March 2010 Archives

8)Static machining

The example processes discussed so far all had some relative movements between the tool electrode and the work piece where the electrodes are fed into the work pieces as the process progressed. One of the variations is called Static Machining where there is no relative movement between each other. Only the electrolytes move through the gaps between the two.

The process shown in [Fig.1] is an example of Static Machining process. A cylindrical electrode with unwanted section insulated is inserted inside a cylindrical work piece, the electrolyte is circulated in the gap and electrical current is applied. The inner surface of the work piece is electrolytically etched and the bore diameter is enlarged. This method is also called "electrolytic bulge" and "electrolytic boring". Enlarging of blind holes can be performed by the same technique. With the static machining methods, there is no gap adjusting involved, and the final accuracy depends on the original machining accuracy.

9)Electrolytic de-burring

This can be considered as a variation of the static machining process. By pre-shaping and placing the static electrode properly, burrs on parts are removed by electrolytic actions only.

Conventionally, de-burring operations are performed using hand tools such as portable die grinders, files, and chisels. However, as more automated machining and assembling are used in recent scenes, the need for precision de-burring and percentage weight of the hand operations has become non-negligible.

Electrolytic de-burring process can not only meet these demands but also enables de-burring of difficult-to-reach areas as well. [Fig.2] shown an example of electrolytic de-burring.

The [Fig.2] shows a de-burring process for branching areas of small cross-drilled holes. The tool electrode is fixed in place in the center bore by the electrode mount fixture, and is insulated other than the areas directly exposed to the branching area of the cross-drilled holes. By insulating, only the targeted areas are exposed to the electrolysis. The tool electrode and the work piece are held fixed together, and the electrolyte flows through the mount fixture to the target area and expelled through the cross-drilled holes.
Electrolytic de-burring is better suited for large scale production schemes since the tools and fixtures must be tailor produced for particular part.

5) Electrolytic turning

This is a process of performing lathe-like turning work by electrolytic process. Unlike the conventional lathe turning where cutting tool bits are to move axially along the part, electrolytic turning produces axially symmetrical part by advancing pre-shaped electrode tools radially towards the center of the rotating work piece, as shown in [Fig.1].

This method can achieve in a single pass what conventional copy lathe can. Also, it possible to perform operations difficult for the conventional lathe associated with part distortion problems sue to tool pressures.

6) Electrolytic cut-off

This method is effective for hard cutting materials such as tungsten and its alloys. Many types of tool electrodes are used for this. One example is shown in [Fig.2].

In this example, thin metal electrode discs are rotated at high speed and the electrolyte is sprayed on to the discs. The sprayed electrolyte is fed into the process gaps by centrifugal force. This example shows two discs but the discs can be of any multiples.

7) Electrolytic face-milling

This is a face-milling process that utilizes the electrolytic effects. An example where the electrolyte is sprayed on a surface of a rotating cylindrical electrode is shown in [Fig.3] This is a non-contacting process and the electrode does not contain any abrasives, which is unlike electrolytic grinding process.

2)Electrolytic cut-out process (electrolytic trepanning)

Electrolytic trepanning is a process of cutting out a desired shaped part from a substrate. The process itself is the same as electrolytic drilling but the product is not the hole but the center material of the hole remaining inside the electrode.

The electrolytic trepanning, therefore, uses the same equipment as the drilling process, with a small modification added to the electrode. The electrodes for drilling was insulated on the exterior for stray current prevention, but the electrodes for trepanning is insulated both on the inside as well as the outside. By doing so, a product with a diameter coinciding the cross section of the electrode can be produced. This is shown in [Fig.1].

3) Cavity machining (electrolytic sinking)

A cavity is a hollow feature. Sinking means "to bore a hole". It can be said that all electrolytic machining processes are variations of this Cavity machining. The notorious example is "EDM die sinking". Die and mold are made of hard cutting material and have complex features, so electrolytic machining is a good candidate and frequently used on forged parts. A notable benefit of this process is that the electrode consumption is low and many molds can be produced with one electrode. An example of a forging mold product is shown in [Fig.2].

4) Electrolytic contouring (shaping)

This is a process to create outside contours of parts. With the other electrolytic processes, electrolytes are supplied via the bores through the inside of the electrodes, but this method tends to leave traces of electrolyte outlet orifices on part surfaces. Special scheme is needed to avoid these traces. An electrode is placed on each side of the work piece, with the work piece held in the middle, and the electrolyte is applied into the gaps. This process is mainly used for contouring of turbine blades. It is most suitable to create surfaces with gradual curvatures that generate small fluid resistances.

(5) Electrolytic etching applications

Electrolytic etching process can create complex shapes with a single pass and is not affected by material's hardness or toughness. The process offers greater benefits for complex applications with materials that are hard to machine by conventional means.

For instance, when the tool radius is small the tool rigidity also becomes small, and the cutting depth must be reduced. This will greatly reduce the process efficiency. Choosing the electrolytic machining in such cases would be quite effective.

Let's examine some applications that relates to the above.

1) Electrolytic drilling

For drilling operations, tubes with various cross sections are used as electrodes.

As shown in [Fig.1], the tube's exterior is insulated with epoxy resin or similar, with a somewhat wide land on the end. The insulation on the tube's sides prevent unintended excess over-cutting of the hole ID by any stray current. This countermeasure is not necessary for certain types of electrolytes. It is said that solution such as NaNO3 with small throwing power can minimize the over-cutting to virtually zero. A product example is shown in [Fig.2].

Curved hole drilling

Electrolytic drilling can be utilized to create curved holes or slots easily. This can be accomplished by using a long electrode shaped in an arch, and feeding it in the circumferential direction as shown in [Fig.3].