How to write assembly instructions for a 3D printed product (BOM, steps, and hardware)

Assembly instructions are finished when a stranger can build without messaging you. That bar is higher than “I remember how it goes.”

Build the prototype once with only the materials you plan to publish: the STLs, the shopping list, and the draft steps. Every time you reach for a screw that is not on the list, or flip a part because the orientation was obvious only to you, that is a hole in the instructions — not a user error.

Serious kits already do this. Voron-style projects ship a BOM and print settings as required reading. Prusa-style manuals keep one action and a clear figure per step. Casual maker uploads often skip both, which is why Reddit threads about bought STLs keep complaining about missing assembly docs.

Use this as the spine of your instructions. If an item is missing, the build is not ready to publish.

1. Title and one-sentence what it is / who it is for 2. License (and what remixers may do) 3. Tools required (included vs bring-your-own) 4. Bill of materials: printed parts with qty + purchased hardware with exact sizes 5. Print notes per STL: material, walls, infill, orientation, supports 6. Named files that match the BOM (no final_final_v3.stl) 7. Mandatory vs optional modules called out clearly 8. Steps in build order — one action each 9. Hardware attached to the step that uses it 10. Orientation cues (“rib faces the wall,” not “correctly”) 11. Checkpoints (“shaft spins freely by hand before the cover goes on”) 12. Three troubleshooting notes for the failure modes you already hit 13. Version + what changed 14. A stable public URL for the manual (not only a zip)

Lists are perfect for this inventory. The rest of the article is why each line earns its place — so you do not ship a checklist that still leaves builders stuck.

Builders open instructions to answer one question first: what do I need in my hands? A pretty explode without quantities is decoration.

Split the bill of materials into two tables in your head even if you publish one list:

Printed — part name, quantity, which file, material, any special walls or orientation. Purchased — screws, inserts, bearings, springs, o-rings, electronics: size, length, qty, and a note when two M3 screws are different lengths.

Repeated fasteners are where releases die. “M3 screws” is not a line item. “M3×8 SHCS × 4” and “M3×16 SHCS × 2” are. If you use heat-set inserts, name the series or outer size you designed the boss for — not only “brass inserts.”

Open hardware projects that survive replication (assistive kits, DIY machines, printer kits) almost always lead with this shopping layer. Your multipart gadget deserves the same respect.

A step is one thing the builder does: press the insert, seat the plunger, drive these four screws. If you need the word “then,” split the step.

Write like someone is holding parts with both hands:

Bad: “Assemble the carriage using the appropriate fasteners.” Good: “Drive two M3×12 SHCS through Carriage Cap into the heat-set inserts in Carriage Body. Do not fully tighten until the belt is routed.”

Put the vitamins on the step that consumes them. Rolling everything into a single hardware bag sorted by type forces builders to hunt; kit feedback is brutal when fasteners are sorted by screw type instead of by chapter. Step-scoped hardware also makes your rolled-up shopping list honest — quantities come from real use, not memory.

Keep orientation in the sentence or the figure: a rib, an arrow, a flat, a chamfer. “Correct orientation” is not an instruction.

Assembly instructions fail upstream when the wrong walls or orientation make a boss crack on first torque. Put print guidance where the STL lives: material, perimeter count, infill, supports, and which face hits the bed.

You do not need a novel. One line per critical part beats a Discord essay after twenty people print the weak version. When a part revises, update the note with the version — builders comparing an old plate to a new manual need to know which world they are in.

Photos, explodes, and slicer assembly exports are useful. They are still not the whole product. A PDF or MakerWorld description freezes the moment you export it. The next bracket revision leaves half your audience on a stale guide.

Prefer a public page that can change when geometry changes: same link, updated step, updated BOM line. Keep the STLs on Printables, GitHub, or wherever your community already downloads files. The instructions layer is what search engines and makerspaces bookmark.

If you only have time for one figure per step, show the part being added and the fasteners for that step. Consistency of camera beats cinematic variety.

Hand the draft to someone who did not design it. Watch without coaching. Every pause is a missing quantity, a missing orientation cue, or a step that tried to do two jobs.

Then publish. Drafts that live forever in private folders do not help the maker world. A clear BOM, step-scoped hardware, print notes, and a stable manual URL will outperform a prettier zip every time.

On Slicebuild that structure is the default: STLs, vitamins on each step, a rolled-up bill of materials, and a public guide builders can open without an account. Use any CAD or slicer you like for the visuals — finish the instruction layer where the build actually lives.