Edge Polishing or Edger Rolling
We introduced our new Edging machine at the annual CLMA Zoom Meeting in January of 2021. It is called an Edge Roller meaning that it contours the lens edge profile. This is different from our Edge Polishing Machine that is intended to polish lathe profiled edges.We developed the Edger Roller in response to labs needing to work with lenses that do not have the edges lathe profiled and require an aggressive action to remove the sharp edge. While it is commonplace to lathe profile rotationally symmetric edges, non-rotationally symmetric edges pose a challenge, especially if the edge follows an undulating surface or has a notch to avoid a scleral elevation.
Many specialty corenal and scleral lenses require some attention to the detail of the edge profile that a lathe cannot accommodate.In the past, most of the edge material that needed to be removed was ground away with a diamond coated beveling radius and/or inverted cone tool to approximate the desired edge profile. Then the lens would be mounted on the edge roller or would be finished by hand. The task required a skilled operator. Since the edger profile affects comfort, it is important to have consistent edge profiles. The manual procedure caused variation from one lens to another and from one operator to another. A consistent profile is especially important for replacement lenses to maintain comfort.Figure 1 shows the edge of a typical lens. The “un-cut” diametered shape is shown. It is clear from the diagram that there is a lot of material to be removed in order to achieve the radiused edge profile.
Figure 2 shows the edge with the bottom of the lens faced off tangent to a radius. Facing off the lens removes about 80% of the lens material on its way to a radiused edge as shown in Figure 3. Even if you do not have full edge profiling capability on your lathe(s), facing the lens off will remove most of the material. Using the lathe is a precision method to remove this material. This method will work well for rotationally symmetric lenses. Non-rotationally symmetric lenses will require more work.
Edge Rolling scleral lenses: The amount of the material to be removed on a scleral lens can be as much as ten times that of a corneal lens. This is due to the increased edge thickness of about three times that of a corneal lens and two times the diameter. So, facing a scleral lens will remove much more of material as compared to a corneal lens. In researching this article, I talked to colleagues to see if they rolled the edge in the un-cut form. The answer was some do; some don’t.
They either lathe profiled (preferred method) or hand ground the edge to the approximate profile before rolling the edge. Rolling the edge for an un-cut edge took several minutes and adjustments to the edging machine were required to achieve the desired profile.What is the effect of polish grit size on cycle time? Most polishes utilize an average grit size of .5 to 1.5 microns. According to my colleagues, a coarser grit size removes more material, yielding a shorter cycle time. In the case of edge rolling a scleral lens, an even coarser grit size, say 5 microns, may be useful since the goal is to grind away the material. There may be some concern about a 5-micron grit polish leaving marks, but it is worth testing that out.Our new edger roller provides a more aggressive process to contour the edge of a lens. While the irregular edge presents a challenge to achieve the desired edge profile, the edge roller will help with that process.