From Handbook of Dimensional Measurement, Fourth Edition

STRAIGHTNESS AND ALIGNMENT MEASUREMENTS

The straight line represents the path of all linear dimensions. Considering the premise that the shortest distance between two points is along a straight line, that path is not necessarily present in a physical sense on the part being measured for size, but it must be incorporated in the length measuring instrument. Straightness, which is a fundamental concept of linear measurements, is also a functionally important condition of many engineering products. As an introduction to the discussion of straightness measurements, a survey of a few basic methods is made in Table 11-2.

Table 11-2: Examples of Methods for Inspecting Straightness 1

OPERATING PRINCIPLES

TYPICAL INSTRUMENTS

DIAGRAM

DISCUSSION

Straightness of a surface element determined by its parallelism to a straight edge of known accuracy

Straight edge and gage blocks

Unequal height of the supporting elements create a controlled deviation from true parallelism. Gage blocks are used to check whether the digressions from the parallel at intermediate points correspond to the calculated values.

The straightness of a surface element inspected by direct contact with a tool of calibrated straightness

Knife edge rule

A knife edge rule brought to bear against the surface element to be inspected will indicate by the presence and width of light gaps the lack of contact caused by deficient straightness.

Straightness of a shaft determined by rotation on fixed supports and measuring the runout of its surface

Bench centers or Vee-blocks and indicator stand

Using the part's axis (between centers) or its surface...

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Products & Services
Form Gages and Form Gaging Systems
Form gages and form gaging systems are used to inspect parameters such as roundness, angularity, squareness, straightness, flatness, runout, taper and concentricity. 
Surface Metrology Equipment
Surface metrology equipment is used to measure the surface finish and/or geometry of engineering components. Surface texture and topology characteristics include surface roughness, contour, form, waviness, and defects.
Precision Shafting
Precision shafting provides the highest degree of overall accuracy, concentricity, straightness and surface perfection attainable in commercial practice.
Autocollimators
Autocollimators are optical instruments that measure angular displacements and are used to align optical components and measure optical and mechanical deflections.
Air Bearings
Air bearings use a thin film of pressurized air to support a load. They do not generate friction.

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