Bi-Concave Lens?

**doctor_who10** · 12-20-2013, 01:19 PM

I got into a...heated discussion with my lab manager about if it would be possible for us to run a bi-concave lens on our machines (without breaking anything). For purposes of the argument, we were going to assume it was a -10.00 sph lens. I don't think our computer system is quite set up to handle it, so it would probably have to be entered manually into the generator. We have a 95w blocker, SGX, and 5056s. Is there a way for us to actually do this or is he correct in calling me insane?

EDIT: We're ignoring (for the moment) any loss of peripheral vision or actual usability of the lens. This is just to know if it can be done, and have the power in the oc come to -10 sph, within tolerances.

**Boldt** · 12-20-2013, 01:38 PM

With an SGX? the generator would handle that with no problems. The blocker I don't know about but I've run a few bi-concaves and had no problems.

**doctor_who10** · 12-20-2013, 01:42 PM

I know the SGX can handle it, but the main problems are blocking it, calculating the curves, and the thickness.

**PRECISIONLAB** · 12-20-2013, 02:22 PM

Processing bi concave requires some good old fashion hand calculating but can be done.

First, your lms should have bi concave lens data in the database. For example we process minus bc 1.67 from x-cell. The lms calculates what is required, btc, thickness etc. What it does not calc properly is thickness.

When blocking i use a specialty block which is convex as oppossed to concave. The generator (in my case an sl-2) is not configured properly for "tool height" for this block. If thickness adjustments are not made the gen may cut into wax and or block..so yes damage can occur.

Simple fix if the bc is -2.25 add 2.25 to thickness, -4.25 +4.25, -6.25 +6.25 and so on. So if gen thickness is 2.0 cut at around 4.25. Fining and polish should be done on slow cycle speed.

You can also make your own bi-concave / myodisc and figure btc by using the compensating factor associated with your material.

Click image for larger version.

Name: BLOCK.jpg
Views: 115
Size: 27.6 KB
ID: 10562

**doctor_who10** · 12-20-2013, 02:41 PM

Something I probably should have mentioned: we're a small (in-store, corporate) lab. We don't do crazy stuff, and I wouldn't be able to get a convex lens blank. I was thinking (after doing a little digging in the forums) of taking a cr39 lens and flipping it.

Originally Posted by PRECISIONLAB

First, your lms should have bi concave lens data in the database. For example we process minus bc 1.67 from x-cell. The lms calculates what is required, btc, thickness etc. What it does not calc properly is thickness.

sadly our lab software is not particularly good. I haven't tried plugging in individual measurements into the system, but I don't think it will handle it very well. We're not set up to run anything exotic server-side.

Originally Posted by PRECISIONLAB

When blocking i use a specialty block which is convex as oppossed to concave.

The picture you posted looks similar to our blocks, but I need one with a fill hole. Our blocker is an older one, and one of the things I'm worried about is the fact that the lens edges on a concave lens actually hit the chiller ring. I don't want wax spillage.
I'm willing to purchase a convex block on my own, if it'll work with our system. Otherwise I'm stuck with the 'red' flat blocks.

Originally Posted by PRECISIONLAB

Simple fix if the bc is -2.25 add 2.25 to thickness, -4.25 +4.25, -6.25 +6.25 and so on. So if gen thickness is 2.0 cut at around 4.25. Fining and polish should be done on slow cycle speed.

Is this fix assuming the front curve is already concave?

**PRECISIONLAB** · 12-20-2013, 03:20 PM

Flipping a standard SVSF blank might be ok but here are some extra steps.

The blank is molded so the Front curve is a True BC while the back may not be. I start with an extra thick blank and usually cut just enough to establish the desired minus bc. You can now flip and calculated curves based on the -bc by using the compensating factor for plastic of 1.06. So on your -10 example make a -2.25 BC with BTC of -8.25.

On the SGX side make sure to input all block and lens data ie edge thickness ct thickness, bc etc. adjust for final thickness as I explained. The minus bc block is a must but blocking on a flat block is possible. Wax shouldn't flow out

**doctor_who10** · 12-20-2013, 03:24 PM

Ok, thanks! I think this may be what I need.

**chaoticneutral** · 12-20-2013, 05:48 PM

Ive run a few bi-concave lenses to. If your going to tape and block using alloy or wax it can be a little tricky to get a good seal on the surface saver tape. I used a straw and put it on the lens before I taped it and vacuumed it to remove the air from the concave lens. Then I blocked it up. It sound complicated but after a few tries you'll get the idea. if you need a pic of what that might look like let me know Ill be happy to post one. I was using a 108 generator. Weeeeeee super fun! Also, I had a mathematical genius/optical demi god do the calculations for me. He told me what thickness settings to use, what laps to fine and polish on and what side needed to be generated. It came out to a beautifull -20.00 -2.00 and promptly exploded on the patterned santinelli I had to edge it down. You know the one, with the blade and the v bevel with the little red line? I forget the model, but yes, it can be done! Be brave and charge forth!

**mshimp** · 12-21-2013, 09:43 AM

Just finished a -22.00 using a -4.00 base curve, trivex material, using the SGX. Came out perfect. The real trick is edging it down. Have to fool our edger that it has a normal base curve.

**PRECISIONLAB** · 12-21-2013, 09:58 AM

Originally Posted by mshimp

Just finished a -22.00 using a -4.00 base curve, trivex material, using the SGX. Came out perfect. The real trick is edging it down. Have to fool our edger that it has a normal base curve.

Can you share if you had to make any adjustments at the SGX. Best way to edge myodiscs is to block on geometric center and let the edger do the decentration. I set bevel to a fixed base curve and allow front side only probing

**mshimp** · 12-21-2013, 11:20 AM

PrecisionLab you are right, figuring out the right thickness is the challenge. Sometimes I go by take off instead of center thickness. I measure the center thickness of the lens, and then set the thickness on the SGX at -.01. then I sneak up to the lens for a "0" cut, and proceed with takeoff from that point. Or I add extra thickness as you have suggested and guess at it(sounds like you have it down to a science). After fining and polishing the first lens you will know what to do for the other eye. Dittos on the slow speed on fining and polishing. Mushroom style tools will also be needed. Make sure you have the proper adapters for the Pfoam, I believe.

**doctor_who10** · 12-23-2013, 12:37 PM

Originally Posted by mshimp

Sometimes I go by take off instead of center thickness. I measure the center thickness of the lens, and then set the thickness on the SGX at -.01. then I sneak up to the lens for a "0" cut, and proceed with takeoff from that point.

Do you mean you enter (ct - 0.01) or set the ct TO -0.01? And again, did you have to adjust anything in the sgx setting screen itself? I will be working more from that screen than from my lab software, so I'd like to know what you did for front/back curve, thickness, ect.

**PRECISIONLAB** · 12-23-2013, 01:57 PM

I'd like to share a few more techiques I use. I made the -10.00 that was first referenced in this post using a SVSF +2.50 blank. I cut a -4.00 minus bc then flipped and block using a convex surface blank. Layout is -4.00 BC with a BTC of -6.50. After grinding the BTC I then set generator to cut a +7.00 set at 18.00 CT. This will reduce blank edge thickness which you will allow for smooth fining/polish as well as edging.