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Form and Contour Tracers are purpose-specific devices that use high accuracy continuous contact sensors with varied styli to obtain small part geometry such as fillet radii and chamfers or that measure roundness, cylinidricity, and other geometric tolerances.(Credit: Q-PLUS Labs)

Size of the Objects Being Measured. You can put a turbofan blade into a machine to analyze its contours, but you cannot do the same with an entire aircraft. Dimensional inspection equipment can also be used to determine metrics such as the space between bolt holes, but only if you use a probe small enough to enter the holes. Knowing the sizes of all the objects you intend to measure will help you select the right dimensional inspection equipment and accessories.

Shape of the Objects Being Measured. Objects that have internal geometry require different measuring methods than those that do not. You may need to purchase accessories that enable internal measurements or use specialized equipment dedicated to that purpose.

The shape of the object can also impact the type of equipment you purchase. For example, if you need to measure the form of a small screw thread you will either need a contact sensor that has a small enough tip (typically knife edge sharp as with a contour tracer) to reach all the geometry, or a noncontact optical sensor with sufficient magnification.

Speed. Do you need to get immediate pass/fail (attribute) quality control results as objects come off a manufacturing line? Or do you require variable data results with more comprehensive data that allows you to understand by how much an object did not pass inspection and possibly why?

Of course there are countless other scenarios, but as with other factors, understanding your requirements will help you make the right choice. If you don't need real-time results, you can often get all the information you need with less-expensive equipment.

Automatic or Manual. Some applications — typically those related to quality control — require dimensional inspection equipment to operate automatically. For example, wall thickness measurements from parts coming off a line can provide real-time pass/fail information for manufacturers. Other applications, such as thread gaging, require manual operation to achieve the desired results.

Ease of Use. Many types of dimensional inspection equipment require programming or other specialized skills to properly operate, such as vector-driven measurements. While training can provide many of the necessary skills, some equipment requires a specialist. If you do not already have, or do not intend to hire specialized staff to operate the equipment, you may find it necessary to outsource some or all of your dimensional inspection needs to a qualified provider.

How Do You Know What Equipment Will Be the Best Fit for a Specific Application?

Here is where knowledge and experience are paramount. Just making the measurement of certain dimensions possible is difficult enough, let alone making the measurements feasible. For every object imaginable, from a nanoscale surface to a rocket engine, there are numerous measurement opportunities. There are also multiple ways to execute each measurement technique, and multiple devices and manufacturers from which to choose.

When investing in dimensional inspection equipment, the primary objectives are to find a device or system that:

  • Allows for the inspection to be performed correctly, accurately and precisely.

  • Allows for the inspection to be performed as quickly as possible.

  • Serves the functions you require.

  • Is reliable and has a long lifetime.

  • Can be easily operated and maintained.

  • Fits the budget.

Types of Dimensional Inspection Equipment

Vision systems are similar to optical comparators, but instead effectively project images directly onto a screen while a camera with interchangeable objective lenses and/or zoom optics relays images to the display. (Credit: Q-PLUS Labs)

There are three primary types of dimensional inspection equipment: precision hand tools, contact sensor systems, and noncontact sensor systems.

Precision Hand Tools. Portable and generally easy to use, precision hand tools are often capable of providing all the information you need. Some of the advantages of using hand tools include factors such as relative low cost and high portability.

However, some of the disadvantages of hand tools include the slow speeds with which readings can be acquired, their relative inaccuracy compared to dedicated systems, and difficulties in obtaining good repeatability and reproducibility among different users compared to automated systems. Here are some of the most common dimensional inspection hand tools:

  • Bore and ID gages measure the internal diameter of an object, either by indicating the deviation from a predetermined standard or by providing an actual measurement.

  • Calipers provide inside, outside, depth, length, or step measurements using various technologies. Certain types can also be used to compare or transfer dimensions from one object to another, or to precisely mark a measurement.

  • Fixed gages are designed to quickly compare specific attributes to a specified standard. They might measure thickness, length, angles, gear teeth, radius, bead size, and a range of other parameters.

  • Micrometers are used for precision dimensional gaging and may use mechanical, digital, dial, scale, and laser technology. Micrometers can be used to measure the thickness, length, depth, internal diameter, outer diameter, height, roundness, or bore of an object.

  • Protractors and angle gages measure the angle between two surfaces. They can be fixed or variable depending on the intended use, and may be designed to provide other functions such as simultaneous depth measurements.

  • Indicators and comparators amplify the movement of a precision spindle or probe and display the results on a dial, digital display, or column. Different levels of precision are available for a range of applications.

  • Air gage instruments use changes in pressure and flow rates to measure parameters such as thickness, depth, internal diameter, outer diameter, bore, taper, and roundness.

  • Plug and ring gages provide a pass/fail assessment for holes and bores, and shafts and pins respectively, based on specified dimensional tolerances. A simple plug gage pin can do one remarkable thing that advanced continuous contact CMMs cannot do: ensure that not a single opposing point reading along the hole or bore is undersize. Since a hole or bore theoretically has an infinite number of diametrically opposed points, a CMM could sample points for a long time, but still only approximate the resultant size.

  • Threaded plug and gages qualitatively measure and/or verify thread size, spacing, shape, geometry, or other parameters.

  • Rules and length gages are used for length measurement, and much like a measuring tape or a ruler, have a flat, graduated surface.

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