L&T: Equipment Quarterly

Sep
2002

On the Block



On the Block
New blocking systems take the operator  and the potential of operator error out of the lens mark-up process

by TIMOTHY HERRICK



They say the first step is always the hardest. When it comes to lens finishing, blocking is the first step. If something goes wrong at this stage, the lab tech can go through the entire edging and mounting process, only to find they have wasted their time. That’s because inaccurate optical center and axis measurements—taken during blocking—can leave patients suffering from vision problems such as aberration and eye strain, rendering a lens useless.

Finishing blocking consists of three steps:
• Inspecting the uncut lens and verifying its Rx.
• Lens layout, the process of decentering the lens within the parameters of the frame.
• Blocking the lens, which is affixing the lens to the finishing block. This assembly is then placed into the edger’s clamp or chuck for cutting the lens to size.
As with all production stages, things can go wrong in blocking, especially in the area of operator error. The end result is lens spoilage, which, of course, is costly to the dispenser. Lab techs agree lens breakage occurs more often in finishing than in surfacing, with lens mounting the stage where spoilage risk is highest. As a result, manufacturers have designed newer finishing equipment with an eye toward reducing blocking errors and making the process less operator intensive.
“In the past, blocking probably accounted for about 10 to 20 percent of our finishing spoilage,” says Dave Finley, lab manager at Dixie Eye Center in St. George, Utah. “It was almost always a human error, where the operator misread the power or marked the lens incorrectly. The new technologies have cut these errors way down, because computers and automation are doing what the technician used to do.”

Today’s blockers—purchased as separate units or as part of an edger—automate what were once manual tasks. These also interface with and/or are housed in the frame tracer.“The tracer unit always gets a more accurate measurement than any tech can with a ruler,” says Jamie Bradford, manager and head optician at Front Range Eye Health Center in Louisville, Colo. “I haven’t had to take a C-size since we got the tracer/blocker. In the past, we would hold the marked lens up to the pattern or frame after we decentered it to see if it would cut out and you still could be wrong.”

In pre-automated blocking, techs would use the lensometer to check the lens, mark the optical center, place the lens in a “Marking Device”—such as the “Project-O-Marker”—and, using frame measurements, manually align the lens for distance decentration, axis and proper seg height. The device would stamp a mechanical marking on the lens, which the tech would use as a guide to affix the block by hand.

According to manufacturers, the latest technology actually includes the lensometer in the blocker, where computer software determines the optical center and aligns the lens accordingly. In some systems, the operator still marks the optical center in the lensometer, allowing the blocker’s on-board computer to guide them to proper lens alignment. The tech inputs pupilary distance (PD) and other data, following computer prompts, into the blocker keypad. The blocker’s computer screen shows a cut out of the final lens shape based on tracing data so the tech can see if the lens will cut on-size and on-axis. Some systems automate lens alignment—meaning the machine moves the lens into proper position—further reducing operator-error, particularly in the area of parallax error.

“The computer screen shows the lens and you just enter the PD and optical center. It aligns it with the tracing data it already has,” explains Finley. Current blocking units also have a small “arm” that holds the block and places it at the precise position on the front lens surface, a procedure the tech used to do by hand.
Techs call the new systems almost fool proof. But, almost is the key word. For the bulk of the finishing jobs, everything goes as planned, but smarter techs emphasize that some jobs require special handling. Layout systems use the same data for both right and left lenses, but some patients’ PD measurements result in optical centers that are different in each lens.

“You have to override the system and align those manually,” says Finley. “It’s a question of knowing the jobs you are doing.”

In other words, automation and computerization can make finishing easier, but will never replace optical knowledge, skill and experience. Blocks, for example, can be problematic. Older blocks, which are metal, are generally made to correspond to base curves. It’s not that each base curve has a distinct block, but each system has three different blocks, with each designed for a specific range of base curves. If the wrong block is used, slippage and other errors can occur in the edger. Newer, plastic blocks conform to the front surface of the lens, eliminating the curve selection issue.

“The plastic blocks are not quite as sturdy,” notes Clarence Covington, dispensing optician at South Coast Eye Care Centers in Laguna Hills, Calif. “Once, I left one near the tinting unit and I think the heat compromised the integrity and the magnet inside fell out. I always have to keep a lot of blocks on hand. They usually last but you want to keep work flowing.”

Metal or plastic, blocks come in two sizes, full and half-eyes, the latter of which are used for half-eye and children-sized frames.

“Some of the newer frame styles are so small, you need to use the half-eye block,” Finley advises. “Knowing what block means knowing how to finish and mount the frames the buyer is bringing into the dispensary.”

Leap pads, a double-adhesive pad that affixes the lens to the block, feature a tab that must be placed sideways on the lens—facing either the temple or nasal—or else the lens can slip in the chuck. The wheel may also cut the pad with the lens, gumming up the wheels and possibly affecting edge integrity. As a result, technicians should test different leap pads and seek the layout blocker manufacturer’s advice before deciding on what leap pad to use.

“Some leap pads are too ‘cushiony,’ or the adhesive isn’t sticky enough,” says Finely. “This also makes some lenses slip in the chuck.”
In addition, lab techs point out that some segmented multi-focals, particularly where the add-power curve is steep, require placing two leap pads on the lens, one on each segment. “The lens can move around in the block because the step is so steep,” says Finley.

Sometimes, after a lens is edged, the finished lens will have marks made by the block. The lens is spoiled, but the block is the indication of a problem, not the cause.

“Putting on the block is actually a straightforward process, especially when it is automated,” says Covington. “When the block leaves marks on the lens, either the chuck pressure was set too high for that material or the machine is in need of calibration or maintenance.”

Still, even with all the details Covington and other experienced in-office finishers believe newer blockers make finishing more efficient.

“The first thing I do is block all the jobs,” he adds. “Then, when I edge them, I can just put it in the chuck, press a button and go do something else. I am pretty picky and I want all the optical centers dead on. Newer systems can usually deliver that.”

 

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