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Slip Sliding Away
 Lens Slippage - Fact or Fiction?

Let's take a look at the problem of lens slippage in the processing of ophthalmic lenses in the optical finish lab. 

Many lab technicians end up farming out their work to an outside lab for fear of breaking an expensive hydrophobic-coated lens. This can be a very costly expense to the business as well as an inconvenience to the patient. The truth is, any edger ever produced can cut these slippery lenses without breakage. Just a few simple steps is all that's required.

ROUGHING WHEEL
The wheel on the left shows a lot of wear, some rust, and polycarbonate debris caked into the wheel. This condition will create excessive pressure to the lens during the roughing cycle usually resulting in lens slippage. The wheel on the right is sharp and clean.

Before we get started, let's take a look back at the recent history of lens edging. A couple decades ago the only options available for ophthalmic lenses were glass and the brand new CR39 plastic material. Edgers during that time were very basic pattern edgers, mainly consisting of a motor and diamond wheels. The one thing that really stands out to me now is the size of the chuck on most of those models. The chucks were very small and were capable of edging as small as a 16-18mm B measurement. Although tiny frames did not exist back then, it would have been possible to cut them in the 1970s.

As our industry moved into the 1980s, patternless technology emerged and the new polycarbonate material began stocking the lens racks. Lens coatings also evolved, as it seemed there was always a new and better scratch coating on the market. Anti-reflective coatings became the icing on the cake for premium lenses. At this time, lens-chucking systems in patternless edgers became much larger. The larger chuck also meant the larger holding surface would be applied to the lens, resulting in a stronger hold. During this period it was impossible for the current patternless edgers to cut anything under a 24mm B measurement. This, of course, was not a problem because there still was nothing that small available. In fact, the minimum segment height on the available progressives in the 1980s was 24mm. It seems funny now because most frame boards today do not have many frames larger than a 24 B and we fit progressives into them.

The 1990s and the new century have brought us amazing advances in lens coating technology and frame styles have been getting smaller and smaller almost every year. Lens companies have been scrambling to make the shortest progressive corridor possible and edger manufacturers are introducing edgers that can cut the smallest of frames. The new "soft grind mode" has become one of the most important advances that edger manufacturers are trying to accomplish.

IMPROPER BLOCKING
Using a brightness filter, I was able to photograph an improperly blocked lens from the backside.

The legendary SANTINELLI LE7070
combined with a groover, handstone, and digital drilling system will perform as well as any current model patternless edger.

Soft grind is an edger function, which brings the lens into the roughing wheel "softer" to prevent the hydrophobic lens from slipping. This is a great feature that will definitely help stop slippage in the edging of lenses, but I cannot help but think back to the 1970s model edgers. These edgers usually had a manual arm that would lower the head and lens into the wheel. Some had manual crank chucks that would let you control the pressure holding the lens. And they also had very small chucks enabling the technician to cut very small lenses.

Knowing this, I had to put my theory to the test. I contacted a few of my colleagues who I know still run old AIT and EdgeMaster edgers. Believe it or not, there are still many older opticians out there that will never give up those old 1970s style edgers. I asked them if they still cut their own work, and as I suspected, the answer was yes. I then questioned them about how they deal with slippage. All three of my colleagues had the same answer, "What is slippage?" I was not surprised at all by this answer. Back in the 1980s, there was no such thing as slippage in an optical laboratory. I bought my first used AIT Grand Mark edger in good condition back in 1985 for $300.00. That particular machine in its lifetime had probably cut over 500,000 lenses. That old edger is still out there just waiting to be fired up again. It has since been replaced by newer technology, but it can still do today what any other edger can do.

As I said earlier, any edger ever produced can cut these slippery lenses without breakage. All that is required is just a few simple steps. First of all, it is very important that you have sharp wheels or blades. If your wheels are worn and dull, this will cause unnecessary pressure during the roughing cycle. The same goes with dry edgers. A dull blade can spin any lens right off the block. Be absolutely sure your wheels or blades are sharp and clean. There are many aftermarket wheel manufacturers that make wheel sets for every edger ever produced. In fact, you can get new updated wheels that will cut all materials including Trivex for any machine.

Secondly, it is essential that you choose the right block and chuck for the lens you are cutting. Many hard or metal blocks come in base curves. It is important to choose the proper block to match the base curve of the lens you are cutting. The closer the match, the more surface area of the lens is being held to the blocking pad. If you are using flexible blocks, this will not be a problem. An 8 base block should never be used on a 2 base lens. The lens will only be making contact around the edge of the block and will probably slip. Also, many modern edgers come with full and half eye chucks. Be sure to use them appropriately. It's easy to get lazy and fall into the trap of using only the half eye chuck. When edging larger frames it is important to use the whole eye chuck. This will give you greater hold coverage, reducing the chances of slippage.

Last and certainly not least, choose the right blocking pads. Twenty years ago there were only a couple choices of blocking pads. There was never really an issue with them. Today, there are far too many choices and all blocking pads are not created equal. I have found that usually you get what you pay for. If you think you are saving money buying the cheapest pads available, you will find that they are more expensive than you think.

One breakage of a premium lens will cost you all the money you saved on the pads and more.

If you follow the above advice, there is no reason why you cannot be confident edging high dollar lenses on any edger you own. Sharp wheels, proper chucking, and quality leap pads will insure your success while delivering excellent customer service to your high-end patients.

Leo Hadley Jr.
Marketing Director / Technical Support
Vision Systems Inc.

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