Stationary and portable metrology probing systems have evolved from traditional single point tactile probes to advanced scanning laser probes capable of reliable, high speed metrology of a wide range of materials in the most demanding of environmental conditions.
THE aim of metrology is quite simple: to ensure that the items manufactured match the original intent of the designer.
Metrology is accomplished by gathering as much mathematical information about the physical dimensional characteristics of the "as built" parts and comparing them against the designer’s specifications. Over time, the ability to do this has become increasingly sophisticated.
Initially, metrology was limited to measuring in just one dimension at a time; height, length, or depth. It was not until the late 1960’s when the first true three-dimensional Coordinate Measuring Machine (CMM) debuted in the market.
A CMM can be used in manufacturing and assembly processes to test a part or assembly against the design intent. By precisely recording the X, Y and Z coordinates of the target, points are generated which can then be analysed via regression algorithms for the construction of features.
The typical “bridge” CMM is composed of three axes, an X, Y and Z. These axes are orthogonal to each other in a typical three dimensional coordinate system. Each axis has a scale system that indicates the location of that axis.
The machine will read the input from the touch probe, as directed by the operator or programmer. As the probe touches the surface of the component, the stylus deflects and simultaneously sends the X,Y, Z coordinate information to the computer.
These points are collected by using a probe that is positioned manually by an operator or automatically via Direct Computer Control (DCC) which can be programmed to repeatedly measure identical parts.
More recent innovations are touch probes that drag along the surface of the part taking points at specified intervals, known as scanning probes.
This method of CMM inspection is often more accurate than the conventional touch-probe method and can be many times faster as well.
Portable CMMs are different from “traditional” CMMs in that they most commonly take the form of an articulated arm.
These arms have six or seven rotary axes with rotary encoders, instead of linear axes. Portable arms are lightweight – typically less than 10kg – and can be carried and used nearly anywhere.
The inherent trade-offs of a portable CMM are manual operation and overall accuracy is in general less accurate than a bridge type CMM.
Non-repetitive applications such as reverse engineering and large-scale inspection of low volume parts are ideally suited for portable CMMs.
In addition, if parts are too large or heavy or if they have to be inspected whilst still in the fixture, a portable CMM that can be brought to the part is often the only solution.
The next generation of scanning probes, known as non-contact scanning, is advancing very quickly. This category includes both point and line scanning probes.
Optical probes – particularly line scanners – work many times faster than the most advanced touch scanning probes to create very large data sets which, due to their large size, are called ‘point clouds’.
These point clouds can be used to not only check size and position, but, due to the high density of point information compared to contact probes, to create a full 3D image of the part as well.
Optical scanners are most suited to complex freeform parts where the shape is not easily represented by simple prismatic entities, examples being car, aerospace, motorcycle bodies, trim and seats; many consumer products, medical components and blade shapes such as those used for power generation.
They have created a whole new range of solutions to common customer problems. Typical applications include:
Target Inspection/Validation – In this application, measurement of specific features contained within the point cloud can be made for the purpose of dimensional inspection or GD&T and the results compared against nominal values. This latter step could of course be performed using conventional tactile measurement but for larger parts where tolerances can be up to 0.5mm or more, an optical scanner is often faster since non-contact measurement is more tolerant to large deviations of the part from nominal.
In addition, due to the high density of information contained in the point cloud, the whole cloud can be compared against the full CAD surface model to give a ‘colour map’ of the deviations across the whole part. Such a tool is extremely powerful for first article inspection and production line tuning processes.
Typically a customer will just measure parts but in many cases the tool that produced the part will also be inspected.
Reverse Engineering – Reverse Engineering is the process of taking a physical part, measuring it to determine its size and creating a CAD design from these measurements. This is most often used in cases where the product design process has significant manual operation, such as automotive design.
Despite the advances in CAD, many designs still start life as a physical model which then needs to be turned into electronic form.
In other applications legacy parts, for which engineering drawings no longer exist, are unavailable or out of date may need to be scanned in order to generate a CAD model to make tooling for replacements.
In both these cases, point clouds can either be processed by special software packages to create 3D CAD models or transferred directly to CAD software where a full working 3D model is created.
Replication (copying/scaling) – For applications where a one-off replica of a part is needed it may not be necessary to create a full CAD model. In this case the point cloud may be turned directly into a polygonal mesh which is a basic form of a 3D model. This mesh can be edited, for example, scaled or a half model can be mirrored to create the desired electronic part.
From this final mesh a physical replica can be created using standard manufacturing techniques like milling or Rapid Prototyping.
Typical copying applications include cultural heritage, for example, buildings where a façade has eroded over time and needs restoration or replication of sculptures in different scales.
The applications described above cover a very wide range of objects with different materials (metal, plastic, clay). Thus the key customer requirements for an optical probe are:
- Repeatability – for series measurement, the results must be highly repeatable
- Reproducibility – independent of the operator programming or running the system, the results should be the same
- Flexibility – the probe should be able to measure a wide range of materials (for example, machined, semi-finished, stamped, forged, casted and painted metals, sand cores, wax, carbon fibre, plastics, clay, rubber, wood, ceramics) and be used easily on a range of different part sizes
- Reliable – must work under a wide range of environmental conditions
- Easy to use – minimal operator training is desirable
- Accuracy – must meet the needs of the application
Stationary and portable CMM probing systems have evolved over the years and the unique technology in the CMS106 and CMS108 scanners now produces an accurate, fast non-contact probe capable of measuring a wide range of materials in demanding environmental conditions whilst being repeatable, reproducible, flexible and easy to use.
[Ian Martin is Managing Director of Hi-Tech Metrology]