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The STEMFIE® Standard

STEMFIE is not a box of parts. It is a standard: a small set of published rules that make every part fit every other part, whoever designed it and whenever it was printed.

1 SU= 12.5 mm
SU HoleØ 7.0 mm
Grid1 SU pitch, halving to ⅛ SU
8 SU= 100 mm, exactly

The STEMFIE Unit

Everything in STEMFIE is measured in one unit, the STEMFIE Unit (SU). At normal print scale, 1 SU = 12.5 mm. Hole spacing, beam lengths, plate sizes, thread pitch, the length of a screw: all of it is a whole number or a simple fraction of SU. That is why a beam designed in 2019 and a screw designed this year fit together without adjustment.

The size was chosen with a desktop printer in mind. Printed with a 0.4 mm nozzle at 0.2 mm layers, 12.5 mm gives holes and walls that are strong and dimensionally stable on ordinary hardware, while keeping the parts small enough for a child's hands.

A five-hole STEMFIE beam: holes 12.5 mm apart, each 7 mm wide 1 SU = 12.5 mm Ø 7.0 mm 1 SU
Beam SU05 — five holes, five units long, one hole every 12.5 mm.

The STEMFIE Grid

Every STEMFIE part sits on the same three-dimensional lattice, the STEMFIE Grid, with a pitch of 1 SU. The centers of the connection holes always fall on grid points. Between whole units the grid divides by halving: ½ SU (6.25 mm), ¼ SU (3.125 mm), and ⅛ SU (1.5625 mm), the finest standard step.

This makes the grid both metric and binary at once. Eight units are exactly 100 mm, one decimeter; a meter is 80 SU. Every step halves cleanly, the way hands and computers both like to count, and the whole thing lines up with any metric ruler. It is a property worth teaching, and one of the reasons STEMFIE suits the classroom.

Any part whose connections sit on the STEMFIE Grid works with every other STEMFIE part. Designers inside and outside the official catalog may build on it, and may say so: built on the STEMFIE Grid.

The STEMFIE Grid as three planes of five by five units meeting in a corner, with a hole on every grid point and a five-hole brace lying on the grid. XYZ
One lattice in every direction. Pitch 1 SU, a hole on every grid point, and every part sits on it.
A ruler in STEMFIE Units with half, quarter, and eighth divisions and the millimeter equivalents. 0123 SU 12.5 mm25 mm37.5 mm ½ SU¼ SU⅛ SU
Metric and binary at once. Whole, ½, ¼, and ⅛ SU; 8 SU = 100 mm.

The SU Voxel and the SU Hole

The conceptual building block of the system is the SU Voxel: a cube of 1 SU, pierced through the center of each pair of faces by an SU Hole of 7.0 mm diameter. It is not a part you print; it is the idea every structural part is made of. This cube is what STEMFIE originally called the block unit, and the name still fits: a beam is a row of these blocks, a plate is a sheet of them, a brace is a row with material removed for lightness, the way a wall is built from bricks.

The SU Hole is the connection interface for every fastener: screws, pins, shoulder screws, and spring pins all pass through or seat in it. It comes in two forms. A free bore is the plain round hole, in which a shaft turns freely. A fixed bore has two parallel flats 5.1 mm apart, which lock onto the STEMFIE shaft profile, a 6.6 mm cylinder with flats at 5.0 mm, so the shaft drives the part instead of spinning inside it. A D-profile shaft turns freely in a free bore and locks in a fixed bore, which is how one shaft can drive a gear and pass through a beam at the same time.

The SU Voxel: a cube of one STEMFIE Unit with a 7 mm hole through each pair of faces. 1 SU
The SU Voxel. A cube of 1 SU (12.5 mm) with a Ø 7.0 mm SU Hole through the center of every pair of faces. Beams, plates, and braces are rows and sheets of it.
The SU Hole in a gear: a free bore in which the shaft turns, and a fixed bore in which the shaft drives; the STEMFIE D-shaft is gray with its 2 mm center hole. Free bore Fixed bore
The SU Hole. Left, a free bore: Ø 7.0 mm, the shaft turns freely. Right, a fixed bore: flats 5.1 mm apart, the shaft drives the gear. The STEMFIE shaft, in gray, is Ø 6.6 mm with flats at 5.0 mm and a 2 mm center hole.

The STEMFIE Shaft

Every shaft and every fastener in the system starts from the same profile: a Ø 6.6 mm cylinder with two flats 5.0 mm apart and a 2 mm through-hole down its axis. The profile turns freely in a free bore and locks in a fixed bore, so the same rod is an axle in one part and a drive shaft in the next.

A fastener is this profile with stopping geometry added: a head, a flange, a threaded section, a groove, or a spring feature that lets a pin snap into place and hold parts together on its own. Threads follow the grid as well. Every STEMFIE thread has a pitch that is a fraction of the STEMFIE Unit, ⅛ SU for everyday screws and nuts, and every pitch exists as a male thread for screws and rods and a female thread for nuts and threaded bores. That is why a screw printed by one person fits a nut printed by another, in another year, on another printer.

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The STEMFIE Shaft profile, and the same profile as a plain shaft with indexing holes, a screw with a round head, and a threaded rod with a nut. Ø 6.6 5.0 Ø 2.0 Shaft Screw Threaded rod and nut
The STEMFIE Shaft. Left, the profile (end view, enlarged): Ø 6.6 mm, flats 5.0 mm apart, 2 mm through-hole. The same profile becomes a plain shaft, a screw with a head and a threaded end, and a threaded rod with its nut. The small indexing holes along a shaft sit 3.125 mm apart and take a pin or a retaining ring.

The STEMFIE Socket

The STEMFIE symbol is not only a logo. Its four-lobed outline, four convex lobes alternating with four concave ones, is a connection profile in its own right, the STEMFIE Socket. As an outer shape it is the silhouette of the nut, the power-transmission hub, and the STEMFIE-head screw. As an inner bore it is the void at the center of wheels, gears, spacers, and other parts that need to be driven. Male slides into female, the lobes lock rotation, and the part can still move along the axis. A gear on a hub, a wheel on a nut: torque passes through the same shape that marks every part as STEMFIE.

The STEMFIE Socket: the four-lobed profile as a male hub and as a female bore in a gear
The STEMFIE Socket. Male on the left: nut, hub, STEMFIE-head screw. Female on the right: the bore of a wheel, gear, or spacer. It slides in and locks rotation.

Counting in holes

A structural part is named by the number of holes along each axis, not by its length in millimeters. A beam with five holes is an SU05 beam; its outer hole centers are 4 × 12.5 = 50 mm apart. Sizes are stated as SU length × width × height, so SU05x01x01 is a five-hole beam one unit wide and one unit high. Children count holes; the millimeters take care of themselves.

A five-hole beam with its holes numbered one to five; the outer hole centers are four units, 50 millimeters, apart. Beside it, the end view: one unit wide, one unit high. 12345 4 × 12.5 mm = 50 mm 0101 end view
SU05 × 01 × 01. Five holes long, one unit wide, one unit high; 50 mm between the outer hole centers.

Any scale you like

Because the unit is a ratio, STEMFIE parts can be printed at any uniform scale. At 50 % one unit is 6.25 mm and the hole 3.5 mm; at 200 % they are 25 mm and 14 mm. Parts of different scales do not fit each other, but a whole set at 200 % is a fine toy for small hands, and a 50 % set makes desktop models.

A hand pinching a five-hole brace printed at 100 percent between thumb and index finger; the same brace at 50 and 200 percent beside it. 100 % 50 % 200 %
Any scale you like. The same five-hole brace at 50 % (1 SU = 6.25 mm, hole 3.5 mm), 100 % (12.5 mm, hole 7 mm), and 200 % (25 mm, hole 14 mm). The 100 % brace sits between the thumb and index finger of an adult hand.

Names that carry the geometry

Every official part has a name that spells out what it is, in a fixed grammar: component, sub-type, modifiers, dimensions, and a permanent serial, the STEMFIE Part Number (SPN). For example:

STEMFIE Beam – Straight – SU–Array – Ends–Square – SU05x01x01 – SPN–BEM–000042

  1. STEMFIE BeamComponent. What it is: a beam.
  2. StraightSub-type. Its form within the family.
  3. SU–ArrayModifier. The standard row of SU Holes, one per unit.
  4. Ends–SquareModifier. The end profile.
  5. SU05x01x01Dimensions. Length × width × height, in STEMFIE Units: 5 holes long, 1 wide, 1 high.
  6. SPN–BEM–000042Serial. The STEMFIE Part Number: BEM for beams, number 42, permanent.

Read from left to right, the name goes from the general to the specific, and every segment is a fact about the geometry. The same grammar names every file in the repository, so a search for SU05x01x01 finds every five-hole part, whatever its family. The SPN never changes and never gets reused, so a part can be referenced in a lesson plan today and found in ten years. Every kit likewise has a permanent STEMFIE Project Set number (SPS).

An open standard, with a history

The Standard is published under open terms: anyone may read it, implement it, teach it, and manufacture to it, without fees or permission; see the license. The STEMFIE name and logotype are trademarks and stay with the project, which is what keeps the standard trustworthy.

The system has been in continuous public development since its first parts were released on 21 December 2019. The 12.5 mm unit was defined in 2018, following work that began in 2015, and the first complete kit, a rubber-band-driven car, was published on 2 January 2020. Since then the standard has grown to thousands of parts across dozens of component families, all on the same grid. The Standard is versioned like software: a major version would mean parts stop fitting, which is precisely what the project exists to prevent; new component families and clarifications arrive as minor and patch versions.

Build on it

This page is the overview. The full STEMFIE Standard, the normative document covering geometry, the component taxonomy, the naming grammar, print parameters, and versioning, is being prepared for publication here. Until then, everything needed to start is already on this page: print a part, check it against the numbers above, and it will fit every STEMFIE part ever made.

If you design parts, write tools for makers, or teach with STEMFIE and want the specification early, or have a question about compliance, write to paulo@stemfie.org. Every question makes the standard clearer. And to hear the moment the full STEMFIE Standard is published, subscribe below.

The Standard is free and will stay free. Writing it, and designing the thousands of parts that follow it, is funded by the people who use it: if it has been useful to you, supporting the project is how the next component family gets made.

⚠️ Contains small parts that are a choking hazard. Not suitable for children under 3. Suitable for ages 3 and up under adult supervision.