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A&M Electric Tools uses LC50 for 3D inspection of plastic tool housings
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A&M Electric Tools, (Winnenden, Germany), manufactures a variety of electric tools such as percussion drills, screwdrivers, demolition hammers, etc. These products are worldwide distributed and better known as “AEG” or “Milwaukee” tools. The housings or shells for this equipment are high precision injection moulded plastic parts with complex geometries. The attractive design, wellthought ergonomics, and basically the fitting of the two half shells requires a thorough inspection. As today’s pressure on manufacturing is higher, A&M Electric Tools decided to invest in a new modern inspection system that enables fast and accurate scanning of full parts.
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To improve the inspection process, A&M Electric Tools installed a Metris LC50 laser scanner on a DEA Global Image coordinate measuring machine (CMM) to verify the full 3D geometry of the housings.
Inspection applications using laser scanning
The main application of the inspection system is fast, full 3D inspection of the housings on assembly issues. A primary task is to inspect whether all features and ribs at the inside of housings are present and complete. Therefore the part is scanned and the resulting point cloud is compared to the CAD model. The results are interpreted in a 3D colour deviation report that immediately reveals deviations out of tolerance or incomplete ribs. Examples of problems that may occur during assembly are misalignment of features in the housings, or left or right pieces of the housings that do not fit together to required standards. Incomplete ribs are often the result of non-optimal fill and packing pressures in tool cavities or improperly designed moulds. These problems are typically due to the complex injection moulding process. Mould shrink, unpredictable warping and dilations when the part absorbs humidity makes it difficult to predict and control the exact dimensional behaviour. Secondly, the 3D deviation colour maps resulting from the part to CAD comparison also indicate if the surface and form of the housing is within the desired specifications. This way, the operator or design engineer can directly see where the product is within or out tolerance.
A third application is to inspect the geometrical features whether they meet the tolerances of the designer specifications. A tolerance match on the dimensions is typically needed to assemble all components of the tool.
The DEA CMM with Metris scanner: a powerful combination
The main advantage of traditional touch trigger probes is the high accuracy and automation of feature measurements. However, the traditional touch trigger probe falls short when it comes to full part measurements. As the part geometries have grown increasingly complicated, often with complex 3D contours, it is required to collect hundreds or thousands points to accurately model the part. Compared to traditional touch probe measurements, laser scanning offers a fast solution to measure a full 3D part with a high accuracy. Therefore Mr. Heintz from A&M Electric Tools wanted an integrated solution including CMM, scanner and application software. “We thoroughly investigated the market for the best integrated solution. A DEA Global Image CMM integrated with a Metris LC50 was the best fit for us.” CMM manufacturer DEA (Italy) is well known for their excellent price-performance CMM’s and good local support. Metris is currently the only laser scanner company that tightly integrates with all major CMM manufacturers. Metris also provides an integrated solution including the downstream applications such as part to CAD inspection and reverse engineering. Also the fact to have one single contact that supports the complete system is an important value for the customer.
The vision of the DEA-Metris integration is also in line with A&M Electric Tools strategy. “Our goal is to operate the Metris scanner directly from within the PC-DMIS environment”, comments Mr. Heintz, “This enables us to easily switch between the scanner and traditional touch probe, combining the best of both worlds.” For certain inspection tasks, such as the measurement of critical dimension with tight tolerances, the touch trigger probe is the best tool. When it comes to measuring of 3D surfaces, the laser scanner is the best solution.
Laser scanning technology
The laser probe projects a line of red laser light onto the surface of the housing. By capturing the reflection of the laser light the CCD camera in the scanner constantly monitors the profile of the laser line on the part. Using triangulation the 3D coordinates of the points on the line are then calculated. The Metris LC50 has a field of view of 50mm by 50mm, with a point accuracy of ± 15mm. The laser scanner captures 768 points per line and scans 25 lines per second, resulting in a scanning speed of 19200 points per second. The data is transferred to the PC through the Renishaw Multiwire that is integrated in the CMM.
While moving the CMM over the part, a dense highly accurate point cloud is generated. As such the time needed to measure complex parts is reduced from hours to minutes.40
In general, the Metris laser scanning technology introduces the following advantages:
- Speed of measurement
The Metris LC scanner measures at a rate of 19,200 points per second. When a sample from a production series is inspected it is necessary to restart the production as soon as possible. As such it is also possible to inspect more housings resulting in a better quality control.
- Digital copy of a housing
Laser scanning enables the complete measurement of a housing including the ribs, features and surface details. The resulting point cloud can also be used for reverse engineering purposes.
- Integration on a CMM
The highly accurate CMM’s are considered the industry standard in the metrology field. The Metris laser scanners are fully integrated with these CMM’s including the Renishaw PH10 2-axis rotational head. The operator can also switch between laser probe and touch probe, so all the existing functionalities of the CMM are still available.
The scanning process
The first step in the inspection process is setting up the laser scanning system. The system is able to scan most materials and surface colours by varying the laser intensity. The part can be positioned on the CMM table without a fixture, but generally a dedicated clamping tool is used.
The scanning software allows the operator to easily calibrate the PH10M orientations that correspond to the different views to scan the part. This calibration is needed to record all measured points in the same co-ordinate system. The automatic calibration takes about one minute per orientation. The quality operator then records the scan path in a macro: the start and end of the scan, the width of the area, and the minimum overlap between different passes determine the scanning area. The recording of a macro to scan a tool housing using 5 different views can be done in less than 10 minutes. The operator also specifies the interscanline distance by setting the desired CMM speed. The operators at A&M Electric Tools usually define an interscanline distance between 0,2mm to 0,4mm depending on the size of the part to be verified. The inside of the housing is scanned with 3 different PH10 angles (-15°, 0, 15°) to capture all details in different directions. A complete housing can thus be scanned in less than 1 hour. Once the macro is written, it can be used to automatically scan similar parts.
Part to CAD verification
The scan result is a dense digital copy of the physical part. This point cloud is processed before verification using the point cloud processing software. First, points that obviously are not a part of the model are removed from the point cloud. The scanning area may contain parts of the scanning environment, such as the CMM table or fixture. Next, scatter reducing and 3D curvature dependent filtering is applied to get a reduced point cloud that still accurately describes the part to be verified. A typical point cloud initially has 6 million points and is reduced with a factor 10 to 20.
The point cloud is then aligned to the CAD model. At A&M Electric Tools, the coordinate systems of nominal and object are aligned through definition of a reference by a touch trigger probe measurement. When this referencing is not possible, a first rough n-point alignment is often used. The alignment can be refined in a second step to use an iterative best-fit alignment. Other alignment possibilities are Reference Positioning System for feature based registration or free form alignment.
Next, the point cloud is compared to the CAD model. The quality engineer sets the tolerances specified by the designer. A&M Electric Tools specifies these tolerances typically in the range of ±0.5mm for global verification and even smaller for detail verification. The result is a full deviation analysis colour plot of the part. The quality engineer can indicate the hotspots with deviation fly-outs for easy interpretation by the designer.
Beside the complete point cloud, also sections in all directions can be analysed and reported. Alternatively, the inspection software can be used as a virtual CMM to check features on the digital copy of the physical model, e.g. the distance between two holes, or the parameters of cylinders.
The reports are typically sent electronically to the incoming inspection operator who accepts or rejects the incoming product batch and liase with the designers, providing them with a detailed analysis of the problem.
Similar to the scanning software, also the inspection of similar parts is automation ready with Visual Basic Automation.
Reverse engineering solution
Another point cloud application at A&M Electric Tools is the use of reverse engineering. Reverse engineering is a fast way of translating the point cloud of a physical model or shape into a digital model so that manufacturing, machining, or repair plans can be produced.
A&M Electric Tools uses the reverse engineering for further refinement of the existing CAD models based on the measured samples. Therefore different sections of the point clouds are imported as STEP files in the PRO-E or CDRS CAD software. The measured sections are then used as a background to create new dynamic curves. This way the designer can rapidly adapt the original CAD models or create new CAD models.
A second application where A&M Electric Tools uses reverse engineering is to re-manufacture new parts that have no existing CAD model. The physical part is scanned and a surface CAD model is created in the reverse engineering software. Further refinement or preparation for manufacturing is done in Pro E software.
Another important reverse engineering application is to reduce the time required to rebuild or improve injection tool moulds based on an existing mould. Often the moulds are iteratively optimised to produce an acceptable first prototype part. Since the modifications of the moulds are often done manually, the original CAD model is no longer aligned with the existing tool. The modified tool is than scanned to update the original CAD model.
Obviously reverse engineering is also applied in tool repair, when a tool needs to be created from an old, fractured or worn original.
Conclusions
Laser scanning is a modern technique that complements the traditional touch trigger measurement. The most important advantage is that in a fast way a full 3D scan of the part is obtained. All form details, previously a tedious task to measure with touch triggers, are now revealed in a clear colour deviation map. Applications such as reverse engineering open new possibilities for rapid product design, tool optimisation and tool repair.
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