The mechanism is the horse
A four-legged Theo Jansen walker with nothing hidden inside it. Every bar in the legs is one of Jansen's own published lengths, cut 1:1 in millimetres, and the linkage that does the walking is the animal's body.
The camera walks round it as you scroll
From the side it is an animal — a long back, a bar-frame head, four legs standing under it.
Step round and the animal is a machine. There is no shell to take off: the body is the linkage.
One leg is nine printed pieces — two triangles doubled into tall and thin plates, four rods between them, a cap over the pin.
Through the legs, the crank line: one bevel wheel between the two eccentrics that swing all four legs.
The horse itself is only what is lit here: a spine with a tail, a head and neck, and the dowels that pin them on.
It stands in the stand it was drawn with, feet 19 mm clear of the desk.
One front leg, solved live from the lengths in the table. Drag the plot or use the slider to turn the crank; the circle is drawn at its true 15 mm throw, the red curve is the path the foot draws, and the readout is how far the foot has got from the fixed pivot.
And the arc it does not have
The foot draws a flat-bottomed oval 67.6 mm long and 33.8 mm tall, holding within 2 mm of level across 23 mm of its travel. It does not go all the way round. Between 170 and 190 degrees the lower chain folds until two joints are 6.77 mm apart and their lugs want the same space, and a rod passes within 3.4 mm of a pin. Drag the crank into that arc and the plot says so.
This is the reference machine's own geometry, not a fault in the print: links f and g are nearly equal, so that chain closes on itself. The leg has 336 degrees rather than a full turn, and the assembly is posed at 90 degrees, in the clear. The test that proves the leg is a linkage at all and not a rocker is that the pivot-to-foot distance changes: on this topology it varies by 42.59 mm.
Eleven of Jansen's thirteen numbers
The Jansen linkage is thirteen lengths that only work together. Change one without the others and the foot stops tracing the path the mechanism exists to trace, which is why they are worth checking rather than trusting. Read out of the pivot bores, centre to centre, this leg carries 11 of the 13 to the last digit Jansen publishes.
The other two are worth saying out loud. Link f is 39.3 mm here where he publishes 39.4: this machine cuts it from a second copy of the c rod instead of a rod of its own. And l, his 7.8 mm offset between the crank axis and the fixed pivot, is not in this machine at all — the two sit on one line.
| a | 38.0 | body, the two fixed pivots |
|---|---|---|
| b | 41.5 | upper triangle |
| c | 39.3 | rod |
| d | 40.1 | upper triangle |
| e | 55.8 | upper triangle |
| f | 39.3 | a second c rod, not 39.4 |
| g | 36.7 | lower triangle |
| h | 65.7 | lower triangle |
| i | 49.0 | lower triangle |
| j | 50.0 | rod |
| k | 61.9 | rod |
| l | 0 | crank axis and pivot are collinear |
| m | 15.0 | eccentric, the crank throw |
Both leg rows, seen low. The feet never touch anything: the stand holds the machine with its whole gait in the air.
The bevel pair sits in the open between the two rows, where the saddle would be on a real horse.
Two rows, half a turn apart
The drive is a 12-tooth bevel pinion against a 24-tooth wheel, 2:1 through a right-angle shaft, and the wheel turns two hex shafts — one for each row of legs. The pinion hangs under the body on its own printed mount, where a motor would drive it. None is modelled here: turn the crank line by hand and the stand keeps the feet clear, so the gait runs on the spot.
Both legs of a row hang on the same crank pin. They are mirror images about the crank plane, 4.4 mm apart, and their four rod ends interleave on that one pin at 2.2 mm centres rather than colliding, so one leg pushes while the other swings. The two rows' cranks are exactly 180 degrees apart, which is what makes the rows alternate.
All of it was checked as geometry before any of it was printed: 120 part pairs tested for interference at the built pose, with 0 clashes, and 15 motion conditions covering both what has to move freely and what has to stay put.
The horse is five parts
The walker came first, and it is a walker, not an animal. What makes it a horse is 3 new printed parts — a spine, a head and neck, and one dowel printed 3 times — which stand in the drawing as the 5 pale solids: a tail, a triangulated girder of 7 nodes and 11 members, a socket at the withers, and a bar-frame head with two pointed ears and a muzzle.
All five lie in one 8 mm plate on the machine's own mid-line. No moving part here changes its depth at any crank angle, which is the whole clearance argument in one line: the legs run in two planes well outside that plate, 26.4 mm of air on either side, and the only movers crossing it are the two hex shafts, which clear the girder by 4.39 mm.
It will not shake off, either. The spine drops onto the body's two pivot rods and a dowel closes each saddle from underneath; the rod sits 0.15 mm above that dowel, so spine to dowel to rod to body is a closed path in the direction the head's weight pulls.
Drawn from the assembly, side on. Pale: the five solids of the dress. Dark: the walker they hang on, and the stand.
What the printer sees
Two filaments: grey for the machine, red for the stand. The three new parts print flat on the plate with no support at all — 18.8 per cent of the spine's surface faces down past 45 degrees and none of it needs propping — and every wall on them clears 0.8 mm at a 0.4 mm nozzle.
The running fits are the reference machine's own: 4.0 mm pins in 4.2 mm bores, 0.2 mm of clearance at every leg joint and 0.4 mm on the rods that interleave on the crank pins. Inside each leg those rods stack on one pin between a 3 mm plate that carries 14 mm of it and a thin plate that closes the top.
- On the plate
- 20.8 h
- PETG
- 5.3 oz
- Bed
- 220 × 220 mm
Specifications
Size and weight are the slicer's, measured over the real mesh in PETG. The mechanism figures are measured off the parts themselves.
| Overall | 8.1 × 4.6 × 7.6 in |
|---|---|
| Weight | 5.3 oz |
| Print time | 20.8 h |
| Solids | 52 |
| Legs | 4 × Jansen, at 1:1 |
| Body width | 84 mm |
| Drive | 12/24 bevel, 2:1 |
| Crank | 336° clear, throw 15 mm |
| Nozzle | 0.4 mm |
| Bed | 220 × 220 mm |
A machine you can read across the room and check from six inches away.
52 solids in two colours, every length in them measured before any of it was drawn.