Case Study Laser Trackers for Production of Automotive Tooling PICO EUROPE designs, manufactures, installs and commissions automotive production lines worldwide for many automotive manufacturers including Ford, Jaguar, Scania, Volvo and many others. PICO EUROPE has been using Leica Laser Trackers in assembly line production since 1996 and now has a total of five Leica Laser Tracker systems. The object of this case study is to give the reader an insight how a laser tracker is implemented for the production of automotive tooling. Also illustrated is how the laser tracker gives the user a greater understanding of the manufacturing process, enabling him to implement proactive improvements from design right through to assembly line final commissioning. Also included is an overview of PICO EUROPE s manufacturing process before the use of laser trackers; enabling the reader to draw a contrast between the measuring processes pre and post laser tracker introduction. Production Objective PICO EUROPE s production objective is to assemble every tool that makes up the production line by bringing together pre-machined tool bases, units, form blocks and location pins in the correct dimensional configuration to reflect the design of the car. This procedure is performed at the PICO EUROPE site. The complete assembly line is then tested. The next challenge is to transport the entire production line piece by piece to the customer s site and reproduce the dimensional configuration achieved at the PICO EUROPE site before final testing and commissioning can commence. Leica Laser Tracker A Laser Tracker is a portable coordinate measuring machine (CMM) for large volume measurements based on a laser interferometer, which tracks and measures the position of a target reflector to an accuracy of 10 microns per metre. The system can measure up to 1 000 points a second and has a radial range of 35 metres. Measurement may be made statically to a tooling hole on automotive tooling, or, dynamically for digitising the surface of a door skin or press tool. Once measured these data can be analysed against discrete nominal X,Y & Z coordinates or free form CAD surfaces. PICO EUROPE
Production line The Global Standard Typically a tooling hole is measured using a precision-machined reflector holder that locates into the tooling hole. The reflector is then located into the reflector holder. The position of the hole is easily measured since the dimensional relationship between the hole, reflector holder and reflector are known. Two sizes of reflector are commonly used with 0.5 inch and 1.5 inch diameters. Procedure at PICO EUROPE before Laser Tracker Implementation On arrival at PICO EUROPE site, tool bases were transported to a fixed solid body CMM and inspected for flatness and the position of unit fixing and doweling locations. If the tool was too large for the CMM s measuring envelope the base was measured in 2 separate halves using the CMM. This process involved measuring one side of the base, then rotating it 180 degrees to measure the other side. The disadvantage of this method was that it was extremely difficult to link both sides of the tool together with any degree of relative accuracy. Smaller tools that fell within the measuring envelope of the CMM could be certified in their fully assembled state. Therefore all the units, form blocks and location pins were positioned on the tool base first, before the tool assembly was positioned on the flat bed of CMM for certification. Having been checked on the CMM the larger bases were transported to another location on the shop floor where the tool assembly began. Another pitfall of moving a large steel plate from one location to another was that its shape changed due to its own weight. Therefore before tool assembly could start the base was levelled again using an optical level. After the complete assembly of the tool with the relative units and form blocks etc; the tool was certified using conventional stick micrometers, steel straight edges and cubes. Employing these traditional methods often made it necessary to move a previously set unit to allow clearance for positioning a straight edge or block. As you can imagine this process took much time and manual effort; removing and replacing units and positioning heavy straight edges and cubes. This process was further compounded if the tool base happened to be at 45 degrees instead of level. Each separate tool assembly was then set up in its position to form part of the production line. Measuring tools used were a combination of piano wires for alignment, stick micrometers for tool spacing and optical levels for levelling the complete line. In moving the tools from the position they were certified to their position in the line, often meant that the tool flexed in some way and the dimensional integrity of the tool was compromised. Measuring the tool in the line proved even more difficult. The completed production line was then tested at the PICO EUROPE site before being transported to the customer s factory for installation and commissioning. Again, moving an entire assembly line from one factory to another poses even more problems in terms of the tools flexing. However, on the customer s site there was no reliable way of verifying or re-certifying the tools. If a unit was damaged in transport, replacing it on site was again difficult as the only way to certify the newly installed unit was to use conventional methods. At this stage removing units would be further hampered due to the installation of pneumatic piping and electric wiring systems. On many occasions a dispute would arise because the customer s panels didn t fit PICO EUROPE s tooling properly. A situation that was extremely difficult to resolve since there was no reliable method of measuring the tooling to verify its compliance to the customer s design. Needless to say much time and money was wasted. Current Laser Tracker Procedure Tool Build PICO EUROPE Site Pre-machined tool bases arrive including all necessary dowel and fixing holes. Datums are also included and can take different forms such as edges, grooves, or holes depending on the customer. These datums provide the initial means of measuring in car-line with the Leica Laser Tracker. If floor space permits the bases can be positioned directly into their relative position in the assembly line. Level datums are established at each end of the production line that the laser tracker is able to use to generate an extremely accurate gravity coordinate system. All the bases on the line can then be levelled co-planar to one another. Piano wires are sometimes still used to roughly align the bases, however final alignment and spacing is performed using a laser tracker since it has a radial range of 35 metres. The laser tracker is then used to check and adjust (via support jacking screws) the flatness of the tool base. Also verified are the position of the tool base s dowel and fixing holes. Tool assembly then commences: first units, then form blocks and location pins. Once complete the tool is ready to be certified. All the components PICO EUROPE
that go together to make up the tooling are pre-machined and their assembly configuration should be close to nominal. The Leica Laser Tracker is then used to measure the tool geometry. Shims are used to make final adjustments. Tool Certification The first step is to measure the datums located on the tool base. Since the position of these datums is known in car-line coordinate system the laser tracker operator is able to perform a coordinate system transformation allowing the laser tracker to measure in car-line coordinate system. The next exercise is to measure all the pass points on the tool in carline. These will now become the primary tool reference in preference to those located on the tool base. Approximately 20 pass points are spread evenly throughout the whole volume of the tool. Once the pass-points are measured each master dowel hole location and every location pin on each unit is measured and adjusted to meet the design specification. The following paragraphs expand on the methods used to measure master dowel holes and panel location pins. Measurements of Master Dowel Hole There are 2 dowel holes to locate each form block but only one is usually measured. A reflector holder along with a reflector is located into the dowel hole, its position is then measured revealing its coordinates in car-line. A simple comparison to the nominal position for this point reveals whether the hole is in the correct position or needs to be moved by adjusting shims. As well as the master dowel hole being in the correct position, the master dowel hole locating face needs to be at the correct attitude in terms of 3 rotations since this will be crucial to the orientation of the form block. This attribute is verified using a bolt on master form block known as a PICO block. Instead of having a free form surface on one side, the PICO block has a series of precisionmachined holes in all of its visible sides that will accommodate a reflector holder. By measuring 3 to 4 points on this block it is possible to determine the attitude of the master dowel hole locating face. Measurements of Location Pins Two methods are used to measure location pins. The first is to employ a 2-point method. A location pin may be perceived by its centre line axis. To define the location pin axis it is necessary to measure at least two points along it. This is easily achieved by having reflector holders at different height offsets. A simple analysis will show the attitude of the pin. Another method is to digitise the pin by moving the reflector over the surface, and performing a cylinder fit. The resulting cylinder axis will represent the pin axis. PICO EUROPE employ the former method for the majority of the time. Once all master dowel holes and panel location pins have been measured and adjusted within design tolerance a report is issued. The report contains actual dimensions, design dimensions and deviations. The pass-point coordinates are also included in the report, which are to become the primary tool references and are essential for the next assembly and recertification stage at the customer s site. Line Assembly Customer Site Once the whole assembly has been certified and tested at the PICO EUROPE site it is dismantled piece by piece and transported to the customer s factory for final assembly, verification and commissioning. Panel sitting on location pin On arrival each tool is positioned into its respective position in the assembly line. Level datums are established at each end of the production line that the laser tracker is able to generate an extremely accurate gravity coordinate system to which all bases are levelled. Piano wires are used to roughly align all the tools. Final alignment and spacing of the tools is performed using the Leica laser tracker. Tool Verification Once the tool has been levelled and is situated in its final position the first step is to measure all the passpoints, approximately 20, using the laser tracker. Pass-points are now the primary tool references. A bestfit coordinate transformation is then performed and its results scrutinised. These results tell the operator much about the dimensional integrity of the tool. Firstly the operator checks that the overall root mean square error (RMS) of the coordinate transformation solution does not exceed a predefined tolerance, usually 0.13 mm. If the RMS value falls within the specified tolerance the tool s dimensional integrity is considered sound enough to warrant only one verification. Verification involves checking all location pins and 10% of master dowel holes. If these fall within tolerance the tool is considered verified. If however the coordinate transformation RMS falls outside tolerance, and any spurious points have been checked and re-measured, a re-certification is performed. Re-certification is required because either damage or flexure has compromised the overall geometry of the tool. Re-Certification Re-certification, as its name suggests involves a repetition of the activities previously performed at the PICO EUROPE site, i.e. 100% inspection of all features and adjustment to within tolerance. All the tools that form the line undergo the process described above before the production line is ready for testing and final commissioning.
Automotive Tooling Glossary of Terms Tool Base Form Blocks Master Dowel Hole Pre machined rectangular steel plate that forms the foundation of the tool to which all other items are assembled. Also premachined are all necessary datums, dowel holes and fixing holes. Pass Point This is the pre-machined dowel location hole on the unit and corresponding form block. Panel location Pins These are small steel blocks with holes in them that are welded to the units. The holes provide locations for reflector holders and will be used later as primary tool datums. Machined steel blocks that locate onto the units via master dowels. The location interface between form block and unit is machined flat, both parts having pre machined dowel holes for location. One of the other sides of the form block represents the freeform CAD surface of the car and will support the actual car panel. The relationship of the form block s freeform surface and the dowel hole are known. Units Shims Individual steel support brackets that are located and bolted to the tool base. Units have their own unique number i.e. Unit 103. Small steel plates for location between form blocks and units to facilitate linear adjustment in X, Y or Z axes. Pins that locate individual panels on the tool, usually 2 per panel. Pins locate onto units.
Checking the Freeform Surface of Form Blocks to CAD Data Some automotive manufacturers place an additional requirement, which is to measure points on the form block s freeform surface and compare them to the CAD design surface of the car. Ultimately it is this surface that will position the panels of the car before spot welding. The measurement results are presented using graphical reports depicting the panel of the car and coloured needle vectors representing the point error direction and magnitude from the CAD surface. Needles are also labeled with point ID and numerical deviation. Software such as Leica s Axyz CAD and Interprog may be used to perform this analysis function. Ultimately the decision whether to use the position and attitude of the master dowel hole or to measure to the freeform surface of the form block lies with the customer. Leica Laser Tracker for Repetition and Reproducibility Measurement At times it is necessary to perform repeatability and reproducibility studies on tooling. A typical example is to locate and clamp a panel into its position on the tooling, then unclamp and remove the panel. The panel is then relocated and clamped for the second time. If the tooling is well designed and manufactured, the panel should locate back into the same position to within an acceptable margin. Repeatability and reproducibility are attributes that are obviously essential for mass production tooling. Laser trackers are used to a great extent for these types of measurements, which often run up to 30 relocation cycles. Up to 8 reflectors are temporarily bonded to the panel with a hot glue gun. The reflector positions are evenly spread over the panel surface. A laser tracker with an Absolute Distance Meter (ADM) is required for this measurement exercise i.e. the Leica LTD500. Each reflector is then measured with the laser tracker. The coordinates from the first measurement cycle become the reference coordinates for all other measurement cycles. The panel is unclamped and removed from the tooling, then repositioned and re-clamped for the second time. For the second and all subsequent cycles the auto-inspect software routine is employed. The auto-inspect function drives the laser beam to each reference coordinate position in-turn and then measures the actual new position of the reflector giving its deviations. With up to 30 measurement cycles of 8 points the auto-inspect function is extremely useful. The auto-inspect function enables many data to be collected in a short time, typically 30 minutes for an 8 reflector, 30 cycle R & R study. Conclusion As mentioned in the introduction, this paper is designed to give the reader an insight and a flavour of how PICO EUROPE uses Leica Laser Trackers for production of automotive tooling. There are always exceptions to the rule in engineering and automotive production tooling can take many different forms. Therefore not every piece of production tooling follows the procedures described. «Since its introduction at PICO EUROPE the laser tracker has revolutionised the way in which we build automotive production tooling. The dimensional integrity of the tooling can now be guaranteed from initial assembly and certification at PICO EUROPE right through to the customer site anywhere in the world. If problems do occur with tooling, like panel misalignments, the possibility now exists for us to examine and precisely pinpoint the cause of the problem and implement corrective action. The laser tracker plays a major role in enabling us at PICO EUROPE to manufacture tooling to comply with the ever-increasing accuracy requirements of the modern motor car.» PICO EUROPE
Leica Geosystems Leica Geosystems is a leading developer, manufacturer and distributor of products, systems and software that capture, visualize and process spatial data through the employment of advanced technologies. Leica Geosystems supplies solutions to the varied users of spatial data. In several of these fields we hold the leading position in the world market. Headquarters Leica Geosystems AG Metrology Division Moenchmattweg 5 CH-5035 Unterentfelden Switzerland Phone +41 62 737 67 67 Fax +41 62 723 07 34 info.metrology@leica-geosystems.com Metrology Division The Metrology Division offers 3D measurement solutions for any demand in industrial processes, setting new standards for procedures in every major industry. Using state-of-the-art laser technology, we have made quality control and construction of large objects or small parts easier than ever before and accurate down to a few microns giving our customers unique advantages and cost savings in every field. Visit us at www.leica-geosystems.com/ metrology For an updated reference list and additional addresses, please refer to our web site.