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Thread: Bi-concave lenses?

  1. #1
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    Bi-concave lenses?

    Hi,

    We have just acquired a surfacing lab, nothing too hi tech, generator is capable of cutting to -13D

    I managed today to work a -16D by first working the front surface concave.

    Another job is floating about needs a -20 (with some cyl) and if i order this in it will cost approx £30, is working the front to -7 optically acceptable or will it throw up problems for the Px.

    What are peoples views on this? Are there limits, should it never be done, is it OK?

    Regards,
    Rick

  2. #2
    Master OptiBoarder Jeff Trail's Avatar
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    Rick,

    One thing you might keep in mind is using different indexed material, even though you may have sold a CR39, by upgrading the index of refraction you can gain in having to use less curve making it optically speaking better (acuity) ...the price difference in the blanks from a cr39 and say a 1.60 would be well spent if it made getting the job out that much easier (saved time in labor) also saved doing something that might not be wearable due to the high amount of tunnel vision induced (that 7^ of reverse curve) ..that is almost to much of a stretch for someone that wears a -20 ...

    Sometimes (as much as we hate to do it) it is more cost effective on the lab side to "eat" a little profit blank wise to save money on the labor and complication of the job.. it is harder to surface the higher concaves and make sure that you maintain the right center thickness, the nightmare of blocking such a beast, etc., etc.

    BTW if you wanted a -20 (not including cyl. here) and you are limited to a 13 curve on your generator you would need more than a -7 front curve, a -4.41 sag (7.25) you would have an opposing curve of -14.62 (beyond the limit range you can do.. a -5.38 (sag) -8.09 curve would give you an inside curve of -13.25, more like a -8.37 would be needed.. that is really pushing the limit in distortion for someone in this RX.. BUT take this same RX use a 1.60 index material, drop it BACK to that -7 you wanted and you end up with a -11 inside curve :) That is well worth the xtra money between the two blanks (cr39 vs. 1.60) ...you might even get by with using a mid index


    I take it when you said you did that -16 it was a SV, but you said you worked the front surface? Why didn't you just take a 4 base and just flip it around sag the inside curve enter that into the equation and surface the front now as the back? I hope that is what you did.. :)

    The amount of money charged on the retail side should make up the cost between the blanks easily and still show a great profit... If I have to go any further than a -6 sh (will push a -7 sometimes) I usually will go with a myodisc as the better option.. than again I usually do not start looking to go bi-concave untill in the upper teens and low -20's unless it is for cosmetic reasons as well (kids and teens who do not want to go in to contacts)

    You will find there is no "rule" to go by and each lab or optical tends to think up their own when it comes to "odd ball" stuff..
    I would keep in mind though taking advantage of the material when possible, if you think you might be doing a lot of this stuff keep your eye out for labs going under and buy up a nice little pile of stock at a lower price.. just my 2 cents worth (or in this case make it a euro :) )


    Jeff "just another option to think about" Trail
    Last edited by Jeff Trail; 07-31-2003 at 01:52 PM.

  3. #3
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    Jeff,

    The generator will work a -13.00D on CR39

    Lens index, that never occurred to me (weve only been doing this for a week and its in my character to wade in with my first idea before a proper think about) ive done some calculations and it seems that using a 1.66 material i can work a -17.00D.
    Cost is not really an issue here i just want to be able to process as many of the orders myself as is possible.

    I take it when you said you did that -16 it was a SV, but you said you worked the front surface? Why didn't you just take a 4 base and just flip it around sag the inside curve enter that into the equation and surface the front now as the back? I hope that is what you did..
    Aargh thats such a good/obvious/labour saving idea that i feel sick at the amount of time i spent blocking and working both sides.

    Thanks for those ideas, 2 things for me to think about.

    Regards
    Rick

  4. #4
    Master OptiBoarder Jeff Trail's Avatar
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    Rick,


    Most of us "lab rats" started out somewhere as well :) might as well take advantage of all of our short cuts and tricks we have learned over the years.. I think you might be at an ADVANTAGE by having to learn things on a limited set of equipment, all of us that came up through the ranks had to learn and understand all the theoretical side of optics and it does make it far more interesting than just entering information into the computer and walking around hitting a "start" button at each pc. of equipment ... knowing the "why and how" makes it that much more interesting. Some of the newer equipment is limited by what it says you can NOT do.. and if you understand the theoretical side of optics in production you can figure out the right way to do it and tell the computer the right lie to make it think it is possible :)

    There are a ton of us "old" lab rats on this board so feel free to pump us for information at any time, shoot this is the stuff that makes a real lab persons blood run... ..OK so I'm 38 but hey in optical years I'm about 242 :) (feels that way)


    Jeff "if you cut me, do I not bleed resin?" Trail

  5. #5
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    If you get the time try a two side ground using xtra thick 6 and 8 base blanks.
    Put your high sphere curve on the plus side and finish other side with lower shere/cylinder.
    Finish layout with plus side ocular.
    Needs practice for good edging technique. Works best in very high minus.
    Joseph Felker
    AllentownOptical.com

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    As you get busier, I think you'll find your real "bread and butter" is not going to be from this kind of Rx. It's great for learning, but you'd be ahead by farming this one out to a surfacing lab and saving your time for working the easier Rxs. This is high risk work and breakage is likely. Just a thought :0)

    shutterbug

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    Jeff

    Most of us "lab rats" started out somewhere as well might as well take advantage of all of our short cuts and tricks we have learned over the years.. I think you might be at an ADVANTAGE by having to learn things on a limited set of equipment, all of us that came up through the ranks had to learn and understand all the theoretical side of optics and it does make it far more interesting than just entering information into the computer and walking around hitting a "start" button at each pc. of equipment ... knowing the "why and how" makes it that much more interesting. Some of the newer equipment is limited by what it says you can NOT do.. and if you understand the theoretical side of optics in production you can figure out the right way to do it and tell the computer the right lie to make it think it is possible
    Ive been a glazing 'labrat' for over a decade now and as a DO im well up on the theory and mathematics of lenses but until recently had never seen surfacing equipment. Nor had i any idea how many new ways there were to bleed.

    Buying a setup and learning to use it over the 'phone was almost biting off more than i could chew but its going OK.
    I agree about the older equipment being more hands on and controllable, i enjoy being able to enter curves and thicknesses and know what im getting, its much more my style, its just a shame the range is limited. I may eventually buy one of those desktop generators for doing the extreme stuff, but you just cant beat knocking out lenses every 30 seconds on an old screaming supermatic 3.
    Its good to know you guys are happly to help the surfacing newbie, makes me feel less vulnerable.
    Regards,
    Rick

  8. #8
    Bad address email on file John R's Avatar
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    Re: Bi-concave lenses?

    rsandr said:
    Another job is floating about needs a -20 (with some cyl) and if i order this in it will cost approx £30, is working the front to -7 optically acceptable or will it throw up problems for the Px.
    To be honest if you were to cost this out you would realize that the risk isn't worth the profit involved.
    I don't know which surfacing lab you use but £30.00 for a -20.00 seems a bit steep (this is one lens right ??)
    You should be able to knock 30% of that price at least form most labs. I know ours would be less than that.
    I would also say that knocking out bi-concave lenses like this, just because you don't have the machinery to make the correct form is opening up a can of worms which could end up losing you work.

  9. #9
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    I don't know which surfacing lab you use but £30.00 for a -20.00 seems a bit steep (this is one lens right ??)
    No thats a pair.
    I would also say that knocking out bi-concave lenses like this, just because you don't have the machinery to make the correct form is opening up a can of worms which could end up losing you work.
    Exactly why i posted originally, my question was one of whether the optics of bi-concave would pose Px problems.
    I have no intention of knocking out chaff that will return problems but neither do i want to see fellow companies charging me 10 times the price i could do the job for.
    I suppose roughing a -13 to a -20 would be a bit optimistic (what do you reckon Jeff)?
    Regards
    Rick

  10. #10
    Bad address email on file John R's Avatar
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    rsandr said:
    I suppose roughing a -13 to a -20 would be a bit optimistic (what do you reckon Jeff)?
    Regards
    Rick
    Not a problem given a nice rough pad. You may have to start it by hand to save a lot of bouncing on the machine.

  11. #11
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    Not a problem given a nice rough pad. You may have to start it by hand to save a lot of bouncing on the machine.
    Really??
    I may just dispense with the generator altogether then, it makes far too much noise!
    Rick.

  12. #12
    Master OptiBoarder Jeff Trail's Avatar
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    Rick,

    As John says you can rough one in, using a very rough grit paper (we in my lab call it the "munchy pad" :) ) ... BUT the problem going that far, over 7^ of curve you run into a few problems..first, where do I "stop" grinding and did I leave myself enough blank to rough it in.. second, running it that long on the cylinder machine knocking and banging..and if you thought the generator is loud wait till you try bring power into a lens by just using lap tools..YIKES sounds like the thing is going to start bouncing across the floor .. another problem is that you have a steep tool and not that much of a curve and leaving it xtra thick, you may not be able to even get the lens block up under the pins.. you can make a bunch of adjustments but is it worth it for one lens ? ...
    If you wanted that -20 and did have the limited curve selection than I would tend to think taking a 4 base flipping it around and making a biconcave might be the better option..
    Now you didn't say what type of generator you had? If it is using a diamond wheel you do know you can change the diameter of the wheel and it changes the radius of the curve :)
    Say you went from a 3 1/2" to a 2 1/2" wheel you would almost double your power range in tha minus.. problem is as you steepen the radius you also shorten the width of the radius so it better be a fairly small frame... also if you do cut a steep curve like this I would either grind away the flat part, I have a grinder as well as a dremal by my cylinder machines just for this, if it is a high plus I'll grind down the sharp edge so it does NOT catch on the pads and chip or crack the lens.. and if you have a big flat area and the stroke of your cylinder machine is set high than it will "rock" up on the flat spot and angle the lens.. you will not get a good finish on the lens and leave swirls..this is stuff you will pickup as you go along on your own.. just might remember you seen it here and save yourself a lens or two somewhere down the road.. because believe me if it is a complicated expensive job it always like to crack, break, chip or not finish out for you..:)
    If you are going to run a three pad system than keep you a roll of pads on the shelf for the "munchie" jobs... a couple of pads is a lot cheaper than farming it out...

    Jeff "oh to be just getting your surface feet wet..gee I miss the fun stuff" Trail

  13. #13
    Bad address email on file John R's Avatar
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    rsandr said:
    Really??
    I may just dispense with the generator altogether then, it makes far too much noise!
    Rick.
    What sort of generator do you have a Supermatic 3 ? Cutting Cr39 should be pretty quite.
    Well if you wnt to spend all day doing one lens then fine have a try but believe me its hard work.....Also you have loads of pitfalls as Jeff explained so well. I'll add another one in as well....Unwanted Prism...Not too bad on S/V but if you doing a bif it can be a killer....

  14. #14
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    If the generator curve and the wanted curve are way different, then getting unwanted prism is usually a problem. Caused by un-equal fining pressures on the mis-match. Plus off-center blocking can also increase the possibility.
    Far cheaper, in some instances, to farm the job out to a lab that can actually cut it correctly the first time. "Doing it yourself" can be the wrong answer if the number of attempts exceeds the profit line, and/or causes un-acceptable delays to your account/patient.
    J. R. Smith


  15. #15
    Master OptiBoarder Jeff Trail's Avatar
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    JR,

    You are probably right, BUT (don't you just love it when the "but gets tossed in there) I take it from Ricks post that this is a small lab in a retail location, chances are the job count is fairly low and you remember how we all were when we were "rookies" we wanted to do EVERYTHING ourselves no matter what the big picture was... and figuring out HOW to do it was actually fun.. :)
    I figure he will learn down the road when and when it is not a good idea to do it inhouse..till than oh to be learning again and not on the newest "hit the go button" and it does it all for you equipment.
    I think it should be everyone in a lab should at the least have cut a slab, a biconcave and a biconvex once in thier life just to see what it is like and all the work involved. I know I get farm work from other labs for the crazy stuff but who wants to pump out .50 sphere's all day long.. give me a dive mask or a 9 cylinder with 12^ of prism any day :) ..keeps ya young at heart...

    Jeff "cut me, do I not bleed resin" Trail

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    Cheers Guys.

    You are all right!
    John, it is too much of a ball ache, Jeff yes it is 'fun' and good experience.
    However i am going to let these jobs go in future, nothing worse than the damned thing falling off the button after insufficient cooling on the second surface damn the heatwave.

    The reason i want to do these in house is to prevent acounts having to use other labs as well as myself (who then try to poach all the work) i have one in particular seems to do 3 jobs/week over 15.00D from a small independent opticians.I will probably see how much money we make over the next couple of months then invest in a SGX or a Vista.

    To put the record straight we are a small manufacturing only outfit, no retail. Approx 100 prs/day

    John, it is a supermatic 3, not very noisy but puts a Briot 6000 to shame which is all we are used to.

    Regards
    Rick

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    Stick out tongue

    Just a note about adding thickness to those "fine in" jobs. You can use a Saggita gauge chart (or formula's are posted in other threds in this site) to determine how much to add. You don't want to fine all day :-). Calculate the sag for the highest curve you can cut on your generator (use the frame ED as the gauge size) and then calculate the sag for the curve you want. The difference will be the amount you'll need to fine off.

    shutterbug

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    Master OptiBoarder Jeff Trail's Avatar
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    Good post shutterbug..... Just two things, first I know I would hate to be the poor lab rat that got stuck trying to rock in 7^ of power on a cylinder machine... second, the poor techs that got suckered into getting the "fast grind" system must know how this feels :)
    Talk about an expensive paper weight

    Jeff "oh what a tangled web we weave when getting into optics" Trail

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    Bad address email on file John R's Avatar
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    Thumbs up Re: Cheers Guys.

    rsandr said:
    You are all right!
    John, it is too much of a ball ache, Jeff yes it is 'fun' and good experience.
    John, it is a supermatic 3, not very noisy but puts a Briot 6000 to shame which is all we are used to.

    Regards
    Rick
    So long as you have the spare time, as Jeff says these jobs are fun to do and keep things in perspective. What you got to remember is not to let them get in the way of the bread and butter jobs. I used to do lots of stuff like this years ago when we had some staff...Now its all on just doing normal work.
    At 100 jobs a day you aint that small by UK standards Rick.
    Happy days using a supermatic but personally i prefer a old hand powered radmaster much better to fiddle (and safer to boot) than any puter controlled machine.

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