Recommended Print settings

Infill: 35%-50% (Higher than normal infill to account for the stresses the tool will encounter.)

Layers: 0.15mm

Supports: [Style: Grid] [Top contact Z distance: 0.25mm]

Material: ABS or PETG since this will likely be left in the car and you'll want the increased heat tolerance compared to PLA. I tested with PETG and not ABS simply because I struggled with shrinkage from ABS and maintaining precise geometry. 



Geometry
The brass knuckles design was implemented to make use of the natural geometry of the force that comes from directly behind the fist. 

The closed fist maintains the vertical alignment while horizontal alignment is achieved by a 7° offset to adjust for the angle of the fist keeping the edge alignment perpendicular to the motion of the arm. This maximizes efficiency by keeping the force directly behind the edge. There are right-handed and left-handed versions for this purpose. An ambidextrous version is included in case that was preferred. 

Through trial and error testing, the preferred edge angle for breaking ice with PETG was approximately 35°. I found this angle was robust enough to maintain an edge through several tests while being able to pry beneath the ice on a glass surface.