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Tap Thread Countersink sizing and angle

Wallaby

Plastic
Joined
Apr 19, 2023
Greetings All,

Our quality engineers want to create a apprenticeship training document for tapping threads in parts. One particular sticking point is countersinks. We have not been able to find any ANSI, ASTM, ISO standard that details the angle, diameter, and depth of a countersink for a hole to be tapped.

Back in the early 80's in trade school, if it wasn't specifically called out on drawing, I was taught to use 82º countersink tool and make the diameter of countersink 10% larger than the major diameter of thread i.e. .190-32 = .207" diameter with approximate depth of .018". It's always worked for me.

Unfortunately, they want to be able to point/reference to a specification. I've searched high and low on the internet, looked through my machinery's handbook (21st edition), read through all of my drafting/design paraphernalia and come up with bupkis. Any guides or personal experiences will be greatly appreciated.
 
You might try contacting tap manufactures for their best practice recommendations.

Edit: I have heard the recommendation somewhere for 120 degree countersink since it matches the flank angle of the threads but I couldn't tell you where it was from. All I could find with a quick web search was this:


If you find something please let us know.
 
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I read the opening post NOT as asking about standards for countersinks for screw heads (i.e. the engineersbible reference), but rather for that little countersink you put on a hole you just drilled with the tap drill before you start tapping the threads. I suspect there is no universal standard, because this countersink is really only there for the convenience of the machinist, as an aid in guiding the tap into the hole (and to help prevent thread tear out at the surface, etc...). That said, the suggestion above about contacting tap manufacturers for best practice guidance is a good one.
 
90° c'sink. Size varies based on application in my experience. Standard for nuts looks like this:

Countersink: Tapped hole shall be countersunk on the bearing face or faces. The maximum countersink diameter shall be 1.08 times the thread basic (nominal) major diameter.

About that 8% larger than nominal is pretty close to what I use if there's no size specified. (And yes, that is in the handbook. Took me 2 minutes to find. If you don't have a PDF version, get one *now.* Searching is WAY faster and easier with one).

And that's not all it's there for a lot of times. Torquing a bolt will often upset metal around a tapped hole. Sometimes a bigger chamfer or even at times a short c'bore is used to circumvent that causing problems.
 
We have not been able to find any ANSI, ASTM, ISO standard that details the angle, diameter, and depth of a countersink for a hole to be tapped.
If such a standard exists, I imagine it would be military,Nuclear, Oil, or aerospace, and both/all would be different from one to the next.

If the customer doesn't specify a countersink size and angle, then I do a 90deg chamfer that is .04" larger than the OD of the thread no matter if it's a #10-32 or a 1"-8.
 
I've had issues with barely over size countersinks. A lot of prints I deal with seem to follow the 10 pct over and I've seen them pull up. It's possible it was from over torquing though or not a long enough screw.
Usually go over generously, by eyeball. (Manual maching ) Unless the tapped hole is using minimum thread engagement (blind hole) over size and deeper tapping, usually doesn't matter.

Dave
 
I prefer form tapping whenever possible, which distorts the top surface as material is displaced. I see the same effect to some degree with cut taps. I countersink at least a hair over the major diameter of the thread, sometimes more depending on the ductility of the metal, to keep mating surfaces flush. No scientific rule of thumb to offer, unfortunately.
 








 
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