Write a brief a print shop can use
Most replacement-part projects stall on a vague brief, not on CAD. Eight fields fix that.
A brief is not a description of the object. It is the set of decisions someone needs in order to make a part that works. If you would hand the job to a stranger, the brief has to survive without you in the room.
Why briefs fail
If the interface is not described, the part will look right and not fit. Measuring the mating part is the step that prevents that, and it takes ten minutes. Three failure modes cover almost everything:
- It describes appearance only. "A black knob, like the photo" says nothing about the shaft, the load or the clearance.
- It hides the hard part. The interface is difficult; the outer shape is easy. Briefs that dwell on the shape and skip the interface produce parts that look right and do not fit.
- It states a solution, not a requirement. "Make it 3 mm thick" is a guess about a decision that belongs to whoever understands the load.
The eight fields
- Object and part. What is it, and which part of it is missing? "Knob on a 1970s teak sideboard, left drawer."
- Function. What must the part do? Pull, turn, cover, clamp, locate, hide a screw.
- Loads and environment. Hand force or body weight? Indoors or outdoors? Near heat, water, sunlight, chemicals?
- Interface. Every dimension where the part meets something else, with the datum for each: shaft Ø, flat, thread, hole spacing, panel thickness.
- Envelope. Overall width, depth and height, plus any collision limits.
- Material and finish. A requirement (must not soften at 70 °C) or a preference (matte black), stated as one or the other.
- Tolerances that matter. Which fits are critical and which are cosmetic. "Press fit on the shaft, 0.1 mm interference" is a brief; "exact fit" is not.
- Quantity, deadline and budget. One part or ten? This week or next month? It changes the method.
Before and after
Before
"Small black knob for a chest of drawers, about 3 cm. Similar to the photo. Can you print it?"
After
"Knob for a teak sideboard drawer, one piece, hand-pulled only. Envelope 32 × 32 × 22 mm, no collision within 6 mm of the drawer edge. M4 pan-head screw through the drawer face into a counterbored hole Ø 8.4 × 6 mm deep in the knob; the screw must not protrude past the drawer face. Indoor use, no heat or sunlight. Matte black PLA is acceptable if it holds at room temperature; PETG preferred. Print the bore slightly undersized and ream to 4.1 mm. One piece, no deadline, budget around the cost of a spare."
The second version can be built without a conversation. That is the test.
What the brief cannot fix
A brief does not rescue a part whose function is unknown. If the original came out of a mechanism you have not seen, a list of dimensions will produce something that looks right and jams. Say so in the brief: "I have not seen the inside; the part must clear 4 mm in every direction" is a constraint a designer can work with, and it is more useful than a guess dressed as a measurement.
The same applies to load. "It broke once" is not a load case. "It broke when someone leaned on it" is closer. "It must survive a 5 kg bag resting on it for a year" is a specification.
Tolerances in plain language
A tolerance is not a quality statement; it is a decision about what is allowed to vary.
- Press fit: the shaft is slightly larger than the hole and the parts stay together by friction. On printed parts, an interference of about 0.1 mm is a reasonable starting point for a small plastic shaft.
- Sliding or rotating fit: roughly 0.2-0.4 mm total clearance on FDM, checked on the first print.
- Cosmetic fit: a millimetre, sometimes two, if nothing collides.
Say which of these applies where. A single "±0.1 mm everywhere" note is usually an expensive way of saying "I have not decided".
Printed holes come out undersized by a few tenths of a millimetre on most FDM setups. Deciding in advance which holes get drilled or reamed is part of the brief.
How to send a brief
One message, one file, no thread to reconstruct. A brief that arrives as six photos and a voice note costs the maker an hour before they can start, and that hour is usually charged or absorbed into a worse part.
- The measurements, in millimetres, each with its datum.
- The interface: shaft, flat, thread, hole spacing, panel thickness.
- The environment and the load in one sentence each.
- Photos of the part in place, plus one from the interface side.
- What you will do with the result: fit it yourself, hand it to a print shop, print ten of them.
If the maker has to ask what the part is for, the brief was not finished. That question is the only test that matters, and you can run it before you send anything.
What to attach
- Photos with a ruler in the same plane as the part, including one from the interface side.
- The mating part or its dimensions: the shaft, the panel, the screw.
- The broken pieces, taped together if possible. Fracture surfaces tell you where the load went.
- A sketch with numbers, even a bad one. A labelled sketch beats a beautiful render for a maker.
Do not attach anything you do not have the right to share. Other people's designs are not yours to reproduce.
A short checklist
- Can someone build this without asking me a question?
- Does every number have a datum?
- Is the interface described in more detail than the appearance?
- Is the environment stated (heat, water, sun, chemicals)?
- Are the critical fits identified, with numbers?
- Is the quantity and deadline stated?
Our project brief builder covers part type, function, three overall dimensions and a local photo reference, and gives you a JSON file you can keep or hand on. It does not generate a model yet. That is the next stage of the product.
Questions
What should a 3D printing brief include?
The part and the object it belongs to, what the part must do, the loads and environment, every interface dimension with its datum, the overall envelope, the material requirement, the fits that matter, and quantity and deadline.
Can a print shop design the part for me?
Some do, many do not: most print shops expect a file. If you need design help, ask before you send dimensions, and expect the brief to matter even more, because the designer cannot see the object.
How do I describe a fit without engineering terms?
Describe the behaviour: "it must stay on the shaft without glue", "it must turn freely but not wobble", "it must not rattle". A good designer converts behaviour into numbers, and will confirm them with you.
Written for the Refabrication preview. Sizing, material and safety notes here are general guidance, not engineering or compliance advice. Check any part against your own equipment, loads and local requirements before you print or install it.