Cylinder machine and lens question

[snowmonster]

Was reading some other posts related to lab equipment and started to wonder about something I've noticed with some of our surfaced lenses.

I've got a Gerber-Coburn 506 cylinder machine and have noticed something with our lenses that usually isn't a problem unless there's a lot of decentration during finish edging or a very large frame.

I notice it when I hold a surfaced lens up to a light and just look through it. When we run any type of lens material, we get something like the attached picture below where the center part obviously is clear but the peripheral portion of the lens just looks "different," as if it were on a different curve. So in the attached diagram, the center part is clear with the correct power, and then the green part would represent a distorted ring 360 degrees without an fining or generator marks. It's not like it's not fined/polished out, just different. And it happens with thicker CR-39 lenses as well as thinner "any other kinds of material" lenses.

Is it possible that it has something to do with the orbit size of our cylinder machine, or perhaps the sweep pattern? It seems to do it fairly uniformally across the lens, not really just at the top/bottom of the lens. So I wouldn't think it would be the sweep pattern.

FWIW, we 1st/2nd fine CR39 at 1:30 on low speed at about 16 psi with a 4:00 polish on low speed. We 1st/2nd fine Polycarb/Hi-index at 2:00 on high speed at about 16 psi with a 4:00 polish at low speed.

Any help would be great!

-Steve

[braheem24]

You need to start with your generator and ensure all curves coming off are accurate, then the strokes on your cyl machines.

When everything is fixed the ultimate test would be to generate a plano sph and have no waves across the entire lens.

Good luck, :cheers:

[snowmonster]

Now that you mention that, I wonder if it is in the generating stage. It's almost acting like the central 80% of the lens gets generated at one curve/thickness and the rest at a different curve/thickness.

I have a 108 CT and my last pass across the lens usually is me pushing the head away from me and not bringing it back. Perhaps this is cutting the central 80% of the lens a little differently than if I bring the head back towards me for the final pass. I'll try a test lens on Monday with the final pass towards me.

We've not had any trouble with actual curve accuracy, everything always 1st fines perfectly (no generator marks after 1st fine).

Thanks for the thought process!

-Steve

[braheem24]

You could also try cutting demo lens about 0.25 too steep so that it fines from the outside first.

If it clears up, adjust your curves, if not check the diamond alignmnt if you have the tool.

[gemstone]

Could the green part be the part not supported by the block? What are you blocking on?

[snowmonster]

Good suggestions, but I've thought of both of those things and the calibration tool is about as close as I can get it to being perfect.

Gemstone's idea of the lens not being supported by the block is also a valid idea that I had thought about, but it even happens on thicker CR39 jobs that I wouldn't think would have any "support" issues.

I still have to wonder if it's the cyl machine either having too large or small of an orbit or sweep, but I have to go to the office this AM to pick something up so I'll just cut two lenses, same material (CR39), same TC, same BC and cross, same thickness, and do the two different sweep ideas and see what happens.

Thanks again!

-Steve

[Fezz]

Bad wobble blocks?

[snowmonster]

Ok, So I ran about a -1.25 DS with the two sweep options and I think maybe it was a little better when I brought the generator head back towards me. But I also cut the lens a little thicker than average so I guess I'll have to try it again on Monday AM.

Fezz - what do you mean about bad wobble blocks? Do you mean the blocks or something with the cyl machine?

-Steve

[shanbaum]

What kind of blocks are you using? I'd guess Coburn aluminum blocks, no?

Fezz was referring to the "wobble bearing" - that's at the opposite end of the spindle from the lap table, which takes the brunt of the force in moving the lap about.

[snowmonster]

Ah - I'll check into that. But yes, we're using standard Coburn aluminum blocks.

-Steve

[gemstone]

You need to find if the problem is front or back. I believe you will find it is on the front or block side. I think it is caused by blocking. Not a support issue but what the heat of blocking does to the lens surface. Check that.

[shanbaum]

Ah - I'll check into that. But yes, we're using standard Coburn aluminum blocks.

-Steve

In that case, you'll probably see that effect no matter what you do. The alloy shrinks as it cools; that produces tension on the lens front - in other words, the front of the lens is sucked into the block cavity. The tension is relieved on deblocking, and you end up with slightly flatter curves than you expect - but only within the inside diameter of the rim around the edge of the cavity.

[snowmonster]

So the only way to avoid this would be to go with a larger diameter block, like the 63 mm? I think we're currently using 55 mm blocks.

So does everyone have this problem, as all blocking techniques work with either heated alloy or wax?

Or is my alloy just too hot? BTW, we're using 117 degree alloy.

-Steve

[gemstone]

Is it a real problem or just something that bothers you?

[snowmonster]

Crap - Gemstone hit the nail on the head.

We really like to be precise in the lab, but sometimes this hurts us by making me waste mental energy on stuff like this.

It's not a problem unless there's a large "A" or large amount of decentration, but I suppose we could just block the lens (in the case of SV, at least) a little off center in the favor of moving the blocked area closer to the actual portion of lens that will be used. Hopefully, this would prevent the weird artifact that I've noticed.

At least somebody here is a "big picture" kind of guy. Thanks, Gemstone.

-Steve

[shanbaum]

So the only way to avoid this would be to go with a larger diameter block, like the 63 mm? I think we're currently using 55 mm blocks.

That will provide a larger area of consistent curvature.




So does everyone have this problem, as all blocking techniques work with either heated alloy or wax?



But only the aluminum blocks have a configuration where the lens is seated at the periphery of the block, with the alloy in a cavity. The rim of the block acts like a fulcrum; the alloy, like a lever.

All other blocking systems produce a blocking media "cookie" which can shrink across its entire face (thus reducing its height - that's why a Step One's reference ring height is 11.8mm, even though the ring itself is 12.0mm).

The shrinkage of the material in such a configuration creates some tension, but nothing like the amount produced in the case of the aluminum blocks.

[gemstone]

A step one would go a long way in improving this, if it becomes a real problem. Then there is dealing with the wax. I like the step one system, but certain things need to be managed for good results. The reason 117 went back up is labs going back to it. Dog! I wish I'd bought a ton at $15 a lb.

[braheem24]

Step ones can setup for alloy or freebond.

BTW, I use coburn 55mm aluminum blocks with wax and get no distortion.

[MIOPE]

In that case, you'll probably see that effect no matter what you do. The alloy shrinks as it cools; that produces tension on the lens front - in other words, the front of the lens is sucked into the block cavity. The tension is relieved on deblocking, and you end up with slightly flatter curves than you expect - but only within the inside diameter of the rim around the edge of the cavity.

You could know if this is the cause of the problem by doing this experiment.
Block the semifinish lens with alloyd as you usually do. Once cool, deblock it but not with hot water. You need to have the alloyd on the block with the curvature copied from the front surface of the lens. Cover the lens with new blue tape. Apply blue tape on the alloyd that is on the block you just use.
Use 5 minute clear epoxy glue (the one that come in two tubes to be mixed) to glue the lens to the block. Since the epoxy hardens due to a chemical reaction, there will be no tension an the surface of the lens.
Work the lens and deblock it by putting it on hot water. If the problem goes then the alloyd blocking is the cause.

Here you can find more informations on epoxy blocking used to try to rework thin lenses.
http://www.optiboard.com/forums/showthread.php?t=15756&highlight=epoxy