Common Pitfalls & How to Avoid Them - Part 1

By Steve Schneider
Today’s optical labs have a lot of similarities and differences from the labs of yesteryear. Our industry continues to change with advancements in lens materials, equipment, processes and consumables, which challenge all of us to stay in step and not fall behind. With the fast pace of today’s lab we tend to forget those small yet common mistakes that can add up to unneeded breakage and extra costs.

Within every lab the common goal is to produce a quality lens every time, the first time. History shows that quality comes from consistency. This can be achieved if we take a closer look at the small yet often-overlooked areas that you may feel are being maintained correctly but in reality are not.

Over the years I have had the opportunity to spend many hours in labs and observe the process of making a lens. I have observed common mistakes from the very first step to the very last. The start of the process may be one of the most important steps in processing an optical lens, protecting the front surface. This is critical because this will affect everything we do from this point forward. I recommend the cleaning of the front surface with the use of air prior to applying the tape. This will remove any small particles that may have attached themselves to the front of the lens during packaging.

The common mistake I see is wrinkles in the tape and air bubbles. Wrinkles can cause unwanted prism because the lens does not seat properly during the blocking process. Air bubbles, especially on a segmented lens, can cause a void between the lens and blocking medium, this can lead to aberrations in the lens. Make sure that the surface tape is being applied wrinkle free and remove air bubbles by pressing them out with the aid of a lint free cloth. Never touch the surface with your bare hand as the oils from your skin can cause poor adhesion between the tape and blocking medium.

As we move on to the process of blocking I find that it does not matter what blocking medium is used because the same common mistakes are made with the use of both alloy and wax. In both cases the leading mistake is incorrect temperature of the blocking medium. Just because your blocking system is set at the temperature based on melting point of the blocking medium doesn’t mean it’s good to go. It still needs to be checked.

When checking this I recommend using a standard digital probe thermometer that can be placed to the bottom of the tank. Infrared thermometers will only give you surface temperature. Your tank temperature should be no higher than two to three degrees of the melting temperature. Increased temps can cause aberrations and waves and decreases the life of the blocking medium.

Another area that needs to be checked daily for correct temperature is at the reclaim tank. It is common to find the temperature to be elevated to increase the speed of reclaiming. I recommend that the blocking medium be added to the blocker in a solid form rather than liquid. This will allow for a more consistent temperature control in your blocker.

Remember that you want to clean any slag or visible dirt from the top surface of the blocking medium prior to placing into your blocker. Adding the blocking medium more regularly when in the solid form will allow for the alloy or wax to melt as you keep blocking. It will also decrease the risk of spilling and contamination of expensive products such as alloy.

In today’s lab you can find an assortment of generator styles that are being used to produce lenses. Before we start to generate we have to ask; did we allow our lens to cool long enough after the blocking phase? Quite often I find that the job has reached the generator before the acceptable cooling time.

I recommend that a 30-minute minimum cool down be used. This allows for the blocking medium to stabilize and provide a true support to the lens. Most calculation programs offer this feature within their system and should be utilized to avoid operators guessing if in fact the lens has cooled long enough.

Now that our lens is cooled, do we have a generator that is dialed in properly for curve and thickness? I recommend that the generator be checked every day for curve and thickness accuracy. This will not take more than ten minutes and will protect against a day of bad lenses. Have three test lenses set aside on the block for this test. This will allow your operator to run a quick check of curve. I recommend a plano, six sphere and nine sphere are cut to cover the full range and any possibility of deviation. Too often we cut just a six sphere and assume we are good to go. As we move up and down the range of curve you will see that you can be right on with your six curve and off on both plano and nine curves. Your curve should be checked at both center and edge of both the base and cross curve.

The allowed tolerance for curve deviation is three one hundredths of a millimeter. This may depend on the type of generator used in your lab. I recommend you refer to the operating or service manual for your generator for this information. By taking these few minutes everyday to check your generator, you will help keep your process consistent and your breakage at a minimum.

Now that the generator is cutting accurately, is our generated surface acceptable or are there pits or other defects caused by dull or damaged blades? I recommend that a visual inspection of your diamond tool take place prior to every shift for damage.

If you are producing a rougher than normal surface you will spend more time fining the lens, which can cause other problems resulting in breakage. If your generator is a wet cut system it is important that you are using a coolant. Coolants provide a number of benefits in the production of a lens. The biggest is lubricity. Water by it self does not provide enough lubricity for the high speed generating done today. Lubricity allows for the diamonds to cut cleanly through the material by reducing friction. Without coolant your diamonds will dull faster causing poor surface quality and additional expense on tooling. There are many coolants to choose from in today’s market. When choosing a coolant make sure it is a product that was developed for lens generation. Many coolants within our industry have actually been designed for other industries and then adapted to ours. Check with your consumable supplier for this information.

Coolant levels need to be maintained within your system for proper mixture percentages. The only accurate way to verify that your percentage of coolant to water is correct is to use a refractometer. This simple measuring device allows you to accurately measure the percentage of coolant within your system. Most labs will just add more coolant or water when the system looks like it needs it. This practice usually leads to wasted coolant or a system that is more water than coolant decreasing your tooling life and rougher surfaces.

In Part Two, I will take a closer look at the mistakes commonly made within the process of fining, polishing and cleaning of the lens.

Steve Schneider has been a part of the optical industry for 19 years. He spent five years working in a lab in Milwaukee, learning all aspects of surfacing and finishing. Steve joined LOH Optical Machinery in February 1993 as a warehouse tech, before moving into technical services and then became a product manager for Rx and finishing consumables in 2001. In 2004 LOH merged with Satis, and his role changed again to sales manager of consumable products.

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Labtalk November/December 2018