Luxonis OAK-D PoE
Stereo depth with on-device inference, looking down from the arch apex. Board-frame coordinates come from four fixture AprilTags, recomputed every frame.
180° ARCH TRACK
01
Parked on the rail
The arm has no base on the bench. It hangs from a carriage that rides the arch itself, and at rest it parks near one foot — folded up, clear of the board, out of your way.
02
180° of travel
A pinion on the carriage drives a rack cut into the arch, carrying the arm from one foot up over the apex to the other. The depth camera and thermal array sit on a mast above the apex, clear of the carriage's path; an RGB camera on a standoff beside each foot covers the work from its own angle, so a hand can't hide the joint from every view at once.
03
Double-tap to designate
Tap the joint twice with the iron tip. A piezo disc under the fixture catches both impulses, the camera confirms the tip was on the board, and that pad becomes the target.
04
Solder, on the joint
The thermal array watches the pad. The moment its trace shows the melt signature, the feeder advances a metered length of wire — then retracts 2–3 mm to break the bridge.
The problem
Soldering needs three hands. You have two.
One hand holds the iron. One holds the board. The solder gets whatever's left.
The technique every tutorial teaches — iron heats the joint from one side, solder feeds into the joint from the opposite side — is exactly the part that gets sacrificed. Solder fed onto the tip instead of the joint loses its flux before it reaches the work, and you get the cold, grainy joint.
Helping-hands clamps and manual solder feeders exist, but they're static and human-triggered: you position them once and work around them, then press a pedal. For someone with the use of one arm, those workarounds collapse entirely. Ferros moves to you instead, and decides when to feed on its own.
Hardware
What's inside.
Depth
Thermal
MLX90640, 55° FOV
32 × 24 long-wave IR, −40 to 300 °C, 16 Hz. Chosen over the AMG8833, which saturates at 80 °C and cannot see solder temperatures at all.
Actuation
6 × Feetech STS3215
30 kg·cm serial-bus servos with 4096-count magnetic encoders, daisy-chained on one wire and one 12 V rail. One drives the rack-and-pinion carriage along the arch.
Feeder
Pancake NEMA17
A silicone-faced drive wheel against a sprung idler — compliant grip instead of a knurled gear, so soft flux-cored wire isn't crushed.
Trigger
27 mm piezo disc
Sandwiched under the fixture plate. Two impulses 80–500 ms apart, gated on the camera seeing the tip near the board.
Travel
180° arch rail
Six F625ZZ flanged rollers carry the carriage foot-to-foot over the apex. Carbon-filled PETG at the carriage, rack, and brackets; standard PETG for the links.
images/ferros-hero.jpg2400 × 1350 px · 16:9
images/ferros-head.jpg1600 × 1200 px · 4:3
images/ferros-pcb.jpg1600 × 1200 px · 4:3
The build
Every part, under $300.
— priced at the stall, not the catalogue
The whole point is that this is reproducible. Sourced from the Huaqiangbei markets in Shenzhen, with a walk-away price on every line so the budget survives contact with a stall.
Budget cap
$300
Excludes tools and compute already on the bench.
Parts total
~$232
Roughly $68 of headroom held for spares and a backup thermal module.
Encoded axes
6
Every joint holds position on its own, on a single serial bus.
| Part | Spec | Qty | Cost |
|---|---|---|---|
| Motion — encoder servo arm | |||
| Feetech STS3215, 12 V | 30 kg·cm, 4096-count magnetic encoder, serial bus — all six axes | 6 | $70.85 |
| STS3215 spare | Identical part, interchangeable with any axis | 1 | $11.80 |
| TTL half-duplex adapter | Feather S2 UART to the Feetech single-wire bus | 1 | $1.40 |
| Pinion gear, metal | 5 mm bore, meshes the printed shoulder-pitch arc rack | 1 | $1.10 |
| Turntable bearing | Steel, 80–120 mm, load-rated for base yaw | 1 | $3.90 |
| F625ZZ flanged bearing | 5 mm bore — arc-rail carriage rollers | 6 | $1.65 |
| 625ZZ / 688ZZ bearing | Elbow and wrist hinge shaft support | 8 | $4.45 |
| Steel shaft rod | 5 mm and 8 mm diameter, cut to length | 1 lot | $4.85 |
| Fastener assortment | M3 / M4 / M5 bolts, nuts, washers, standoffs | 1 | $4.45 |
| Threadlocker | Blue, medium strength | 1 | $1.10 |
| Subtotal | $105.60 | ||
| Solder feed — cassette feeder | |||
| Pancake NEMA17 | 20–25 mm body — kept as a stepper for step-counted linear metering | 1 | $2.20 |
| TMC2209 driver | UART, 2 A, silent — StallGuard doubles as jam detection | 1 | $1.95 |
| Shaft coupler / hub | 5 mm bore, set-screw — mounts the printed drive wheel | 1 | $1.40 |
| Silicone O-rings | Stretched over the drive and idler wheels as a soft grip surface | 1 pack | $1.40 |
| Compression spring | Light rate, 10–15 mm — idler tensioner | 1 | $0.70 |
| 625ZZ bearing | Idler and drive-wheel support | 2 | $1.10 |
| PTFE guide tube | 1.0–1.2 mm ID — tight fit is what stops the wire buckling | 2 m | $1.40 |
| Blunt needle kit | Stainless 14–18 ga, Luer-lock — the last 25 mm of the path | 1 kit | $2.50 |
| 608ZZ bearing | 8 mm bore — spool spins free in the quick-load bay | 2 | $0.55 |
| Subtotal | $13.20 | ||
| Perception & trigger | |||
| MLX90640 breakout | 55° FOV, 32 × 24, −40 to 300 °C, 16 Hz — verified module, not a parts-bin clone | 1 | $29.20 |
| Piezo disc | 27 mm — tap trigger under the fixture plate | 3 | $0.20 |
| K-type + MAX31855 | Thermocouple ground truth for the melt-signature calibration | 1 | $4.20 |
| AprilTag sheet | 36h11 family — fixture plate and end effector | 1 | $1.10 |
| Subtotal | $34.70 | ||
| Electronics & power | |||
| 12 V 5 A supply | Servo bus draws 1.5–2.5 A realistic, ~4 A worst-case transient | 1 | $3.90 |
| Buck converter | 12 V to 5 V, 3 A — logic rail | 1 | $0.70 |
| E-stop, NC mushroom | 22 mm panel mount — breaks the 12 V rail in hardware, not in software | 1 | $1.40 |
| Wire and shielded pair | Servo bus, piezo run, power — the iron is the loudest noise source on the bench | 1 kit | $3.05 |
| Perfboard | Hand-soldered carrier circuit | 2 | $1.10 |
| Subtotal | $10.15 | ||
| Filament, fixture & consumables | |||
| Bambu PETG-CF | 1 kg spool — ~430 g into the joints, carriage, rack, and brackets | 1 | $32.00 |
| Bambu PETG | 1 kg spool — links and non-load-bearing brackets | 1 | $22.00 |
| Fume extraction | 120 mm 12 V fan and carbon pad — interlocked, no extraction means no feed | 1 | $2.75 |
| Solder wire, SAC305 | 0.8 mm (~100 g) and 0.6 mm (~50 g), lead-free by choice | 2 | $3.90 |
| Flux pen, brass wool | Brass wool, not a wet sponge | 1 ea | $2.10 |
| Mat, ties, sleeving | ESD-safe silicone mat, zip ties, heatshrink | 1 lot | $5.15 |
| Subtotal | $67.90 | ||
| Total against a $300 cap | ~$232 | ||
Prices are goal figures at roughly 7.2 RMB to the dollar; each line also carries a walk-away maximum, and the first quote at a stall routinely runs 30–80% high. Filament is ordered online rather than bought at a stall.
Already on the bench — not in the budget
Raspberry Pi 5
OAK-D PoE + injector
Feather S2
3D printer
Soldering station
Hand tools
Where it stands
Build log.
-
DONE
Engineering package
An 18-page write-up covering architecture, the sensor trade study, feed mechanics, safety and risk analysis, a phased build plan with exit criteria, and a test plan built for ARKSEF.
-
DONE
Sourcing and BOM
Full parts list costed against a $300 cap, with a per-line walk-away price and a market-by-market sourcing map.
-
PHASE 1 · NEXT
Feeder bench prototype
Characterise commanded versus actual feed length, slip against idler tension, and jam modes. Exit criterion: ±0.5 mm over a 10 mm feed, 50 consecutive cycles jam-free.
-
PHASE 2
Melt detection
The emissivity study across SAC305 and Sn63, against thermocouple ground truth. Exit criterion: fire within ±0.4 s of confirmed liquidus on at least 18 of 20 trials, with zero false fires on 20 heat-without-melt trials.
-
PHASE 4
Ferros Zero
The full sensing chain with the nozzle positioned by hand — tap designates, thermal gates, feeder feeds and retracts. Deliberately a complete result on its own, so nothing downstream is load-bearing on the arm being finished.
-
FERROS 2
Roadmap
A second arm carrying the iron itself, turning the bench into a fully automated point-soldering cell at roughly a third the price of the cheapest machine that removes the human trigger.
Ferros is in the planning-to-build transition. Photos, source, and measured results go up here as each phase lands.