Common Engineering Terms and Their Meaning

As an engineer, there can be a lot that you need to remember. From machine functions to complex equations to specific terms, it’s important to keep informed. Whether you’re a new engineer working on a project or just need to be refreshed, we took some common engineering terms and broke them down for you.


  •  Common Engineering Terms and Their Meaning - circle1Circularity (Roundness) - Used to describe how close an object is to a true circle.


  •  Common Engineering Terms and Their Meaning - CylindricityCylindricity - This is used to describe how close the object is to being a true cylinder. Being a 3-dimensional tolerance- this controls the overall form of a cylindrical feature. This ensures that the object is round enough and straight enough along its axis.


  •  Common Engineering Terms and Their Meaning - RunoutRunout – Used to describe the inaccuracy of rotating mechanical systems. There are two main forms- radial Run-out: Caused by the tool or component being rotated off center. And axial run-out: Caused by the tool or component being at an angle to the axis.


  • Common Engineering Terms and Their Meaning - Flatness1 Flatness - A form control which defines how much a surface on a physical part may vary from the ideal flat plane. This is a crucial geometric condition workpieces and tools.



  • Common Engineering Terms and Their Meaning - Straightness Straightness - Is described as a condition where part of a surface (or edge) is in a straight line.


  • Common Engineering Terms and Their Meaning - Parallelism Parallelism - This is an orientation control that outlines how much surface on a physical part could vary from being parallel to a specific datum.


  •  Common Engineering Terms and Their Meaning - PerpendicularPerpendicularity - Depending which reference is specified; perpendicularity can have two different definitions. Surface perpendicularity (the normal form) is a tolerance which controls perpendicularity between two 90° surfaces. Also, it’s important to know that surface perpendicularity is controlled with two parallels which act as its tolerance zone. Now, axis perpendicularity is a tolerance which controls how perpendicular specific axis’ need to be to a datum; this is controlled by a cylinder which is around a theoretical axis that is perfectly parallel.


  •  Common Engineering Terms and Their Meaning - Surface_Finish Surface Finish - This is the nature of a particular surface which is defined by three characteristics of texture, roughness and waviness. When controlling friction and transfer layer formation during sliding, surface texture is one of the most important factors.


  • Common Engineering Terms and Their Meaning - Concentricity Concentricity – Describes a condition in which to or more features (cylinders, cones, spheres, etc.) have a common axis. Measurement requirements for concentricity involve the complex task of mapping the referenced feature by way of opposed point measurements.


  • Size tolerance- A size tolerance is the allowed limit or limits of variant in either a physical dimension, a measured value or physical property of a material manufactured object, system or service. It also includes other measures values like temperature or physical distance.


  • Cycle time- This is a simple definition as it is the period of time to complete a cycle of an operation or the amount of time to complete a job or task from start to finish.

Lastly, in the CNC machining world, precision + repeatability=accuracy. Below, we describe which each part of that equation means.

  • Precision- Indicates the closeness of two or more measurements to each other.
  • Accuracy- The degree of closeness a measured value is to the true quantity of what is being measured. Accuracy is the combination of precision and repeatability.
  • Repeatability- Describes how well a system can reproduce an outcome of the same item and under the same conditions.


As technology continually changes, new products and machines emerge, these terms are here to stay. With engineering, it’s important to never stop learning as this can only benefit you more in the industry- being knowledgeable in your field is the key to being a successful engineer. 


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