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
Best Sellers Rank	737,004 in DIY & Tools ( See Top 100 in DIY & Tools ) 668 in Countersink Bits
Brand	Aircraft Tool Supply
Cutting Angle String	9E+1 Degrees
Cutting angle string	9E+1 Degrees
Finish Types	Uncoated
Finish type	Uncoated
Item Dimensions	18.3 x 9 x 2.9 centimetres
Item Weight	1 Pounds
Item dimensions L x W x H	18.3 x 9 x 2.9 centimetres
Manufacturer	Aircraft Tool Supply
Material	metal
Minimum Bore Diameter	0.06 Inches
Model Number	Aircraft Tool Supply [SOMETHING]
Number of Flutes	2
Point Style	Taper Point
Shank Type	Straight
Shank type	Straight
Surface Recommendation	Metal
Tool Flute Type	Spiral
Unit Count	1.0 Count

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

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