Precision Engineering – Accuracy vs. Precision

Precision engineering concerns the manufacture of items that have a wide range of sizes, from those that are as large as a satellite rocket launcher to ones that are as small as a microchip. Of course, the absolute dimensions of the size of precision-engineered products vary widely, but the reality is that the relative accuracies involved can be comparable. Precision engineering is therefore thought of as being heavily dependent on metrology parameters such as length and angle. Its objective, in the widest sense, is the manufacture of materials and components, the development of manufacturing processes, the design and the manufacture of high-precision machine tools, measuring devices and their control systems. Features of everyday life and those relating to some products are set out in dimensional size, measurement accuracy and manufacturing tolerance. One of the goals of precision engineers is to achieve a high relative accuracy, i.e. the ratio of dimension to tolerance.

Precision engineering CMM measurement

The initial drive for high precision is firmly rooted in the industrial revolution. In the intervening century and a half, the precision of conventional machine tools has improved by orders of magnitude. Today, some types of components can be produced on a routine basis with an accuracy (ratio of dimension to tolerance) of one part in 100,000 or more. This is an improvement of perhaps as much as 1000x from those early beginnings.

It is nevertheless impossible to manufacture workpieces without deviations from the nominal shape. Workpieces always have deviations of size, form, orientation and location. When these deviations are too large, the usability of the workpiece for its purpose will be impaired. Control and measurement of these deviations is therefore critical.

Measurement is a procedure in which an unknown quantity is compared with a known standard, using an accepted and consistent system of units. A measurement provides a numerical value of the quantity of interest, within certain limits of accuracy and precision.

As people tend to generally consider accuracy and precision as having one and the same meaning, it is important to highlight the distinct difference between these two terms in order to comprehend the discussion on precision engineering.

Accuracy vs precision sketch
Accuracy vs. Precision

A measurement procedure is accurate when it is absent of systematic errors, which are positive or negative deviations from the true value that are consistent from one measurement to the next. Precision is the degree of repeatability in the measurement process. Good precision means that random errors in the measurement procedure are minimised. Random errors are usually associated with human participation in the measurement process. Examples include variations in the setup, imprecise reading of the scale, round-off approximations, and so on. Nonhuman contributors to random error include temperature changes, gradual wear and/or misalignment in the working elements of the device, and other variations.

At CNC Proto we specialise in the machining of non-ferrous metals, mostly 6000 and 7000 series aluminium alloys, and engineering plastics. On our Quote Request form, we offer by default 3 different choices to our customers for linear and radii dimensions general tolerances (+/- 0.10mm; +/- 0.07mm; +/- 0.05mm). Of course, if your parts include features with tighter tolerances than thesefeel free to include a 2D drawing or let us know in the additional comments field of the form!

For more articles on CNC machining, materials and design of machined parts, be sure to check our Resource Library.

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Fernando Duarte Ramos

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With an MSc. in Mechanical Engineering, Fernando has worked 8 years at CERN designing and building the next generation of particle detectors alongside top European engineers and physicists. Being passionate about CNC machining, he started CNC Proto, an online CNC machining service in Europe dedicated to providing fast turnaround, high quality parts at a competitive cost. Learn more About Us

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