Parametric Servo Bracket

derived from this... http://www.thingiverse.com/thing:13850

which was derived from http://www.thingiverse.com/thing:9915

existing parametric variables

servo width - servo_width

added and/or fixed parametric variables for;

servo length - servo_length
base width - base_width : the width of the base plate of the bracket
base length - base_length : the length of the base plate of the bracket
base thickness - base_thickness : the thickness of the base plate of the bracket
hole distance - hole_distance : distance between servo mounting hole
hole diameter - hole_diameter : servo mounting holes sizes

improved the math for positioning the mounting holes; made the holes a fixed distance from outside edge of the base plate rather than the center.

made the servo_width variable an argument to the 'arm' module