Positive SSL
1 欲攻1/4-20UNC之螺絲孔,則攻螺絲之前的鑽孔直徑應為
A 1/5”
B 1/4”
C 5/16”
D 3/4”
攻螺絲之前的鑽孔直徑=螺紋外徑-節距=1/4-1/20=1/5”