The Complexities of Electrochemical Grinding

January 22, 2019 Joshua Jablons

What makes it such a specialized — and rare — grinding method?

One underlying principle of electrochemical grinding is that it’s a version of surface grinding and yet a category unto itself. That’s because of some inherent realities of the electrochemical process.

In fact, one of the paradoxes of electrochemical grinding is it’s a terrible way to do traditional surface grinding!

So, why does anyone do the electrochemical method? And what makes it such an interesting niche?

Removing Material with Electrochemical and Abrasive Actions

One could say the electrochemical grinding process is a combination of surface grinding and chemistry. The process “grinds” metal using both electrolytic activity and the physical action of a charged grinding wheel.

Specifically, the electrochemical method combines light abrasive action with anodic dissolution of the material in the presence of a conductive fluid. In effect, the metal is decomposed by a DC current flowing between a negatively charged grinding wheel (cathode) and the positively charged workpiece (anode).

Unlike many traditional grinding methods, electrochemical grinding works on a wide range of metals regardless of material hardness or strength. However, it does require the metal being ground to be conductive and electrochemically reactive.

Variables in the Electrochemical Grinding “Recipe”

Each different type of metal you may want to grind has its own specific chemistry and conductivity requirements. The precision of the electrochemical grinding process depends on getting the recipe right.

That includes striking the right balance in four key variables:

  • Voltage — A low voltage DC power supply gives you less electrochemical action and more abrasive action. A higher voltage gives you the opposite — more electrochemical and less abrasive action — but beware! Too much voltage can cause arcing.
  • Electrolyte Flow — There must be enough flow to create an electrochemical cell and complete the grinding process. However, too much electrolyte flow can remove metal beyond the area targeted for grinding.
  • Feed Rate — As with low vs. high voltage, the rate of material feed has an impact on the electrochemical and abrasive actions. In this case, a slower feed results in more electrochemical action and less abrasive force, while a faster feed increases the abrasive action.
  • Wheel Type — While different wheels can be used in electrochemical grinding, the wheel must be conductive. Thus, the grinding wheel is similar to traditional bonded abrasive particle wheels but unique to the process, the wheel is infused with high amounts of copper to make it conductive.

Comparison with Other Methods

As with the electrochemical cutting (ECC) method sometimes used for 2-axis cutoff, with electrochemical grinding there is a ratio to be controlled and manipulated between the material that is actually removed abrasively and erosion of the electrochemical reaction.

That means the process is not affected by issues such as wheel loading or glazing. In addition, the grinding wheel has a longer life — as much as 50 times longer — and there is less frequent need for truing and dressing than with traditional abrasive grinding methods.

The electrochemical grinding process bears some resemblance to creep-feed grinding, in that both feed material more slowly than conventional surface grinding. However, while creep-feed can be used to enhance surface finish, that method was designed mainly for removing large amounts of material in one pass.

Unlike traditional mechanical grinding, electrochemical grinding produces little to no heat or stress that can distort delicate components and harden or damage the metal being ground. The method is also generally more efficient and cost-effective than non-traditional processes such as wire and die-sinker electrical discharge machining (EDM).

But as we said above, electrochemical grinding is primarily an erosive process, which decompose the workpiece. In addition, the material that’s removed is left behind in the conductive solution. Therefore, the fluid must be changed frequently to maintain the proper chemistry and conductivity.

Electrochemical grinding can achieve a 16 Ra microinch surface finish. However, the process results in a matte finish rather than the highly polished finish of abrasive grinding. That’s because there is no smearing of the metal as there is in conventional grinding.

That may be an advantage or a disadvantage, depending on the application. For example, in glass to metal sealing, smearing is a desirable characteristic — in fact, it’s the whole point. So, electrochemical grinding would never be used for this application.

Uses for the Electrochemical Grinding Process

If the work can be moved, electrochemical grinding can be used to achieve different planar surfaces. For example, for the medical device industry, the process is used to create products such as:

  • Multiple-bevel needles
  • Stylets
  • Trocars
  • Lancets

However, the process requires a lot of fixturing along with the proper recipe for each metal’s chemistry and its specific conductivity. That adds time (and cost) to the process.

How quickly the process is completed also depends on the part diameter and wall thickness, as well as factors such as the complexity of the end configuration and how much the edges may need to be rounded.

In addition, there is another paradox if you are trying to make something sharp, whether it is a corner, with minimal or no radius, or an actual cutting blade. And that is, by virtue of its chemical and physical properties, any electrochemical machining process removes any sharp point on the metal.

The combination of the electricity being attracted to it and the chemistry acting on the thinnest diameter will literally round any acute angle at the metal’s final tip.

The interesting work-around in the electrochemical grinding process is create a slight burr and then electropolish the burr off, leaving behind as sharp a point as possible. This step must be done as a secondary process in order to not remove the acute angle feature.

The Complexity Grows

Now we see that the electrochemical grinding process is really a combination of three things: surface grinding and chemistry and fixturing. And the three are unavoidably intertwined.

There is no good reason to use an electrochemical grinding machine for its surface grinding aspect alone. For instance, you’d never use it to achieve surface finish and parallelism on a block of metal, since the electrochemical process would basically erode the block.

Probably the only reason you’d do that is because you could remove more material using the electrochemical action as an accelerant. However, for various reasons, creep-feed grinding would be a far more economical way to achieve the same effect.

In fact, one of the disadvantages of electrochemical grinding is its very complexity. It becomes very complicated to bring together all of its requirements:

  • The mechanical action of complex surface grinding and movable fixturing
  • The recipe for the perfect chemistry and the right current, voltage, and amps — a recipe that is different for every metal
  • The inherent erosion of the process on the machinery itself

In addition, the process is not highly automated, so it is not highly efficient. For instance, it generally requires multiple operators or, perhaps these days, robots to load and unload cartridges to keep the process moving along.

However, one of the advantages of electrochemical grinding is if you can manipulate a part using moving, automated fixturing on the surface grinder bed, then you can do some interesting things.

And the results are truly unique to electrochemical grinding.

What’s Your Best Choice?

The electrochemical grinding process is highly specialized method, occupying a very small niche in machining. In fact, the process is practiced by very few companies and has limited applications.

However, your manufacturing partner can work with you to help determine what method — whether it’s electrochemical or some other process — is the best choice for your precision grinding application.

 
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