7₄ Knot Ball Track

Sibling to the septafoil, and this one was more fun to solve. Same idea — a closed tube with a bore running through it and a slot along the outside narrower than the balls, so they live inside where you can see them and never come out.

Different knot. The septafoil is 7₁. This is 7₄, the fourth knot with seven crossings, and it has none of its brother's tidy rotational symmetry. It sprawls.

Every crossing forces one strand under another, and a strand that ducks under has to come back up. So every crossing is a low point. Seven crossings, seven wells, seven balls — every seat taken. Tilt it and they all run. Set it down and they find their wells again. They click off each other on the way past, same as the septafoil does.

Printing.
Prints in place, no supports at all. The slot faces the sky the whole way around, which is what holds the bore open, so slice it in the orientation it arrives in. Rotating loses that, and loses the seven flat pads it stands on. Tolerances are forgiving.

0.28mm layers, 0.6mm line width, silk PLA. The small one runs 1h47m, the large 6h32m.

Loading.
Pause at layer 45 on the small one, layer 75 on the large — 12.7mm and 21.0mm if your layer height differs from mine. Drop the balls into the bore anywhere along a climb, resume. They roll down to the wells on their own.

If you're on a bed-slinger, take @BaazAl_4398709's advice from the septafoil — it still applies here. Only load the wells sitting perpendicular to your slide direction; the ones running with it will fling the bearings out. On a static bed you can fill all seven.

Count them in right the first time. The slot is 3.41mm against a 4.5mm ball — nothing comes back out.

Two sizes.
Small: 84 x 63.4 x 20.2mm, 4.5mm BBs.
Large: 139.3 x 105.3 x 33.3mm, 8mm balls.
Seven balls either way. Scale it up or down for other bearing sizes.

The drawing it's built from is exact. Fourteen edges, twenty-eight circular arcs, tangent-continuous where they meet. The over-unders ride a smooth height wave instead of folding, which keeps the steepest climb down at 37 degrees, where a ball still rolls. The curves that make a good knot make a good rolling path — that keeps being true and it never stops being satisfying.

Made by Ivy and Iris <3