Optical Glass

Optical Glass: Advanced Rotary Surface Grinders Deliver “Endless Possibilities” and Tighter Tolerances

To combat the loss of more seasoned operators to retirement, glass job shops are turning to more modern, automated surface grinders.

For job shops that prepare a variety of glass and crystal workpieces for optics and photonics-related applications, modern, automated rotary surface grinding equipment can not only achieve tighter tolerances in less time but also provide “endless possibilities” compared to traditional grinding equipment.

This is particularly important as expert machinists retire, leaving rotary surface grinders that often have been in service for many decades in the hands of less experienced operators. With limited control of spindle speeds as well as manual controls, “old-school” equipment requires sophisticated operators that can factor in complex calculations and understand the nuances and quirks of each unit.

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“When I started almost 20 years ago, we were still using [old rotary surface] grinders from the 1940s and 50s. They are tried-and-true, but not very accurate without an experienced machinist using them,” says Brennan Cipro, Chief Engineer of Worcester, MA-based Howard Glass Company, which specializes in glass grinding and polishing for industries such as optics, biomedical, electronics, and aerospace.

Fortunately, today’s automated rotary surface grinders offer advanced sensors and controls that can reliably achieve tighter dimensional tolerances, flatness, parallelism, and surface finish in much less time. The equipment can be used to grind flat glass to precise dimensions before polishing, significantly reducing intermediate lapping steps as well as preventing breakage of what is often a high-value product.

“The possibilities are endless with the new automated grinders,” says Cipro. “Operators can enter the specific requirements, for example, 712 RPMs on the spindle, 22 RPMs on the table, with a down-feed rate of .003 inches a minute, with a certain dwell cycle. Essentially, operators can program the machine to do whatever they want.”

Grinding Challenges 

Older flat glass grinding equipment has limited mechanical controls and can require significant expertise as well as time and labor to accomplish a task. For instance, such machines often use large wheels and dials to control the grinder’s movement. This means that the soft touch of an expert machinist is required to run them – instead of programmable machine controls.

Since Howard Glass focuses on two-dimensional glass shapes, factors such as thickness, parallelism, and surface condition are very important. As a job shop, applications are very diverse with some orders as small as a single piece.

Cipro notes that the glass materials provided by factories have varying degrees of standard thickness. This means that glass materials often must be ground to smaller, precise dimensions, which can be challenging with older conventional grinders.

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“Some customers adapt their process to utilize glass that is a stock thickness, but many require us to grind glass down to specific sizes with tight tolerances. This often means removing significant amounts of material to get it to size. In a recent job, we needed to deliver glass in 0.385-inch thickness and the closest [standard size] from the supplier was 0.5 inches. So, we had to remove almost 50% of the material to meet the customer’s requirements,” says Cipro.

The main problem, however, he explains is that the experienced operators who could successfully run such equipment are in increasingly short supply.

“Decades ago, expert operators were trained on the equipment when they first entered the field at a young age and over time became skilled at running them. They had to be exceptional at math, run the algorithms in their heads, and usually spent half a day setting up the machines for a job,” says Cipro.

Because the traditional grinding process often lacked necessary precision without expert intervention, this contributed to a rather inefficient production.

Cipro notes, for example, that if 0.25-inch-thick glass stock had to be ground to 0.125-inch thick, a traditional surface grinder had to typically stop 0.020-inch short of the required removal, and it lacked a polished surface.

“Previously, when we went from grinding down to polishing on our older equipment, we required an interim process to get that last 0.020-inch. So, we used another machine that provided an easy grind and polish, but that took hours and hours,” says Cipro.

Advanced Rotary Surface Grinders 

Three years ago, Howard Glass had the opportunity to purchase a used vertical spindle rotary table surface grinder from another shop that had completed a project and no longer needed it. The IG 280 SD from Winona, MN-based DCM Tech, a designer and builder of industrial rotary surface grinders, has a 24" variable speed table and a 20HP variable speed grinding spindle motor.

With this type of vertical spindle, rotary table surface grinder, the table rotates with the workpiece held firmly in place underneath a vertical spindle. With this approach, the grinding is not performed by the peripheral edge of the wheel, but rather by the entire diameter of the abrasive surface.

Today such surface grinders have much more advanced sensors and controls that automatically maintain very tight tolerances, removing material down to within one ten-thousandth of an inch of the final thickness. Digital technology allows for an interface with easy-to-use touchscreen controls. When combined with automation, this means that surface grinder operators no longer need to be highly trained individuals.

“The IG 280 from DCM Tech had a digital readout, memory, and could remember where ‘zero’ was, so it was an important upgrade that helped with efficiency, accuracy, and surface finish,” says Cipro. “It enabled us to grind the glass very close to the final dimensions so only a tiny bit had to be removed in the polishing process, which saves tremendous time.”

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After a decades-old traditional surface grinder stopped working, Howard Glass decided to invest in a new IG 280, and shortly thereafter a larger format IG 380, which comes with a 36" variable speed table and 30HP spindle motor. Cipro says he was immediately impressed with the automation and refinements made by DCM since the early version of the IG 280.

Such upgrades make it possible for virtually anyone to successfully operate a unit for glass grinding without much training. One example of innovation involves the automation of the initial contact between the wheel and the part, which typically had to be finessed by the operator.

With this new option, the advanced sensor technology senses vibration. In addition, it automatically fine-tunes not only the pressure of the spindle motor but also how quickly it moves the abrasive wheel down onto the part.

When the machine senses the abrasive wheel has contacted the part, it automatically begins the grind cycle, which helps to minimize the potential breakage of sensitive glass or crystal parts. This capability is important in loud manufacturing facilities where operators cannot rely on listening for the sound of initial contact between the abrasive wheel and the part. Given that many such parts are high value, an operator coming in too aggressively and breaking a part can cost the company hundreds or thousands of dollars.

“I was amazed at the refinements and tighter tolerances now possible,” says Cipro. “Previously, when precision down to ten-thousandths of an inch was required, it could take three hours to remove the excess on our interim machine. Now, we can grind down to ten-thousandths of an inch quickly and effortlessly without extra steps.”

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Eliminate Additional Finishing Processes

Another benefit for job shops is the reduction or elimination of other finishing steps. Typically, glass is cut close to the final dimensions before going through a series of lapping and polishing processes to achieve the desired finish.

However, because such grinders are now so much more precise, job shop operators can dramatically reduce finishing processes. Previously, some may have lapped and polished parts six times before they were finished. Now, with a precise grind to start, they may only have to do so two or three times. This saves a tremendous amount of time and money.

At the same time, the process repeatably achieves high throughput and eliminates variability, which enables job shops to achieve high-quality final parts, batch after batch.

Making parts in less time does no one any good if half of the parts do not pass the final inspection and cannot be used. So, the more job shops can optimize the upfront glass grinding process, the less polish time is required. This not only improves the cycle time but also lowers costs and increases revenue.

Perhaps even more important to Howard Glass were improvements in flexible processing, which allow operators to enter virtually any requirement into a touch screen with programmable Human Machine Interface (HMI) controls.

Cipro adds that with this kind of flexibility if a piece of glass breaks it is easy to back any factor down a little to prevent the issue from reoccurring.

He points out that for routine processes the use of different grind “recipes,” with sets of parameters for specific parts, can further speed production, enhance quality, and aid in quick changeover.

“If the glass is a little off in the first pass, the DCM grinder can be programmed to take corrective actions on subsequent passes. There is no need to pick up the glass and measure it after every move, as with older machines,” says Cipro.

As the tolerances for glass and crystal grinding become stricter and production requirements more demanding, job shops that take advantage of advanced, automated rotary surface grinders will stay competitive even as experienced operators retire.

“Every day I hear my operators discussing ways to improve our glass grinding process because of the versatility of the advanced equipment. We are still discovering its potential,” concludes Cipro.

DCM Tech