I was wondering if anyone had any advice on how to fix a lens that has unwanted prism (about .25 - .50 degree) after surfacing (from something like a blocking error or clamping into the generator incorrectly). We've tried resurfacing the lens by blocking it correctly and skimming it on our LOH v50, but we always have the lens come out with the power off and the prism isn't always corrected. Any help is greatly appreciated.
Sometimes you just can't win all the time.
Any specifics would be helpful.Originally Posted by G.M.
Is this a progressive? Most PAL's tilt in blocking rings, more so in larger ones. 0.25 to 0.50 prism diopters base down is nominal.
I couldn't understand if your lenses were off power only after the skimming operation, or if all of your lenses are off power. In any case, it's probably not tightly coupled to your unwanted prism issue, not at that magnitude of prism.
The type of lens that turns out with prism issues is pretty much at random. I think the prism issue is caused by clamping a lens into the generator incorrectly/slightly tilted one way or the other (alloy under blocking rings also i suppose), so the powers are correct but have .50 degrees or so prism. After resurfacing/skimming the lens to remove the unwanted prism, the power simply slides about .50 diopter off. I was hoping some one had a better way of removing the unwanted prism or knew what we are doing wrong (that made the power change) when we resurface the lens.
Thanks for the responses by the way.:o
What kinds of blocks are you using? 43mm blocks with fiber rings?
We're using the standard aluminum blocks/rings for a LOH PLA Layout Blocker.
That's the alloy blocker, right? The heat from the alloy (there's a lot of it) could be at least part of your problem when resurfacing. Make sure the temperature is as low a possible, both in the tank, and the feeder tube (I'm assuming there are discrete heaters), and that your chillling fluid is as cold as practical (the limit usually being the point at which condensation becomes excessive, or the lines freeze up).
As I mentioned, a small amount of prism on progressives is inevitable - but it's not on lenses with rotationally-symmetrical fronts. You may need some additional measuring equipment to figure out what's going on; a gauge that measures the prism on a blocked lens would be useful. Together with a sensibly structured test, it could help you figure out from which element of the process the prism is arising.
Sounds good, I'll do some checking into the temperatures and the like. Thanks for you help.
GM
Hey there! We just recently switched from wax to alloy, also using the PLA's. Your problem is HAUNTIngly familiar! Did you get it straightened out? I'm very interested in how your situation turned out.
Iīm intersted also in your results, we use the LOH PRA blockers.
Its the temperature of the alloy why you are off power.
I have many times tried to resurface lenses, sometimes it works sometimes
not. The reblocking deforms the lens...
If someone have an idea how to make resurfacing possible, telle me
I tried many things, but the no better output as 50% so i stopped.
We use Alloy 60 degrees C.
wow forgot about this thread :shiner:
That prism problem comes and goes randomly- no idea what it is other than operator errors.
We had a guy from satisLoh come in and give a few pointers, he suggested the same thing as shanbaum did, keeping the alloy as cool as possible and the like. I dont remember the minumum temperature off hand.
For resurfacing- we've had the best luck double taping the lenses and blocking on the smallest ring (48mm) possible. (Important to let them cool afterwards to- deblocking the lens with loose alloy in a machine is a nightmare.) Fining with the same tools a little seems to help the power move a .25 one way or the other and polishing with the same tool seems to clear it up pretty well too. Lens manufacturers seem to be sending badly labeled blanks lately as well- lots of front curves are mislabeled, I'd suggest using a sag gauge to verify a type of lens if you have a to much trouble, we've seen a lot of that too.
I was wondering also if anyone has problems with transition lenses- no specific brand, they just seem to become wavy more often than any other. Is it the transition coating they put on the blank?
First check the temperature. Next on the Loh block holder, (where you place the block in the PRA), check to see if there is any harded alloy moving from around the locating pin. This pin that locates the block to allow axis placement sometimes gets small alloy pieces that can become lodged under the block on during the blocking of one lens then slide back behind the pin. If most of the problem lens are blocked near the 180 point and the good ones are between 30 and 120 degrees then I think you may have this floating alloy. A soldering iron will melt the alloy and you can get all of the small pcs off of the pin. Then either use a vacuum or once cool simply use scotch tape sticky side down to remove these unwanted particles.
If you are sure this is not the problem then the chucking issue maybe the problem. Make sure the block is seated in the chuck before generation of the lens surface.
Let us know if this helps.
G.M.
Just wondering, are you using a different generator when the prism occurs? You mention the V50, but that generator wasn't causing us as much problem as the SL2's we have. Turns out that it's the combination of machinery. Everything was peachy with our step one wax blocker, but when we started using LOH blocking....good grief, what grief THAT gave us.
Last edited by rhondaboman; 12-29-2005 at 12:40 PM.
We've allways used the V50- sounds like the alloy blocker is a pain in the butt all around. Always a good idea to clean the blocking rings and surrounding area, clean them again, and when your done clean then again. I've found the smallest specks of alloy creating prism. Also, make sure the block on the lens is smooth after you block it, sound obvious, but its easy to miss. Quite often we get little runs of alloy from the feeder tubes that sit on the front of the block and get in the way when you stick it in the generator. Keep a soldering iron handy- helps clean up unexpected slip ups ;)
As mentioned earlier too much pressure can/will distort a lens when reblocking causing power problems - try double taping the lens (this will also help insulate from heat). Do not use a chilling method let the lens cool gradually.
Last edited by Slug; 12-30-2005 at 11:56 AM.
I have worked with alloy as well and find that when you are redo a lens the pressure and heat leda to alot of the problems. The trick for the heat is tape the surface twice, this helps with the heat and when you are reblocking hold the lens under the ring not in the center, this will eliminate some of the flex from the pressure.
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I am a spanish speaker but will try to explain what we do.
You surface a lens and when you inspect it, it is off prescription, scratch, etc and it a expensive one. You want to resurface it.
Wax blocking does not work.
Go to your stock and take a semifinished lens equal to the one you used to grind this one.
All of the following is done with the semifinished lens.
Mark the axis and descenter it as if you where to make this lens again.
Put the blue tape on the lens ans with a marker, paint the axis over the blue tape.
Block it with alloid on the largest block you can use according to the size of the finished lens you want to save. Let it cool and then deblock the semifinished lens. To deblock it i grip it in the old generator (coburn 113) and hit the lens of with a ruber hammer.
Now you have a mold of the front surface of your lens and the axis should also was tranfered to the surface of the alloid. If not, mark the 0 axis over the alloid.
Return the semifinished lens to the stock.
Take the lens you want to save and mark the axis as you did with the semifinished lens.
Put blue tape on the lens and on the alloid mold .this is important for easy deblocking.
Use epoxy glue preferible clear. The one that sets in 5 minutes.
Mix a few drops of each tube over the block and when mixed, place the lens over the block aligning the axis of the lens with the mark of the 0 axis over the tape on the alloid. Excert some presure to make the lens sit and if there are small bubles is ok. If dealing with PAL, make sure the reading part of the lens correspond to the reading part on the alloid mold.
Let it set until the epoxy is hard.
Now you can regrind it.
To deblock it, try to bend the lens so that one of the blue tapes would deblock or if it is too hard, place it in hot water so that the alloid will melt.
My teory is that when the alloid or wax cools, it contracts a bit. If the lens is thin it will bend copying the new shape of the alloid. The epoxy will not contract and will fill any contraction the alloid had. this contraction should be small since you had blocked a semifinished lens to make the mold.
Try it. It really works and will save you many lenses.
dear G.M.
try this:
measure the front curve of a semifinished blank
mark a 90 and 180 lines on the lens
block it with your regular alloy blocker on either one of the lines
set your generator curves (base curve and cross curve) to match the front cuve of your blank.
grind your lens to about 4mm thick
measure the four ends of your 90 and 180 lines and any difference in thickness should tell you where your prims base problem is coming from.
also if your lens powers are usually off, it might be your sag. gauge that is off and needs to be calibrated. or if only certain lens materials then the lens parameters data is incorrect ( exp. index of refraction of lens material)
gil
Hi Gola,
itīs a great idea for recurfacing, i will try it out.
thanks
good idea's, we'll try em out- thanks for the input
I am relatively new in surfacing Poly lenses and am in the process of making the stock of laps. Some of my laps are out of curve so i use the epoxy glue method i just described every day. I keep some block with alloid for curves 4, 5 and 6. I will never melt them just reuse. This is specially for single vision lenses. If the front curve of the lens to resurface is aberrated, you probably wonīt be able to save the lens but if not, it does not mater if the front curve of the single vision lens does not match perfectly the curve of the mold made of alloid.
Just do not put too much pressure on the lens while glueing it with the epoxy.
Help it sit on the block and let it harden The epoxy hardens due to a chemical reaction so it will not contract at all.
I do not know why i has been having problems with wax blocking and even allid blocking and transitions and Sunsensors. They very often tend to become wavy on the front surface if the center thikness is less than 2.5 mm. This are expensive lenses specially PAL. In the last week i started to block with epoxy those lenses and i have not have the problem again. I know that mi problem is heat during the process that cause this wave but we live in a warm wheather 12 month a year and it would be too expensive to have the air conditioner keep a low temperature.
Just a few hours ago we had to make a +.50 -1.00x125 and 10 diopters of prism base down for a customer that had lost his only pare of lenses. Form some reason i does not know, the SGX generator did not want to cut more than 8 diopters of prism.
I took one of this molds made of alloid, place a spacer on the border of the alloid block in the axis where the edge of the prism should go so that the lens would be glued with epoxy tilted or separated making 2.4 diopters of prism. Then i just told the SGX generator to cut the 7.6 diopters of prism.
This is the kind of job that we do rarely and is the first i have to do since we have the SGX.
enjoy it as i do.
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