The tapered milling is performed by gradually immersing and raising in and out of the etchant.