The shape is either "hex" for hexagonal nuts or "square" for square nuts ... undersize amount to decrease screw diameter, a scalar to apply to all parts, or a 2-vector to control shaft and head.
bevel if true, bevel the outside of the nut. Default: true for hex nuts, false for square nuts bevel1 if true, bevel the outside of the nut bottom. bevel2 if true, bevel the outside of the nut top.