I'm sure this info is published elsewhere, I can't possibly be the first to figure this out. But since I was unable to find it anywhere online I figured I might as well post it here just in case anyone finds themselves in the situation I was in. Yesterday a patients glasses were stalled at the lab due to equipment failure. The patient was seriously ****** off and wanted a refund, so to save the sale I offered to FastGrind the lenses in our small onsite lab. The patients order was a sunsensor FT-28.
The RX was as written below:
+0.50 - 1.75 X 175
+0.50 - 0.50 X 165 Add +2.50
Unfortunately I had neglected to call the 1-800 number to get my new "Key" for the Fastgrind software. And it being 3:30pm on the west coast the office was of course closed for the weekend. The patient was coming back in an hour and I had no way to get into the Fastgrind software. I realized I needed to figure out the mathematical formula for determining which Fastgrind tool to use and a way to use the machine without the software.
So I started tinkering around and came up with the following procedure for Fastgrinding a lens without the software.
The Fastgrind I'm using is the FG 2200 model.
First, determine which base curve you want to use and pull the appropriate semi-finished blanks. There is a mathematical formula for doing this. But a good rule of thumb is if its within +/- 2.50 diopters use a 6 base. If its a high plus use an 8 Base and if its a High Minus use a 4 base. It's also important to look at what base the patient is currently wearing, blah blah blah you know this stuff already. For this job I used the 6.12 base lens.
Second, use the following math formula to determine which tool you need.
First subtract the Sph power of the OD lens (in this case 0.50) from the base curve (in this case 6.12) that sum is the first set of numbers you need. Then subtract the Sph from the Cyl (1.75) and subtract THAT sum from the base curve as well. That will give you the second set of numbers you need.
Put simply.
1) 612 - 050 = 562
2) 050 - 175 = 125 - 612 = 737
Thus the tool you need for the OD is 562 - 737.
Repeat these steps for the OS and begin the Fastgrind procedure as you normally would (break out your diamond pads, etc) the only difference is you have no computer to run the machine, time the process or keep track of thickness. Thus you're going to have to do things a little unconventionally.
First you have to trick the machine into running. This is not hard. Simply press the BIG RED stop button in and then pull it back out. You can now press the Start button. The machine will run on low speed for about 30 seconds. When it stops push in the big red stop button and pull it back out for another 30 seconds worth of runtime.
Doing this you can FastGrind a lens at a slightly slower than normal pace. On the upside I've noticed the slower grind speed greatly decreases the chance of aberration in the lens. Use this same procedure to polish the lenses with the appropriate pads. As far as measuring thickness goes you can use a pair of calipers and measure the thickness of the block then subtract that thickness from the gauge reading. But thats too technical for my taste and I was in a hurry. Normally you can simply eyeball the lens and tell if its still too thick.
Use your gut. Remember plus lenses have a greater center thickness with super thin edges. Meanwhile minus lenses have very thin centers with thick edges. The higher a plus prescription the thicker the center the higher the minus prescription the thicker the edges. Blah blah.
Worst case scenario the lens comes out a little thicker or thinner than you would like. It's not the end of the world and hopefully you'll only have to use this technique once in a blue moon.
-Inouye
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