How to make assembly instructions from CAD

Good assembly instructions are written for someone standing at a bench with a box of parts, not for the engineer who designed it. Keep that person in mind at every step below.

1. Start from the parts list

Export or copy the bill of materials from the CAD assembly: every part, how many, and its part number. Repeated parts are where instructions go wrong ("which of the three screw lengths?"), so check the counts against the model tree, not memory.

• Give every part the name the reader will use, not the CAD file name. "M3 × 8 screw" beats BTS2_M2_6X8:1.
• Group fasteners by size. Many manuals print fasteners at 1:1 so the reader can hold a screw against the page.
• Decide which sub-assemblies arrive already built. Those are one part in the instructions.

2. Work out the order

The order in the CAD tree is the order someone modelled it in, which is rarely the order anyone can build it in. Two ways to find a buildable order:

• Take it apart in the CAD. Hide parts one at a time from the outside in. Reverse that list and you have a candidate build order.
• Build one. If a prototype exists, assemble it and write down each action as you do it. This catches the screw you can't reach once the cover is on.

3. One action per step

A step is one thing the reader does: fit this bracket with these four screws. If a step needs the words "and then", split it. A step that adds twelve identical parts in the same way is still one step.

4. One drawing per step, showing what changed

• Keep the viewpoint the same from step to step unless the reader has to turn the assembly over. Then show it turned over.
• Show the part being added moving into place, along the axis it fits on, with a thin trail line back to where it goes.
• Make the new parts stand out: a heavier line, or a single accent colour, with the existing assembly in a lighter weight.
• Leave out what doesn't matter for the step. Hidden lines usually don't help here.

Both FreeCAD and Fusion can keep several exploded arrangements of one assembly, which suits a drawing per step. In FreeCAD 1.0 one assembly's exploded views container holds one or more exploded views; in Fusion you create a storyboard per step:

5. Words: short, in the imperative, next to the picture

• Start with the verb: "Fit", "Tighten", "Push until it clicks".
• Give quantities and torques as numbers, not "firmly".
• Put a warning before the step it applies to, not after.

If the product is machinery sold in the EU, the Machinery Regulation says what the instructions must contain, and assembly instructions with drawings are on that list:

For the general craft of instructions, the horizontal standard is IEC/IEEE 82079-1:2019. Its abstract says the 2019 edition gives consideration to instructions for self-assembly products. What the EU Machinery Regulation changes about instruction manuals covers the regulation in more detail.

6. Test it on someone who hasn't seen it

Give the instructions and a box of parts to someone who didn't design it and watch without helping. Every place they hesitate is a step to fix. Then keep the drawings tied to the CAD: when a part changes, you want to know which steps show it.