Straightness is a measure of how straight an object is, usually compared to a theoretical straight line. In metrology, straightness is an important characteristic of linear dimension. An object’s straightness can change over time due to external factors such as temperature, humidity, and force.
Straightness is measured using comparison to a fixed reference line, and this reference line is typically a mean straight line. The comparison between the two lines is made directly or indirectly, typically by machine with a comparison jig, or a laser system. Linear displacement measurements provide the data necessary to determine the deviation or straightness of the part to the reference.
Straightening of parts typically involves the application of force to create a desired shape or dimensional geometry. Force can be applied by means of a rotation device, vibration, or stretching. Heat can also be used to relax the materials properties which can cause parts to become straight. Heat can be applied in thermal furnaces, induction heaters, or even with a blowtorch.
For precision measuring, the parts are compared to a mean straight line of known accuracy. If the part deviates from the line the part is measured straightness against the reference line using calculations for both the radial runout rate and the in-line step frequency.
The radial runout rate is the rate at which a part deviates from its reference or imaginary line and is typically measured with a dial indicator. The in-line step frequency is the number of steps at which the part deviates from its reference or imaginary line and is typically measured with an optical comparator or an automated linear scale system.
The degree of straightness depends upon the application or the customer’s needs and can range from several thousandths of an inch to a few millionths of an inch. Quality systems will use statistical sampling techniques and mathematical controls to determine the acceptable straightness limit that has been established by customer request.
For highly precise applications, such as aerospace, optical and medical industries, a device called a straightness machine is utilized which mounts parts on a rotating
ring, raising them to a predefined height above a staff and measuring the lateral movement of the parts.
Straightness is a critical factor for many manufactured products, including those used in aerospace, medical, and optical industries. It is important for manufacturers to ensure parts conform to the customer’s straightness requirement, as failure to meet specified tolerances often result in customer dissatisfaction and costly measures such as having to rework or scrap parts.
