Application Note - Diameter measurements
The measurement of diameter, ID or OD, is one of the most usual applications of industrial metrology. Holes and circular profiles can be found on almost any machined or injected part, creating the need for inspection of diameter tolerances, form tolerances (circularity, cilindricity), position tolerances (concentricity and coaxility) as well circular profiling tolerances.
Many measurement systems may be used to check these dimensional features. Traditional metrology has a very wide choice of instruments for performing these measurements in laboratory, varying from simple dial gages to highly sophisticated 3D coordinate machines. These options, despite the fact of having excellent accuracy and repeatability, are usually not suitable for use in shop-floors. Among the main factors that limit or difficult their use on industrial harsh conditions is important to highlight:
- Instrument's low tolerance to environmental contaminants (specially tooling and lubrification fluids);
- Requirement of basic technical knowledge of the instrument for correct operation, result analysis and reduction of breakdowns;
- Requirement of controlled environment conditions (temperature and humidity) for proper instrument accuracy and repeatability.
- Long measurement time required for each single inspection;
On this context Metrolog offers many measurement systems designed specifically for use on shop-floor. Despite the different technologies used on each model, all solutions have the same basic features: high throughput measurements (volume production), high accuracy and repeatability on harsh environments.
Air-electronic technology
The air-electronic measurement systems apply compressed air to detect very small variations dimensional features. Its physical principle is based on the fact that the flow and pressure of compressed air are directly correlated to the reduction of the escape area. Based on that, if compressed air is forced through a small hole to the atmosphere, it is possible to detect small changes on the flow and pressure if it closes to a surface.
This measurement system is suitable for measuring diameters from 3 up to 300 mm, and tolerances from 0.001mm up to 0.080 mm. Typical applications allow use of resolution of 0.0001 mm and repeatability better than 0.002mm. The measuring process has a very low thermal drift, not being susceptible to normal environment temperature changes. In special cases where tight tolerances must be met or materials with high dilatation coefficient are in use, it is possible to add temperature feedback to compensate the errors caused by the thermal drift.
A standard measurement system has a measurement tool, two masters, and one air-electronic equipment, as well a clean compressed air supply.
To measure internal diameters and correlated profiles, it should be used an air spindle. This measurement tool typically has 2 or 3 jet (where the compressed air flows out) and is inserted into the hole it will inspect. Picture 1 shows a complete measuring system, including one air-electronic M20-2P column, one dual air spindle (used to measure two concentric diameters) and two masters for calibration.
Picture 1 - M20-2P and air-spindle used to measure two concentric internal diameters |
After a initial setup (nominal value, limits setup, etc.) and equipment calibration, the measurement process is very straightforward, requiring only the insertion of the air spindle inside the measured part and read of the resulting value on the equipment LCD or color check of the bargraph (green for approved, red for reproved). Optionally external signals may be connected in and out the equipment, allowing external indications, setup selection or online feedback to tooling processes.
To measure outer diameters a similar setup is used, only replacing the measurement tool and calibration masters. In this particular case an air ring or air 'C' snap is used, and master rings are used for calibration.
Picture 2 - M20-1P and air 'C' snap for inspection of outer diameter |
Due to the limited measurement range of the air-electronic system (typically ±0.040mm), the measure tool must be build for a specific size. Also it is important to check some additional factors, such as surface roughness and part geometry, before selecting this measurement system for a particular use.
In essence the air-electronic system is suitable for inspections of machined parts with a good surface finish. Due to its high roughness, low thermal drift, high precision and high repeatability, as well the ability to clean up the measured surface this measurement technology is very well suited to high production lines where 100% part inspection is desired.
LVDT Electronic Technology
The electronic measurement systems used LVDT sensors to detect very small dimensional differences. The typical gage head transducers is build from two parts - inner core and a outer body - and features exceptional linearity and repeatability and are long life. These transducers manufactured in two types: open core and gage head or 'pen' type.
Outer diameter measurements typically employ two LVDT gage head sensors, because these probes have a non-rotating plunger guided by a bearing assembly. On this application the probes are installed on a ring shape or "V" shape fixture where the part is inserted or laid down for inspection. The most common probe models are LBB and GCA series. This setup, know as differential mode, allows the transducers to work together, return only the real dimensional difference between both signals. This eliminates errors due to misalignment, wrong part positioning or vibrations that may be present during the measurement process.
Picture 3 - M20-4S column with electronic ring fixture (assembled with two LBB315PA-040 probes) for outer diameter inspection |
The measurement of inner diameters typically employs an electronic plug, for example the Metrolog DS20 series. Electronic plugs have a mechanical system that translates the axial movements of small contacts tips to an axial movement, detected by a high precision LVDT transducer. Typical measurement range of ± 0.15 mm allows hole inspection during coarse and fine tooling processes.
Picture 4 - M20-4S column with electronic plug DS20 and master discs for system calibration |
In essence, electronic measurement system employing LVDT transducers are suitable for inspections in-process and general dimensional high precision measurements. A
In essence, electronic measurement system employing LVDT transducers are suitable for inspections of machined parts and in-process dimensional control. Due to its high roughness, low thermal drift, high precision and high repeatability, as well setup flexibility is very well suited to high production lines and mixed production lines that requires periodic setup for new parts.
Laser Technology
The laser measurement system is based on highly advanced optical components and state of the art digital processing circuits, allowing non-contact ultra high precision inner and outer diameter inspection. Typical application employs a laser micrometer and a M20-2L measurement column. The laser measurement system creates a continuous laser beam that scans the part silhouette and detects its outer diameter with resolution up to 0.000033mm. Among the main advantages of this non-contact measurement system, it is important to point its high thermal stability and continuous self calibration.
This unique non-contact method of measurement allows diameter size inspection where other technologies cannot be applied, such as output at extrusion lines, parts at high temperature, inspection of fragile parts or parts with very fine surface finish.
Picture 5 - Laser column M20-2L and XLS Micrometer |
Outer diameters may also be measured with use of laser heads. Based on a different method of measurement than the laser micrometers, laser heads measure distances by the principle of triangulation. On this method of measurement a laser beam touches the part surface and reflects back to the laser head where the return beam angle is measured by a high definition CMOS sensor array. With proper use criteria, laser heads may be used to measure large parts, roll run outs, extrusion continuous profiling, plastic injection lines, among many other applications.
Picture 6 - Laser columnM20-2L and Atlas laser head |
How to select the best measurement technology
Many factors must be weighted before selecting a measurement technology for a specific application. In many cases one may use different equipments and get the same results. In general, the following factors must be considered before selecting the most suitable equipments/sensors for a specific application:
- Considered part dimensions and tolerances
- Production measurement throughput or maximum cycle measurement time
- Part material and surface finish
- Measurement environmental conditions (temperature, contaminants, etc)
- Overall investment cost on development of special measurement tools and masters