Bed and motion upkeep
Everything above the first layer is built on it, and everything in the XY plane is placed by the motion system. Those two subsystems — the plate the part grows from and the belts and rails that move the nozzle across it — are the quiet infrastructure of dimensional accuracy. When they're right, you don't think about them. When they drift, they don't announce themselves as "the bed" or "the belts"; they show up disguised as a part that won't stick, a first layer that's fat on one side, or a hole that measures round in the model and oval on the plate. This is the routine that keeps that infrastructure from moving under your calibrated numbers.
The bed: a flat, clean, level start
A first layer is only as even as the surface under it. Two things spoil that surface, and they fail in opposite directions.
Level and mesh. If the plate isn't level — or, on a machine with mesh compensation, if the stored mesh no longer matches reality — the nozzle rides closer to the bed in some regions than others. Where it's close the first layer squashes wide; where it's far it lays down thin and barely sticks. Either way the base of your part is uneven, and since every dimension grows from that base, an uneven first layer taxes everything above it. Refresh the level (and re-probe the mesh) after you move the printer, change the plate, or notice the first layer going inconsistent.
Cleanliness. Adhesion is chemical and mechanical contact between plastic and plate, and a film of skin oil or dust breaks that contact completely. A plate that looks clean but was handled with bare fingers will refuse to hold a part in exactly the spot you touched. Wash it with soap and water or wipe it with isopropyl alcohol, and handle it by the edges. Grease is the single most common reason a print that used to stick suddenly won't.
The motion system: honest XY, no ringing
The belts and rails decide whether the nozzle actually lands where the model says it should.
Belt tension. A slack belt has backlash — the carriage lags when the motor reverses — and it lets the toolhead ring and overshoot on fast direction changes. That shows up as ghosting on the surface and, more importantly, as dimensions that come out subtly off-size and out-of-square. A belt tensioned right (firm, with a low even hum when plucked, not a loose flap) is what keeps a modelled 40.0 mm reading 40.0 mm on the part. This is the "blurred dimension" failure from When your tolerances drift, and belt tension is its usual root.
Lubrication. Linear rails and lead screws need a light, appropriate lubricant so the toolhead moves smoothly and repeatably. A dry or gritty rail moves in tiny stutters — inconsistent positioning that scatters your dimensions — and wears faster besides. A clean, lightly oiled rail moves the same way every time, which is the whole point.
| Task | How often | What it protects |
|---|---|---|
| Wipe / degrease the plate | Every few prints | First-layer adhesion |
| Re-level / re-probe mesh | After moving printer, new plate, or bad first layer | Even first layer, honest Z base |
| Check belt tension | Monthly, or on ringing / off-size parts | Accurate, square XY dimensions |
| Lube rails & lead screws | Every few weeks of use | Smooth, repeatable positioning |
| Inspect for play / wobble | Occasionally | Overall dimensional stability |
Why this is the upkeep that keeps your promises
When you draw a clearance and calibrate it, you're making a promise: this fit will print at this number. That promise is only keepable if the machine can still place plastic where the model says, layer after layer. A greasy plate, a stale mesh, a slack belt or a dry rail each erode the machine's ability to hit the geometry you drew — not by breaking, but by drifting. The tolerances you promised in the model are achievable exactly as long as the hardware that produces them stays in tune. This routine is how you keep that true. Reach for a test coupon whenever a first layer or a dimension looks off before you go re-drawing anything (Validate before you print), and when a part fails outright, work the surface it started on first (When a print fails).
Which is the thread that ties this whole hardware topic together. The machine and the material impose the real tolerances — the nozzle's bore, the plastic's shrink, the bed's flatness, the belt's tension — and knowing those numbers, and keeping them from drifting, is what lets a Kapy model print right the first time. The dimensions you own in the model are only ever as trustworthy as the hardware and material underneath them, which is why it's worth circling back to Filaments and materials and Choosing a filament with fresh eyes: the model and the machine are one system, and now you can read both.