OpenSCAD source code: MPCNC knife adapter modified for cheap eBay collet pen holder

MPCNC Drag Knife Adapter - Collet Pen Mod.scad

// Adding clearance for eBay collet pen holder

MPCNC_OD = 12.0;      // pen holder OD (matches STL curvature)
MPCNC_Z = 8.9;        // Z offset of pen axis

Pen_OD = 11.5;        // actual pen body OD

ID = 0;
OD = 1;
LENGTH = 2;

Flange = [11.5,15.7,2.2];   // actual pen body flange

Locknut = [16.0,16.0,2.8];  // knurled locknut
Locknut_Offset = 4.5;       //   flange center to locknut

Wall = [41.0,4 * 0.4,9.0];  // thicker walls for more spring and better fill

$fn = 32;   // default cylinder sides

difference() {
  translate([-(101.3 + MPCNC_OD/2),-111.9,0])   // put pen axis above Y axis, flange centered on X axis
    import("/mnt/bulkdata/Project Files/Mostly Printed CNC/Accessories/Tool Holders/MPCNC_525_Drag_Knife-1860310.STL",
           convexity=5);

  if (true)                                    // improve holder-to-mount fit if needed
    translate([0,60/2,8.90])
      rotate([90,0,0])
        cylinder(d=MPCNC_OD,h=60);

  translate([0,0,MPCNC_Z])                      // improve flange slot clearance
    rotate([90,0,0])
      cylinder(d=Flange[OD] + 1.0,h=Flange[LENGTH] + 0.5,center=true);

  translate([0,Locknut_Offset - 0.5,MPCNC_Z])  // add locknut clearance
    rotate([-90,0,0])
      cylinder(d=Locknut[OD] + 1.5,h=Locknut[LENGTH] + 1.5,center=false);
}

#  translate([-(27.0 + Wall.x/2),0,0]) {         // embiggen walls for higher spring constant
    translate([0,-24.4,0])
      cube(Wall);
    translate([0,20.6 - Wall.y,0])
      cube(Wall);
  }

More details on my blog at https://wp.me/poZKh-7LG