Inside Story Issue 8

Issue 8
4th Quarter
A Mitutoyo America Publication

Toughened glass is made from annealed glass, either by thermal tempering or chemical processing. Chemical processing results in increased toughness compared with thermal toughening and can be applied to glass objects of complex shape. Toughened glass is typically used architecturally. Safety glass, which comprises toughened (tempered) glass reinforced with polymer laminations is, of course, standard in autos. Aerospace applications generally employ combinations of polymer, glass and other materials.


When toughened glass becomes scratched, there are three possible outcomes: the scratch may not affect the application at all; it may affect only optical performance by intefering with vision but not having any further implications; or, the scratch may compromise mechanical performance by concentrating stresses resulting in “webbing out” and eventual fracturing.

When scratches are problematic, two remedies are available: Discard the glass and replace with new (a costly option as major architectural or vehicular, especially aerospace, applications can run into many thousands of dollars) or polish out the scratch. However, polishing to remove a scratch is possible only if it is not too deep, generally no more than 0.002”. If a scratch is deeper than that, polishing may cause either a “fish bowl effect” with unacceptable optical distortion, and/or the glass may thin out sufficient to adversely affect its mechanical properties.

Traditionally, two methods have been used to determine if a scratch is not too deep to be polished out. The first is by “feel”, generally with a finger nail. The second is to compare the scratch visually and by feel to sample sheets carrying scratches of known depth. Either method is less than ideally accurate. And here’s the catch: if a scratch is actually deeper than estimated, the technician won’t find out until it is too late (by polishing to the point of estimated depth and finding the scratch is still there)! The result: a waste of time and materials.

These were the facts when David Gittins, President of Innovative Polishing Systems, Inc., of Stuart, Florida (, decided that his business would benefit from a fool proof, accurate and repeatable method for measuring the depth of scratches on glass.

Measuring for a GO/NO GO decision
According to Gittins, “Getting good information on scratch depth would enable a solid GO/NO GO decision on whether to proceed with polishing. This, in turn would improve productivity by eliminating false starts.” And, said Gittins, “There would be other benefits as well: We provide training seminars, train-the-trainer courses, and equipment and materials to many types of organizations who polish and restore glass – from field agents and installers working for architectural glass manufacturers to manufacturers of autos, boats and aircraft. Instead of trying to teach how to judge scratch depth by feel – something that can really only be learned through experience – I thought that if there were an instrument capable of gaging that information, it would be possible to document its use as part of formal training. Such an instrument would also make it possible to keep records of scratch depth vs. polishing time on a job-by-job basis. Finally, knowing exact scratch depth would make it much easier to generate accurate cost estimates.”

Gittins, who has a background in chemistry and technical experience with DuPont, Boeing and the Navy, conceived a solution: A dial indicator fixed to a frame that would enable placement on either curved or flat glass surfaces. To operate, the instrument’s frame would be placed so that the indicator’s stylus would contact the glass immediately adjacent to the scratch. Next, the indicator would be zeroed out. Finally, the gage frame would be moved slightly so that the stylus could enter and bottom out in the scratch, providing a measurement of depth.

Application engineering
The idea seemed workable, so Gittins started calling measuring instrument manufacturers. None were enthusiastic about the project. Then Gittins attended a trade show where he learned of Mitutoyo. He immediately contacted Pat Harkness, Product Specialist at Mitutoyo America (and an engineer with extensive tool design experience). According to Gittins, “Pat was open minded enough to hear me out.”

Harkness picks up the story, “I discussed David’s [Gittins] idea with him. I also apprised him of alternative technologies that might work, including an ultrasonic approach, but ultimately, I agreed that David’s concept was the right approach. It would be simple to train on, easy to use in the field and affordable enough to include in the kits Innovative Polishing provides trainees and other customers.”

Harkness integrated a number of elements to bring Gittins’ concept to fruition. Utilizing a Mitutoyo IDS Digimatic® indicator, Harkness first designed a very fine, almost needle-like probe of a size and shape such that the force of the indicator’s plunger, about 150g, would be enough to ensure that the probe bottomed out in the scratch. Also, the frame in which the indicator was fixtured had to provide steady support while moving smoothly across curved surfaces like windshields. This was accomplished with U-shaped, radiused feet. A lift handle was devised to ease positioning of the gage frame. The frame was fabricated of aluminum and black oxided for cosmetic appeal.


Documentation for best practices
The ability to correlate scratch metrics against all aspects of glass polishing performance – plus having the ability to reference standards established by the process engineers responsible for a specific glass item under consideration – enabled Innovative Polishing Systems to take the lead in development of glass polishing best practices. The company discovered it could slash the time needed by many procedures to a fraction of what they had been. For example, Innovative Polishing Systems’ methods are now providing a national maker of shower doors with annualized savings of $2.4 million, while a boat manufacturer saves more than $1 million per year in restored windshields that otherwise would have been scrapped.

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