Advancements in manufacturing equipment such as Swiss screw machines and high speed CNC-milling systems have revolutionized precision part production. Some Swiss screw machines lathes have guide ways which hold bar stock at lengths of up to 12 feet (300mm) of steel, polymer materials, and composites. Part features are generated simultaneously by moving the bar stock and the cutting tool to create the component. This capability has introduced considerable speed advantages and optimized cutting of shapes to create with higher throughput than ever before. Part accuracy is superb (+/- .00002 in) and part-to-part consistency is greatly improved.
With the evolution of this machinery and their related processes, manufacturers of high-to-medium volume turned parts are facing increased inspection requirements in terms of frequency, feature measurement, and production statistics, as well as rising accuracy specifications. Manufacturers are seeking advancements in measurement technology, such as the speed of measurement cycles, the ability to check parts with greater accuracy, and the ability to precisely measure geometric features and free-flowing forms.
Metrology technology has come a long way in the last five years, and there are tools available to meet such exacting inspection requirements. The right solution of course, depends on the particular needs of the end user. We are going to explore three different, but effective, approaches to accurate measurement of turned parts in the medium-to-high volume production environment.
Non-Contact Turned Part Measuring Centers
The dental implant industry is a compelling example of an industry making use of opto-electronic non-contact measuring systems. Manufacturers are producing many size and shape variations of the main 'core' components of a dental implant via turning/thread cutting as performed using CNC turning technology in a high volume production environment. The implants, also known as prosthetics, are human spare parts, and therefore quality is of utmost importance; all functional dimensions must be inspected on a 100% basis.
The components are very small, typically less than 4mm in diameter, with many dimensions that are very difficult and time-consuming to measure using traditional gauging methods like optical projectors, toolmaker’s microscopes or hand-held tools like micrometers. With increased levels of production and quality demands, and the variety and volume of components produced with increasing measurement criteria, non-contact measuring systems were the next frontier in metrology for this industry.
Today, a single non-contact automatic rotary profile measurement device such as a TESASCAN 25 can be dedicated to a cell of 2 or 3 CNC lathes producing one product type. The object to be measured is fixed in a rotary mandrel and turns (if required) while a light projects the profile onto a collection sensor array, which digitizes the image. The software then measures the pre-programmed features using the digitized image as a guide. Typical cycle time for 12 dimensions on a part is 28 seconds. Often, production engineers are responsible for part programming, which is performed on and off-line, while the operators use the measurement system to automatically inspect the components. CNC operators can check all critical external dimensions with a single piece of equipment. Profile devices such as the TESASCAN offer advantages over laser systems both in terms of overall accuracy, speed and the types of features that can be measured.
Once each component is measured, the results can be displayed numerically and graphically together, with the facility to analyze measurement data statistically in the form of histograms, control charts, and capability reports. This capability, previously impossible with manual methods of inspection, has provided a statistical base for determining process trends and adjustment of individual machine tools. An added bonus is that measurement data is traceable to individual machines using either a batch number or machine tool identification number entered by the operator.
Automatic thread measurement can be accomplished if the machine incorporates a slewing device which tilts the part for better measurement of machined threads, known as helix angle compensation.
Another advantage of this type of tool is scalability. If a shop producing turned parts has the capability for various sized end products, turned part centers can be purchased that will accommodate much larger turned parts; up to 500 mm long and 80mm in diameter. However, this type of tool does have limitations. Generally they can only accommodate a single part at a time, and must be loaded and unloaded manually, even though the measurement cycle is automatic. If the feature for measurement cannot be seen in profile, such as a machined channel, then it cannot be measured.
For production machining, one of the first benefits found in vision measurement technology is the throughput speed as found in a piston valve automotive application. For example, when measuring via a conventional method, piston valves can take up to an hour or more just to collect the measurements of the part. Plus, that inspection time does not include the data compilation and statistical analysis necessary for many of these components. With the introduction of rapid vision measurement systems, the complete inspection process can be cut down to less than five to ten minutes per part.
A component like a piston value also has very exotic shapes and radius blends that require inspection. Once again, the vision measurement technology provides the right solution. By using profiling calculation tools, the task of measuring very complex curves is cut down to a simple profile trace. Adding simplicity to the use of the calculation tools is the ability of supportive CAD referencing with a software package such as PC-DMIS Vision. With some simple steps, the CAD model is adjoined to the physical part to enable point-and-click support in measuring the complex profiles. This process is possible for many types of parts such as small medical components with profile tolerances of microns or electronic components with gap and height measurements in the submicron levels.
There are many other types of parts which are ideal for vision metrology. Part data is generally collected in seconds and supported by pick-and-place trays which minimize operator handling. Feature location and feature form can be measured from the data acquired. The price range of these systems is normally based on capacity, speed and accuracy, resulting in systems that fit a wide spectrum of budgets. A vision system like the Brown & Sharpe Optiv 1 Classic fits an entry level budget. At the other end of the spectrum, a Brown & Sharpe Optiv 3 Performance provides extreme precision with submicron accuracy. Vision metrology can truly be called the micro metrology tool of the future.
And the vision metrology world continues to evolve. The introduction of interactive multiple sensors have truly revolutionized how 3D measurements can be performed in vision based systems. The various sensors provide the building blocks to expand or enhance a vision system. These enhancements are either tactile, like touch probes holding tolerances down to a couple of microns and providing articulation when necessary. Articulation permits the measurement of features that may not be in-line with the vision sensor. Additionally, non-contact sensors such as laser and white light scanning probes can drive precision down to submicron levels and in some very special cases, angstrom levels of precision. Lasers provide selective support for measurements of form using a real-time rotary or tilt axis. White-light sensor technology is ideal for certain very tiny features such as small steps and 3D forms. The correlation of these sensors in combination with the optical sensor provides an unparalleled benefit for speed and agility of vision systems.
Touch Probe Systems
The final solution for production machining offers the highest throughput option, and involves incorporating measurement touch probe on the turning center itself. The following company has reaped the benefits of such a system. Davromatic Precision Ltd. of Rugby,UK, is a second tier supplier for aerospace, defense and heavy machinery industries. On any given day, the company faces a balancing act as a manufacturer of precision turned parts. On one hand, they need to produce precisely turned and milled parts with tolerances of only +/- 8 microns. On the other hand, they need to keep costs down to a minimum. The investment of a turn-milling-center with an integrated touch probe turned out to be the most economical investment.
With very high volume production, each pause in production and each manual adjustment of the turning center affects the productivity and the profitability of the job. In order to ensure the quality of parts with tight tolerances, and prevent process drift, an integrated metrology solution was needed for permanent monitoring and adjustment of the machining parameters.
Davromatic implemented a hardwired infrared touchprobe system by M&H Inprocess Messtechnik GmbH that allows the turning lathes with movable heads to inspect turned and milled contours, while the part still is on the machine in its counter spindle. The touchprobe is mounted on a mounting bracket fixed near the main spindle-head, in order to move the cut off workpieces for dimensional measurement to the touchprobe by the counter-spindle.
Measuring critical dimensions such as the outside diameter, length, width of hexagon cross-sections, and milled surfaces is accomplished in just seconds, and since the measuring process takes place on the counter spindle, it can be done independently of the main spindle, reducing the impact to production even further.
Davromatic has also realized secondary benefits of the system in terms of monitoring tool wear and premature failure. Some alloys used in production were contributing to inconsistent and uneven tool wear, which caused production to drift out of tolerance quickly. Sampling every part allowed real time monitoring of this situation so that inserts could be changed before a lot of expensive material was scrapped and production time lost.
Overall, the ability to achieve a 100% sampling rate automatically and do in-process adjustments to the machine tools has enabled Davromatic to increase productivity by approximately 20%, while reducing scrap to virtually zero.
This scenario represents an extraordinarily successful result of in-process measuring. However, this technique does have some important limitations. First, since this method employs a touch probe, everything that has to measured must be able to be touched. Depending on your part, some features may be too small to be touched, or a single size touch probe cannot adequately inspect everything that needs to be inspected. Complicated geometry may be beyond the capability of such a system. Second, it does not represent an independent verification of the quality of the part. If the machine itself is inaccurate, the results might be suspect. Some quality programs or customers require independent verification of results. One option might be to use a touch probe system to monitor and control production in combination with an offline solution for final verification.
The good news is that there are more choices than ever to inspect turned parts, from off-line to in-process and even hybrid solutions incorporating a combination of techniques and equipment.
In conclusion, as the manufacturing of cylindrical components continues to evolve and expand, a variety of metrology systems are meeting the quality requirements and the specific budget needs of this industry. Everything from bone screws to valves to plastic components can be measured with confidence and reliability. Yes, today's manufacturing standards demand speed and greater accuracy, and the measurement industry is ready to provide an affirmative solution. The end result can be significant reductions in inspection time and improved process control - both of which contribute heavily to any manufacturer's bottom line.
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