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newSchoolWorks.com › VoTechWorks › Manufacturing & Marine › CNC Machining › Metrology: Precision Measurement for CNC Machining › Lesson 12
MFG-CNC14-07 · Lesson 12 · 20 clock hours · lab

Calibration Awareness, Measurement System Analysis, and Capstone Inspection Exercise

Objective. Students demonstrate the complete shop measurement workflow—instrument selection, zeroing and calibration check, measurement, SPC data entry, and inspection report—on an unknown part under timed, exam-like conditions.

⚠ Pending professional review

This lesson, its worked examples and its practice questions has not yet been verified by a licensed engineer or certified journey-level tradesperson or by a certification or approval body. It is a study draft and may contain errors. Do not use it as instructions for work on a job site or on live equipment. Always follow your instructor, your supervising professional, and current official references and regulations.

VoTechWorks program approval and accreditation are still in progress. Credentials are issued only by the certifying bodies named in each program, after their own exams.

Calibration Awareness, Measurement System Analysis, and Capstone Inspection Exercise

This lab pulls together every metrology skill you have practiced into one timed, exam-like workflow. By the end you will have selected and zeroed instruments, measured an unknown part against its drawing, entered data into a Statistical Process Control (SPC) template, calculated a Gage Repeatability and Reproducibility (%GR&R) result, and handled an out-of-calibration instrument correctly. Each of these steps mirrors what you will perform on the shop floor and what NIMS evaluators expect.

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Why Calibration Matters

A measurement is only as trustworthy as the instrument making it. Every gage in a professional shop is part of a gage recall system: a documented schedule that assigns a calibration interval to each instrument, records who calibrated it, and flags instruments whose calibration has expired. The calibration due date is printed on a tag attached to the instrument. When you pick up a micrometer, the first thing you check—before you zero it, before you touch the part—is that tag.

If the due date has passed, the instrument is out of tolerance for trust purposes, even if it appears to read correctly. Your immediate responsibilities are:

1. Stop using the instrument.

2. Attach a quarantine label that says something like "DO NOT USE – Calibration Expired."

3. Physically segregate the instrument (a dedicated red bin or locked drawer) so no one else picks it up.

4. Notify the quality or tool-room supervisor and log the action on the gage recall record.

Using an out-of-calibration instrument on production parts can mean those parts must be placed on hold until traceability is re-established. The cost—in time, material, and customer confidence—is significant.

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Zeroing and Reference Checks

Before measuring any part, zero or datum your instrument to a known reference:

Record your zeroing step on the inspection form. If a micrometer refuses to zero correctly even after adjustment, treat it the same as an expired instrument: quarantine and tag.

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Measurement System Analysis and Gage R&R

A measurement system has two sources of variation beyond part-to-part differences:

Gage R&R combines both: GR&R = √(EV² + AV²).

The industry-standard way to express the result is as a percentage of the tolerance (or sometimes the total study variation). The guideline most shops follow:

In today's exercise, three operators each measure ten parts twice. Your worksheet guides you through calculating the range within each operator's repeated readings (for EV) and the range between operator averages (for AV). Follow the template step by step and carry at least four decimal places until the final answer.

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Independent Part Inspection Workflow

When given an unknown part and its drawing, work through these steps in order:

1. Read the drawing first. Identify every dimension callout, tolerance, and surface finish symbol before touching the part or any instrument.

2. Select instruments. Match instrument resolution to tolerance: as a rule of thumb, your instrument's smallest graduation should be no larger than one-tenth of the bilateral tolerance.

3. Inspect the instruments. Check calibration tags; zero or datum each one.

4. Measure systematically. Go callout by callout; record the measured value, the nominal, and the tolerance on your inspection form. Note in/out-of-tolerance disposition for each feature.

5. Complete the inspection report. Include part ID, drawing revision, date, your name, instrument IDs, and a final accept/reject disposition for the part.

6. Present findings. Be prepared to explain why you selected each instrument and how you zeroed it.

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Safety Points for This Lab

Key terms9

Gage Recall SystemA facility's documented program that assigns calibration intervals to each measuring instrument, tracks due dates, and provides a procedure to retrieve and re-calibrate instruments before or when their interval expires.
Calibration TraceabilityAn unbroken chain of comparisons linking a shop instrument's accuracy back to a national or international measurement standard, typically through an accredited calibration laboratory.
Repeatability (EV – Equipment Variation)The variation in repeated measurements made by the same operator using the same instrument on the same part under the same conditions; reflects the gage's inherent precision.
Reproducibility (AV – Appraiser Variation)The variation in measurements made by different operators using the same instrument on the same part; reflects differences in operator technique.
%GR&RGage Repeatability and Reproducibility expressed as a percentage of the part tolerance or total study variation; the primary metric for judging whether a measurement system is adequate for a given tolerance.
QuarantineThe act of physically segregating and labeling a measuring instrument that is out of calibration, damaged, or of unknown status so it cannot be used on production parts until its status is resolved.
Inspection ReportA formal document recording each drawing callout, the measured value, the tolerance, in/out-of-tolerance disposition, instrument used, and a final accept/reject decision for the part.
Measurement System Analysis (MSA)A structured evaluation—including gage R&R studies—of all sources of variation in a measurement process to determine whether the system can reliably distinguish good parts from bad parts within a given tolerance.
Datum (Zeroing)The act of setting an instrument to read a known reference value (often zero) before measurement so that all subsequent readings are referenced to that known standard.

Worked examples4

1. Three operators each measure ten parts twice in a gage R&R study on a diameter with a bilateral tolerance of ±0.002 in (total tolerance = 0.004 in). After completing the worksheet, Equipment Variation (EV) = 0.00072 in and Appraiser Variation (AV) = 0.00054 in. Calculate GR&R and %GR&R.
  1. Step 1 – Combine EV and AV using the Pythagorean formula: GR&R = √(EV² + AV²) = √(0.00072² + 0.00054²).
  2. Step 2 – Square each term: 0.00072² = 0.0000005184; 0.00054² = 0.0000002916.
  3. Step 3 – Sum the squares: 0.0000005184 + 0.0000002916 = 0.0000008100.
  4. Step 4 – Take the square root: √0.0000008100 = 0.000900 in. This is the combined GR&R.
  5. Step 5 – Divide by total tolerance and multiply by 100: %GR&R = (0.000900 ÷ 0.004) × 100 = 22.5%.
  6. Step 6 – Evaluate: 22.5% falls in the 10–30% range, meaning the measurement system may be acceptable depending on the application but warrants investigation into operator technique and gage condition.
Answer: GR&R = 0.000900 in; %GR&R = 22.5% — conditionally acceptable, requires investigation.
2. You are measuring a shaft diameter with a nominal of 1.2500 in and a bilateral tolerance of ±0.0005 in (total tolerance = 0.0010 in). You have a 0–1 in micrometer (0.0001 in resolution) and a 1–2 in micrometer (0.0001 in resolution) available. Which instrument is correct, and does its resolution meet the one-tenth rule?
  1. Step 1 – Determine the measuring range needed: the nominal is 1.2500 in, so the instrument must measure in the 1–2 in range.
  2. Step 2 – Select the 1–2 in outside micrometer (the 0–1 in micrometer physically cannot close on a 1.25 in diameter).
  3. Step 3 – Apply the one-tenth resolution rule: instrument resolution ≤ total tolerance ÷ 10 = 0.0010 ÷ 10 = 0.0001 in.
  4. Step 4 – Compare: the micrometer resolution is 0.0001 in, which equals the required 0.0001 in — the instrument meets the rule exactly and is appropriate.
Answer: Use the 1–2 in outside micrometer; its 0.0001 in resolution satisfies the one-tenth-of-tolerance rule for this callout.
3. During the capstone inspection, your micrometer's calibration tag shows a calibration date of March 10 and an interval of 6 months. Today is October 3 of the same year. Is the instrument in calibration? What do you do?
  1. Step 1 – Calculate the due date: March 10 plus 6 months = September 10.
  2. Step 2 – Compare to today's date: October 3 is after September 10, so the calibration has expired.
  3. Step 3 – Stop using the instrument immediately. Do not take any measurements with it.
  4. Step 4 – Attach a quarantine label ('DO NOT USE – Calibration Expired') to the instrument.
  5. Step 5 – Move the micrometer to the designated quarantine area (red bin or locked drawer).
  6. Step 6 – Notify the quality technician or tool-room supervisor and enter the action in the gage recall log.
Answer: The instrument is 23 days past its calibration due date. Quarantine it, label it, segregate it, and notify the supervisor before using any other instrument for the inspection.
4. A gage R&R study returns EV = 0.00030 in and AV = 0.00000 in (perfect operator agreement) on a feature with a total tolerance of 0.006 in. Calculate %GR&R.
  1. Step 1 – Calculate GR&R: GR&R = √(0.00030² + 0.00000²) = √(0.00000009) = 0.00030 in.
  2. Step 2 – Calculate %GR&R: (0.00030 ÷ 0.006) × 100 = 5.0%.
  3. Step 3 – Evaluate: 5.0% is well below 10%, so the measurement system is fully adequate for this tolerance.
Answer: %GR&R = 5.0% — measurement system is acceptable (below 10% threshold).

The math in this lesson2

Learners compute %GR&R from equipment and appraiser variation against total tolerance in a gage study.
S-ID.2N-Q.3
Brush up in the math halls at newSchoolWorks.com:
In class: Perform a gage R&R (repeatability and reproducibility) exercise on one critical diameter: three operators each measure ten parts twice; enter data into a provided template and calculate %GR&R; discuss whether the measurement system is adequate for the tolerance. · Given an unknown machined part and its drawing, each student independently selects instruments, measures all callout dimensions, completes an inspection report, and presents findings to the instructor—simulating a job-shop quality check. · Review facility gage recall procedures: students trace a mock calibration tag on a micrometer, determine if it is past its calibration due date, and follow the written procedure to quarantine and tag the instrument.
Materials: Unknown machined parts (instructor-selected), complete instrument set (calipers, micrometers, indicators, surface plate), gage R&R worksheet, mock calibration tags and quarantine labels, gage recall procedure SOP (facility or simulated), inspection report forms, NIMS Measurement, Materials & Safety competency checklist.
Check: Complete independent inspection report accepted by instructor; %GR&R calculated correctly; gage recall procedure followed without prompting; pass = inspection report disposition correct, %GR&R within ±5% of instructor value, and recall procedure executed correctly.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

NIMS Measurement, Materials & Safety
1. A micrometer's calibration tag shows it was due for recalibration two weeks ago. What is the correct first action?
NIMS Measurement, Materials & Safety
2. In a gage R&R study, Equipment Variation (EV) = 0.00060 in and Appraiser Variation (AV) = 0.00080 in. What is the combined GR&R value?
NIMS Measurement, Materials & Safety
3. A gage R&R study on a critical bore yields %GR&R = 35%. Which statement best describes the measurement system?
NIMS Measurement, Materials & Safety
4. The one-tenth resolution rule states that an instrument's smallest graduation should be no larger than what fraction of the feature's bilateral tolerance?
NIMS Measurement, Materials & Safety
5. You pick up a 1–2 in outside micrometer to measure a 1.500 in nominal diameter with a ±0.001 in bilateral tolerance. Before measuring the part, what is the correct zeroing reference?
NIMS Measurement, Materials & Safety
6. In the capstone inspection, what information must appear on a completed inspection report? Select the most complete answer.
NIMS Measurement, Materials & Safety
7. Which Personal Protective Equipment (PPE) is most appropriate when handling a freshly machined part with unfinished, potentially burred edges during a metrology lab?
NIMS Measurement, Materials & Safety
8. Reproducibility (Appraiser Variation) in a gage R&R study measures variation caused by which factor?
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