Geometric dimensioning and tolerancing (GD&T) is the language shared by design, production and metrology: applied consistently, a part assembles and works as intended; misread, it produces conflicting measurements, scrap and repeated exchanges between teams. Metrologic DCS notes that as product manufacturing information (PMI) is embedded in CAD files more often and standards keep changing, the openings for error multiply. Its four practices run from reading design intent to the reports given to the shop floor.
Start from the PMI
PMI must first be decoded — which feature is called out, which references apply, which tolerance is specified — a manual step where a mis-keyed value or a misread datum can send a part to inspection against a requirement that was never specified. Metrologic recommends software that reads PMI directly rather than having tolerances re-entered, so features and evaluations come from the digital part definition: no re-keying, fewer interpretation gaps and time saved at the start of program preparation.
Build the datum reference frame automatically
The datum reference frame (DRF), assembled from reference features such as A, B and C, fixes where measurement begins and which degrees of freedom the evaluation constrains. Metrologic warns that a frame built in the wrong order, or with poorly chosen degrees of freedom, can leave the same part conforming for one operator and non-conforming for another — a major source of GD&T misinterpretation. Deriving the frame from the drawing rather than from operator choice, as Metrolog does when a GD&T is evaluated, is presented as the route to repeatability across operators, machines and sites.
Reuse the references already in the model
References A, B and C are already labelled in the CAD model by the design office; rebuilding them in program preparation wastes time, invites discrepancy and forces rework whenever the definition changes, and on heavily annotated parts tracking what has been handled is difficult. Metrologic advises starting from the PMI-derived frames and keeping progress visible; its Scan GDT function reads the decoded PMI and assigns a status so decoding and measurement can both be checked.
Report for the reader, not just pass or fail
A bare pass-or-fail verdict tells the shop floor nothing about which defect it faces, Metrologic argues, leaving operators to adjust by trial and error and generating scrap and correction loops. Reports should suit their audience: numeric tables for quality control, a go/no-go sheet for a quick decision and a decomposition analysis for the operator. Metrolog splits the deviation of an evaluated feature into form, size, position and orientation alongside the overall figure, showing which type dominates and therefore what to correct first.