coined countersinks

We frequently coin countersinks in sheet metal. The process is done on
a turret by punching a thru hole and then restriking the hole with a
countersink punch and die set. The material is offset into the thru
hole and that has to be figured into the thru hole punch diameter
because the material moves and closes up the hole size.

Does anyone have a rule of thumb for figuring the initial punch size?
Trial and adjustment is our current method. Various material types and
thicknesses have an effect.

Any help is appreciated. Diego

 

I've designed coined countersink holes with coin relief on the
backside. I found that "conservation of volume" is a sound approach.
Volume displaced by the countersink has to go somewhere (steel is
effectively incompressible at that scale). An equal volume is moved
into the coin relief volume Coin relief geometry: cylinder of known
height, OD, and ID concentric with hole and ID same as hole.

You could use the same approach, oversize your holes based on the
volume of steel which will be displaced by your countersink cone.

 

Outside of making a test piece, volume is the only guesstimate I know
of. However, it never fills the hole as much as it should. Metal
doesn't compress, but it finds a place to go. Most of the time that
place is up, the top of matl around the outer dia of the csink will
bulge. We usually make the csink punch with a ledge to spank the top.

 

Thanks for the suggestions. Mate Precision Tooling sent an excel sheet
that suggests a punch size for a given countersink and material. I can
forward it to anyone interested, or you can request it from them.

diego

 

Hi Diego

I could use a copy of this, it's bound to come in handy in the new year.

John Layne
www.solidengineering.co.nz

 

I'm having trouble decifering the email's - maybe it's the holiday
punch from the company party. Please send an email to
and I'll forward the xls.

 

Another rule of thumb from Wilson Tool

Approximate pre-punch size = ((countersink size - thru hole size) x
..75)
--subtract this amount from the original countersink dimension

Example: 82 degree x .507 x .266 thru hole:

..507-.266 = .241
..241 x .75 =.181
..507 - .181 = .326 approximate pre-punch.

Use the next size-up punch diameter available for the first try.

(thanks to Wilson Tool for this rule of thumb)