May 2010 Archives

#043 Electrolytic Etching - Electrolytic Grinding Applications - 1

(6) Surface finish roughness of electrolytic grinding

General steel alloys can easily be machined and ground by conventional means and seldom require electrolytic grinding. But electrolytic grinding is a excellent alternative in case of heavy cutting high speed steel alloys where heat damage risks are present, for easily bent and damaged thin plate materials and flat grinding of honeycomb structure material, and for vanadium rich high speed steel alloys that cause heavy wear on grinding wheels. Also, the process does not cause any burrs rendering quite attractive for hypodermic needles and multi-layer materials.

[Fig.1] Electrolytic grinding of hypodermic needles - Before (left) and After (right)

The most applicable example for electrolytic grinding is for carbide cutting tools. Such hard-to-grind high hardness material grinding can gain the most from increased processing speeds.

[Fig.2] Electrolytic grinding example of a reamer tool

[Fig.2] shows an example of electrolytic grinding of a reamer tool. As can be seen in [Fig.2], the carbide and high speed steel blades, 1.5mm thick/2.1mm wide, are ground in a single path. Usage conditions of a diamond grinding wheel with 60~80 grain size and electrolyte containing NaNO2 and the results are shown in a chart within [Fig.1].

With conventional grinding methods where removal amount is small per one tool path, multiple positioning is needed for multiple tool rotations. But for the electrolytic grinding, cutting depth can be made deep and all the material can be removed with a single path. Tool position corrections can be made less frequent since the diamond wheel wear is only 15% or so of the standard wheels.

In addition, the electrolytic grinding generates little heat. For materials that are problematic with conventional grinding methods due to heat distortion and work hardening, such as turbine blades for jet engines, bucket material, nuclear reactor components, and various magnetic materials, electrolytic grinding offers advantages.

#042 Electrolytic Etching - Electrolytic Grinding - 4

(6) Finished surface roughness of electrolytic grinding

[Fig.1] shows a mechanism that determines finished surface roughness. The figure shows a relationship between the grinding wheel and the ground surface. The area "b" is subject to electrolytic grinding.

[Fig.1] Mechanism of electrolytic grinding

However, the area "b" is not the only area filled with electrolyte. The electrolyte supplied by the nozzle will fill the "c" area while traveling to the "b" area, as well as overflowing to the "a" area. The electrical current will in all the areas and etching action will occur in all three areas.

The "a" and "c" areas not touched by the abrasives are affected only by electrolytic etching. The "a" area will be removed as the process advances so the surface roughness is not affected.

The "b" area is affected by etching as well as the mechanical grinding. Since this area will remain as "finished" and the surface roughness is somewhat affected.

In the "c" area, the effects of etching on the surface will gradually lessens as the wheel's contact point pulls away from the surface. But since the area is the final finish that remains after the process, the effects on roughness is the greatest.

When all three areas are compare for roughness, it shows that area "a" is the roughest, and the area "b" is the least rough. The area "c" is rougher than "b" and this is the area of the final finish roughness. It is thought that the area "c" becomes rougher than "b" because of some stray current.

If the etching occurring at area "c" can be prevented, best finish can be obtained as the final finish. It is not possible to completely eliminate the etching at "c", but some measures can be applied to reduce to a minimum. In practice, the applied voltage is increased and table feed rate is increased to reduce the "c" time duration to a minimum.

#041 Electrolytic Etching - Electrolytic Grinding - 3

(5) Grinding wheel electrodes for electrolytic grinding

The grinding wheel electrodes for electrolytic grinding are made of non-conductive abrasive particles bound by conductive binder material. Diamonds are typically used for the abrasive particles for processing cemented carbides and other steel alloys with hardness of HRC65 or more. The particle sizes seem to be in a range from 100~120 mesh. The conductive binders are copper alloys and bronze impregnated resins. For general purpose steel materials, corundum particles for the abrasives, and bronze alloys (for metal bound wheels) / copper impregnated resins (for resin bound wheels) are frequently used. The corundum particles are of 60~80 mesh for metal bound, and 100 mesh for the resin bound wheels.

When the abrasive particles wear and protrusion amount is reduced by grinding use, they need to be dressed. The metal bound wheels can be dressed by reverse electrolysis. By connecting the wheel as the anode, the work piece as the cathode, and applying a low voltage DC to dissolve the metal of the binder surface, the abrasive particles can be exposed on the surface again. The resin bound wheels can be easily dressed mechanically so the reverse electrolysis is not required.

Other than the general purpose wheel types mentioned above, there is a type called Single Layer Diamond Wheel. The wheel is composed of a metal base, (mainly copper) with a layer of diamond abrasive particles bound to the surface by nickel plating. This type offers higher performances in comparison to the general purpose wheels since the diamond abrasives protrude from the bound surface very uniformly. It is suitable for use as profile forming grinding wheels since the metal wheel base can be made into final shapes required very easily.

However, the use of this type is limited to some special cases only since it is difficult to correct/repair when the diamond particles fall off from the surface, and the costs compared between the two, one with just a single layer of diamonds versus the one with a 1~2mm layer of diamonds, shows little difference. Important characteristic required for profile forming grinding wheels is the shape formability. For this reason, graphite bonding material is frequently used. Graphite bonded corundum wheels are in use for profile forming of high speed steel. This type includes alumina, WA, C, GC grits bound by graphite with a small amount of clay.

#040 Electrolytic Etching - Electrolytic Grinding - 2

(4) Electrolytes for electrolytic grinding

Electrolytes for electrolytic grinding are mostly of conductive water solution containing inorganic acids, and since prevention of passivated layer formation is not required, emphasis is on the following two points.

(1) Has corrosion prevention properties.
(2) Has high electrical conductivity.

Corrosion inhibitors used are: NaNO2, NaNO3, KNO2, NaCO3, Na3PO4, borax (Na2B4O7), and other salts that cause high PH properties. High electrical conductivity is obtained by adding KNO3, NaNO3, KNO2, NaNO2, etc. Amines or other organic corrosion inhibitors are sometimes added.

For cemented carbides, WO3 and TiO2 are eluted from WC and TiC initially, so chemicals that dissolve them must be added.
The chemicals used for this are potassium sodium tartrate (Rochelle salt), phosphate, carbonate, and etc. Chemicals frequently used for hard-to-grind materials are shown in [Table 1].


[Table 1] Electrolytes used for various metals
Metal materialElectrolyte composition
Tool steel, stainless steelNaNO2+NaNO3 +
Amines
TungstenKCL+KNO3 + Rochelle salt
High speed steelNaNO2 or KNO2 or combination of both + organic corrosion inhibitors

For metals with high resistance to chemical erosions, electrolytes that include chlorides must be used. Although this type of electrolytes are highly corrosive to the processing system components, they are frequently used for rough machining where high removal speeds are required. Mixture of NaCL and boric acid is used for this purpose.

Electrolytes used for cemented carbides may also be used for other tool steel materials to avoid replacing the electrolyte when changing over from one work to another. If both material types are frequently mix ground on a system, both electrolyte types may be used as a mixture.

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