Model Based Definition: What a Machine Shop Needs Beyond the 3D Model
You send a STEP file and a PDF: the model carries geometry, the PDF carries tolerances. It works because each document has a clear job.
Model based definition collapses that split. The tolerances, datums, surface textures and notes live in the 3D model itself, attached to faces and features, and the 2D sheet becomes optional or disappears. For a machine shop this is either a clean improvement or a slow-motion quality disaster, depending on how the data is authored and transmitted. Here is what actually reaches the shop floor, and what to check before you release a model-only package.
What MBD really means on the shop floor
There are two ways to attach product and manufacturing information (PMI) to a model, and the difference decides whether a shop can use it automatically.
Graphic PMI is annotation placed in 3D space, positioned by hand, readable by a human who rotates the model. It is fine for a person, useless for software.
Semantic PMI is annotation stored as structured data: a tolerance zone with a type, a value, a datum reference, and a link to the faces it applies to. CAM and inspection software can read it without a human interpreting a leader line.
The relevant standards are ASME Y14.41 in the US and ISO 16792 internationally, with STEP AP242 (ISO 10303-242) as the exchange format that can carry semantic PMI. A model built that way is genuinely machine-readable. A model with graphic PMI exported to a neutral format is a 3D picture of a drawing, and the shop will print it, measure it and retype it — which is where the efficiency claim quietly dies.
How a shop consumes it
PMI is used at three points.
Programming. CAM software imports the model and uses tolerances to set stock allowance, choose tools and decide where to leave material for a second operation. Semantic PMI on a bore with a ±0.01 mm tolerance tells the programmer to plan a separate finishing pass. The same bore with the tolerance only on a PDF gets discovered by the operator, at the machine, at 7 pm.
Inspection planning. Model based inspection compares measured point clouds to the nominal model and evaluates tolerances directly. This is where MBD delivers the biggest saving: probing paths can be generated against the model, and the report references the same datum scheme the designer intended. Without semantic PMI, the inspector rebuilds the datum scheme by hand and any misreading is invisible until a part fails at your receiving dock.
Verification and records. A first article on an MBD part should reference model revision and feature ID, not a balloon number. If your quality system expects ballooned drawings, agree up front how the inspection report maps back to your numbering.
See also: Sending CAD Data to a Machine Shop: What to Control and Chamfer vs Fillet: Which Edge Treatment Belongs on the Drawing.
What still belongs on a 2D sheet
Very few shops receive a drawing-free package. Certain information has no natural home in geometry.
| Element | In the 3D model | On the 2D sheet | Why it belongs there |
|---|---|---|---|
| Feature tolerances and datums | Yes, semantic PMI | Optional | The model is the authority; the sheet is a convenience copy |
| General tolerance block | No | Yes | Applies globally, not to a feature |
| Material and condition | No | Yes | Not geometry; must survive any CAD translation |
| Surface finish on non-critical faces | Yes, per face | Yes as a general note | Easier to state once than annotate 40 faces |
| Marking, etching, laser text | Partly | Yes | Font, depth and location are process rules, not geometry |
| Thread callouts | Yes, as modelled or as PMI | Yes | Modelled threads bloat files; callouts are cleaner |
| Heat treatment and finish spec | No | Yes | Process instruction, outside CAD scope |
| Packaging, cleanliness, traceability | No | Yes | Contract terms |
| Critical-to-quality flags | Yes | Yes | Belt and braces on the two or three features that matter |
The practical rule most shops prefer: the model is the authority for geometry and tolerance, the sheet carries process, material, finish and commercial notes. If the two disagree, a written precedence statement should say which wins; without one, the discrepancy is discovered when parts are already made.
Where translation fails
Most MBD problems are not authoring problems. They are translation problems, and they cluster in four places.
PMI dropped entirely. Exporting to AP203 or an older STEP schema strips PMI. The geometry arrives perfect and the tolerances vanish. A shop that receives the file will machine to general tolerance and be technically compliant with what it received, and commercially exposed to what you meant.
Datum features renamed. Translation tools sometimes rename or renumber datum identifiers when the assembly structure changes. Datum A becomes Datum A1 on three faces. The inspector notices; the CAM programmer does not.
Tolerance zone modifiers lost. MMC (maximum material condition) and projected tolerance zones are exactly the modifiers that translation engines handle worst, and exactly the ones that affect whether a hole is accepted. If your design uses MMC on a pattern of holes, verify it survived.
Units and scale. A model in inches imported as millimetres is obvious. A scale factor applied twice is not obvious until the first cut.
A practical mitigation: ask the shop which CAD and CAM systems it uses, export native if an NDA is in place, and always send STEP AP242 alongside. Then ask for written confirmation of what PMI was received. Our intake accepts files up to 50 MB and confirms which tolerances we can read from the model.
What to put in the RFQ package
For an MBD part, the package that produces a fast, accurate quote contains five things:
- The native CAD file plus a STEP AP242 export, both at the same revision.
- A single-page PDF that states revision, material, finish, general tolerance, critical features and precedence.
- The datum scheme in writing, especially if it differs from the model's default.
- Any feature you consider critical to quality, flagged in both model and PDF.
- The inspection expectation: full dimensional report, first article to AS9102, or a sampling plan.
The fifth item is the one most often left out and the one that moves price most. A ±0.01 mm profile on a large thin aluminium face may need different fixturing and a temperature-controlled inspection step. If the model says it and the RFQ does not, the shop may quote a process that cannot deliver it.
If you want to see how we handle model-based packages and finishing callouts, our capabilities page covers the formats we accept, and our materials page lists the grades and conditions we hold in stock. Quotes come back within 24 hours when the package is complete, which is the practical argument for getting the five items above into one folder.
Frequently asked questions
Do I still need to send a 2D drawing with an MBD model?
In most cases yes, as a one-page control document for material, finish, general tolerance, marking and precedence. It need not duplicate feature tolerances that are semantic in the model. Customers who genuinely ship drawing-free usually have a mature PLM setup and a supplier trained on their standard.
Which STEP version carries PMI?
AP242 is the schema designed to carry semantic PMI, and the one to request. AP203 and AP214 carry geometry and assembly structure but not semantic tolerance data. Even with AP242, survival depends on exporter settings, so ask for the export profile rather than assuming.
Is a 3D PDF an acceptable substitute?
It is a reasonable human-readable backup, but it is not machine-readable. A shop receiving only a 3D PDF still types tolerances into CAM and inspection software by hand, so you keep the labour cost and the transcription risk. Use it as a convenience copy, not the controlled dataset.
How do we handle a revision change on an MBD part?
Change the model revision and the PDF together, and state the effective date and which purchase orders are affected. Because the model is the authority, a stale model in a supplier's CAM folder is a real risk. Ask how long old revisions are retained and whether the ERP blocks release against a superseded model.
Related reading
Send a drawing for a quote and DFM review.
STEP, IGES, X_T, PDF or DWG up to 50 MB. Tell us quantity and finish, and the quote comes back with tolerance and process notes, not just a price.