Customized version of http://www.thingiverse.com/thing:193647

use with this bolt http://www.thingiverse.com/thing:938128

Works better to print these two at a time. You need to give the model time to cool or eventually the hot end just stirs around molten plastic and you wont get any good thread definition.

Using the following options:

thickness = 2
slot_width = 1
resolution = 0.5
step_shape_degrees = 45
thread_step = 2
thread_outer_diameter = 7.5
inner_diameter = 8
outer_diameter = 14
drive_type = socket
nut_resolution = 1.0
non_thread_diameter = 0
facets = 6
drive_diameter = 5
type = nut
head_height = 5
nut_step_shape_degrees = 45
slot_depth = 2
countersink = 2
head_type = hex
texture = exclude
wing_ratio = 1
nut_thread_step = 2
nut_thread_outer_diameter = 8.5
nut_diameter = 12
nut_type = normal
thread_length = 8
head_diameter = 12
nut_height = 5
non_thread_length = 12.0