BMAR-PH1-001 Rev B — Engineering Drawing Set

// PROJECT: BMAR — BIOMECHANICAL ROBOTIC ARM | DOCUMENT: BMAR-PH1-001 REV-B

PHASE 1: UPPER ARM
COMPLETE ENGINEERING DRAWING SET

DOC NO.BMAR-PH1-001

REVB — 2025

MATERIALSEE BOM SHT-00

VOLTAGE12–24V DC

UNITSALL DIMS IN mm

PA6/PA66 NYLON

NICHROME Ni90

POLY. THREAD

KEVLAR/ARAMID

PET SLEEVE

TPU ELASTIC (optional)

NYLON TENDON (30-50 strand)

OIL FILL

Al/BRASS BONE

FINE FASCICLE

COARSE FASCICLE

SHT-00
ASSEMBLY OVERVIEW

START HERE — HIERARCHY + BOM + OVERALL DIMENSIONS

FIGURE 00.1 — PART HIERARCHY (LEVEL 1 → LEVEL 5)

FIGURE 00.2 — MASTER BILL OF MATERIALS (PHASE 1: UPPER ARM)

ITEM	DESCRIPTION	MATERIAL / SPEC	QTY (PH1)	UNIT	SOURCE / SEARCH TERM	DWG REF
P-01	Nylon monofilament — coarse fiber	PA6 or PA66, ∅0.30mm	5	m	AliExpress — "nylon fishing line 0.3mm PA6" — CONFIRM NOT PVDF	DWG-01
P-02	Nylon monofilament — fine fiber	PA6 or PA66, ∅0.10mm	3	m	AliExpress — "nylon monofilament 0.1mm"	DWG-01
P-03	Nichrome wire — coarse heater	Ni90 (NiCr9010) AWG34, ∅0.15mm, ~17.2Ω/m	5	m	AliExpress — "Nichrome 90 AWG34" or "resistance wire 0.15mm"	DWG-01
P-04	Nichrome wire — fine heater	Ni90 AWG40, ∅0.08mm, ~138.8Ω/m	3	m	AliExpress — "Nichrome 90 AWG40 0.08mm" — HANDLE CAREFULLY	DWG-01
P-05	Sewing thread binder	Polyester, thin gauge, T_rated ≥180°C	1	spool	Any haberdashery — standard polyester sewing thread	DWG-01
P-06	Kevlar/Aramid thread	Braided Kevlar, ∅0.3–0.5mm approx	5	m	AliExpress — "Kevlar braided thread" or "Aramid fiber cord"	DWG-03 (fascicle core only)
P-07	PET braided sleeve	4mm ID expandable PET mesh	2	m	AliExpress — "PET expandable braided sleeve 4mm"	DWG-03
P-08	Oil fill — fascicles	Light mineral oil or silicone oil, 50cSt	50	mL	Pharmacy (mineral oil) or AliExpress (silicone oil 50cSt)	DWG-03
P-09	TPU elastic cord — optional ligament	TPU, ∅0.15mm — NOT rubber (creep-resistant). Optional: for side ligament panels only. NOT used in main tendon.	1	m	AliExpress — "TPU elastic cord 0.15mm"	DWG-05 (ligament panel, optional)
P-24	Nylon tendon bundle	PA6/PA66, ∅0.104mm monofilament — 30–50 parallel strands per tendon	20	m	AliExpress — "nylon fishing line 0.1mm PA6" — CONFIRM NOT PVDF	DWG-05 (primary tendon)
P-10	Aluminum tube — bone	6061 Al, OD 16mm, ID 12mm, wall 2mm	300	mm	AliExpress — "aluminium tube 16mm OD 12mm ID" or local metal supplier	DWG-04
P-11	Resin for joint brackets	Standard photopolymer resin, 3D printed	2	pcs	3D print — PTFE spray after curing (see DWG-04 for geometry)	DWG-04
P-12	PTFE spray — joint coating	PTFE aerosol, dry lubricant	1	can	Any hardware store or AliExpress	DWG-04
P-13	M3 pivot bolt + nut (joint)	M3×20 stainless steel hex bolt	2	sets	Any hardware store	DWG-04
P-14	MOSFET — per fascicle channel	IRLZ44N (logic-level) or IRF540N	8	pcs	AliExpress — "IRLZ44N N channel MOSFET"	DWG-06
P-15	Flyback diode	1N4007, 1A/1000V	8	pcs	Component kit or AliExpress — "1N4007 diode"	DWG-06
P-16	Gate resistor	10kΩ, 1/4W, ±5%	8	pcs	Component kit	DWG-06
P-17	Decoupling capacitor	100nF ceramic, 50V	8	pcs	Component kit	DWG-06
P-18	ESP32 microcontroller	ESP32 DevKit v1 or equivalent	1	pcs	AliExpress — "ESP32 DevKit"	DWG-06
P-19	Heavy-duty connector	32A/250V, 2-pin (or multi-pin)	4	pcs	Already sourced	DWG-06
P-20	Lab PSU	30V/10A adjustable, current-limiting	1	pcs	Already sourced	DWG-06
P-21	CA glue (cyanoacrylate)	Standard CA glue, thin viscosity	1	tube	Any hardware store — for securing thread ends	DWG-01
P-22	Syringe — oil fill	10mL plastic syringe, blunt needle	1	pcs	Pharmacy	DWG-03
P-23	304 SS wire (alt heater)	∅0.06mm, soft temper — optional alt to P-04	2	m	AliExpress — "soft stainless steel wire 0.06mm 304"	DWG-01

BUILD SEQUENCE

STEP 1Make + test individual fibers (DWG-01)

STEP 2Pair fibers into 2-ply (DWG-02)

STEP 3Bundle pairs into fascicles (DWG-03)

STEP 4Machine/print bone tube + joint (DWG-04)

STEP 5Make tendons + ligaments (DWG-05)

STEP 6Assemble arm: route fascicles, attach tendons (DWG-04)

STEP 7Wire circuit, test each fascicle (DWG-06)

STEP 8Commission: tune PWM per fascicle, calibrate

OVERALL ARM DIMENSIONS

BONE TUBE OD16mm — Al 6061

BONE TUBE ID12mm (wall = 2mm)

BONE LENGTH250mm (humerus segment)

PIVOT BOLTM3 × 20mm stainless

JOINT BRACKET3D printed resin, see DWG-04

MUSCLE OFFSETFlexor: 15mm from bone CL Extensor: 12mm

FASCICLE COUNT4 coarse + 4 fine (8 total, phase 1)

ELBOW ROM0–150° (set by ligament length)

STEP	OPERATION	PARAMETER	VALUE / NOTE
01	Cut materials	Length	200mm for prototyping. Confirm PA6/PA66 on nylon packaging.
02	Lay parallel	Arrangement	Nichrome alongside nylon — NOT wrapped. Direct contact across full length.
03	Thread-wrap P-05	Wrap pitch	Contiguous — no gaps. Thin polyester T_rated ≥180°C. Secure both ends CA glue P-21.
04	Pin + weight	Pre-load weight	5–10g hanging weight. Pin top end to board. Weight keeps tension during twist.
05	Twist (CW)	Turn count	80–120 turns per 200mm until strand springs into helix spontaneously.
06	Heat-set	Temp / Time	150°C / 10–15 min OVEN. Keep weight attached. Full cool before removing.
07	Inspect + measure	Resistance	Multimeter across Ni ends. Target: 10–25Ω / 10cm (coarse). Reject if <5Ω or >60Ω.
08	Test actuation	Voltage	Apply 5V for 1s. Fiber should visibly shorten. If not, check resistance and Ni wire continuity.

PARAMETER	COARSE (P-01/P-03)	FINE (P-02/P-04)
Nylon OD	0.30mm ±0.02	0.10mm ±0.01
Nichrome OD	0.15mm AWG34	0.08mm AWG40
Ni resistance	~17.2Ω/m	~138.8Ω/m
Target R/10cm	10–25Ω	35–50Ω
Power @ 12V	~5.8W (25Ω)	~3.6W — use 20% PWM
Coil OD (approx)	~1.2mm	~0.4mm
Heat/cool time	0.5–2s / 3–8s	~0.3s / ~1s
Thermal mass ratio	1× (reference)	~1/9× (faster)
PWM mode	40–100% burst	15–30% continuous
Purpose	Force + movement	Pre-tension + hold
AWG40 handling	—	⚠ Abrade+flux before solder

PARAMETER	VALUE	CRITICAL NOTE
Individual fiber twist direction	Clockwise (CW) — consistent	Both fibers in the pair MUST be twisted in the same direction during fabrication (DWG-01)
Inter-pair twist direction	Counter-clockwise (CCW) — OPPOSITE	If individual coils are CW, pair twist MUST be CCW. Mixing up directions defeats the torsion balance.
Inter-pair twist density	3–5 turns per 100mm length	Not too tight — just enough to interlock. Excessive twisting reduces working stroke.
End fixation	Both ends of both fibers fixed	Crimp with small metal sleeve or tie with Kevlar thread + CA glue. All 4 ends (2 per side) must be captured.
Electrical connection	Ni wires in PARALLEL at each end	Twist all Ni+ wires together → 1 terminal. Twist all Ni− → 1 terminal. One MOSFET channel per pair.
Return spring	NOT REQUIRED	Torsion balance provides this automatically. An antagonist muscle group or gravity provides extension.
Reference	Haines et al., Science 2014	University of Texas Dallas — canonical TCP paper. Coiled polymer actuators.

COMPONENT	ITEM	MATERIAL	DIMENSION	NOTES
Bone tube (humerus)	P-10	6061 Aluminum alloy	OD 16mm / ID 12mm / L 250mm	Cut to 250mm. Deburr both ends. Drill 2× M3 clearance holes for joint bracket at each end, 8mm from tip.
Joint bracket	P-11	Standard photo-resin, 3D printed	H=16mm / W=30mm / wall=3mm / pivot hole ∅3.2mm	Print at 100% infill. PTFE spray all surfaces after curing. M3 clearance hole through center.
Pivot bolt	P-13	M3×20 stainless steel hex bolt	M3 × 20mm + M3 nut + M3 washer	Nyloc nut preferred. Tighten finger-tight + ¼ turn. Should rotate freely but without slop.
Flexor fascicle offset	—	—	15mm from bone centerline (anterior)	Fascicle centers at 15mm from bone CL. Use 3D printed guide clip or adhesive mounting.
Extensor fascicle offset	—	—	12mm from bone centerline (posterior)	Extensor slightly closer to bone than flexor — follows biological proportion.
Tendon attachment hole	—	—	∅2mm, 8mm from pivot CL	Drill through bracket. Kevlar/aramid tendon looped and knotted through hole. CA glue the knot.
Elbow ROM	—	—	0–150° nominal	Set by medial + lateral ligament panel length. Adjust panel size to change ROM.

TYPE	MATERIAL	FUNCTION	STRETCH	CONSTRUCTION
Tendon (PRIMARY)	30–50× Nylon ∅0.104mm P-24	Force transmission from muscle to bone	~3–5% (target ~human 1–5%)	Parallel strands, no twist → ferrule at one end → capstan wrap + washer + backup ferrule at bone
Fascicle core	Nylon + Kevlar/Aramid (P-06) twisted	TCP contractile element + heat resistance	Contracts ~40% when heated	Twisted nylon+kevlar core, nichrome coil, PET sleeve, ferrule at one end
Medial ligament	Kevlar/Aramid 3-layer woven panel P-06	Prevent valgus (lateral) dislocation at elbow	~0% by design	Weave 15mm × 20–25mm × 3 layers, anchor ∅1.5mm holes at 4 corners
Lateral ligament	Kevlar/Aramid 3-layer woven panel P-06	Prevent varus (medial) dislocation at elbow	~0% by design	Identical to medial, installed opposite side of joint
Optional TPU element	TPU ∅0.15mm P-09	Stretchy side ligament if desired (NOT main tendon)	High (~20–30%)	Bare cord or sparse Kevlar wrap. For joint limit only.

CH	GPIO PIN	FASCICLE TYPE	MUSCLE GROUP	PWM MODE	R (APPROX)	NOTES
CH1	GPIO25	FINE (∅0.1mm)	FLEXOR — anterior	15–30% continuous	40–80Ω	Always active. Holds flexor nylon tendon under baseline tension.
CH2	GPIO26	FINE (∅0.1mm)	FLEXOR — anterior	15–30% continuous	40–80Ω	Redundant fine fascicle for precision holding.
CH3	GPIO27	COARSE (∅0.3mm)	FLEXOR — anterior	40–100% burst	15–40Ω	Burst-recruited for elbow flexion movement.
CH4	GPIO14	COARSE (∅0.3mm)	FLEXOR — anterior	40–100% burst	15–40Ω	Second coarse fascicle — max load handling.
CH5	GPIO12	FINE (∅0.1mm)	EXTENSOR — posterior	15–30% continuous	40–80Ω	Holds extensor nylon tendon under baseline tension.
CH6	GPIO13	FINE (∅0.1mm)	EXTENSOR — posterior	15–30% continuous	40–80Ω	Redundant fine fascicle for extensor group.
CH7	GPIO15	COARSE (∅0.3mm)	EXTENSOR — posterior	40–100% burst	15–40Ω	Burst-recruited for elbow extension movement.
CH8	GPIO2	COARSE (∅0.3mm)	EXTENSOR — posterior	40–100% burst	15–40Ω	Second coarse extensor fascicle.

#	STAGE	WHAT	FERRULE
1	Origin (proximal)	3 fascicle ends + NiCr leads + tendon start	M0.8
2	Mid-muscle	Fascicles in separate PET sleeves, tendon in guide channel	—
3	Insertion (distal)	3 fascicle ends + 30-strand tendon → ferrule	M1.0
4	Bone anchor	Capstan wrap → rubber washer → backup ferrule → bolt	M1.0

LOCATION	CONTENTS	BUNDLE OD	FERRULE
Origin	3 Nylon + 6 NiCr	~0.7mm	M0.8
Insertion (30 strand)	3 Nylon + 30 Nylon tendon	~0.65mm	M1.0
Insertion (50 strand)	3 Nylon + 50 Nylon tendon	~0.85mm	M1.2
Capstan backup (30)	30 Nylon tendon	~0.65mm	M1.0
Capstan backup (50)	50 Nylon tendon	~0.85mm	M1.2

JOINT	GROUPS	MAIN FERRULES	BACKUPS	TOTAL
Elbow	2	4	2	6
Forearm	2	4	2	6
FULL ARM	4	8	4	12

05 DRAWSTRINGS