$fa = 0.5;
$fs = 0.5;

channel = 7.7;
bearing_d = 22;
bearing_w = 7;
frame_to_channel = 10;
bolt_head_w = 6;
bolt_head_d = 14.7;
nut_w = 7;
nut_d = 14.7;
wheel = 40;

difference() {
    union() {
        translate([0, 0, -(channel - 0.2)/2]) cylinder(h=channel - 0.2, d=wheel);
        translate([0, 0, (channel -0.2)/2-1]) cylinder(h=4, d=nut_d+15);
    }
    union() {
        translate([0, 0, -(bearing_w + 1) / 2])  cylinder(h=bearing_w+0.5, d=bearing_d);
        translate([0, 0, (bearing_w + 1) / 2 - 1]) cylinder(h=nut_w + 3, d=nut_d + 2);
        translate([-wheel, -wheel, -wheel * 2]) cube(size=wheel*2);
    }
}