Creating Precision Surfaces

To effectively remove stains or scratches from window glass we must create a new “precision surface”.  The goal is to remove as little glass as possible, in the least amount of time, and end up with the most optically precise surface.  This is done with a host of different superabrasive particles.  Which can be either bonded or loose.   Bonded superabrasives are held together within or on the surface of a solid matrix.  Which could be metal, resin, or plastic.  An example would be diamond lapping film used in the optics industry.  Such films are coated with diamond particles, and are usually applied with either a proprietary liquid, water, or a simple light oil.  Loose superabrasives can be applied dry or mixed with water or a light mineral oil.   Or we might create a compound where loose particles are suspended in a thick carrier fluid.  The thicker the carrier fluid the longer the particles will remain in suspension. For the product to be commercially viable they must remain in suspension for at least six months.  One simple way to test a compound for shelf life involves the use of a centrifuge.  I developed one with a variable speed up to 25,000 rpms, and an eight inch diameter. One example of a commercial compound is polycrystalline diamond.  It comes in different colored oral syringes.  Each color indicates a different particle size.
The micro-abrasive machining process of the optics industry is accomplished by literally driving the particles into the glass surface so that they cut away small bits of glass.   In so doing they become dull and create a smoother surface.  Some are friable and will break up into smaller particles revealing new sharp edges.  These will also generate a smoother surface. In general smaller particles result in a more smooth surface.  It really is all about the particle.  Any particles chosen to resurface glass must be harder than glass.  They might however have different hardnesses (one from another), sizes, shapes, chemistries, purities, and even surface functionalities.  All of these different characteristics can be exploited to attain different results.  Not to mention the different ways they are employed whether bonded or loose.  Further;...different abrasives might be blended together.  Certain compositional abrasives which are typically softer than glass, might even be added to obtain different results.  The possibilities are virtually endless!
Lets begin with cerium oxide.  This particle/powder, “reacts with glass  to create a complex new surface of cerium, oxygen, and silicon.  This is softer than a soda lime glass matrix , and is therefore more easily removed by the abrasive action of the polishing particles”.  As of recent this superabrasive has become rather expensive.  Most of the commercial hard water stain removal products sold to the window cleaning industry are not based on cerium oxide.  Rather they use crude silicas. A pure optical grade cerium is very effective at removing most stains and light scratches.  It is however critically necessary to use the correct application method.  Some people will use a hard felt pad on a rotary drill motor.  They dip the edge of a wet pad in a pail of dry powder.  Then they rip into the glass ’edge on’!  This creates very intense waves of zillions of microscopic scratches.  This ‘abrasion burn’ is completely invisible except in direct sunlight at just the right viewing angle.  The solution lies in creating the correct ’pad interface’. It is true that we want to attain the highest stock removal rate possible.  Yet we can’t overlook the fact it is quite easy to do serious damage to the ’optical surface’.  Enough damage at least to win a lawsuit. While there are many pad interfaces to choose from my favorite to date is a hard felt ring on a special aluminum polishing wheel.  I developed this system which is based on what I call the Wobble Wheel.  It uses a universal joint and a failsafe collar.  The hard felt pad remains totally flat on the glass at all times preventing what I have described here as abrasion burn.  This is what I like to call a fusion technology which joins the optics world with ours.

Written by Henry Grover Jr.

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