Pre-pierced hole diameter before countersinking(Sheet Metal) 2

[Ash30]

Good day people!

I would like some guidance in determining the diameter of a hole which is pre-pierced before countersinking by a coining process.

I deal with sheet metal materials like cold-rolled steel, EG steel, stainless steel, etc.

Is there a formula or standardized reference tables I could use?


Thanks!

 

[notnats]

As a pretty good rule of thumb, unless you are reducing the material thickness by drawing, maintain a constant volume of metal. ie, calculate the volume of metal in the finished countersunk hole, and size the pierced hole in the blank so that the same volume of metal is present before forming the countersink.

 

[toolmantwo]

Ash30, I agree with Notnats above. We have done this for several years on several materials and sizes. You look at just moving the same volume of steel because it does not compress, you are just moving the steel to a new place.

If you want me to spend some time, I can give you some examples of starting size, final size and countersinking or counterboring. If you need and very good tight size tolerance on the final hole size, we re-pierce to clean out the hole. The tricky part there is to leave the burr large enough or heavy so it will not be sucked back up into the hole.
Good luck, John

 

[Ash30]

Many thanks Notnats & Toolmantwo! :-D

I get it now. Really appreciate your advice. B-)

Toolmantwo, it would be great if you could provide us with an example, please.

Thanks! :->

 

[toolmantwo]

Ash30

One example:
We run one job now where it is .110 thick material of C1075 steel. We first pierce a hole through the full thickness with standard percent die clearance to Diameter .200 punch, then we counter bore to Diameter .270 by .055 deep. This will almost close off the top of the hole prior .200 hole because it fills into the open hole. Then we re-pierce to Diameter .170 punch with nearly zero percent clearance where the result is a thin shave very thin scrap slug removed. This is all done with progressive tooling. This is all inch system. The punch for the counter bore is dead sharp on the corner. The counter bore punch acts with a flat die surface.

Hope this helps you
Have a great 2004, John

 

[reubj]

Ash30
When I ran a precision sheet metal shop I developed a reference chart for pre-punch diameters for coining countersinks. We used AMADA punch presses. If I can have your e-mail address I would be happy to send you a copy.