April 2010 Archives

(1) Principle of electrolytic grinding

Electrolytic machining is a method of removing material from metal surfaces by electrolytic etching, and electrolytic grinding is a process that adds mechanical grinding process to this.

Typically, with the electrolytic elution processes, some anodic byproducts that inhibit the elution will be formed and in some cases the elution can completely stop due to the metal surface becoming passivated. In order to prevent this, non-passivating solutions are selected for electrolytic machining. In contrast, the electrolytic grinding employs mechanical grinding by abrasive grinding media to scrape away the passivated layer so the etching process can continue on to the freshly exposed metal surfaces.

Electrolytic grinding was originally developed for grinders to produce electrolytic machining tools, but eventually was widely applied for many hard-to-grind materials since the method offered lower grinding heat and forces in comparison to the conventional methods.

(2) Grinding wheel electrode

In electrolytic grinding, the total amount of material removed can be divided into the amount removed by mechanical grinding and electrolytic elution. The amount removed by the mechanical grinding is very small as 1~2% for carbon steel, and 10~16% for carbide, while most material removal is by the electrolytic elution. [Fig.1] shows the cross section view of the grinding wheel.

The grinding wheel is composed of non-conductive abrasive particles and binder material (mostly metallic). The non-conductive abrasive particles protrude by a small amount (typically 0.02~0.05mm) from the surface of the conductive binder material. The insoluble anodic byproducts on the surface is ground off by the abrasive particles to allow the conductive binder material to continue the electrolysis on freshly exposed metal surfaces. This also sets the machining gap.

(3) Electrolytic grinder system

[Fig.2] shows an example of a electrolytic grinder system. Electrical current flows from: positive side of the DC power supply > work table > work piece > electrolyte (working gap) > brush > negative side of power supply. The electrolyte is sprayed onto the frontal surface of the wheel, then the centrifugal force of the rotating wheel feeds the electrolyte into the working gap.

(2) Acidic solutions

Various inorganic acid solutions have relatively high electrical conductivity, and there is no electro deposition risk on tool electrodes since cathode reaction is mainly of hydrogen discharge. However, the acidic solutions are typically highly corrosive and only used for special cases only.

In addition, when using the acidic solutions, the electrical conductivity becomes lower as the process progresses and the hydrogen is depleted. At the same time, the PH value increases and the metal ions are likely to electro deposit on cathode surfaces. The eluted metal ions will remain in the solution.

(3) Alkaline solutions

Normally, alkaline solutions are not used for most metals, since insoluble anodic products are generated and inhibit dissolution of work piece materials.

However, the alkaline solutions are effective for tungsten and molybdenum. These metals can firstly be oxidized by anodization, then tungstates and molybdates can be eluted using NaHO, and etc.

For these reasons, the alkaline solutions are used for electrolytic machining of cemented carbide material. Cemented carbide is essentially a mixture of tungsten carbide and cobalt, and they have different elution mechanisms. In order to obtain good surface finishes, at least two components that promote WC and Co elution. NaCL and NaOH are used for this purpose. Co is eluted with NaCL by Co+2CL > CoCL2+2e as cobalt chloride.

WC is first anodized into WO3, then reacts with NaOH as...

and eluted as sodium tungstate.

Alkaline solutions require care in handling but since they are non-corrosive to metals, selecting of component material for the processing system becomes very easy.

(4) Roughness of electrolytically machined surfaces

Since the electrolytic machining processes utilize electrolytic etching (elution) to remove an atom at a time from the material surfaces, mechanical forces and heat related effects typically associated with mechanical machining do not apply. Therefore, the resultant surfaces are different from that of mechanical machining.

Mechanical machining removes the material using contacting cutters that leave cutter marks, and that shows as the surface roughness, but non-contacting electrolytic machining does not. However, most metals are of polycrystalline in nature and each will have varying electrolytic elution speed, therefore some surface roughness will show.

Following characteristics are required for electrolytes for electrolytic machining.

There are electrolytes that generate precipitation while others do not. The former includes neutral salt solution such as NaCL, and the latter includes solutions of various acids and alkali.

(1) Neutral salt solutions

Neutral salt solutions are in common use since they are less corrosive than others, though have lower electrical conductivity. As anode reaction, compounds of metal and negative ions are formed, and as cathode reaction, hydrogen gas and hydroxyl ions are formed. These byproducts make chemical bounds with each other in the solution to form insoluble hydroxides.

The hydroxides will float in a colloid-like form in the electrolyte, and do not largely affect the electrolyte's conductivity, process speed, finish roughness, and accuracy up to 2wt% (weight percentage) of concentration. If the concentration value exceeds the above, the liquid's viscosity will sharply rise and inhibit the electrolyte from flowing freely. The concentration must be kept below 2wt%.

Typically, removal of precipitated byproducts is performed by combinations of filtration, centrifugal separation, and sedimentation. It is more economical to clean and reclaim even the lowest cost electrolytes (such as NaCL) rather than disposing and replacing.

Most popular neutral salt based electrolyte is the NaCL solution. It is used for almost all metals including steel alloys and nickel-chrome alloys.

This solution contains chloride ions that inhibit passivation of anodes, and since Na ions are the positive ions that do not electro-deposit on cathodes. These characters make for excellent as electrolyte solution but it is somewhat corrosive and has low machining accuracy as shortcomings also.

The reason for the low machining accuracy of the NaCL solution is because of the solution's high throwing power. The "throwing power" is the ability of an electroplating solution to deposit metal uniformly. Since "high throwing power" means good metal elution even when the gap is large, the machining accuracy would be lower as the result.

Electrolytes such as NaCLO3 and NaNO3 are used as low throwing power types. These are non-corrosive and have good machining accuracies, but with low electrical current efficiencies.

8) Electrolytic micro-hole machining

Although micro-holes can be created with electrodes made of small diameter metal tubes with outer insulation, the insulating of this outer surface is difficult and the insulation at the tip is easily broken during the process.

In order to overcome this shortcoming, a scheme shown in [Fig.1] is used. The surrogate electrode is made of an insulating material such as glass tube with a fine tip, connected to the cathode camber with a flexible tube.

The actual electrode (metal rod or plate) is immersed in the electrolyte within the cathode chamber. Approximately one atmospheric pressure is applied to the cathode chamber to cause the electrolyte to fill and flow from the fine tip (fewμm) of the surrogate electrode.

The tip is placed very close to the work piece and the electrical current applied. The electrolytic etching occurs only at around the tip, and by advancing the tip position into the work piece as the etching progresses, the micro hole can be bored.

Since a constant electrical current will remove a certain amount of material in a certain amount of time, a hole with a constant diameter can be created by advancing the tip at a constant speed.

9) Electrolytic transcription

This is a process of transcribing patterns on a rotating drum onto a flat substrate, and can be applied to create printed circuits and etc.

The original pattern on a metallic plate is created by photo etching, first. Recesses of the original pattern is filled with non conductive material such as epoxy. This plate is affixed to a rotating drum, and the work piece is mounted on a slide table that reciprocates in linear motion under the drum. The gap distance between the two is maintained at 0.15mm and electrolyte is introduced into the gap while an electrical current is applied. By synchronizing the drum's rotation and the slide table, the pattern on the drum will be transcribed onto the work piece.

The used electrolyte carrying the electrolysis process byproduct will travel on the work piece and is collected by a tank below. Dissolved metals in the collected electrolyte are removed by an ion exchange resin filtration system and the electrolyte is put back into a reservoir tank for reuse.