[ATM] grinding questions

["Bill and Dorothy Morgan" <mountainclan12@msn.com>]

Hello everyone,

I am working on a 8" f/5 as my first mirror I started this project years ago, and ended the project with the blank shaped like a pringle's potato chip when I put it away. I have reground the potato chip surface flat, and ground the back flat. This has been a learning experience, as I have played with the different strokes to learn the different grinding processes.

 I have had to make two (dental stone) plaster tools, only to learn that plaster tools do not hold up well during the rough grinding stages. I corrected the "hole in hole" error mentioned in a previous posting on another ATM list, and brought the two focal points together into one. Once the sun finally came out and I re-tested the focal length by using sun light against a wall. I was able to focus a point of light to the smallest diameter possible (approx. the size of a dime) and measured the focal length at 33 inches, which is 7 inches short of the 40 inches needed for the f/5. The center of the blank is at the proper depth, and curve goes to the edge of the blank. 

** I think I understand what caused the "hole in hole" problem of two separate focal points. I hogged out the hole and tried to make the blank spherical at the same time, hence, giving me a short and long focal point. I corrected this by using TOT. After doing this, I reread the books and found that I should have hogged out the central hole to depth and then measured the focal length, and from that point worked the glass to the edge using the normal strokes. Am I understanding this right?

Where I am to date:

I'm currently out of the 80 carbo, and my second tool is shot. I think I can salvage some grit from the wash basin and hopefully get enough grit to finish the hogging. I have a 2.5 inch pipe end cap and tried to use it, but I'm not sure of the strokes and methods to use. Should I use the end cap to finish the rough grinding or make another tile tool. I have enough tile and dental plaster to make another full sized tool to finish the rough grinding with the #80 carbo, or should I start grinding with the #120 carbo to lengthen the focal length the seven inches that I need?.

It's good to be back friends, please be patient with me as I will have many questions,

Bill 
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