Re: Making a punch die?

> side cuts through the aluminium. At the very least you would need

to shape

> the bottom of the punch so that the cut is progressive i.e. it

starts at the

> middle and finishes at the ends or vice versa.

A couple of more comments to help you decide to CNC machine them if
possible.

Punch/die construction and effectiveness depends on some things being
done right.

#1 and absolutely critical is that the punch have a sharp-sharp
edge. You CANNOT mill this edge. It MUST be a ground edge, or you
will make snaggletoothed holes. That means that both the sides and
the end must be ground. Grinding the end is easy, and you just grind
two angles to help reduce the force required at any point in time, as
posted above. You might even get away with the finish from a disk
sander. However the sides are another matter. Grinding them is pretty
much out of the question for home/hobby equipment. You might get
away with a milled surface, but should at least polish it smooth
before heat treating. A steel punch should be heat treated to at
least 55-60 Rc before grinding. Anything less and if you try to
punch steel you will have a swedged cutting edge and the punch will
be useless. Aluminum is very abrasive(many cases are annodized), and
it is just as critical to harden the punch and die for aluminum to
prevent premature wear on the edges.

#2 most punches and dies also have a side relief. This helps prevent
the punch and die from becoming jammed together after the punch
enters the die. It also reduces the chance for a tiny piece of swarf
from getting jammed between the two and preventing the removal of the
punch. Complex shapes like a D hole or a connector clearance hole
punch can sometimes be made by precision machining segments and then
pressing the finish ground pieces into a retaining pocket or ring.


As far as machining them on a CNC machine, remember, you may be able
to cut out a rectangular section and screw or pop-rivit a machined
flat plate into place. It is much easier to cut an Rs-232 hole into
a flat piece of 1/16th aluminum, than into the side of a 2x6x8 folded
box. A hand nibbler will do a nice job of cutting out the large
hole, and if you have several penetrations for components, they will
still be nice and neat, with fairly tight locations.

If you are cutting holes for rectangular components, like switches or
power plugs, watch out for the corner radii. Some of them have
square corners. It might turn out to be easier to mill a small
chamfer or radius on the component's plastic case, rather than to
fuss around trying to mill a square corner on a mounting hole.

Been there, done that, what a pain.

Best Regards,

Fred Smith IMService