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.

What is the Thread size of the cutter's shank? (IE 1/4-28 ?)

It looks like 1/4" 28. I don't have any fine thread nuts around. It catches the first thread on 1/4 20 but won't go any further.

Is it 60 degrees or, like most countersinks, 80 degrees? They don't say in the description.

100 degree counter sinks -- for standard aviation rivets AD426-xxxx

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Aircraft Tool Supply Countersink Kit W/Pilot Cutter

Product ID: 6310901

Details

Brand
Aircraft Tool
Finish Type
Uncoated
Item dimensions L x W x H
7.2 x 3.54 x
Material
metal
Shank Type
Straight

🎯Includes pilot cutter for pinpoint accuracy

⚙️2-flute spiral design for smooth chip removal

🛠️Stainless steel for corrosion resistance & durability

Description

🎯 Drill with precision, countersink with confidence—upgrade your toolkit today!

BUILT TO LAST - Robust stainless steel construction resists corrosion and wear for lifetime use.
PINPOINT ACCURACY - Pilot cutter guarantees exact starting points, eliminating guesswork.
PRECISION PERFECTED - Professional-grade countersink ensures flawless, clean holes every time.
EFFORTLESS EFFICIENCY - Spiral flute design minimizes binding and speeds up your workflow.
VERSATILE METAL MASTERY - Ideal for countersinking in all metal surfaces—your go-to for any project.

The Aircraft Tool Supply Countersink Kit features a professional-grade, 2-flute spiral bit made from corrosion-resistant stainless steel. It includes a pilot cutter for precise hole starting points, delivering clean, accurate countersunk holes in metal surfaces. Trusted by professionals and rated 4.6 stars, this durable tool combines efficiency and longevity for superior metalworking performance.

Specifications

ASIN	B005VR821G
Best Sellers Rank	#222,962 in Tools & Home Improvement ( See Top 100 in Tools & Home Improvement ) #199 in Countersink Drill Bits
Brand	Aircraft Tool Supply
Brand Name	Aircraft Tool Supply
Customer Reviews	4.6 out of 5 stars 107 Reviews
Finish Type	Uncoated
Finish Types	Uncoated
Item Dimensions	7.2 x 3.54 x 1.14 inches
Item Weight	16 ounces
Item dimensions L x W x H	7.2 x 3.54 x 1.14 inches
Manufacturer	Aircraft Tool Supply
Material	metal
Material Type	metal
Model	Aircraft Tool Supply [SOMETHING]
Number of Flutes	2
Part Number	133KIT
Point Style	Taper Point
Set Name	Countersink Bit Set with Pilot Cutter
Shank Type	Straight
Surface Recommendation	Metal
Tool Flute Type	Spiral
Unit Count	1.0 Count

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