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Machine accuracy by the numbers.

Basically, there are two ways to test machine accuracy: either dimensionally check finished workpieces, or try to measure the positioning accuracy and repeatability of the machine itself. Since the entire work envelope of a machine is important and a typical workpiece requires only a portion of the envelope to complete, machine tool builders use accuracy testing techniques that involve traversing each axis of the machine while holding the others, then measuring the variation of the actual position from a theoretical position.

The trouble is, builders can choose from several different procedures to measure and report the accuracy of their equipment. Being aware of the differences when you're shopping for new equipment is important, says AMT vice president for technology Charlie Carter. "In comparing one builder to another, what the buyer needs to do is make sure the builders are quoting the same accuracy."

Four standard procedures often used by machine tool builders to measure accuracy are the German VDI/DGQ 3441; the NMTBA (AMT) guideline in the US; JIS 6336 (for machining centers) in Japan; and an ISO standard, 230-2. Recommended procedures vary somewhat--the Japanese standard is less rigorous than the others--but the main difference, says Mr Carter, is in how test results are reported.

"In the German, US, and ISO standards, you take the data from multiple tests, apply a standard deviation calculation, and state the accuracy based on |+ or -~3 sigma. In the JIS, you take the data and then you report just what the maximum spread was. When you take a set of data and compare |+ or -~3 sigma to the actual spread, in machine tool applications the |+ or -~3 sigma will typically double the spread," he says.

Yet another US standard, ASME/ANSI B5.54, is due for publication March 15. It includes a section on machine installation to minimize temperature, vibration, and other environmental influences that can affect accuracy--something not addressed in the other standards, says Bob Hocken, PhD, a University of North Carolina (Charlotte) professor of mechanical engineering, who has worked on the standard since its inception in the mid-1980s.

For testing linear accuracy, ASME/ANSI B5.54 is similar in concept to the other standards, but mandates use of a laser interferometer. The total spread is then reported, says Dr Hocken. The standard also includes two other tests for linear accuracy, one for periodic error and one for bidirectional repeatability that uses very small steps about two or more points on each axis.

The linear accuracy tests are actually a small portion of the procedures spelled out in ASME/ANSI B5.54, says Dr Hocken. The standard is much more comprehensive than any of the existing standards, and includes accuracy tests for rotary axes and indexing tables, machine parametrics, spindle tests, contouring performance tests, tests for linear displacement accuracy along machine diagonals, and cutting tests.

Because the standard was developed by a consortium of university, industry, and government representatives, Dr Hocken expects it to be used not only in the US but globally. "It will be adopted probably relatively quickly by the big corporations that sat on the committee, such as General Motors and Texas Instruments," he says. "As soon as they use it, that means their suppliers will go along." Machine builders who participated on the committee include Cincinnati Milacron, Giddings & Lewis, and Monarch Machine Tools.

Dr Hocken believes adoption of ASME/ANSI B5.54 will eventually "improve the accuracy and performance of machines, because builders are going to have to address some of the issues raised (by the standard)."

The document will be sold through ASME's Codes and Standards Div; as of press time, the price had not been finalized.

Make accuracy a top priority

Speaking of accuracy, top-of-the-line H-series universal milling machines from Hermle Machine Tools, Gosheim, Germany, are gaining a reputation in the US for accuracy in applications ranging from machining of components for a particle accelerator ring to parts for Indy cars. Marketed through Hermle USA Inc, Menomonee Falls, WI, the machines incorporate many accuracy-enhancing features--such as a polymer concrete base to minimize vibration--that are becoming more common throughout the industry.

The centerpiece of the company's drive for accuracy is a direct digital drive and "dual feedback" control system developed in cooperation with Heidenhain, the Traunreut, Germany, manufacturer of linear and rotary encoders and machine controls. Heidenhain supplies the linear and rotary encoders and the control used on the machines.

Hermle's direct digital drive system communicates the control's digital signals directly into the drive system, which then generates the amperage needed to drive the motor. A Heidenhain rotary encoder with 512,000 increments per revolution of the motor shaft provides precise position control.

The system is especially effective in providing exacting control for contouring work, says technical adviser Wolfram Hermle. Interpolating a circle, for example, requires that axes come to a complete stop and then reverse motion every 90 degrees. The dual-feedback control minimizes backlash when that occurs, says Mr Hermle.

The control, which uses three 32-bit processors, also incorporates an algorithm that automatically compensates for thermal expansion of the spindle and other machine components during operation, using inputs from thermocouples placed in strategic locations on the machine. The result, according to Mr Hermle, is a five-fold improvement in thermally dependent accuracy on the Y-axis.

Hermle H-series machines are tested according to the VDI/DGQ 3441 test method and are guaranteed accurate to a position tolerance of 0.0015 mm.

Soldering seminars

Schunk Automation Systems, Old Saybrook, CT, and ERSA GmbH, a European manufacturer of soldering equipment, have scheduled a series of four one-day seminars covering various aspects of soldering.

The courses are designed to update anyone involved with soldering operations on the latest technologies, methods, quality control, and environmental issues. Topics to be covered are:

Seminar 1: Basics of soldering

Seminar 2: Soldering/surface mount technology

Seminar 3: Cleaning alternatives

Seminar 4: Statistical quality control

The series is scheduled for March 9-12 in Chicago and March 16-19 in Dallas. For registration information, contact Heinz Bockard of Schunk at 203/399-2441.

Control assures weld quality

The resistance welding process has been around for 100 years or so, and consistent welding has pretty much always depended on "seat of the pants" adjustments by an experienced operator. Process and workpiece variables that adversely affect weld quality include electrical current, resistance, and power; electrode force; and workpiece thickness, surface condition, and thermal expansion.

Because of these variations, destructive testing has been used to try to assure weld quality. The problem with that approach, says Robert Cohen, president of WeldComputer Corp, is it provides no information about any weld except the one that's destroyed.

The Albany, NY, company has developed a system that monitors and adaptively controls process variables to assure consistent welds without destructive testing. The device can detect potential problems as the weld is being made to provide automatic compensation to maintain weld consistency. If adequate compensation is impossible, the system aborts the process, identifies the problem, and notifies the operator. The system also automatically records weld process data for statistical process control or other quality recordkeeping purposes.

The WeldComputer is built around an 80386-based personal computer with multiple processors. It is particularly useful for manufacturers who need to be able to document weld reliability, or for those using difficult-to-weld materials such as aluminum alloys, says Mr Cohen.

"Aluminum alloys tend to oxidize fairly rapidly after cleaning, and material that's been cleaned an hour ago is different from material that's been sitting half a day," he says. "A study by NASA, for example, indicated use of the adaptive control could extend the weldability of aluminum-lithium alloys to about 15 days after cleaning. Without adaptive control of the welding process, the most you could extend without cleaning is about two days."

According to Mr Cohen, the reliability of resistance welding using the adaptive control exceeds 99.9999%--far above the 99.5% required by the Defense Contracting Administration Services (DCAS). Using documentation from the control system, user Allied-Signal was able to obtain DCAS approval to eliminate destructive testing of resistance-welded jet engine components, says Mr Cohen.

Gearmaking training begins

With a curriculum aimed at increasing the productivity of operators with less than one year of experience, the American Gear Manufacturers Association (AGMA) Training School for Gear Manufacturing is under way at the Instrumented Factory for Precision Gears (INFAC).

Located at the Illinois Institute of Technology Research Institute (IITRI), Chicago, INFAC donated the space and equipment needed to run the training sessions.

The five-day course includes three days of hands-on training in hobbing, shaping, and inspection and two days of classroom training and discussion. Cost per session is $525 for AGMA members and $675 for nonmembers. Sessions are scheduled for April 19-23, June 14-18, and September 20-24, 1993, and registration is limited.

Milacron to acquire Valenite

Cincinnati Milacron, Cincinnati, OH, has agreed to purchase GTE Valenite, Troy, MI, for $80 million in cash and assumed debt of about $10 million.

Valenite, one of the world's largest suppliers of carbide inserts and other metalcutting products, posted 1991 sales of $275 million. It will become a wholly owned subsidiary of Milacron, which expects to retain Valenite's approximately 2700 employees.

The acquisition should push Milacron's annual revenues above $1 billion and provide a better balance of revenues among its main businesses of machine tools, plastics processing equipment, and industrial products, says Milacron chairman and chief executive officer Daniel J Meyer.

Valenite's former corporate parent, GTE Corp, Stamford, CT, is in the process of spinning off divisions not directly related to its core telecommunications businesses.

MMCs challenge machining

Metal matrix composites (MMCs) are increasingly being considered as substitutes for materials ranging from cast iron to graphite-reinforced epoxy. They pose their own set of challenges for rough and finish machining, as Charles T Lane, Duralcan USA, explained at an advanced materials symposium held in conjunction with BIMU in Milan. Here's why:

Generally, MMCs can be machined on the same machine tools that are used for cast iron or aluminum. As with all materials, a rigid machine with adequate horsepower is desirable. MMCs are similar to aluminum in that they require about only one-third the power per unit volume machined as cast iron. The higher material removal rates produced by higher recommended speeds, feeds, and depths of cut (relative to cast iron), however, require a robust machine tool for full-time manufacturing.

Although most commercial MMCs are aluminum-based materials, approaching machining as if they were aluminum is a mistake. Most aluminum alloys are quite soft relative to ferrous materials and are commonly machined with high-speed steel (HSS) tools. The exceptions to this are aluminum casting alloys that contain large amounts (greater than 12%) of silicon. These materials generally require polycrystalline diamond (PCD) tooling.

The problem with MMCs is the ceramic reinforcing materials used, such as silicon carbide, are harder than commonly used carbide cutting tool materials. Trying to machine MMCs with HSS or carbide tools, in fact, results in the MMC serving as a metal-bonded grinding wheel relative to the tools. Even titanium nitride (TiN) coated carbide or HSS tools are ineffective. Use of carbide tooling for MMCs is therefore limited to relatively low surface speeds (less than 100 m/min).

Interest in machining MMCs is running high as the materials are becoming more competitive in terms of cost and applications are continuing to grow, says Alcan Enterprises president Ian Rugeroni. "There are serious development programs under way to establish the broad use of (MMCs) in brake rotors and drive shaft tubes," says Mr Rugeroni. Another automotive application being evaluated in cylinder liners for aluminum-block engines, he adds.
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Copyright 1993 Gale, Cengage Learning. All rights reserved.

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Title Annotation:Manufacturing Update; includes related articles
Publication:Tooling & Production
Date:Mar 1, 1993
Words:1929
Previous Article:NC software - how it stacks up.
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