Advanced metrology software lets shops probe almost any part shape while the part remains in its fixture on a machine.
Lockheed Martin conducts on-machine, finished-part inspections with PC-Dmis/NC 3D validation software.
Metrolosys software uses G-code programs to drive probes to part features for on-machine measuring.
Lockheed Martin's Missiles and Fire Control facility in Orlando, Fla., does "true on-machine" part inspection. Special server-based metrology software lets the shop run complex 3D part-measurement programs on multiple machine tools, correlate the results to analyze individual components, and compare and document the results from one machine to another. The software provides Lockheed Martin an all-encompassing process-control package that performs beyond the capabilities of standard probing macros.
Lockheed Martin's inspection program is based on PC-Dmis/NC on-machine 3D validation software from Wilcox and Associates Inc. (www.wilcoxassoc.com). Wilcox and Associates is a Hexagon Metrology company.
Lockheed Martin conducts complete finished-part inspections on the machine tools that cut its parts, eliminating the need to remove parts from fixtures and transfer them to a coordinate measuring machine (CMM). On-machine part inspections also allow the shop to reduce or skip some of the quality steps that were previously required and relieves workload bottlenecks at the shop's CMMs.
According to Jan de Nijs, senior staff manufacturing engineer for Lockheed Martin, PC-Dmis/NC gives the shop's machine tools true CMM functionality. This is a must because, after machining a part, finished-part inspection requires touching off on all the part datums and evaluating measured features to the datum structure. All Lockheed Martin's parts are dimensioned per the ANSI Y14.5M1982 standard, so the software has to be able to properly define those datums on machine tools to complete finished-part inspections.
At Lockheed Martin, PC-Dmis/NC resides on a standalone server/PC connected to multiple machine tools that run concurrently. To start its on-machine part-inspection process, the shop creates a separate probing program using PC-Dmis/NC off-line. That software post processes the program into machine tool G and M codes and sends it to the appropriate machine. Operators then select the program from the machine's control. Because the probing program is created in PC-Dmis/NC, when it is executed, it automatically announces itself to the server that is running PC-Dmis/NC.
When the probing program is launched, the server starts to collect the measurement results. It "listens" to those results to perform analysis, generate geometric dimensioning and tolerancing (GD&T) reports, flag errors or print out dimensions. It also can update machine offsets when they are needed.
Wilcox and Associates said the PC-Dmis/NC for machine tools is actually the metrology software it developed for CMMs. The program offers the same capabilities whether it is used for measuring or for geometric dimensioning and tolerancing functions. The machine tool version is applicable to machining centers, multitasking machines and lathes, and any other machine tool that can be equipped with a probe.
Lockheed Martin is using the program to reduce the workload it places on its CMMs by increasing part-sampling sizes; it is not trying to eliminate its CMMs. After doing first-part inspections on a CMM, the shop can verify its machining processes are running within specs by using on-machine inspection. Rather than checking, for example, every third part with its CMM, Lockheed Martin could stretch its sample sizes to every tenth part or have the CMM inspection focus on just critical features. Another benefit is that fewer inspection requirements streamlines part flow through the shop.
Like Lockheed Martin, a lot of shops are doing some type of on-machine measuring. Whether it is for finished-part inspections, adaptive machining (see "Adaptive machining" sidebar) or validating part positions in fixtures, recent developments in metrology software are making those jobs easier.
For example, NX5 software from UGS (www.ugs.com) gives NC programmers the power to program probes. This software makes on-machine probing an NC operation within the part program, as opposed to manually inserting probing routines into the G and M code files, a practice that is subject to errors.
The NX5 software moves probes around workpieces like cutting tools do. And, because it actually uses CAM toolpath technology, the software can maneuver probes into much more complex and intricate positions than are achieved by manually driven probes.
NX5 software users can determine exactly how they want a probe to move to a particular position or positions before they take a measurement. In fact, NC programmers not only gain the ability to define the probing operation — the probe's approach and where to measure — but also the ability to simulate the operation before it runs. Unlike manual probing, NX5's simulation offers accurate visualizations of the probing process to pinpoint potential collisions ahead of time.
Probe simulation happens in an interactive, step-by-step environment within the NX5 software. When the programmer adds a probing step, he sees that step. There is no lag between programming and simulation.
According to Sharad Mundra, a product manager at Marposs Corp. (www.us.marposs.com), what shops have been doing on CMMs can now be done on machine tools because of advancements in probes and in metrology software. Advanced probes work beyond two-and-a-half axes to move to any part vector and to measure it. They are designed and capable of repeating to tolerances of one micron. These capabilities are essential to conduct on-machine part inspections with programs such as Marposs's 3D Shape Inspector metrology software.
Typically, shops run probing macro programs from a machine tool's control, but Mundra said that such probing routines only work in two dimensions. His company's program is designed to work in three dimensions to allow shops to measure any type of odd part shape on the machine tool that cut them. That includes slopes and tapers. The program is designed to touch — or probe — as many as 20 point locations to accomplish the measurements.
The software resides either in a machine tool's control or in an external PC. It is a CMM software program that Marposs converted into a G-code-qualified inspection and verification tool that can be used on moving machine tool axes. At the part-programming stage, the Marposs 3D Shape Inspector software generates any style of report that a CMM would, including GD&T.
Shops can also change report styles to suit their needs. For example, they can produce GD&T reports to rework out-of-tolerance measurements immediately, without having to remove the workpiece and send it to a CMM.
Metrolosys metrology software from Creative Products Co. (www.metrolosys.com) also uses G-code programs to drive touch probes on CNC machine tools. The Metrolosys software does not require special software or modification to a machine's CNC and operates as a standard "skip" cycle (usually G31) and an output statement (usually DPRNT) to send data through an RS-232 port or other external port on the machine tool.
Because the software does not rely on special programming macros and because it does not change a machine tool, it is compatible with all probe-ready CNC machines. Its capabilities include probing G-code, web-ready HTML inspection reports, collision avoidance for surface models, built-in probe-calibration routines, and DMIS traceability, and it adheres to ASME U14.5M standards.
| ADAPTIVE MACHINING |
Delcam's (www.delcam.com) on-machine verification (OMV) version of its PowerInspect software is geared toward adaptive machining. PowerInspect OMV allows shops to detect errors earlier and at all stages of the manufacturing process.For example, operators can check that the correct amount of stock has been left on a component after a roughing operation, rather than waiting until all machining operations are complete only to discover an error has been made. Similarly, with the software, shops can accurately assess the extent of any damage caused, for example, by tool breakage and can determine immediately whether the part can be completed within tolerance or scrapped.
PowerInspect OMV also offers off-line programming of inspection sequences for machine tools along with fully integrated simulation and collision checking. The software's multi-axis option supports four, five or six-axis equipment, and the options main advantage is its ability to check under cut regions without having to reposition workpieces.
According to Delcam, PowerInspect OMV most benefits those shops that do not have existing inspection capabilities but have CNC machine tools.
For UGS, adaptive machining is the wave of the future. The company strives to apply logic at machine tools in real time after adaptive-machining data has been collected in the registers of machine controllers. Basically, machines would understand "if-then" scenarios and self-adjust their own offsets. Or as UGS foresees, machine tools could refer back to the CAM software and actually change to or delete a specified toolpath or re-compute a new one based on previously made on-machine probe measurements. And all this functionality would be packaged in an off-the-shelf software.
RELUCTANT TO TRY ON-MACHINE MEASURING
During these conversations, Mundra talks to his customers in terms of process control and how much time a machine tool will sit idle if the shop's CMM department finds a part is bad. Also, he does not suggest on-machine checking every part, but instead, tells shops to determine a part-measuring schedule that takes up little cutting time. For instance, they could check every tenth, twentieth or fortieth part in a job.
In the long run, shops that spend a few minutes doing on-machine gaging avoid a lot of potential machine downtime resulting from machining errors, scrapped parts and broken or worn tooling.
"Shops need to think of on-machine measuring as a control process situated between the machining and inspection processes to develop a ‘first part, good part' strategy," advised Mundra.
NOT JUST ANY MACHINE TOOL WILL DO
A shop can't just pull out a machine tool that has been working in the back for the past five years and start doing finished-part inspections on it. Jan de Nijs of Lockheed Martin said: "You have to know the machine, what it can and can not do as far as accuracy is concerned."
Lockheed Martin regularly calibrates the machine tools that run finished-part inspections using the same calibration routines (lasers, ballbars and such) it puts its CMMs through.
"While some may argue that the same machines cutting parts can't be used to measure them, it is possible as long as a shop has processes/systems in place to keep machines maintained, calibrated and accurate," de Nijs said.
According to Sharad Mundra of Marposs, shops doing on-machine measuring should at least conduct ballbar testing on those machines every three to six months.