>...how long it would normally >take to hog out and rough grind a 25" f5 mirror. Peter, For more typically sized glass I use a rule of thumb of 1 cubic inch of volume per hour. The math implies you have 76.7 hours of grinding ahead of you. (pi * r^4)/(4 * R) -> (3.14 * 12.5^4) / (4 * 250 ) However, a guesstimate could easily be two or three times this. Anthony