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newSchoolWorks.com › VoTechWorks › Manufacturing & Marine › CNC Machining › Metrology: Precision Measurement for CNC Machining › Lesson 5
MFG-CNC14-07 · Lesson 5 · 14 clock hours · lab

Dial Indicators, Test Indicators, and Surface Plates

Objective. Students set up dial and test indicators on magnetic-base stands over a surface plate to measure runout, flatness deviation, and part-to-surface-plate squareness on assigned workpieces.

⚠ 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.

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Dial Indicators, Test Indicators, and Surface Plates

Precision measurement is the backbone of CNC machining. A part is only as good as your ability to verify it. In this lab you will use dial indicators, test indicators, and a granite surface plate to measure flatness deviation, total indicated runout (TIR), and squareness — three of the most common inspection tasks on a shop floor.

The Granite Surface Plate

A granite surface plate is your reference datum — a flat, stable platform against which all measurements in this lab are compared. Surface plates are graded by flatness tolerance; Grade B (also called Laboratory Grade B or Inspection Grade) is the minimum for this course. Grade A is flatter and used in calibration labs. Handle the plate with care: never drop tools on it, always clean it before and after use with a lint-free cloth and appropriate stone cleaner, and replace the cover when not in use. A damaged surface plate passes errors into every measurement made on it.

Dial Indicators

A dial indicator measures linear displacement through a spring-loaded plunger. The plunger movement is amplified by a gear train and displayed on a circular dial. Common resolutions are 0.001 inch (one-thousandth) and 0.0001 inch (one ten-thousandth). Always select the resolution that matches the tolerance on the drawing — using a 0.001-inch indicator to check a 0.0002-inch tolerance is inadequate.

Key setup rules:

Measuring Flatness Deviation

Flatness is a GD&T form control that limits how much a surface may deviate from a perfect plane. To measure it, you sweep the indicator across the workpiece surface in a pattern — for this lab, a 9-point grid (three rows of three points). Record the reading at each point. Flatness deviation equals maximum reading minus minimum reading across all points.

Compare your calculated deviation to the flatness callout on the drawing (for example, a flatness symbol with a value of 0.003 inch means the surface must fall within a 0.003-inch tolerance zone).

Measuring Total Indicated Runout (TIR)

Total Indicated Runout (TIR) — also called Full Indicator Movement (FIM) — is the total range of indicator readings recorded as a cylindrical feature is rotated 360 degrees about its axis. It captures both out-of-roundness and eccentricity in a single value.

Setup:

1. Support the cylindrical part between centers or in a V-block on the surface plate.

2. Position the dial indicator plunger perpendicular to and contacting the cylindrical surface.

3. Rotate the part one full revolution while watching the indicator. Record the maximum and minimum readings.

4. TIR = maximum reading − minimum reading.

5. Repeat at a second axial position and record both values on your inspection report.

Compare TIR to the GD&T runout callout on the drawing. If the callout is 0.005 inch total runout and your TIR is 0.003 inch, the part passes.

Test Indicators

A test indicator (also called a lever-type or dial test indicator) uses a pivoting contact arm rather than a plunger. The arm sweeps through an arc, making test indicators ideal for reaching into tight spaces and for aligning features parallel or perpendicular to a reference. Resolution is typically 0.0001 inch. Because the contact arm pivots, the cosine error is more significant than with a plunger indicator; keep the arm angle within the manufacturer's recommended range (often within 15 degrees of perpendicular to the surface being measured).

Use your test indicator to:

Safety

Completing the Inspection Report

Every measurement in industry is documented. Your inspection report must include: part identification, date, instrument used and its resolution, all raw readings, calculated flatness deviation or TIR, the drawing callout, and a pass/fail determination. A measurement without documentation does not exist in a quality system. Have your instructor sign off before you disassemble the setup.

Key terms10

Dial IndicatorA spring-loaded, plunger-type measuring instrument that converts linear displacement into a rotary needle reading on a graduated dial; common resolutions are 0.001 inch and 0.0001 inch.
Test IndicatorA lever-type precision instrument with a pivoting contact arm used to detect small deviations in surfaces; especially useful for alignment and checking in confined areas.
Surface PlateA precisely flat reference surface, typically granite, used as a datum for layout, inspection, and measurement setups; graded by flatness tolerance (Grade A, B, etc.).
Total Indicated Runout (TIR)The total range (maximum minus minimum) of indicator readings taken as a part is rotated one full revolution; captures both eccentricity and out-of-roundness in one value.
Flatness DeviationThe difference between the highest and lowest indicator readings swept across a surface; compared to the flatness tolerance callout on the drawing to determine conformance.
Magnetic-Base StandAn adjustable indicator holder with a switchable permanent magnet base that secures to ferrous surfaces such as machine tables and surface-plate accessories.
Cosine ErrorA measurement error introduced when the indicator plunger or test-indicator arm is not perpendicular to the surface being measured; the reading appears smaller than the true displacement.
GD&T Runout CalloutA geometric dimensioning and tolerancing symbol (circular or total runout) that specifies the maximum allowable TIR of a feature relative to a datum axis.
Pre-loadDeliberate compression of an indicator plunger (typically one full revolution) before zeroing, ensuring the plunger stays in contact and reads accurately throughout a sweep.
Grade B Surface PlateA surface plate whose flatness meets the ASME B89.3.7 Grade B tolerance; acceptable for inspection use in production and training environments.

Worked examples4

1. You sweep a flat workpiece in a 9-point grid and record the following indicator readings (in inches): 0.0000, 0.0012, 0.0008, −0.0004, 0.0015, 0.0009, 0.0002, 0.0011, 0.0006. The drawing flatness callout is 0.002 inch. Does the part pass?
  1. Step 1 — Identify the maximum reading: scan all nine values. The highest is 0.0015 inch.
  2. Step 2 — Identify the minimum reading: the lowest is −0.0004 inch.
  3. Step 3 — Calculate flatness deviation: 0.0015 − (−0.0004) = 0.0015 + 0.0004 = 0.0019 inch.
  4. Step 4 — Compare to the callout: 0.0019 inch < 0.002 inch tolerance.
  5. Step 5 — Make the pass/fail call: the part passes because the deviation is within the 0.002-inch flatness tolerance.
Answer: Flatness deviation = 0.0019 inch; the part passes the 0.002-inch flatness callout.
2. While measuring runout, your dial indicator reads a maximum of 0.0047 inch and a minimum of 0.0009 inch as the part makes one full revolution. The GD&T total runout callout is 0.005 inch. Calculate TIR and determine whether the part is within tolerance.
  1. Step 1 — Apply the TIR formula: TIR = maximum reading − minimum reading.
  2. Step 2 — Substitute values: TIR = 0.0047 − 0.0009 = 0.0038 inch.
  3. Step 3 — Compare to callout: 0.0038 inch < 0.005 inch.
  4. Step 4 — Make the pass/fail call: the part passes because TIR is within the 0.005-inch runout tolerance.
Answer: TIR = 0.0038 inch; the part passes the 0.005-inch total runout callout.
3. A test indicator sweeps a side face from bottom to top and reads −0.0003 inch at the bottom and +0.0005 inch at the top. What is the total deviation of the face from true squareness over that height, and what does a positive reading at the top indicate about the lean of the face?
  1. Step 1 — Calculate total deviation: 0.0005 − (−0.0003) = 0.0005 + 0.0003 = 0.0008 inch.
  2. Step 2 — Interpret direction: the reading increased (became more positive) moving upward, meaning the top of the face leans away from the precision square (the face is not perpendicular — it leans outward at the top relative to the datum).
  3. Step 3 — Report the result: total squareness deviation = 0.0008 inch; the face leans outward 0.0008 inch over the measured height.
Answer: Total squareness deviation = 0.0008 inch; the top of the face leans away from the square (outward tilt).
4. Your 0.001-inch resolution dial indicator passes the instructor's check, but your reading differs from the instructor's value by 0.0003 inch. Is this within the ±0.0002-inch acceptance band for the lab check?
  1. Step 1 — State the acceptance criterion: the student's measurement must be within ±0.0002 inch of the instructor-measured value.
  2. Step 2 — Calculate the difference: |student − instructor| = 0.0003 inch.
  3. Step 3 — Compare: 0.0003 inch > 0.0002 inch.
  4. Step 4 — Conclusion: the reading falls outside the ±0.0002-inch band. Additionally, a 0.001-inch resolution indicator cannot reliably detect 0.0001-inch increments; switching to a 0.0001-inch resolution indicator is recommended for this level of precision.
Answer: 0.0003 inch exceeds the ±0.0002-inch acceptance band; the measurement does not pass the lab check, and a finer-resolution instrument should be used.

The math in this lesson2

Learners subtract signed indicator readings, such as 0.0015 − (−0.0004), to find flatness deviation.
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Brush up in the math halls at newSchoolWorks.com:
In class: Set up a dial indicator on a granite surface plate to sweep a flat reference surface; record readings at a 9-point grid and calculate flatness deviation; compare to drawing callout. · Mount a cylindrical part between centers (or in a V-block) and measure total indicated runout (TIR) at two axial positions; record on an inspection report and interpret against a GD&T runout callout. · Use a test indicator to align a square workpiece parallel to the surface plate and verify squareness of a side face with a precision square; document the setup with a sketch.
Materials: Granite surface plate (Grade B or better), dial indicators (0.001 and 0.0001 inch), test indicator with swivel mount, magnetic-base stands, precision squares, V-blocks, parallels, reference cylinders, inspection report forms.
Check: Instructor observation: student correctly sweeps flatness and reports TIR within ±0.0002 inch of instructor-measured value; documented inspection report signed; pass = both measurements within tolerance and report complete.

Practice check8

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

NIMS Machining L1 — Measurement and Quality Control
1. When setting up a dial indicator on a magnetic-base stand to measure flatness, you should pre-load the plunger approximately one full revolution before zeroing. What is the primary purpose of this pre-load?
NIMS Machining L1 — Blueprint Reading and GD&T
2. You record nine surface readings and calculate a flatness deviation of 0.0026 inch. The drawing flatness callout is 0.002 inch. What is the correct disposition?
NIMS Machining L1 — Measurement and Quality Control
3. Total Indicated Runout (TIR) is calculated as:
NIMS Machining L2 — Precision Measurement
4. A test indicator's contact arm is set at a 30-degree angle to the surface being measured instead of the recommended range near perpendicular. What type of error does this introduce?
NIMS Machining L1 — Measurement and Quality Control
5. Which surface plate grade represents the minimum acceptable standard for the production inspection work performed in this course?
NIMS Machining L1 — Measurement and Quality Control
6. You are measuring runout on a cylindrical shaft supported in a V-block. The indicator reads 0.0062 inch at its highest point and 0.0014 inch at its lowest point during one revolution. What is the TIR?
NIMS Machining L1 — Safety and Shop Practices
7. While preparing to use a granite surface plate, you notice a small chip on the corner and metal chips scattered across the surface. What is the correct first action?
NIMS Machining L2 — Quality Systems and Documentation
8. When writing an inspection report for a runout measurement, which piece of information is LEAST critical to include?
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