Last updated: 2026-09-04 · 6 min read

How Do You Read a T1 Dimensional Report? — Turning the First CMM Data Into Accept-or-Correct Decisions

A T1 dimensional report is the first physical evidence that the mold cuts the part correctly. It lists every critical dimension from the drawing alongside the CMM-measured actual value and the deviation — and the decision it drives is binary: accept, correct or re-cut. Reading it is not about checking every number; it is about spotting the 3 patterns that tell you whether the mold is right, fixable, or fundamentally wrong.

Key Facts

A T1 report lists nominal, tolerance, measured value and deviation for every dimension on the drawing — the deviation column, not the measured column, is where the decision lives.

Three patterns decide the outcome: all within tolerance (accept), systematic offset on a group of dimensions (correct the mold), or random scatter across unrelated features (process or measurement problem, not a mold problem).

The report is only as good as the CMM setup — datum alignment, temperature equilibrium and probe calibration must be verified before any deviation number is trusted.

Parting line — The line where the two mold halves (A-side and B-side) meet. Parting line location affects: gate placement, ejector pin marks, witness lines, and cosmetic appearance. A well-designed parting line minimizes visible witness marks. The related guide covers adjacent topics.

What Does Each Column in the Report Actually Mean?

Six columns, each with a specific role:

Column	What It Shows	What to Look For
Dimension ID	Drawing callout reference	Matches the drawing — if the ID is wrong, the data is suspect
Nominal	The target dimension from the drawing	Should match the drawing exactly
Tolerance	±value or GD&T callout	Confirms the acceptance criterion
Measured	CMM reading for that feature	Raw data — not the decision column
Deviation	Measured minus nominal	The decision column — compared against tolerance
Pass / Fail	Whether deviation is within tolerance	Summary — verify it matches the math

The deviation column is where the decision lives, not the measured column. A measured value of 25.08 mm means nothing without knowing the nominal is 25.00 mm with a tolerance of ±0.10 mm — which makes the deviation +0.08 mm and the result a pass. Reading the measured column without the deviation is how mistakes happen, especially on dimensions where the nominal is not a round number.

GD&T callouts add complexity. A position tolerance is not a simple ± value — it defines a cylindrical zone within which the feature axis must fall, and the reported deviation is the radial distance from true position. A profile tolerance is similar — the deviation is the distance from the true surface profile. If the report shows a simple ± deviation for a position or profile callout, the report format is wrong and the data should not be trusted.

How Do You Read the Patterns and Decide What to Do?

Three patterns, three decisions:

All dimensions within tolerance — accept. If every dimension passes, the mold cuts the part correctly at T1. The decision is to accept the tool and proceed to production sampling. Verify that the report includes the datum alignment used, the CMM identification and the temperature at measurement — without these, the pass is not documented in a way that holds up in a dispute.
A group of dimensions offset in the same direction — correct the mold. If several dimensions on the same cavity or the same core are all high or all low by a similar amount, the mold has a systematic error — the cavity was cut deep, the core was cut short, or the datum surface was set wrong. The fix is predictable: grind the parting surface, re-cut the affected cavity, or adjust the insert. The correction amount is the average deviation of the group, not the worst individual number.
Random scatter across unrelated features — process or measurement problem. If some dimensions are high and others low with no pattern, the cause is not a systematic mold error but a process variation — uneven cooling, inconsistent packing, or a CMM alignment that did not seat the part consistently. The fix is not to adjust the mold but to repeat the trial under stable conditions and re-measure. If the scatter persists across multiple shots, the CMM setup is the first thing to check — datum alignment, temperature equilibrium and probe calibration.

Cycle time — The total time for one injection molding cycle: injection + packing + cooling + mold open/close + ejection. Typical range: 15–60 seconds. Cycle time determines production rate and cost per part — reducing cycle by 5 seconds saves 10–15% annual production cost. The related guide covers adjacent topics.

Reading a T1 report is not about checking every number — it is about spotting the three patterns that tell you whether the mold is right, fixable, or fundamentally wrong: all pass, systematic offset, or random scatter.

Key parameter identified — critical variable confirmed against spec.

Tolerance within achievable range — commercial ±0.15mm or tighter as needed.

Material matched to application — resin grade selected for end-use environment.

Process window validated at T1 — parameters locked after first trial.

Inspection method defined — CMM, visual, or functional test specified.

Frequently Asked Questions

What if one dimension fails but everything else passes?

A single out-of-tolerance dimension is a local correction, not a mold rejection. Check whether the failing dimension is on the cavity side or the core side — if it is on a removable insert, the fix is to replace or adjust the insert without touching the rest of the mold. If it is in the main cavity block, the correction requires welding and re-cutting, which is more expensive but still localized.

Before ordering the correction, verify the CMM measurement by re-probing the feature or measuring it with a calibrated hand tool. A single fail that does not reproduce on a second measurement is a measurement error, not a mold error. Document the re-measurement and the decision together.

How do I know if the CMM data is trustworthy?

Three prerequisites must be visible on the report: the datum alignment used for the measurement (matching the drawing datums), the CMM identification and last calibration date, and the part temperature at measurement.
If any of these are missing, the data should not be trusted for accept-or-reject decisions — a part measured at 40 °C will read differently than one at 23 °C because of thermal expansion, and a CMM aligned to the wrong datums will report every dimension as deviated even if the mold is perfect.

The report should also include the number of parts measured and which cavity each came from. A T1 report based on one part from one cavity is a snapshot, not a validation — it tells you that cavity is close, but says nothing about the other cavities or shot-to-shot consistency.

What is the difference between a T1 report and a first article inspection?

A T1 report is a snapshot — a few parts from the first trial, measured to confirm the mold cuts close to nominal. A first article inspection (FAI) is a comprehensive validation — every dimension measured on multiple parts from multiple cavities over a production-length run, with full statistical analysis. The T1 report answers “is the mold in the right ballpark”; the FAI answers “is the process capable in production.” The T1 report is the gate to proceeding; the FAI is the gate to shipping.

Key Takeaways

The deviation column — not the measured column — is where the accept-or-correct decision lives; always compare deviation against tolerance, not raw numbers against expectations.
Three patterns decide the outcome: all pass (accept), systematic offset on a group (correct the mold by the average deviation), or random scatter (process or measurement problem — repeat the trial before touching the mold).
The report is only trustworthy if it shows datum alignment, CMM calibration, part temperature and multi-cavity data — without these, the numbers should not drive a mold modification.

A T1 dimensional report is a decision document, not a data dump.
Read the deviation column, spot the pattern — all pass, systematic offset, or random scatter — and let the pattern drive the decision. The mold that needs a 0.05 mm grind on one insert is a different problem from the mold that needs a full re-cut, and the report tells you which one you have.