How to turn an empty filament spool into an extension cord reel (and how much cord fits)

Measure your spool first: bore, width and core

Four numbers decide whether a hub fits and how much cord the spool holds:

• Bore: the diameter of the hole in the middle. The hub’s snap fingers hook its far edge.
• Width: across the spool, outside flange to outside flange. The hub’s sleeve is sized to it.
• Inside width: the gap between the flanges, where the cord actually winds. The capacity formula below needs it.
• Core: the diameter of the barrel the filament was wound on. Cord winds on it too.

There is no standard for any of them. The hub’s sliders cover bores of 50–58 mm and widths of 55–75 mm. The table shows where common 1 kg spools land, and how sure we are of each figure.

The drive peg and the hole it needs

The hub’s sleeve sits in a round bore, so on its own the crank would just spin the hub inside the spool. The drive peg is what locks them together: an 8 mm peg on the crank paddle that reaches into the flange resting on the paddle, 39.5 mm from the centre (the build log’s peg radius). The engine checks it in a 10 mm hole.

• If that flange has an opening at least 10 mm across, 39.5 mm from the centre of the bore, the peg drops into it.
• If not, mark a point 39.5 mm from the centre and drill a 10 mm hole through the flange. That is the one tool the reel needs.

The build report doesn’t record which spools have a matching opening, so check yours before you print. While you are there: at the default 60 mm crank the paddle reaches 73 mm from the centre, inside the 100 mm rim radius of a 200 mm spool.

How much extension cord fits on a filament spool

The engine estimates capacity as the ring between the core and a line 5 mm under the rim, times the winding width W between the flanges, times a packing factor of 0.75, divided by the cord’s cross-section. The packing factor accounts for round cord leaving gaps between turns. For a round cord:

The build report doesn’t state the winding width it uses. Worked backwards from its 19.4 m result, the formula gives about 59 mm, and the cord the model draws on the spool is 58 mm wide. With the 67 mm outside width instead you would get 22.0 m, so measure inside the flanges.

Which cord is which

Extension cords are sold by gauge, not by diameter. Cord makers and distributors publish a nominal outside diameter for each type; these are typical values and vary by maker. Measure your own cord where it is round, away from the plug.

Starting the wind

The hub fills the bore, so the first plug can’t go through the middle, and the build report lists no slot or clip for the cord end. Tape the plug flat against the core, or feed it out through an opening in the flange, then wind the first turns over it.

Real reels hold less than the estimate. The plugs take room, the first turns won’t lie tight against the core, and a hand-cranked cord winds looser than a machine-wound one. The designers who publish figures for their spool reels land below these numbers: the DarkerView design on Cults3D says 50 ft of light-duty or 25 ft of heavy-duty cord on a 55 mm-bore spool, and the BusinessCat Overture-spool reel says 50 ft of 14-gauge. Plan on a 50 ft cord of 16/3, or 25 ft of 12/3, and you won’t be fighting the last few turns.

How the hub holds in the bore without screws

The hub is a sleeve that goes through the bore. A fixed lip stops it on one side; six snap fingers on the other side flex inward as you push, then spring out and hook the far edge of the bore. No screws, and nothing to undo when you want the spool back.

A snap fit has two ways to fail, and they pull in opposite directions. A smaller bore bends the fingers further, so strain goes up; a bigger bore leaves less of the hook behind the edge, so the catch goes down. That is why the engine rebuilt the hub at both ends of the bore slider, 50 mm and 58 mm. Both built and passed every check. The arithmetic behind the strain figure is in the snap-fit design guide.

The hub is designed in PETG, for two reasons. The engine’s material profiles give PETG 10% elongation at break against PLA’s 4%, 2.5 times as much, so a PETG finger bends much further before it cracks. And a reel often lives in a garage or a car boot that gets warm. The PLA vs PETG vs ASA comparison covers the heat side.

A print-in-place grip that turns while you crank

To wind, you hold a handle still in one hand while the spool turns around it. Here that handle is a fluted grip on an axle that prints inside the hub’s sleeve, already assembled, on a captive bearing.

• Running gap: 0.3 mm between the axle and the sleeve at the default; the slider runs 0.2–0.5 mm, and the hub builds and passes at both ends.
• Captive: a 1.77 mm capture flank blocks the axle both ways along its length, and every captive body is held by at least 0.8 mm, two perimeters.
• Motion: the engine swept a full winding turn in 145 poses, at most 5° apart, and the knob through a full turn on its pin in 73. Nothing touched.

Pick the gap the way you would for a hinge: print the tolerance test in the PETG you’ll use and take the first gap that turns freely. The print-in-place hinge guide explains why tight print-in-place gaps fuse, and why the first layer is usually the culprit.

The hub prints standing up with the grip axle inside it, which makes it 177.6 mm tall at the default 67 mm spool width. That is the number to check against your printer:

On an A1 mini that leaves 2.4 mm at the default width. The report gives the height only at that width, and its slider-end builds ran on the A1 profile with 256 mm of height. The axle runs through a sleeve sized to the spool, so a wider spool will most likely make the print taller: on a printer with 180 mm of height, set your width in the model and check the height before you start the 2 h 15 min hub print.

Will the crank snap? The load checks

Winding a cord isn’t a heavy job, but a crank turns a small pull into a large twist, and the parts that carry it are small. The engine checks three of them against PETG’s strength across the layers, where printed parts are weakest:

Why across the layers? The grip axle prints standing up, so when the reel’s weight bends it, the bend opens its layer lines, and the drive peg and knob pin take sideways loads the same way. The lower figure is the honest one to check against. The method, and why a jolt counts double, is in the guide to how much weight printed parts hold.

The crank slider runs 40–80 mm. A longer crank turns the spool with less effort, but the same pull at a longer arm puts more twist through the drive peg, so its stress rises with the crank length. The engine rebuilt the hub at 80 mm and the peg still passed.

Printing it: parts, grams and time

Two prints make four parts: the hub with its grip axle inside, and the knob on its pin, which then snaps into the crank arm. Nothing needs supports.

The defaults quoted in this guide are at 2 walls and 15% infill. The drive peg and the grip axle carry the crank load, and in bending the walls do most of the work, so 3 walls is a reasonable step up for 30 g and 53 minutes more.

Limits, and using a cord reel safely

This is a storage reel. Nothing touches the wiring: no outlet moved into the hub, no cut cord. Unwind the cord fully before you plug anything into it.

Bayco’s cord-reel manual says to pull the cord out to full length when using it. Brennenstuhl, which makes cable reels, explains why: a wound cable can’t shed the heat the current makes, so the core of the coil gets very hot, with a risk of insulation damage and fire. Its own reels can be used partly wound only within a rolled-up load rating marked on the reel, and they carry a thermal cutout that trips at about 65 °C. An ordinary extension cord on a printed spool has neither, and the engine’s PETG profile puts the hub’s heat deflection at 70 °C.

What the engine didn’t check:

• Continuous motion, friction and wear: the motion checks are sampled rigid poses.
• Loads while moving, flex, and any hardware outside the model, including your spool.
• Dropping a full reel, or a spool flange cracking around the drive peg.
• Cardboard spools, spools other than the 200 mm one it was checked in, and the print height at widths other than 67 mm.

The model also went through Loftsmith’s Workshop review. It passed on the first round with nothing it had to fix, and scored 4 of 5 for usefulness, 4 for correctness, 5 for honesty, and 3 of 5 for looks: it is a functional part, and it looks like one.

Sources and method

Written by the Loftsmith team. The hub numbers come from the Loftsmith engine’s build report of 8 October 2026, made with the Bambu Lab A1 profile, PETG and the default sliders (bore 55 mm, width 67 mm, crank 60 mm, gap 0.3 mm, cord 8 mm, core 90 mm). The report holds 16 checks, all passed, and 12 rebuilds at the ends of the six sliders, all of which built and passed. The capacity tables scale the engine’s 19.4 m result with its own formula; the 59 mm winding width is worked back from that result, not read from the model.

Spool and cord sizes come from the maker, retailer and distributor pages linked in the tables, and the spool table says which of those pages we could open. Competitor capacities are those designers’ own claims. Safety advice comes from the reel makers linked above.

Not tested: nothing in this guide has been printed, wound or load-tested. When it has, the results will go here.

Tolerance testerFind the running gap your printer makes in PETG before you set the grip’s gap slider.71 × 35 × 10 mm · 5 g · 21 min

Captive rotary fidgetA captive ring that prints in place, the same trick as the grip on a smaller scale.32 × 32 × 8 mm · 4 g · 15 min

Cable organizerSnap-in clips sized to your cable, for the cords that stay plugged in.74 × 16 × 11 mm · 5 g · 11 min

The hub itself is a free Workshop model: open the filament spool cord reel hub, enter your spool’s bore and width and your cord’s diameter, and the engine rebuilds and rechecks it.