what does the number stand-for for the countersink cutter? sometime is #40, #21, sometime is 1/4", I need 1/8" or 5/32", do you have?

The number is standard drill size. #40 being slightly larger than 3/32" and #30 slightly larger than 1/8". A quick google will give you a conversion chart. They do also sell them in standard sizes like 3/32 or 1/4, but you may have to contact ATS directly.

Although this seems to be used for rivets on aircraft, can it also be used on solid aluminum and steel millwork projects for flat head machine screws?

Hello, Unfortunately this item will not work for the purpose you are proposing. These pilot cutters are 100 degree 3 flute cutting edge and I believe if you are using AN screws they will require a 82 degree cutter which we do not sell.

ASIN	B005VR821G
Batteries Required?	No
Batteries included?	No
Best Sellers Rank	730,633 in DIY & Tools ( See Top 100 in DIY & Tools ) 654 in Countersink Bits
Date First Available	9 Sept. 2014
Finish	Uncoated
Item Package Quantity	1
Item Weight	454 g
Item model number	Aircraft Tool Supply [SOMETHING]
Manufacturer	Aircraft Tool Supply
Material	metal
Part number	133KIT
Product Dimensions	18.3 x 9 x 2.9 cm; 453.59 g

Aircraft Tool Supply Countersink Kit W/Pilot Cutter by Aircraft Tool Supply

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