"On-Machining Inspecting," Feature Article, April 2005

on-machine inspecting

Turning ... and milling ... time into cash.

by Paul Sharke, Associate Editor

If you aren't making chips, you aren't making money. So goes the reasoning that keeps on-machine inspecting out of many shops. It's a popular, though flawed, objection, according to Barry Rogers, director of sales for Renishaw Inc., a Chicago-based metrology equipment maker.

Although measuring a part when it's clamped into a machining center uses up metal-cutting time, doing so can actually save minutes upstream and downstream of the machining cycle, Rogers said. That can lead to faster process cycles overall.

On-machine inspecting has helped to turn around business for at least one machine shop, Tech Machine in Colorado Springs, Colo., where it's made a big dent in scrap rates. Tech Machine manufactures tight-tolerance stainless steel and titanium medical parts that have many complex curves and very smooth finishes. According to its owner, David Wiggans, the company has now fitted all 13 of its vertical machining centers with probes.

On-machine inspecting requires a bigger user commitment to reap its rewards.

Probing checks setups and tells the machine exactly where the workpiece is before a single cut begins. It also checks the tools themselves for diameter and z-axis depth, Wiggans said. Machines can automatically inspect fragile small-diameter drills and flag any part that breaks one so that it receives no further machining. Probes are also used to detect an error, such as an unclamped work pallet, which, left unfound, could ruin every tool, part, and fixture on the machine as it starts up.

Of course, Wiggans knows of the objections to on-machine inspections, that they eat into metal-removing time. Of this, though, he's found the opposite to be true: "For a probing routine that takes 30 seconds and saves me from ruining one part, the time it takes is immaterial," he said. A part that takes 20 minutes to make but goes to the scrap pile is actually a 40-minute part, he added.

macroeconomics

A typical just-in-time order for Tech Machine might total 500 pieces in all, but consist of multiple small-quantity lots, some only a few parts long. The company typically deals in parts "families," Wiggans said, where one common shape might have several different variations in length, numbers of holes, and so on. Thanks to on-machine probing, a pallet of parts from one family can be loaded into the machine and then probed to identify, by shape, what each part is and which sequence of machining each requires. What used to require 18 different programs on the computer numerically controlled, or CNC, machine—one for each part—is now handled by a single master program, Wiggans said.

On-machine inspecting may just now be trickling down to small shops like Tech Machine, although it has been embraced quite favorably by bigger aerospace and automotive manufacturers, Renishaw's Rogers said. Rogers was a bit surprised when he paid a visit to the Colorado manufacturer to see what Wiggans was doing with all the probes he'd ordered.

Wiggans, a self-professed computer guy, had written all the macrocode himself to integrate the probing routines into his machining centers. That's something which may be a little beyond the calling of the average machine shop operator, he suggested. But after taking on the task, Wiggans is now sold on the benefits of on-machine probing.

A coordinate measuring machine brings flexibility and precision to inspecting, while working with commercial software.

"The whole idea behind on-machine probing is to eliminate variation in a process," Rogers said. "Anytime variation shows up there it turns into repairs, rework, or dollars lost."
Some of those variations arise in the machine itself. Everyone is familiar with the Monday morning syndrome, Rogers said.

That's where a machine that's running off parts on Monday morning has to have its offsets changed manually as the machine—and the shop itself—warms up. Thermal expansion is one of many variables that creep into a machining process.
Renishaw recently introduced artifact, or reference, probing as a way of dealing with transient thermal errors.

The artifact is a dimensional master, mounted next to the part in process, that grows or shrinks with the workpiece. Probing the artifact just before making a critical cut lets the CNC machine compensate for any variations that may have developed. Artifact probing is one of several techniques that enable parts to be bought directly off the machine that makes them, without their having to pass through a coordinate measuring machine or other quality control check.

Renishaw uses artifact probing in its own manufacturing operations to enable its machining systems to run unattended 140 hours a week, Rogers said. At certain points in the machining cycle, a spindle-mounted probe references a dimension off a nearby artifact, then the machine compensates for any scale, geometric, or thermal errors. Renishaw reports that a large commercial jet engine manufacturer uses artifact probing as well to maintain accuracy on large-diameter fan containment cases.

Artifact probing is one method of defusing another popular argument against on-machine probing: that you can't measure parts on the same machine you cut them with.

One very important step in using machine tools as coordinate measuring machines is making sure they are calibrated and able to produce accurate measurements. Laser and ball bar systems help with this critical step.

lost in translation

Still, a machining center is built stout for handling its principal task of shaving metal. It is not designed with the flexibility or the accuracy of a CMM in mind. Even the code used to govern its motions is fairly crude.

The very first numerically controlled machine, a three-axis milling tool built in 1952 for the U.S. Air Force, took its instructions from punched tapes in what would come to be called M&G code, according to Ed Red, a professor of mechanical engineering at Brigham Young University in Salt Lake City. Today, the typical path that a part follows from design to manufacturing involves modeling it in CAD, using CAM to plan the tool paths for making it, then generating files—such as CL for "cutter location" and APT for "automatically programmed tool"—which are ultimately post-processed into M&G code to run the machine.

In many ways, the current state of CAD/CAM embraces the barriers that have sprung up between engineering and manufacturing over the years, Red said. Engineering designs products, establishes tolerances, and chooses materials. Manufacturing selects machines, picks tools, and plans cutting paths.

Information flows only downhill, Red said. A change instituted at the machine tool—a frequent occurrence—has no direct route back up to the CAD drawing, other than via a complete regeneration of all files and the M&G codes. A fillet that's not right or a hole that's misplaced loses any association with the original design, he said.

It was recognizing that controllers were increasingly digital that led the university's researchers to develop Direct Machining and Control along with its adjunct, Direct CMM. M&G code at the machine represents something akin to a "raw text file," according to Red, and it is very good at making radius and line moves, but not nearly as adept at moving over complex surfaces. The situation has had little transformation over the past four decades and is now holding back new machining methods, he said.

Direct Machining runs tool paths that are based on four different CAD/CAM systems—from ContourCAM (above) and Catia (below), as well as on systems from UGS and Alias.

Indeed, the communication on a typical numerically controlled machine is "primitive," according to Karl Sipfle, a senior interface developer at Wilcox Associates of Danville, Calif., which makes the metrology software PC-DMIS. Compared with a coordinate measuring machine, a numerically controlled machine is an infant in its capacity for real-time conversing.

The idea behind Direct Machining is to use the geometry of a CAD file rather than some interpreted version of it to operate the machining servos. The structure comprises three parts, including the CAD/CAM system, a motion planning system, and the servo controllers themselves. All reside on a PC, Red explained.

Red and his colleagues have demonstrated the capabilities of the systems by machining a test surface chosen from a Ford GT automobile. The system developed tool paths from the commercial versions of four different CAD/CAM systems— UGS, Catia, ContourCAM, and Alias. He discussed his findings last November at the ASME Congress in Anaheim, Calif.

A direct, reversible link between a machining center and a CAD system ought to attract the interest of auto and aerospace design studios where designs can change several times a day as they are machined. Any tweaking of tool paths shows up on the CAD model.

Another advantage of direct machining is the way it opens up on-machine probing to the model itself. Direct CMM, according to Red, addresses one of the two major obstacles standing in the way of widespread application of machine tools as CMMs. Those two obstacles are accuracy and communication. Hurdles to accuracy are being scaled by companies such as Renishaw. Direct CMM addresses the communication obstacle, he said.

One of the difficulties shop owner Wiggans encountered in setting up an on-machine probe system was writing the macros for the machine tools to work as CMMs. He had to rely on his understanding of M&G code to make the machine act in the manner he wanted.

Direct CMM would have enabled him to use a commercially available CMM software package, such as PC-DMIS, to instruct his machine tools as though they were CMMs. That would have made his task easier, he said.

Red envisions a time when a CAD/CAM program will reside on the same computer with a coordinate measuring program and both will share control of a machine. For shops that cannot afford a CMM, this will provide a low-cost metrology solution.

But the real strength of such an advance will be in the way it allows process updating of the manufacturing as it happens. This will be a big advantage for automakers and aerospace manufacturers who can afford CMM equipment but look for any way to improve cycle times, Karl Sipfle of Wilcox said.

A part that can be measured at the machine and corrected there is one that avoids moving between the numerically controlled machine and the coordinate measuring machine. Small cuts can be made and then checked immediately. A tendency for an operator to overcompensate on a final cut is lessened, Sipfle said.

Sipfle, whose company makes only software, talks with all the major CMM makers and many of their users. For many users, the coordinate measuring machine is only one of a variety of inspection techniques, such as photogrammetry and vision, that they are using currently, or may use one day. On-machine probing is coming, he said—it's inevitable—but it may be a bit early to start seeing its wholesale adoption.