Just a quick experiment in printing K'nex-compatible parts

knex_female.js

/** Hovis's experiments in K'nex-compatible parts for home fabrication
 * Author: Hovis Biddle
 * Contact: [email protected]
 * License: ISC
 * Url: http://openjscad.org https://gist.github.com/hovissimo/ce5366a35834c3690488
 *
 * All units in inches/degrees unless noted or obviously wrong.
 * These models fit well on MY printer, which is almost certainly calibrated differently than your printer.
 * Use at your own risk.
 */

var shaft_radius = 0.127;
var shaft_hole_radius = 0.130;
var wall_thickness = 0.062;
var peg_insert_depth = 0.366;
var shaft_hole_to_peg_seat_radius = 0.369; // distance from center of connector to the outer limits of the coupler jaws

function frame_bounds() {
   /* This shape is used to form the frame, and also to trim at the end of rendering. */
   return union(
      cube({
      size: [peg_insert_depth + shaft_hole_to_peg_seat_radius, 2*shaft_radius + 2*wall_thickness, 2*shaft_radius ],
      center: [ false, true, true ]
   }),
   CSG.cylinder({
      start: [ 0, 0, -shaft_radius ],
      end: [ 0, 0, shaft_radius ],
      radius: wall_thickness + shaft_radius,
   })
   );
}

function knex_female_coupler() {
   function frame() {
      // The main structure of the female connector clamp
      return difference(
         frame_bounds(),
         frame_cutter().translate([shaft_hole_to_peg_seat_radius, 0, 0])
      );
   }

   function frame_cutter() {
      /* Subtracting this cutter shape from the frame_bounds forms a U shape.
       * The legs of the U form the jaws of the connector.
       */
      return cube({
         size: [ peg_insert_depth, 2*shaft_radius, 2*shaft_radius ],
         center: [ false, true, true ]
      });
   }

   function groove_bump() {
      /* The groove bump is the cylindrical feature that is perpendicular to the joined peg near the base of the jaws.
       * This feature serves to lock the peg at the right depth.
       */
      return cylinder({
         r: groove_bump_radius,
         h: 2*shaft_radius,
         center: true
      }).rotateZ(90);
   }

   function capture_bump() {
      /* These are the bumps in the "jaws" of the connector which have a parallel axis to the joined peg.
       * These bumps serve to prevent the peg from sliding out of the jaws laterally.
       */
      return CSG.cylinder({
         start: [ height_to_groove_focus, 0, 0 ],
         end: [ shaft_hole_to_peg_seat_radius + peg_insert_depth, 0, 0 ],
         radius: capture_bump_radius
      });
   }

   function distance_to_capture_focus(offset) {
      // calculate how far away from the origin the center of the capture bumps should be.
      // Positive offset goes into the region between the jaws
      return shaft_radius + capture_bump_radius - offset;
   }

   var capture_bump_radius = 0.120;
   var capture_bump_offset_from_center = shaft_radius/2 + 0.015;
   var radius_to_capture_focus = 0.202;
   var capture_bump_protrusion = 0.013;

   var groove_bump_radius = 0.076;
   var radius_to_groove_focus = 0.166;
   var height_to_groove_focus = 0.126 + shaft_hole_to_peg_seat_radius;

   // add up all the parts!
   return union(
      frame(),
      groove_bump().translate([ height_to_groove_focus, radius_to_groove_focus, 0]),
      groove_bump().translate([ height_to_groove_focus, -radius_to_groove_focus, 0]),
      capture_bump().translate([
         0,
         -distance_to_capture_focus(capture_bump_protrusion),
         capture_bump_offset_from_center,
      ]),
      capture_bump().translate([
         0,
         -distance_to_capture_focus(capture_bump_protrusion),
         -capture_bump_offset_from_center,
      ]),
      capture_bump().translate([
         0,
         distance_to_capture_focus(capture_bump_protrusion),
         capture_bump_offset_from_center,
      ]),
      capture_bump().translate([
         0,
         distance_to_capture_focus(capture_bump_protrusion),
         -capture_bump_offset_from_center,
      ])
   ).intersect(frame_bounds()); // trim to original frame dimensions
}

function main() {
   return difference(
      union( //positives
         knex_female_coupler(),
         knex_female_coupler().rotateZ(45),
         knex_female_coupler().rotateZ(90)
      ),
      union( //negatives
         CSG.cylinder({ // perpendicular shaft_hole
            start: [0, 0, shaft_radius],
            end: [0, 0, -shaft_radius],
            radius: shaft_hole_radius
         }),
         peg_cutter(),
         peg_cutter().rotateZ(45),
         peg_cutter().rotateZ(90)
      )
   ).scale(25.4); // inches to mm
}

function peg_cutter() {
   return difference( frame_bounds(), knex_female_coupler() );
}