The Order a Movement Comes Apart

A watchmaker taking down a movement follows an order, and it is not arbitrary. The balance cock, with the balance hanging from it, has to come off before the pallet cock can; the train bridge holds three wheels and cannot lift until its screws are out and nothing above it is in the way. Tick Again makes you follow that order, and refuses a part that cannot come off yet. The interesting thing is that the order is not written down anywhere in the game.

Covered, or not

Every part in the model has a footprint, its outline seen from above, and a height above the plate. Wheels are circles at their tip radius; bridges are outlines of up to three lobes joined by tapered arms, drawn as signed distance shapes. A part is covered, and cannot come off, if any other part on the same side sits higher and overlaps it:

public func isCovered(by other: Part) -> Bool {
    guard other.side == side, other.kind != .stem, kind != .stem else { return false }
    return pieces.contains { mine in
        other.pieces.contains { theirs in theirs.height > mine.height && theirs.footprint.overlaps(mine.footprint) }
    }
}

Overlap here means by more than a hair, 0.05 millimetres, so two parts that merely touch at their edges do not hold each other. That one rule, with the real heights and outlines of C1’s parts, is enough to produce the real order. The heights come straight from the movement: on the arbors, the centre wheel at 0.35, the third at 0.45, the fourth at 0.55, the escape wheel at 0.6, the pallet at 0.75, the barrel at 0.9 and the balance at 1.0; above them the pallet cock at 0.9, the centre and train bridges at 1.6, the barrel bridge at 2.2 and the balance cock highest at 2.4.

The rules, in order

Overlap is the main rule but not the only one. When you try to take a part off, the model asks five questions, in this order, and the first one that says no is what you are told:

public func refusal(_ id: String, wound: Bool) -> Refusal? {
    guard let p = part(id), !removed.contains(id) else { return .alreadyOff }
    // The dial side's parts carry none of the mainspring's force.
    if wound && p.side == .back { return .wound }
    if let screw = present.first(where: { $0.holds == id }) { return .screwed(by: screw.name) }
    if let over = covering(p) { return .under(over.name) }
    if let waiting = p.after.compactMap(part).first(where: { !removed.contains($0.id) }) {
        return waiting.side == p.side ? .first(waiting.name) : .otherSide(waiting.name, waiting.side)
    }
    if p.kind == .stem, !stemFree { return .stemHeld }
    return nil
}

Is there still power in the spring? Then nothing on the back comes off, because a train released under a wound mainspring spins itself to pieces. Is a screw still holding it? Even a loose one counts. Is something above it? Is it waiting for a part that is not a matter of geometry? There are only two of those: the centre wheel cannot come out until the cannon pinion is off its arbor on the dial side, and the keyless works wait for the stem, which in turn is held until the setting lever screw is backed out a turn and a half.

The order that falls out

From those rules, a let-down C1 comes apart from the back like this: the balance cock with its balance, the pallet cock, the pallet fork, the ratchet wheel, the crown wheel, the barrel bridge, the centre bridge, the barrel, the train bridge, the escape wheel, the fourth, the third. Then the centre wheel is refused, because its cannon pinion is still on the dial side. Turn the movement over, take the hands and the dial and the motion works off, and it comes out. A test checks exactly that sequence, and another checks that “take anything that will come” empties both sides completely.

Some of the order is less obvious than it looks. The centre wheel bridge reaches over the barrel’s teeth, where they drive the centre pinion, so it has to come off before the barrel can. The train comes out top down, the escape wheel before the fourth and the fourth before the third, because each overlaps the one beneath it, and the third’s rim also runs under the centre wheel bridge. The barrel bridge waits for the keyless works because the ratchet and crown wheel sit on top of it.

Going back

Reassembly uses the same rules backwards. A part cannot go back if something it would sit under is already in, and it cannot go in until everything it sits on is there, the lowest missing part named first. So the order back is the order off, reversed, without a second list. Screws are the one wrinkle: a screw goes back into its hole loose, with its whole thread still to turn, three turns for the ratchet screw and two and a half for a bridge screw, and the watch is not whole until every screw is driven home. The crown wheel’s screw is left-handed, as on real movements, so that winding tightens it rather than backing it out; turn it the usual way and the model tells you so.

The same rules power the hints. Tomás, the watchmaker whose bench you inherit, will tell you the next thing to do if you ask, and his advice is computed from the very functions that refuse the work: he never names a part the rules would refuse. A test follows his advice alone through every watch in the drawer, down to the last screw and back up to a running watch within tolerance.

What the tests hold

• Wound, every part and screw on the back is refused.
• The real order works, and the centre wheel is then refused for the cannon pinion on the other side.
• A bridge waits for its screws, even loose ones; the balance comes out before the pallet cock; the train comes out top down.
• Everything goes back in the reverse order, a screw only goes in on its part, and a screw left loose leaves the watch unfinished.
• Every caliber in the game comes apart completely and goes back together.

Taking a movement apart is usually how you find out what is wrong with it. The last article is about what can be wrong, and how each fault is a change to the physics rather than a flag.