Imagine a scenario where your CMM programmer imports the CAD model, builds the alignment, and starts pulling nominals for a hole pattern controlled by true position. Then they check the drawing and the boxed basic dimension says 2.500. But the model says 2.4985.
Now they have a choice to make. Should they overwrite the CAD nominal with the drawing value, or trust the model and move on? The answer matters more than it looks, because true position is calculated from those basic dimensions.
What a basic dimension is, and why a few ten-thousandths matter
Under ASME Y14.5, a basic dimension is a theoretically exact value. It sits in a rectangular frame on the drawing and carries no tolerance of its own. Its job is to tell the inspector where a feature should be relative to the datum reference frame, so the geometric tolerance in the feature control frame can say how far the real feature is allowed to wander.
That makes basic dimensions the zero point for the measurement. A CMM computing true position compares the measured location against the basic location, then doubles the radial deviation to get the reported position value. If the basic location is wrong by 0.0015 in one direction, the position error picks up as much as 0.003 before the part has done anything wrong. On a pattern toleranced at ⌀.005, that can be the difference between a clean FAIR and a nonconformance.
Which document is the authority?
For most aerospace and defense manufacturers, the 2D drawing is the controlling document. It is the document the customer released, and the one your purchase order and first article should match. The CAD model is a convenience for programming the CMM and the machine, unless something in writing says otherwise.
Some primes have moved to model-based definition, where the annotated 3D model is the authority and any drawing is a reduced or reference document. ASME Y14.41 covers how that data is presented, and in an MBD program a queried model value can be exactly what you are supposed to inspect against.
So before touching a nominal, find out which one your contract names by checking the following:
- The purchase order and its quality clauses or flowdowns
- Title block and general notes on the drawing (look for language that names the model as the authority or calls the drawing reference only)
- The customer's supplier quality manual
- Your own contract review record, if the question was answered at quote time
If none of those settle it, the drawing governs by default and you should be sure to let the customer know about the discrepancy.
Four reasons the model and the drawing drift apart
Once you know which document wins, figure out why they disagree. The cause decides who has to fix it, and it is not always the CMM programmer.
| Cause | What it looks like | What to do |
|---|---|---|
| Display rounding | Drawing shows 2.500, model holds 2.4998 or 2.50004. The difference may be the result of display precision or rounding rather than a different design requirement. | Check the model's true value with a measure or query tool. If the drawing governs, program to the drawing value. |
| Overridden dimension | The drafter typed a value over the associative dimension, so the drawing no longer reflects the geometry. Often shows up as a round number next to an odd model value. | Most CAD packages flag overridden dimensions (a different color or a symbol). Send it back to the design authority; do not pick a side yourself. |
| Model and drawing at different revisions | An engineering change updated one file and not the other. Common when the model is supplied separately from the released drawing. | Compare revision letters on both. Request the matching set before the FAI is performed. |
| The measurement itself | The nominals are fine, but the alignment, datum setup or probe compensation is off, so actuals look like they disagree with the model. | Re-verify the datum alignment against the drawing's datum reference frame before blaming the nominals. |
How this affects first article inspections
A basic dimension mismatch could end up on Form 3 of the AS9102 report, where the customer's reviewer compares your characteristic list and results against the drawing they released.
If the CMM was programmed to a model value that differs from the drawing, the position results on Form 3 could be computed against a location the drawing never specified. Or if someone "fixed" the nominal without telling anyone, there is no record of the decision. When the same part is re-inspected six months later with a fresh program, the results will not match and nobody will know why.
Treat the resolution as part of the FAI record. If the customer confirms which value to use, note it with the FAIR, along with who confirmed it and when. Basic dimensions are verified through the geometric tolerances that reference them, so how they appear on Form 3 varies by customer. Check the customer's requirements before deciding whether to list them as separate characteristics.
A checklist for when CAD and the drawing disagree
- Stop before editing any nominal in the CMM program.
- Confirm the drawing and model revisions match.
- Find which document the contract names as the authority. With no written answer, the drawing governs.
- Query the true model value and compare it to the drawing at full precision, so you can rule rounding in or out.
- Check the drawing for overridden dimensions.
- Re-verify your datum alignment against the drawing's datum reference frame.
- If the difference is real and not rounding, send the question to the customer's design authority in writing.
- Program to the governing value and record the decision, with the customer's response attached, alongside the FAIR.
Where GroundControl fits
GroundControl's FAI Generation Tool starts from the released 2D drawing. It balloons the drawing and builds the Form 3 characteristic list from what is printed there, so the basic dimensions and feature control frames in your FAIR come straight from the controlling document instead of a model export.
