The STEP and the drawing disagree: which does the shop build to?
It depends on the shop, so don't leave it to them. Many shops program from the STEP and inspect to the drawing, so a mismatch can make parts that fail inspection or pass the wrong check. Export both files from the same revision, put the revision in both file names, and add a note saying which one governs.
Why it happens
A drawing and a model start in sync. Then a feature moves late in a revision, someone exports a new STEP for a quote, and the PDF in the folder is still from last week. Or the drawing is updated, and the STEP attached to the PO is the one from the first RFQ.
Shops then resolve the conflict their own way. One online manufacturing service says that it generally uses the 3D model as the basis for making the part and inspects against the 2D drawing; when the two disagree during production, it puts the order on hold and asks the customer, who pays for any extra work (Fictiv). Another notes that there are times when the drawing may supersede what comes from the CAD model (Xometry). A local machine shop may do it differently again. Engineers on forums don't agree either.
Worked example: 150-0336 sensor mounting plate
Rev C of the plate moves one M4 tapped hole 6 mm to clear a cable. The engineer updates the model, exports a STEP for the quote, and gets pulled into a review before updating the drawing. The PO goes out with both files:
150-0336.step exported after the hole moved (Rev C geometry)
150-0336.pdf still the Rev B drawing
The shop programs from the STEP and inspects to the PDF. The hole is "wrong" on every part, by the drawing. Production stops while everyone works out which file was right.
Step 1: Export both from the same saved revision. At release, save the drawing and model, then export the PDF and STEP together, in the same sitting, from the released files.
Step 2: Put the revision in both file names.
/150-0336 Sensor mounting plate/
/released/
150-0336_RevC.pdf
150-0336_RevC.step
A file called "150-0336.step" says nothing about which revision it is.
Step 3: Say which file governs. Add a note to the drawing. A common form is: "In case of conflict, this drawing governs. 3D model 150-0336_RevC.step supplied for reference geometry." If your team uses the model as the authority instead, say that, and say what the drawing is for.
Step 4: Name both on the PO. List the drawing and the STEP by file name and revision on the PO line, as in How do I make sure my supplier builds the right revision?
Step 5: Ask the shop how they work. One question at quote time saves an argument later: "Do you program from the model or the drawing, and what do you inspect to?"
Ways to set precedence
| Approach | Good for | Main risk |
|---|---|---|
| Drawing governs, model for reference | Most small teams; tolerances live on the drawing | The model must still match, or the shop programs from wrong geometry |
| Model governs, drawing for critical tolerances | Complex surfaces that the drawing can't fully dimension | Untoleranced features fall back to the shop's assumptions |
| No statement | Nothing | Each shop decides for itself, differently |
| Send only the drawing | Simple parts a shop can program from dimensions | Slower to quote; more room for interpretation |
| Send only the model | Quick quotes for prototype parts | No tolerances, finish or notes unless written somewhere else |
When this stops working
- Mismatches turn up at the shop rather than before files leave.
- Several people export files, and nobody knows which export went out.
- Shops ask "which one is right?" more than once a month.
- The released folder holds files without a revision in the name.
At that point the drawing and model need to be released together as one revision, so they can't drift apart. Checking that a new revision's drawing lists every change helps too: How do I check that a drawing revision lists every change?
FAQ
Which takes precedence, the 3D model or the drawing?
Whichever your drawing or PO says. If it says nothing, each shop decides for itself, and online manufacturing services publish different rules, so always state it.
Should I send a STEP file with every drawing?
Usually yes, because many shops program from a model. Export it from the same revision as the drawing and put the revision in its file name.
What should the precedence note on the drawing say?
That the drawing governs in case of conflict, or that the model does, and which file and revision it refers to. Keep it to one or two lines in the drawing notes.
What should I do if a shop finds a mismatch?
Answer in writing, with corrected files from the same revision, and log what happened. If parts were already made, treat them as nonconforming and decide what happens to them.
Internal links
- How do I make sure my supplier builds the right revision?
- How do I check that a drawing revision lists every change?
- What do I do when a supplier's parts don't match the drawing?
Sources
- Fictiv, Discrepancies between 2D drawings and 3D models: https://www.fictiv.com/help/production-and-fulfillment/discrepancies-between-2d-drawings-and-3d-models
- Xometry, Model, drawing and quote discrepancies: https://community.xometry.com/kb/articles/690-model-drawing-and-quote-discrepancies
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