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Finding the pluses in high-precision accuracy.

Ratios and processes hold the key to understanding limits.

The concept of high precision in machining isn't too different from tunneling under the English channel. Failure to achieve the desired results--tunneling from one side to meet the other in the middle--can be devastating, especially to traffic between France and England.

In manufacturing discrete parts, missing the accuracy specs will result in the same dead-end. The need for high precision is more important today than ever before. One reason is that precision and accuracy, which are the keys to improved performance for everything from automobiles to medical devices, have become the rallying cry in our quest for quality.

Pictured here and on the cover is the strut of a Ram Air Turbine (RAT) being machined on a Mitsui Seiki JIDIC machining center at Sundstrand's Aerospace Div, Rockford, IL. The RAT is designed to provide emergency hydraulic and/or electrical power in case of engine failure on a jetliner. The strut was machined from a 36" long 7075 aluminum forging fixtured to a two-axis rotary table on a JIDIC machining center, which delivers positioning accuracy TABULAR DATA OMITTED of 60 millionths on XYZ axes and 40 millionths repeatability.

Manufactured products, whether they are airplanes or toasters, simply work better and last longer if the components from which they are made fit better and meet designed specifications. It isn't always clear-cut what the spec ought to be--for example, the clearance of the cylinder in its bore--that's a challenge facing the product designers.

In assessing the challenge that ever-tightening tolerances is placing on US manufacturers, Dr Dennis A Swyt of the National Institute of Standards and Technology's (NIST) Precision Engineering Div iterated the many aspects of dimensioned geometry. For shapes (such as blocks, spheres, cylinders, and cones), the key considerations are:

Size (length, width, diameter);

Positional relations (true position of a feature with respect to a specified coordinate system within the part and distance between any two features);

Form (including circularity, cylindricity, and sphericity of rotationally symmetric features);

Finish (smoothness, flatness);

Angular relation of features (including orientation, parallelism, and perpendicularity);

For non-prismatic shapes, curvatures complicate the machining of the part's geometry.

According to Dr Swyt, the National Center for Manufacturing Sciences' assessment of state-of-the-art conventional machining indicated that tolerances had decreased by a factor of five during the decade of the 1980s. His own estimate is that the overall trend to tighter manufacturing tolerances is showing a decrease by a factor of three in the size of tolerances every ten years.

Relativity in accuracy

Charles F Carter, vice president-technology, the Association for Manufacturing Technology (AMT), Washington, points to relativity in the concept of accuracy and high precision. "If you're making a 1000-ft long supertanker and you would like to control total length to an eighth of an inch, that's probably high precision. However, if you're making a valve spool that's a half inch in diameter, high precision there might be better than 10 millionths. Similarly, high precision for the surface of a mirror used in a laser might be 2 millionths."

People have defined high-precision accuracy differently, explains Dr Robert Hocken, professor of mechanical engineering at the University of North Carolina, Charlotte. "The concept of high precision is not that hard to understand if you look at its two aspects.

"First, it is the ratio of the size you're making to the tolerance. Suppose you want to make an inch-long part within a thousandth. The ratio is 1000. That's not really high precision. But if you try to make it to a tenth, then you are talking about a ratio of 10,000, or one part in 10,000. That's higher precision, but one part in 100,000 would be ultraprecision," explains Dr Hocken.

The first rule, however, doesn't apply to every situation. The second consideration is the process that is being used. Is the tolerance of your machining or forming process approaching the state-of-the-art?

According to Dr Hocken, in forming an auto fender, with all the variables, like springback, the state-of-the-art used to be a tenth of an inch. Today, it's getting closer to a hundredth of an inch, so that you can roll your ball bearing down them. "That's precision," says Dr Hocken. "It's the ratio of the size to the tolerance, with the process folded in when needed."

In the world of conventional manufacturing, however, machining takes on a whole different view from that done in laboratory or clean-room conditions. Conditions include everything from the ambient temperature and humidity to the characteristics of the machine's components and structures.

Put another way, Dr Hocken says that trying to do an honest tenth in a normal shop on a part a foot long is a serious bit of precision work. "You walk up to a machinist and ask him if he can do two holes true position on this milling machine to a thousandth, and he says...well, yeah...maybe." Maybe that's another reason why machining a prismatic part is like "chunneling" under the English channel.

Knowing exactly where the hole or channel is at any point in the process is extremely important. That's why true position measurement is becoming the standard for two- and three-axis and dynamic accuracy for contouring measurement, explains Scott J Walker, vice president-sales, Mitsui Seiki (USA) Inc, Franklin Lakes, NJ.

"True position measures your ability to bore one hole in one location and then bore a hole at a different XYZ location in a prismatic part and know how close the center of one hole is to the center of the other hole." It goes beyond positioning and repeatability, which are measures of single-axis accuracy. Locating machining cuts by true position is critical for machining prismatic parts such as twin screw compressors, engine blocks, and a host of other products that depend on the proper and accurate alignment of machined slots, channels, holes, and grooves.

A new test called telescoping ball bar permits measuring machine tool precision exactly in three axes, says Mr Walker. Described in ANSI/ASME Standard B5.54 1993, the test provides an extremely accurate way of testing and calibrating a machine tool's accuracy in X-Y-Z positioning in a workpiece envelope.

Just how critical?

AMT's Mr Carter believes that both assembly and performance would benefit if variability of parts were minimized. He argues that if you could make ball bearings--the inner and outer races and all the balls--exactly the same, you wouldn't need selectivity in assembly, where components are classified and grouped so that they match up more precisely. A NIST research project on manufacturing pistons is intended to do exactly that--find the optimum design parameters of a part which changes shape, from oblong to round, when it is functioning.

What is obvious in this example is that making parts to print tolerances, however tight, still allows for some variation--within the specification's parameters. It's not necessarily a poorly written spec, either. The state-of-the-art in design engineering may not have caught up with every variable in the product's performance.

There's no way of knowing how a new part or a replacement part is going to function in the application or on the production line without testing a production sample. In the auto industry, this testing is usually done with a carefully-crafted prototype part. Olofsson's Prototype Machining Div, West Branch, MI, now offers production- quality turned prototype parts for transmissions in quantities sufficient for production and performance testing.

The service duplicates a high-accuracy, high-volume manufacturing environment while producing low-volume quantities of as many as 10,000 turned parts for process evaluation. The high-precision parts can be run at the various operation stations of the production line in run-offs or used by builders of the machine tools to run-off machines being built for the production line, explains Tony Buckett, sales manager of the Prototype Machining Div.

Process capability

One of the most important ways of looking at process capability has come out of the auto industry's penchant for statistical process control and analysis. Cpk, which is a measure of the ratio of the specification range to the machine's capability, is the key. A Cpk of 2.0 means that the spec range is twice as wide as the machine's current capability, which is very good. All parts are well within tolerance. As the machine's capability changes, the ratio will drop, with a Cpk of one meaning that the machine's capability exactly matches the specification.

The Cpk is a good measure of the machine process because it takes into account everything about the machine, its tooling and fixturing and all its components. "It's something you can measure when the machine is new and measure periodically as the machine begins to age and wear with use to see how your process capability is affected," says Mitsui Seiki's Mr Walker.

At Cincinnati Milacron's Plastics Machinery Div manufacturing plant in Mt Orab, OH, machining centers are now routinely producing |+ or -~0.001 tolerances on parts cut on Milacron machining centers. This ability to hold a tighter tolerance eliminates the need for a long and costly trip to a dedicated boring mill operation after the machining center.

Though machine slides would position to tenths accuracy on just about all machining centers, Milacron explains that thermal expansion and roll/pitch/yaw effects cause errors which can add up to several thousandths at the toolpoint. Formerly, experienced machine operators realistically knew the best accuracy they could promise was |+ or -~0.002" on small machining centers or |+ or -~0.003" on large ones.

A Milacron-patented probing technique uses touchpoints on the fixture to virtually eliminate the geometric and thermal errors of the machines.

Metalforming, too

As a general rule, tightening tolerances apply equally to metalforming operations. As recently as 1991, Japanese cars exhibited customer-perceived superiority in operation of an automobile car door when compared with US cars. According to the account by Taguchi cited by Mr Swyt, "quality in the operation of an automobile door as perceived by customers correlates with the variations in, the force required to open the door. A comparison showed that to open them, US cars required forces of 76|+ or -~58 N (17|+ or -~13 lb), while Japanese cars required 31|+ or -~9 N (7|+ or -~2 lb)--the Japanese companies holding a six-to-one advantage over US companies.

How fine can a metal stamping be, and how accurately can holes be positioned? According to Baltec Corp, Carnegie, PA, part accuracy of the one-hole-at-a-time variety is directly related to the accuracy of press ram movement. One of its CNC stamping cells features punch and die clearances of 0.0008" per side maximum, without regard to material thickness, hole dimensions, and type of material. In one application, the punch and die pierced holes in 301 stainless, half hard and 0.018" thick, with a web between holes of only 0.006".

On a larger punching scale, Finn-Power International Inc, Schaumburg, IL, has introduced a standardized accuracy test, LKP-7100, for its machines. Turret press accuracy is tested by dynamic punching of holes in a 1 m x 1 m sheet with 100% speeds and measuring the location (X/Y) and angle (CNC Index Tool) of the punched holes from the sheet. Accuracy of Finn-Power presses will now be given in both the LKP-7100 standard for punching accuracy and the commonly-accepted German standard VDI/DGQ 3441 for positioning accuracy.

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Author:Lorincz, James A.
Publication:Tooling & Production
Article Type:Cover Story
Date:May 1, 1993
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