Working documentation
This page is generated from the project's working notes. It records a real retrofit in progress, corrections included. Anything marked 🔴 or ⚠️ is unresolved or carries a caveat - read those before acting.
The contactor sequence — what closes, in what order, and why¶
9 September 2026. TuChop has thirteen contactors and control relays. They close in a deliberate order, and almost every step in that order exists for a reason worth understanding.
⭐ The whole thing is hardware. Almost none of it appears in the PLC, and all of it survives the retrofit — LinuxCNC inherits the lot by driving one output.
The order, in one list¶
| What closes | Only if… | |
|---|---|---|
| 0 | -QM1 main isolator, by hand |
80 A, door-interlocked — you cannot open the cabinet with it on |
| 1 | 🔴 The auxiliaries | Start button pressed, E-stops out, air pressure present, and nothing already running |
| 2 | -KA master relay |
The controller says CNC OK and the key switch is turned |
| 3 | -KAX, -KAY, -KAZ, -KAF |
Each axis drive says it is healthy |
| 4 | -KMX, -KMY, -KMZ axis mains contactors |
Their own axis relay pulled in at step 3 |
| 5 | -KM560 carousel contactor |
|
| 6 | -KMI inverter contactor |
KMX is already in, and a timer has elapsed |
| 7 | -KM601 vacuum pump |
Commanded — star first, then delta |
| 8 | -KM200 spindle motor |
Commanded by OF.48. Latches mechanically |
| 9 | -KM101 drills motor |
Commanded by OF.9 |
Step by step, and why¶
0. The main isolator¶
-QM1, Telemecanique V4, 80 A, with a door lock and extension shaft.
⭐ You physically cannot open the cabinet without turning the power off. The handle is the door catch. That is not a convenience feature — it is the reason nobody has to remember.
At this point the transformers are live, 110 V control exists, both 24 V supplies are up. Nothing has moved and no contactor has closed.
1. 🔴 The auxiliaries — a series chain with a clever condition in it¶
Sheet 11, AUSILIARI INSERITI. From rail 2, everything below is in series — any one open and nothing starts:
| In the chain | Meaning |
|---|---|
-SB1 |
The start button |
🔴 -KM101 NC auxiliary |
The drills motor contactor is NOT closed |
🔴 -KMI NC auxiliary |
The inverter contactor is NOT closed |
-SE1 |
Mushroom E-stop (ZB4-BS844) |
-SE2 |
Second mushroom, in the =BM+X3 branch |
-SQF1 |
Guard switch |
🔴 -SP1 |
Air pressure switch |
-KAA, -KA2 |
Latching and interlock relays |
⭐ Two of those deserve attention¶
🔴 You cannot start the auxiliaries while the drill motor or the spindle inverter contactor is already closed. Those two NC auxiliaries force a start from a known dead state. If a contactor has welded shut, or something is still energised from a previous fault, the machine refuses to come up rather than starting on top of it.
🔴 -SP1 means no air, no machine. Every clamp, stop and ram on this machine is pneumatic, and a machine with electrical power but no air can hold nothing. Putting the pressure switch in the auxiliary chain means the machine will not even come alive without the ability to grip.
2. -KA — the master relay, and it takes two keys to turn¶
Two conditions, deliberately:
OF.1 |
The controller is alive and saying so |
-SA1 |
A physical key switch, removable both sides |
⭐ The key is a maintenance lock. Turn it off and the machine is electrically dead downstream no matter what any software does — and you can take the key with you.
🔴 OF.1 is a master enable, not a status light. Drop it and every axis contactor opens and the Z brake applies. Mesa's OF.1 must therefore be a genuine hardware watchdog output, not a software-set one, or a hung PC leaves the machine powered.
3. -KAX, -KAY, -KAZ — each drive is in its own enable circuit¶
From rail 8A, each relay's coil returns through its own drive's DRIVER OK contact:
| Relay | Returns via | Wire |
|---|---|---|
-KAX |
X drive's alarm contact | 72 |
-KAY |
Y drive's alarm contact | 86 |
-KAZ |
Z drive's alarm contact | 100 |
-KAF |
🔴 Z drive's CN1 pin 27 — the brake output |
35 |
⭐ A drive that faults drops its own contactor. No software is involved and nothing has to notice. The drive is wired into the circuit that feeds it.
✅ And you get fault indication free. The other side of each relay's changeover lights -HLX/-HLY/-HLZ, so a lit red lamp names the faulty axis directly.
⚠️ -KAF is the Z brake. It follows the Z drive's own brake output, so the brake releases when the servo is on and applies when it is not — which is why Z will feel seized with the machine off, and that is correct.
4. -KMX, -KMY, -KMZ — only now does 400 V reach the drives¶
KAX contact 10-6 → wire 195 → -KMX coil, and the same for Y and Z.
🔴 So the drive's mains supply is connected only after the drive has reported healthy.
⚠️ Which raises a fair question: how does a drive report healthy before it has power? The most likely answer is that the DRIVER OK contact is closed when unpowered and opens on alarm — the standard fail-safe arrangement, and it makes the whole chain work: unpowered drive → contact closed → relay in → contactor closes → drive powers up → any later fault opens the contact and drops its own contactor.
Marked as the likely mechanism, not a reading. Settling it needs the drive manual or one observation at power-up.
5. -KM560 — the carousel¶
From 8A, straight to the coil. The Panasonic gets its 220 V from -T1.
⭐ Nothing gates it beyond KA, because the carousel cannot hurt anything on its own — it only turns.
6. 🔴 -KMI — the inverter, and the only timed step¶
Two conditions, one of them a deliberate delay.
⭐ Why a timer? Inrush. A VFD's DC bus capacitors are effectively a short circuit at the instant of switch-on — the drive draws a large surge while they charge.
🔴 Staggering the inverter behind the axis contactors means the surges do not all land at once. The incoming supply is protected at 40 A; three servo drives plus an inverter all charging simultaneously is exactly the kind of coincidence that trips a supply breaker on a machine that is doing nothing wrong.
✅ And KMX being a condition means the inverter cannot come up on a machine whose X axis has not come up — so the spindle can never be live on a machine that is not otherwise ready.
7. ⭐ -KM601 — the vacuum pump, star-delta¶
Three contactors, not one: -KM601L, -KM601T, -KM601Y, with a LAD-9R1 mechanical interlock and a LAD-T2 timed contact.
Why not just switch it on? A 7.8 kW motor started straight across the line draws six to eight times its running current — well over 100 A on an incoming supply protected at 40 A.
| Star-delta | |
|---|---|
| Start in star | Each winding sees about 58 % of the voltage, so roughly a third of the current and a third of the torque |
| Then switch to delta | Full voltage, full torque, once it is already turning |
🔴 The LAD-9R1 is a mechanical interlock — a physical bar that makes it impossible for the star and delta contactors to close at the same time. If they ever did, you would short the supply phase to phase.
⭐ Note that this is metal, not logic. The designer did not trust a control circuit with a fault that severe.
8. ⭐ -KM200 — the spindle motor, mechanically latched¶
Siemens 3TF3500 with a 3TX4090-0GF3 mechanical latch, and OF.48 drives its SET coil.
A latching contactor stays closed when its coil is de-energised. You pulse it closed and pulse it open.
⭐ Why? Because a spinning spindle should not be stopped by a momentary dip in a control circuit. A normal contactor drops out on any brief interruption. A latched one holds, so the spindle stops when something deliberately says stop.
9. -KM101 — the drills motor¶
OF.9 drives it. One 1.5 kW motor spins all ten vertical drills; the individual OF.10-OF.15 and OF.25-OF.28 outputs are pneumatic selects that push a spindle down.
🔴 Which is why the motor output is separate and must lead: a drill pushed into the work with its motor stopped will jam or snap.
🔴 -KAMF — a flagged interlock that does not add up¶
-KAMF's coil is fed through -KM200's NC auxiliary, so KAMF is energised when the spindle contactor is OPEN. A KAMF contact then gates rail 8B, which feeds UF2 and UF3 — the drills, the saw and the router ram.
An earlier note read that as "the drill, saw and router outputs are inhibited while the spindle contactor is closed" and flagged it as worth verifying.
⚠️ It does not survive scrutiny. The router plunge ram OF.24 is on rail 8B. If 8B were dead whenever the spindle contactor is closed, you could never plunge the router while its motor was running — which is the machine's main job.
✅ So the sense is almost certainly the other way round: the KAMF contact in the 8B path is one that closes when the spindle contactor is IN. The interlock is real, but it enables the cutting outputs when the spindle is running rather than inhibiting them.
🔴 Still worth one meter check rather than swapping one assumption for another. But the old reading can be set aside: the machine's own function contradicts it.
What the retrofit inherits¶
✅ All of it, by driving one output.
| Untouched | Every contactor, the star-delta starter, the mechanical interlocks, the latch, the timer, the per-axis drive gating, the fault lamps, the Z brake |
| 🔴 Mesa's one job | Drive OF.1 as a real watchdog. Everything downstream then works exactly as built |
| ⚠️ And must not | Bypass the key switch, the pressure switch, or the NC auxiliaries in the start chain — those are the interlocks that make a cold start safe |
Related¶
safety-and-enable-chain.md— the same chain in the diagram's own termspower-in-plain-english.md— where the voltages come fromfuses-and-protection.md— what protects each of these circuits