Polishing Insides of Scleral Lens
“I wonder if you have made any big bladders to polish the periphery of scleral lenses. I am doing some 18-20 mm lenses now and must polish the periphery on a blending machine. I have not had to do that since we got your bladder polisher.If you had a 10 mm R. bladder, that might work well.
Dear ----: I have run into this with a few other labs that are doing scleral lenses. Going back to the original operation of the bladder polisher, it is the machine geometry not the bladder radius that determines how the lens is polished. If you recall, using the elevation to set arcing axis at the bladder center will theoretically polish the optic zone only. When you set the arcing axis below the center of the bladder, the lens tracks a larger radius and polishes the peripheral curves, also.
Remember that the bladder radius only matters for cone lenses where a small enough radius is required to ''reach" into the base curve. A small radius bladder will polish any back surface much like a buffing pad that you polish a car with.We and labs have found that setting the stroke range to maximum and lowering the arcing axis to get the lens far enough out to polish the periphery works very well.
Plastic Blocking Arbors
Dear Erik: I am still having random prism in our lenses. I checked the blocker calibration with your setup tools and have no more than .008 mm radial runout and .004 mm on the face. But when I block with our production arbors, I get up to .030 mm runout. The face stays pretty good at up to .009 mm. Sometimes I get lower readings than with your setup tool.
We are using arbors that are hollow and have a head for bar coding. What do you think is going on?
Dear ---: Clearly there is a difference between the setup tools and your production arbors.I have come across this before with all types of blocking arbors, both metal and plastic. I have visited labs that use the arbors that you described and found that the shanks held in the collet varied between 12.67 mm and 12.71 mm and sometimes were not round.
As you may have seen from my articles and presentations, the arbor shank must be 12.7 mm in diameter for the collet to fully engage the surface of the shank. A smaller shank will cause only the front of the collet to close on the arbor. A larger shank will cause only the back of the collet to close on the arbor. The 12.67mm arbors are not being held straight. The result is that the lens is not centered in the lathe, and you may see both radial and face runout.
Another thing that we have found is that for plastic tools, solid is better, at least for front turning. We have made hollow tools for customers who have insisted on having a tapered arbor to fit their polishing spindles, but these arbors are just not stiff enough and the lathe surface quality suffers. For those who have our polishers, we convert the spindles to 12.72 mm I. D. socket spindles to accommodate the solid arbors.
Our experience has shown that the best way to get precision 12.7 mm arbors is to centerless grind them. This achieves the diameter AND surface quality necessary to make a long-lasting arbor. I have seen other suppliers make similar arbors to ours, but they were only lathe turned. It turns out that the shank surface finish affects how the arbor is held in the collet.
Some labs like the head to act as a shoulder. We supply an “O” ring for this purpose that also works well on autoloading machines.
Unfortunately, we have not found a way to make a one piece “headed” arbor that can be centerless ground. We can press a ring onto a standard arbor to make a “headed” tool. Of course, these tools are individually manufactured (not molded) and the cost reflects that.
I recommend that you measure the arbors that you are using and let me know what you find.