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Precision Engineering and Accuracy

precision targets example

Precision engineering operations require both accuracy and precision.

•  Accuracy refers to the closeness of a measured value to a known value, such a
    specific linear measurement.

•  Precision refers to the closeness of two or more measurements to each other
    —often called repeatability.

•  The movement caused by thermal changes affects both the accuracy and precision
    in any engineering operation. 

Largest Source of Error

Thermal effects in a precision system represents the largest single source of systematic error and non-repeatability for nearly all ultra-precision manufacturing processes. As a result, minimizing and controlling thermal influences also offers the largest single source of improvement in a precision system and most often at a fraction of the cost of the overall system.

How Much Temperature Control Do I Need?

Most precision systems show dramatic performance improvement with temperature control 100 times better than ambient. However, the more error that you eliminate, the more accurate your manufacturing processes will be. So, in general, you want to eliminate as much error as possible in as many places as possible, and providing as much temperature control as possible is generally the easiest and most economical way to achieve improved accuracies.

Errors in a manufacturing process are cumulative. Therefore, error reduction in one area of your process will allow you to accommodate greater errors in other areas. For example, if you create greater accuracies by using precision temperature control, you may have more latitutde in part and machine setup.

Error Budgets

Error budgets are useful tools to categorize and predict the errors in precision manufacturing operations. Error budgeting is a methodology that allocate errors to components and processes of an instrument, and predicts the total error of the instrument's action.

When creating an error budget, precision determinism states that all error values—both systematic (straightness, squareness, positioning error, etc.) and non-systematic (thermal errors) are cumulative and additive. Therefore, eliminating as much error in any area of the process will contribute to the overall accuracy of the endeavor.

What Accuracy Improvements Can I Expect from Precision Temperature Control?

A Præcis Environment system can improve the accuracy of a manufacturing or metrology process by a factor of two up to ten. For example, if you have a well-designed machine operating in a poor environment (e.g., +/- 1.0 °C), you can expect a factor of ten accuracy improvement (+/- 0.01 °C).

The example below shows an interferometer measurement of a 200-mm long aluminum gauge block as it changed length in typical room temperature variation. A one-degree temperature change caused a 0.0046 mm change in length.


What is Thermal Expansion?

Everything Changes Size When the Temperature Changes.

Bridge Divider

One of the most noticeable examples of thermal expansion is the spreading of joints in bridges. These joints allow for seasonal variations in the length of the bridge span as temperature vary throughout the year. In precision engineering, optics, metrology, and other ultra-precise engineering applications, small differences in size are absolutely critical.

When a material is heated the distance between individual atoms will change. For most materials the atoms get further apart and the total length change depends on how many atoms are in the length. This makes the temperature change proportional to length. For example, if a one-meter piece of metal changes length by some small amount, a two meter piece would be expected to change by twice the amount.

Changes in length is proportional to the temperature change. The constant of proportionality is called the coefficient of thermal expansion (CTE), denoted by the Greek letter alpha (a).

Predicting Thermal Expansion

Precision instrumentation is built from components of a variety of materials—which, with different CTE’s, expand and contract by different amounts with temperature changes. To further complicate the problem, different materials have different rates of thermal conductivity, making the individual instrument parts and the work piece fluctuate in size at different rates.

Because of the complexity of the system and time to execute the activity, the combined thermal changes in a precision manufacturing process are impossible to predict.

To prevent the effects of thermal expansion from harming ultra-precision measurements, Praecis Inc. has developed an ultra-precision air temperature control system. By stabilizing the air to +/- 0.003 °C from a set temperature, Praecis ATCU (Ultra Precision Air Temperature Control Unit) can virtually eliminate any thermal instabilities and makes even the slightest precision temperature control and thermal management easier. 

Learn more about stabilizing the temperature of your instruments and measured parts with precision temperature control.