QMT Features: October 2010
Making quick work of quality.
Model Based Definition for the shopfloor - portable CMMs makes this a reality bringing big quality benefits. By Ron Branch, manufacturing engineer.

For the machine shop that supplies the aerospace industry, PCMM (portable coordinate measurement machine) and MBD (model based definition) are everyday terms that are integral parts of its daily inspection routines. In the effort to drive quality and control processes, aerospace OEM’s have altered the inspection methodology and the tools of their entire supply chain.

Many of these suppliers are small or mid-sized machine shops that have invested in inspection hardware, software and processes to satisfy strict quality standards. With the success of these supply chain efforts, portable CMM’s and the Model Based Definition Inspection technology are expected to spread to all industrial segments.

Now, it would appear that these new inspection tools and processes would place a big burden on the machine shop while benefiting only the aerospace manufacturer. It is true that this approach to quality control and inspection does demand more than what the usual assortment of hand tools and gages can deliver. It places much more emphasis on CMM-based inspections, which could mean a big investment in quality control. However, the new quality practices actually benefit the machine shop, fabricator or tool maker and it costs a lot less then they might expect.

By deploying PCMMs to the shop floor, these small businesses are realizing great gains without staffing up in the quality control department. These companies are performing thorough and accurate measurements much quicker than previously possible. They have adopted a convenient and efficient measurement process that covers everything from incoming inspection through final part inspection. And they have gained greater control over their processes.

So, how do these small shops respond to the demands of aerospace manufacturers and realize all of these advantages? They have adopted a rapid shop floor inspection approach: taking 3D measurements on the floor at the source for immediate feedback and reporting against the Engineering quality specifications.
This approach is a growing trend outside of the aerospace industry and a strategy that many non-aerospace machining and fabrication shops are using today.

The tools
There are three items needed to implement rapid shop floor inspection: hardware, software and engineering data.

To conduct inspections on the shop floor, a portable CMM is needed. Although there are several options, the common PCMM choice for the small machine shop is typically an articulated arm with a positive contact probe. Temporarily mounted on any rigid surface, these lightweight devices are easily transported anywhere in the shop. Arms have joints that let the machinist extend and rotate the measurement probe into every channel, bore or pocket. With a reach of 2 to 12 feet, the arms do not have physical size limits common with stationary CMMs. PCMMs can also be outfitted with laser scanners to capture dense point clouds of measurement data. (For large items and structures, PCMMs take on other forms, such as laser trackers. Using lasers and infrared light, long-range PCMMs can capture measurements to distances of several hundred feet.) These features make PCMMs versatile inspection tools.

The second component of a rapid shop floor inspection system is the software that receives all of the information from the arm. In the most basic mode, the software will log and report measurements taken with the arm. However, the big gains in time and efficiency happen when the software’s full functionality is leveraged. For example, the software will create prompted inspection plans as datums and features are selected while in teach mode. It will also provide real-time inspection data with visual, on-screen references. To really expedite the inspection process, it can import a CAD reference model for direct comparison of the manufactured part to its design intent.

Since the arm will be used in a variety of applications, the software will allow the user several modes of operation.

• Direct measurements for on-the-fly spot checking as an alternative to a hand tool.
• Measurement to drawings in accordance with datums, dimensions and GD&T callouts.
• Direct comparison to a CAD model that contains the dimensional requirements.

The final piece of the rapid shop floor inspection solution is the quality specification and inspection plan. As just stated, the inspection work can be performed by referencing a part’s engineering drawings. This approach works fine, but the inspection process becomes much quicker when that data is assigned to a digital reference within the PCMM’s software. Start by importing the 3D CAD model and then assign the quality specifications from the drawings to the digital model. This allows the software to prompt where measurements are to be taken and immediately report a pass/fail condition.

Model Based Definition (MBD)
With model based definition (MBD), a 3D CAD model is the sole reference object for all details and specifications regarding a part, sub-assembly, product or tool. In its purest form, no other documentation is referenced when describing the characteristics and qualities of an object. So, MBD is an approach where the CAD file provides much more than a description of the design intent.

In the realm of inspection-related activities, MBDs supply all of the dimensional and tolerance information. There are no drawings. The CAD model contains all of the specifications, including the GD&T callouts. The dimensional specifications may be either annotations within the model file or digital specifications linked to a feature. With the latter approach, inspections are completed simply by referencing the CAD model and picking the appropriate points on the object. This eliminates all manual documentation and data entry and expedites the inspection process.

An advanced concept linked to MBD is minimum dimensioning. With this approach, a global profile tolerance is applied to the CAD model and only those features that are critical to function are explicitly defined through GD&T callouts.

For shops whose clients have adopted MBD, the process becomes even faster. Since the model contains all the quality specifications, it is simply imported, and a few moments later inspections can be performed. This eliminates all the manual effort to interpret drawings, document measurements and report against the prints.

The PCMM goes to the source of the inspection, which can be on the loading dock, manufacturing floor, machine shop or tool room. All that is needed is a little space and a stable work surface to mount the arm and place the part. With battery operations and optional wireless communications, the truly portable systems do not even require a wall outlet or a cabled connection. After a quick calibration routine, the system is ready for operation.

With these three components—PCMM, software and quality specifications —machine shops have a portable inspection tool that is the equivalent of a cart full of hand tools and gauges and a roll of engineering drawings. Everything that is needed to determine go/no go is bundled in a rapid shop floor inspection tool that travels to the work piece.e the portable CMM to confirm first articles before they are loaded onto the truck. In these instances, the shop gets early detection of quality problems and a thorough set of data information that helps to diagnose a process that is no longer in control.l

Author: Ron Branch, Manufacturing Engineer, V & M Precision Machining and Grinding.
email: rbranch@vm-machining.com
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