CNC Quality & Inspection Glossary

Accuracy is how close a measurement is to the true value. A machine with high accuracy makes parts very close to the target size. Do not confuse accuracy with precision.

Acceptance criteria are the rules that decide if a part passes or fails. They list the measurements, surface finish, and other features the part must meet. If a part meets all criteria, it is accepted.

AS9100 is a quality standard for the aerospace industry. It adds extra rules on top of ISO 9001. Shops with AS9100 must track parts, control processes, and keep detailed records.

An audit is a formal review of a quality system. An auditor checks if the shop follows its own procedures and meets the required standards. Audits can be internal or done by a customer or outside body.

A bilateral tolerance allows a dimension to vary in both directions from the nominal value. For example, 1.000 ± 0.005 means the part can be between 0.995 and 1.005. Most CNC tolerances are bilateral.

A bore gauge measures the inside diameter of a hole. It has a spring-loaded contact that presses against the bore wall. The gauge reads how big the hole is so you can compare it to the drawing.

Calibration adjusts a measuring tool so it reads correctly. You compare the tool to a known standard and fix any error. Shops must calibrate tools on a set schedule to keep measurements trustworthy.

A CMM is a machine that measures parts by touching them with a probe. The probe records X, Y, and Z positions. A computer calculates the part dimensions and compares them to the drawing. CMMs can measure very tight tolerances.

A CoC is a document that states the parts meet the customer requirements. The shop signs it to certify the parts were made and inspected correctly. Many contracts require a CoC with each shipment.

Concentricity measures how well the center of one round feature lines up with the center of another. If two holes share the same center axis, they are concentric. It is a GD&T control used on shafts and bores.

A control chart tracks measurements over time. It shows if a process stays within limits or starts to drift. Operators use control charts to catch problems before parts go out of spec.

A corrective action is a fix for a quality problem. It finds the root cause and changes the process so the problem does not happen again. Many quality systems require a formal corrective action report (CAR).

Cpk is a number that shows how well a process stays within spec limits. A Cpk of 1.0 means the process barely fits. A Cpk of 1.33 or higher is considered capable. Higher is better.

A datum is a reference point, line, or surface on a part. All measurements are taken from the datum. In GD&T, datums are labeled A, B, C and set the order of how a part is held.

A deviation is the difference between the actual measurement and the target value. A small deviation means the part is close to spec. A deviation beyond the tolerance means the part fails.

Disposition is the decision made about a part after inspection. The three common choices are accept, reject, or rework. If a part fails, the disposition determines what happens next.

An FAI checks the first part from a new setup or process. You measure every dimension on the drawing to verify the process works. In aerospace, FAI follows the AS9102 standard.

Flatness is a GD&T control that limits how much a surface can curve or wave. A flatness of 0.001 means the entire surface must fit between two parallel planes 0.001 apart. No datum is needed for flatness.

A feature is any physical part of a component that can be measured. Holes, slots, surfaces, and edges are all features. Inspectors check features against the drawing specs.

Gage R&R stands for Gage Repeatability and Reproducibility. It is a study that checks if a measurement system gives consistent results. It tests both the tool and the people using it.

GD&T is a system of symbols on engineering drawings. These symbols tell the machinist how flat, round, or straight a feature must be. GD&T goes beyond simple length and width. It controls form, orientation, and location.

A go/no-go gauge is a simple tool that checks if a part is within tolerance. The "go" end should fit. The "no-go" end should not fit. If both pass, the feature is in spec. Fast and easy to use on the shop floor.

A hardness test measures how hard a material is. Common methods are Rockwell, Brinell, and Vickers. The test presses a ball or point into the surface and measures the indent. Harder materials resist the indent more.

A heat lot is a batch of metal made in one furnace run. All the material in a heat lot has the same chemistry. The heat lot number lets you trace a part back to the original batch of metal.

In-process inspection checks parts while they are still being made. It catches problems early before the whole batch is finished. Operators may measure key dimensions after each setup or at set intervals.

An inspection report is a document that lists the results of a quality check. It shows what was measured, the spec, the actual value, and whether each result passed or failed.

ISO 9001 is an international quality management standard. It requires a shop to document its processes, train employees, and track customer satisfaction. Most CNC shops start with ISO 9001 before adding industry-specific standards.

ITAR stands for International Traffic in Arms Regulations. It controls the export of defense items. Shops that handle ITAR parts must restrict access to U.S. persons only and follow strict security rules.

The LSL is the smallest value a measurement can be and still pass. If a part measures below the LSL, it is out of spec. The LSL and USL together define the acceptable range.

A lot is a group of parts made together under the same conditions. Inspectors often check a sample from the lot instead of every single part. The lot number helps trace parts back to when they were made.

A micrometer is a hand tool that measures very small distances. It can read to 0.0001 inches. You place the part between the anvil and spindle and turn the thimble until it is snug.

An MTR lists the chemical makeup and physical properties of a batch of metal. It proves the material meets the required spec. Aerospace and defense jobs almost always require an MTR.

MSA studies how much error comes from the measurement system itself. It checks the tools, the method, and the people. Gage R&R is one type of MSA. A good MSA shows the measurement system is reliable.

NADCAP is an accreditation for special processes like heat treating, plating, and NDT. Aerospace customers often require NADCAP-certified vendors. The audits are very detailed and happen on a regular schedule.

An NCR documents a part or process that does not meet requirements. It describes the problem, the root cause, and the corrective action. NCRs help track and reduce quality issues over time.

NDT checks for defects without damaging the part. Common methods include X-ray, ultrasonic, dye penetrant, and magnetic particle testing. NDT finds cracks, voids, and other hidden flaws.

NIST is the National Institute of Standards and Technology. It sets the standards that calibration labs use. When a tool is NIST-traceable, its accuracy can be traced back to a national reference standard.

The nominal value is the target dimension on a drawing. It is the ideal size the part should be. The actual part may vary from nominal within the allowed tolerance range.

An OGP machine uses optics and light to measure part features without touching them. It takes a magnified image and software measures distances and angles. Good for small or delicate parts.

Out of spec means a measurement falls outside the allowed range. The part does not meet the drawing requirements. An OOS part triggers an NCR and a disposition decision.

Parallelism is a GD&T control that limits how much one surface can tilt relative to a datum surface. A parallelism of 0.002 means the surfaces must stay within 0.002 of being perfectly parallel.

Perpendicularity limits how much a surface can lean away from 90 degrees relative to a datum. It is a GD&T orientation control used on walls, bores, and pins.

PPAP is a process used in automotive to approve new parts or changes. The supplier submits samples, data, and documents to prove the process can make good parts consistently. There are five levels of PPAP submission.

Precision means getting the same result each time you measure or make a part. A machine can be precise but not accurate. You need both precision and accuracy for good quality.

A probe is a touch sensor used on CMMs and CNC machines. It contacts the part surface and records the position. Probes can be used for in-process checks or full inspection after machining.

Repeatability is how consistent a tool or process is when the same person uses it the same way. If you measure the same part 10 times and get the same number, the tool has good repeatability.

Reproducibility is how consistent a measurement is when different people use the same tool. Good reproducibility means any trained operator gets the same result.

Root cause analysis finds the true reason a problem happened. It goes deeper than the symptom. Common methods include 5 Whys and fishbone diagrams. Fixing the root cause prevents the problem from coming back.

Runout measures how much a surface wobbles when the part rotates around a datum axis. It combines roundness and concentricity into one check. Runout is measured with a dial indicator while the part spins.

A sampling plan sets how many parts to check from a batch. Instead of inspecting every part, you check a sample. AQL (Acceptable Quality Level) tables tell you how many to check based on batch size and risk.

A specification (spec) defines what a part must look like and how it must perform. It includes dimensions, tolerances, materials, surface finish, and any special requirements. The drawing and purchase order together form the spec.

SPC uses statistics to monitor a process. Operators track measurements on control charts and look for trends. If the data starts to drift, they adjust the process before parts go out of spec.

Surface finish describes how smooth or rough a surface is. Ra is the most common unit. It measures the average roughness in microinches. A lower Ra number means a smoother surface. Typical CNC finishes range from 16 to 125 Ra.

A surface plate is a very flat granite or cast iron block used as a reference for measurements. Inspectors place parts on the surface plate and use height gauges or indicators to check dimensions.

Tolerance is the allowed variation from the target dimension. A tolerance of ±0.005 means the part can be 0.005 above or below the nominal size. Tighter tolerances cost more because they need more care and better tools.

Traceability means you can track a part back to its raw material, machine, operator, and date. Lot numbers, serial numbers, and heat lot numbers make this possible. Aerospace and medical parts almost always need full traceability.

True position is a GD&T callout that controls where a feature (usually a hole) is located. It measures the actual center versus the ideal center from the datums. True position uses a cylindrical tolerance zone, which gives more room than ± X/Y tolerances.

A unilateral tolerance allows variation in only one direction from the nominal. For example, 1.000 +0.000 / -0.005 means the part can be 1.000 or smaller, but not larger. Used when a feature must not exceed a limit.

The USL is the largest value a measurement can be and still pass. It is the top of the acceptable range. Together with the LSL, it defines the tolerance window for a feature.

A vernier caliper measures outer dimensions, inner dimensions, and depth. It has jaws that slide along a beam. Digital calipers show the reading on a screen. Resolution is typically 0.001 inches.

A vision system uses cameras and software to inspect parts. It checks dimensions, surface defects, and label presence. Vision systems are fast and can inspect 100% of parts on the line.

Visual inspection is checking a part by eye. The inspector looks for scratches, burrs, cracks, and other visible defects. It is often the first step in any quality check. Good lighting and magnification help.