BIZOMAT MAKEOS · Quality
Measurement Error from Part and Gauge Temperature
How far off is my measurement because the part and gauge are not at 20 °C, and what is the true size?
Size error when a part and the gauge or machine scale measuring it are at temperatures other than the 20 °C reference: the apparent deviation from their different expansions, the size corrected to 20 °C, the share of the tolerance the error uses, and the temperature difference that would keep the error within a chosen share of tolerance.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Nominal size | mm | Nominal length being measured. |
| Part expansion coefficient | µm/m·K | Expansion coefficient of the part (23 aluminium, 11.5 steel). |
| Part temperature | °C | Temperature of the part when measured. |
| Gauge or scale expansion coefficient | µm/m·K | Expansion coefficient of the gauge, scale or CMM (11.5 steel, about 0 for glass-ceramic scales). |
| Gauge temperature | °C | Temperature of the gauge or scale. |
| Measured size (0 = nominal) | mm | The reading taken (0 uses the nominal size). |
| Total tolerance band | mm | Total tolerance band of the dimension. |
| Share of tolerance allowed for temperature | % | Share of the tolerance you allow for temperature effects. |
What each input means
Open an input to see what it is, its units, the worked example and the common mistakes, with a small diagram.
What this means: Nominal size
- What it is
- Nominal size of the dimension being measured, from the drawing.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 100000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 500 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The apparent deviation is proportional to size.
What this means: Part expansion coefficient
- What it is
- Linear expansion coefficient of the part material, from a materials reference or measurement for the actual alloy.
- Units and format
- Enter a number in µm/m·K: micrometres per metre per kelvin. The form accepts 0.000001 µm/m·K to 200 µm/m·K. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 23 µm/m·K as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
What this means: Part temperature
- What it is
- Temperature of the part when it is measured, taken with a contact thermometer on the part itself.
- Units and format
- Enter a number in °C: degrees Celsius. The form accepts -50 °C to 300 °C. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 28 °C as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Measure the part, not the room air; parts fresh from machining are often warmer.
What this means: Gauge or scale expansion coefficient
- What it is
- Linear expansion coefficient of the gauge, scale or measuring machine reference that the size is read from.
- Units and format
- Enter a number in µm/m·K: micrometres per metre per kelvin. The form accepts 0.000001 µm/m·K to 200 µm/m·K. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 11.5 µm/m·K as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
What this means: Gauge temperature
- What it is
- Temperature of the gauge or scale when the measurement is taken.
- Units and format
- Enter a number in °C: degrees Celsius. The form accepts -50 °C to 300 °C. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 22 °C as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
What this means: Measured size (0 = nominal)
- What it is
- The size you actually read on the instrument, which the tool corrects back to the 20 °C reference temperature.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0 mm to 100000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- 0 = use the nominal size as the reading.
What this means: Total tolerance band
- What it is
- Total tolerance band on the dimension: upper limit minus lower limit.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 100 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.04 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- The whole band, not the plus or minus half of it.
What this means: Share of tolerance allowed for temperature
- What it is
- Units and format
- Enter a number in %: percent: the number is read as a percentage, so type 8 for 8 %. The form accepts 1% to 100%. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 10% as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Type the percentage, not a fraction.
Method and formulas
- Both expand from their 20 °C sizes: apparent deviation = L [α_p (T_p − 20) − α_g (T_g − 20)].
- Size at 20 °C = measured − deviation.
- For part and gauge at the same temperature, the error per kelvin is L |α_p − α_g|; allowable offset = share × tolerance ÷ (L |α_p − α_g|).
- Example: a 500 mm aluminium part at 28 °C measured with steel at 22 °C reads 80.5 µm large.
Limitations
- Uniform temperatures; parts straight from machining have gradients.
- Coefficients vary with alloy; use measured values for tight work.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Correct measurements taken on the shop floor
- Decide how long parts must soak before inspection
- Explain disagreements between shop and inspection measurements
Who it is for
- Quality engineers
- Tooling engineers
- Metrology technicians
What you get
- Apparent deviation from temperature
- Size corrected to 20 °C
- Share of tolerance used and allowable temperature difference
Interpreting the result
- Equal temperatures do not remove the error when part and gauge are different materials.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 375–378, 2660. Topic reference; no source tables or recommended settings are reproduced.
Learn the Concepts Behind This Tool
These BIZOMAT Library readings explain the method this tool uses, with worked examples.
Method explained here · BIZOMAT original book
Grinding on a magnetic chuck
In Magnetic Workholding by the Numbers
What it covers: Surface grinding forces against magnetic holding, why a bowed part ground flat on the chuck springs back, thermal bow from grinding heat, chuck heating, and measuring at the right temperature.
BIZOMAT tools are screening and decision-support utilities. Verify important financial, engineering, safety, legal or operational decisions using appropriate professional standards and your organisation’s approved data.
Open the Measurement Error from Part and Gauge Temperature calculator