Our standard draft angle ranges from 5 to 7 degrees. We use 7
the most. If a part has a fairly deep, narrow cavity (deeper than it
is wide) we may go to 10 to prevent sticking. The draft angles
don't really vary with the part size.
Fillets & corner radii vary a lot more. Our standard here is to
have .094" corners and .125" fillets, but .063" for both is also fairly
common. Some of our smaller parts have corners as small as .031"
where necessary (clevis legs, for instance, or the flats on a hydraulic
fitting) but this is best kept to shallow features where the stress
concentrations in the die aren't going to be a huge problem, or
where a thin rib leaves room only for that.
Fillets are usually more critical because they become corners
in the die, which metal must flow around. Picture a rib standing
from a flat part, with fillets at the base. As the dies close, metal
flows around these fillets into the rib. If the fillet is too sharp,
won't quite make the corner, but will leave a gap, and as the rib
fills, the metal may turn back on itself, leaving a crack at the rib
Tougher material also calls for larger radii, because it flows less
This has not been the most orderly post, unfortunately, but I
hope I have answered your questions.
Lakeview Forge Co.
P.S. Cliff: True enough that we have some good alloys nowadays.
Unfortunately they aren't always easy to get, but that wasn't my point,
really. I have on my desk a forged brake part that has been cut in
half and etched to bring out the grain patterns, which follow the
part's contour quite closely. With a hogged-out part, you can't get
this; the grain follows the original block. This isn't so good for
resistance. Of course the parts were probably heinously overdesigned
to begin with, if they are government parts...