you need a column in the cart to set the drill bit size, lets call this column "bitSize". for all 1mm drill bits, set that column to 1 in the add to cart button, for 2mm bits, set the column to 2, etc.
Then when setting the discounts, you can use the Number Of Items With a Specific Value trigger, For Example:
Total Number of items where "bitSize" has a value of "1" is ">" a value of "100"
and in the calculation, use Based on Items with A specific Value, for example:
Total of "Quantity" where "bitSize" has a value of "1" "times" "2.50"