Need help sizing a countersink bit

[buening]

I need to countersink some #8 flathead machine screws into some angle iron and need to source a countersink bit. I'm finding that a #8 countersink bit is much too large for a #8 machine screw, so I don't think the two size terminology meshes. Can anyone tell me what size I need? I don't do much machine/fabrication, so I'm not up to snuff on sizing this stuff.

 

[gorilla]

The angle of the countersink must match the angle of the machine screw which should be 82*. Sounds to me like you have a 100* countersink most commonly used on aircraft. You should be able to get what you need from McMaster_Carr.

 

[bggrnchvy]

Diameter is too large?

Usually a machining countersync bit is in a 1/4", 3/8", etc dia as it's common to ask for a countersync done to a deeper depth (more aptly to a large OD) to recess the head to some measure. I usually make sure I go at least .030 deeper than I need to on my drawings to make sure my screws fit below flush.

As long as you have the right degree, 82 or 100 as mentioned and not a 90 chamfering bit, set you're drill press or mill depth stop after you've figured what you're recess has to/can be.

 

[buening]

From McMaster Carr (drill-point countersinks): http://www.mcmaster.com/#countersinks/=gnu9of

It mentions a trade size #8 has a 5/16" drill diameter with 3/4" body diameter, wayyy too big.

 

[bggrnchvy]

Ah, not very familiar with drill point countersinks.

IIRC clearance on a #8 screw clearance hole is .170-.193 and the countersink should be .330 in diameter.

The trade size #5 seems to be about right.

 

[bad_idea]

Go to home depot or lowes and buy a countersink bit. Hold it beside one of your screws to make sure the taper is the same angle as the screw. The outside diameter of the countersink is going to be much larger than the screw head. Just don't run the countersink in until it bottoms out. Run the counter sink in some and check to see if the screw fits flush. If not, drill a little more. No need to over-complicate this, I assume this is a home project?

 

[gorilla]

The drill point countersink you are looking at is for center drilling round stock in a lathe for a center. A number 5 drill tip countersink would be what you need.

 

[jetmech89]

If we're talking aviation, a #8 countersunk screw is a #21 hole, 8-32 threads, it requires a #21 countersink. If this is what you're talking about try the yard store out of wichita kansas, they have all the sheetmetal tools you would ever need and if they don't brown tools will.