Velocity and 1.0 Center Lenses

[willsaake]

I was wondering what everyone thinks of these lenses that we grind down to a 1.0 center. At work, the MD's have all their lenses surfaced at an independent lab. Then we edge them on our Briot Accura edger. The "Velocity" lens is a favorite among the optician's at work. Problem is that it breaks in the edger all the time. Even on the "gentle" cycle. I check each pair with calipers to make sure that they are not being ground any less than 1.0 but they still break. We use standard plastic block and 3M blocking pads. I can't figure this out and I wonder if everyone has this problem. Is there any other reason other than the extra .5 millimeter most hope to gain on the edge that people use this lens?

Will Saake


"I don't really have a cool quote yet"

[Bev Heishman]

Just to make sure I understand, these are the ASL lenses by SOLA? You are talking about - lenses?

We haven't had this problem yet. We have a Santanelli 7070LE.

[Bev Heishman]

Have you tried round center dot blocking circles by 3 MM on the
front and back surfaces? We use these on our AR and mirror coated lenses. That should help.

[JRS]

Although you didn't say how they 'broke' Will, I would assume you mean either cracked or what appear to be lines (like scratches) running through the thin areas of the center.

If this is the case, you could be stressing (bending) the lens with the blocks you are using. Even on "gentle" cycles, the edgers still produce over 100psi on a small, thin, and very flexible area.

[Steve Machol]

Will,

One question I have is whether Velocity can be safely ground to 1.0mm centers and still pass FDA impact requirements. My recollection is that the minimum thickness is 1.5mm, unless you use a 'buffer' coat on the back surface. Have you checked with Sola to see if that thickness is within their recommended range?

Sola Technical Services: (800) 358-8258, press 3

[Mike Fretto]

Will;

Theres one thing you might want to check, some edgers come with different size chucks, and lense holders. You have to make sure there both the same size, for example if the lense holder is larger than the chuck it causes stress to the lense and it could break. I'm not familiar with the Briot edgers, just a thought, good luck.

[willsaake]

Thanks guys,

I will try some of these suggestions. I'm not sure about the thickness requirements of this and some of the other lenses I see; I just know that I see some poly's and hi-index come in that are in between 1.0 and 1.5. Usually around the 1.20 to 1.25 range; especially in these Velocity lenses. Do all lenses have to be 1.5 or is it legal for some lenses to be 1.0? I know when I worked in the surface lab we use to grind certain brands down to a 1.0 if possible. I haven't worked in surface for a while so I am unfamiliar with the brands that are popular today and their exact spec's. All I see are mainly Perceptas, Varilux Comforts, and theses Velocity lenses.

[JRS]

I wasn't aware you could go to 1.0 center on Spectralite Steve. This a new thing - especially with AR applied?

[Steve Machol]

Originally posted by JRS
I wasn't aware you could go to 1.0 center on Spectralite Steve. This a new thing - especially with AR applied?

Actually JR, that was my point. I don't think Spectralite can be ground to 1.0mm centers and still meet impact requirements. The only way I see this happening is if the lab is willing to go through the trouble and expense of putting on a buffer coat. Even then, they'd have to prove statistically that their process produces FDA compliant lenses.

I strongly advise anyone to check with the lens manufacturer before attempting to grind ultrathn (<1.5mm) lenses

[Darryl Meister]

Yeah, what Steve said...

Best regards,
Darryl

[Duane Metcalfe]

Lots to consider...edger chuck pressure (manual chuck, air or electro/mechanical); edger head pressure; elongated ed's with too thin a lens creating flex; material surfaced to thickness not recommended by mfg., but did it any way cuz that 's what they want (always wanted to know who 'they' was...patient or dispensing optician); bad edger wheels; inappropriate HC for material used with AR; Cure time too long, too short on HC; lenses cooled (cold shocked) inappropriately; on UV cure, machine...not set up/maintained well.

My vote is: lenses are too thin to begin with and add any of the rest to compound the issue.

[willsaake]

I havent varified this thorugh SOLA yet but according to the opticians here the Spectralite lens can be ground to a 1.0 center. I am going to go to SOLA's website and find out if it meets impact req. at 1.0 or not. If a lens does not meet req.'s does that means you can not dispense the lens or is that for children 16 and younger?

Thanks again for the replys, sorry it took so long to respond,

Will Saake

[Steve Machol]

Will,

Note what Darryl said above. Being that he's currently the Technical Marketing Manager for Sola. I'm sure his answer is definitive. :)

[harry a saake]

Regarding lens thickness and our constant quest for thinner lenses in the center. One must remember that in just about all cases ,going for an extra .5 mm thinner center is almost meaningless. First whatever you save in the center on a minus lens is all you will save on the edge.:hammer: You have to think nobody is going to notice this. What you will accomplish is compromising the structual integrity of the lens, possibly making it dangerous to you and the wearer. The lens may very well not pass a standard drop ball test.
Years ago when i was with B&L we had the famous 1.8 high index flint glass lenses. They were almost always ground to a 1.0 center. Reason, there was no law yet and you could get away with it. However upon glazing you could quite often put your thumb through the center, as i can tell you from personal experience i have.
If you want to decrease edge thickness,use aspheric lenses, fit frames with same pd as the patient, use rounder or oval shapes, use high index, use ar coat.
Probably in most cases if you do all or some of the above you will save more edge thickness then if you ground the lens .5mm thinner. Also bear in mind 1.0 ct,s will invariably produce more warpage. Personally i think 1.0 should go:finger:

[Clive Noble]

Hi Will,

I don't really know what the 'Velocity' is but let me talk about 1.0 plastic in general.

Because there are no restrictions regarding CT here and because every supplier wants their lenses to be the thinnest we see a lot of thin lenses.

I'm also using a Briot Accura and have found the best way to stop breakages (period) is to reduce the weight of the head on the wheel.

Now, there is no way this can be done by adjustments to the setup of the machine, so I do it manually, in fact we now use this method for virtually every lens we cut and every strain and breakage problem has gone.

I use a small pin hammer, holding the hammer head in my hand and directing the wooden handle under the lens rotating axle giving pressure upwards.

Depending on the lens, I reduce the weight of the 'cut' for 2 to 4 full lens rotations and never let the lens cut on push/pull action.

Of course, we only do this for the first cut, not on the grooving or bevelling actions.

I have mentioned the problem of head 'weight control' to the Briot guys, but it seems that like all the litle things I've discovered about the design of this machine, I'm the only one who has the problem........!!!!!

Good luck, it works!