View source for Off-world Power Generation Air-Bearing Testbed

The '''air-bearing testbed''' is the ground analogue for the project's in-space
self-assembly work: identical robotic modules float on a near-frictionless planar
surface (3 degrees of freedom — x, y, yaw) and rehearse relative navigation,
docking, and growth logic at hobby cost. It does '''not''' reproduce orbital
relative dynamics; it validates the planar kinematics, docking, and autonomy. The
shared apparatus below underlies [[Off-world Power Generation Lab 3: Propulsive navigation|Lab 3 (propulsive navigation)]],
[[Off-world Power Generation Lab 4: Moment control|Lab 4 (moment control)]], and
[[Off-world Power Generation Lab 5: Robotic self-assembly|Lab 5 (robotic self-assembly)]].

== The running surface ==

Modules float on a sheet of '''float glass''' (flat to tens of microns by
manufacture) — the smooth running surface. The glass is supported continuously by
a stiff, flat '''bed'''; the bed is leveled on '''3 adjustable points'''. Full stack:

 stock 4-leg table (rough-leveled, no drilling)
   -> 3 leveling adjusters (top-side, at working height)
     -> torsion-box bed (stiff + flat)
       -> float glass (continuously supported)
         -> air-bearing puck (the module)

'''Glass:''' ¼″ (6 mm) clear '''annealed''' float, cut to ~'''20″×35″''' to sit just
inside the 20.5″×36″ table (no overhang — a cantilevered annealed sheet cracks).
Ask for '''seamed''' edges (safe to handle); don't pay for polished — the bearing
never runs on the edge. Specify '''annealed, NOT tempered''' (tempering's roller-wave
distortion ruins the ~10 µm fly height). Source locally and pick up — don't ship
glass. Get ≥2 quotes; small custom cuts vary widely on shop minimum + edgework.

== Leveling — the #1 build variable ==

Residual tilt projects gravity into the plane as phantom gravity: '''a = g·sin θ'''.
At 0.10° that is 17 mm/s² of drift; the target is '''< 0.05°''' (8.6 mm/s²), ideally
0.01°. Use '''kinematic 3-point support''' — three points define the plane exactly
(no rock, both tilt axes trimmable); 4 supports over-constrain and rock.

'''Level the surface, not the table.''' The table has fat 4×4 legs; drilling and
turning feet under a heavy table is awkward, and table-leg leveling only fixes
overall tilt — not the wood top's local waviness. Instead, stand '''3 adjustable
bolts on top of the table, under the bed''', and trim there at working height beside
the digital level. A stiff bed on 3 points also '''bridges over''' the table top's
waviness. Rough-level the table once with shims (~1°), then trim < 0.05° on the 3
bolts, and '''re-check loaded''' (glass + module shifts it). The air bearing exerts
negligible horizontal force, so gravity holds the bed; optional cone / V-groove /
flat (Kelvin) seats under the 3 bolt tips make the mount non-sliding and repeatable.

== Torsion-box bed — build plans ==

A torsion box is two thin skins glued to an internal rib grid: stiff and dead flat
for low weight and ~$40 of materials. A plain ¾″ panel on 3 points would sag at the
overhangs (0.3 mm over 10″ ≈ 0.07° slope — over budget); the box stays flat. It is
'''only as flat as the surface you glue it on''' — so assemble it on the float glass
itself (a micron-flat reference), covered with wax paper so glue won't stick. Target
box ≈ '''20.5″×36″ × ~2.5″ thick'''.

[[File:Torsion box walkthrough.png|800px|center|Torsion-box bed build walkthrough: rib grid, exploded sandwich, glue-up on the glass jig, leveling-insert placement, final stack, and the buy/step list.]]

=== Buy ===

* '''Skins:''' ¼″ MDF or tempered hardboard — two pieces '''20.5″×36″'''.
* '''Ribs:''' ¾″ × 2″ strips, ~16–20 linear ft (rip from ¾″ stock, or 1×2 furring).
* '''Glue:''' PVA wood glue (Titebond); brad nails/staples or clamps + even weights.
* '''3× threaded inserts''' (fine-thread 3/8″-24 or M10×1.0) + 3 bolts / leveling glides.

Buy raw and cut yourself, have a store make the straight panel cuts, or order the
panels '''cut-to-size online''' (e.g. SendCutSend, Cherokee Wood Products) and glue
at home. A light box (~10–15 lb) is what makes the 3-bolt leveling easy — avoid
heavy ready-flat plates (granite ~100 lb, MIC-6 aluminum) here, since you hand-turn
the leveling bolts.

=== Build ===

# Cut the two skins flat and square (20.5″×36″). Cut the rib grid: a perimeter frame + internal ribs forming '''~5–6″ cells''' (≈5–6 cross ribs along the 36″ length + 2–3 long ribs across the 20.5″ width). Butt joints are fine.
# Lay the '''float glass on the table, cover with wax paper / plastic''' → your flat assembly jig. Lay the '''bottom skin''' on it.
# Glue the rib grid to the bottom skin; tack with brads or weight down. The flat skin forces every rib into one plane.
# Glue the '''top skin''' on. '''Weight the whole sandwich evenly''' and let it cure on the flat reference (overnight). It cures locked-flat.
# Seal / edge-band the edges (humidity stability); wipe the top clean.
# Install the '''3 threaded inserts in the bottom skin, each directly over a rib''' (so the adjuster load goes into structure, not an unbacked ¼″ skin) — two along one long edge, one centered on the opposite edge, inset ~3–4″. Thread in the 3 bolts; their tips stand on the table top.
# Set the box on the table, drop the glass on top (continuous support), and level on the 3 bolts against the digital level. Re-check loaded.

'''Key gotchas:''' assemble on the flat reference (step 2 is the whole game); the
inserts must land over ribs (step 6); rough-level the table first so the bolts only
do the fine trim.

''Lazy fallback (no box):'' doubled ¾″ MDF (1.5″) on the 3 adjusters — stiffer than
single ¾″ but marginal vs the 0.05° budget at this span; build the box if you can.