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Thread: Minus BC surface blocking/distortion with Loh PRA.

  1. #1
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    Minus BC surface blocking/distortion with Loh PRA.

    Howdy! Long time lurker, first time poster. In the last 4 months I've taken over the surfacing manager role. Come from a background of final QC.

    I'm tackling a lot of issues regarding best practices. Established "best practices" in this lab under previous management unfortunately err'd a little too much on the side of "close enough" for my tastes, and one of them is on our minus base curve lenses. I'm seeing significant first surface deformation on our minus base lenses. The SOP that has existed has been to adjust the tooling numbers in our generators to compensate. Clearly this is horrendously wrong and I immediately put a stop to it.

    We have a small collection of Precision Tool Technologies' cataract blocks for high plus lenses. Notches were cut out of them as a fill point for use in our LOH PRAs. There's a TON of alloy in there and the first surface deformation is readily apparent to me just on visual inspection alone. These jobs were only ever QC'd by the previous surfacing manager and never made it to my desk in my previous role as I'd have immediately rejected them. Generating as calc'd results in a slightly off-power lens so operators have been trained to adjust the tooling to compensate.

    In the mess I've adopted I've found a whole collection of Precision Tool's color coded "perfect power" blocks. I cannot for the life of me determine how they are to be used with the PRA system when the blocking ring does not contact the lens surface. Can anyone advise on how these should be used in conjunction with the PRA system?

    So far I have personally been using the unmodified cataract blocks and filling them by hand very slowly in a cool water bath to reduce alloy contraction as much as possible. This has completely eliminated the distortions, and the resulting lenses nuetralize exactly where they should, no more of this "its within ANSII, send it" non-sense. However doing this is tedious and a waste of 20 minutes of my day to block only 4-5 jobs.

    Thanks!

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    Minus base curve blocks (block itself is convex) is similar to the one I use on a step one blocker for minus base lenes.

    See page 85 of Precision Tool catalog as you may have to use a specifically designed ring as well with the PRA blocker.

    Specifically, part #PTT-1717 might be a possible solution.

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    First why are you having so many minus bc orders every day. I would think that you have decided to have minus front to reduce the back curve not a good solution for the wearer.
    Use the convex block to reduce the alloy volume do not chill the block this can cause curve problems. Allow the lens to cool naturally might take 1 hour. You can experiment different cool times by Deblocking the lens and measure concave curve for variations that would before it is surfaced.

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    Quote Originally Posted by Lensman11 View Post
    First why are you having so many minus bc orders every day. I would think that you have decided to have minus front to reduce the back curve not a good solution for the wearer.
    What would you advise for the -18, -19, and -23 diopter jobs instead? I don't believe we have too many minus BC jobs. These only amount to 0.2% of our volume.

    Use the convex block to reduce the alloy volume do not chill the block this can cause curve problems. Allow the lens to cool naturally might take 1 hour. You can experiment different cool times by Deblocking the lens and measure concave curve for variations that would before it is surfaced.
    That is the whole point of my question. Namely: HOW do I use the convex "Perfect Power" blocks on the PRA? I have been unable to find any blocking rings in our inventory that sit proud of the PRA and seal against the first surface that would contain the alloy. I have used the convex block but the entire concave portion of the lens filled with alloy and to top it off: got stuck to the blocking ring.

    The approach they have with the cataract blocks with the notch cut out was a kludge developed by the previous manager and was "good enough" in his eyes. I want to do it the correct way but I cannot find a blocking ring that works and doesn't get trapped with the 5 ounces of alloy that fills up a -6.

    Quote Originally Posted by PRECISIONLAB View Post
    Minus base curve blocks (block itself is convex) is similar to the one I use on a step one blocker for minus base lenes.

    See page 85 of Precision Tool catalog as you may have to use a specifically designed ring as well with the PRA blocker.

    Specifically, part #PTT-1717 might be a possible solution.
    Forgive me: I do have PTT-1717, but they do not protrube from the blocking ring enough to seal against the front and alloy just spills out.

    I'm about ready to go to a local machine shop and have something made.
    Last edited by SurfaceDummy; 07-23-2022 at 07:42 PM.

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    Two points to remember the lens will bank against the outer part of the ring so your blocking diameter will no longer be the ring size. Use putty to seal the ring that is common practice.

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    Thx. I'll give that a try on Monday. I'm going to have to definitely disable cribbing then.

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    Sorry but I forgot the name of the putty it came in a ribbon with cuts in it so you pull a string off the ribbon. Place the putty around the periphery and push the lens down to seal. I think it was used for glazing windows. Been retired for 7 years and forgot tiny details.

  8. #8
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    I found it for you on Amazon. Coitak Butl sealant putty tape. $12.98 for 13 ft.

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