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Innovation sparks grinding advances.

While some people may regard grinding as a mature technology, in which grinding's potential is pretty well established, even incremental advances continue to expand grinding's technological envelope into new territory. In this article, we take a look at three recent grinding advances that are typical of many advances taking place in grinding: one that improves accuracy; another that improves efficiency; and one that reduces capital investment.

Optical projection and CNC

For roughly a century, precision grinding has required highly skilled operators--grinding by hand is nearly an art. Adding CNC, many argue, has failed to overcome the inconsistencies in grinding that make it so resistant to programmed control. Some builders have tried to solve the problem, largely with varying success in toolroom applications, by programming compensation for wheel wear.

More recently, several builders have had much greater success by combining two systems in one machine: CNC and optical projection. Optical projectors have been used as an aid to manual grinding since the turn of the century. The optical system makes it easy to see and correct for grinding-wheel wear and for minute positioning adjustments. But optical grinders still require a skilled operator and full-time attention, as well as great concentration. The necessary adjustments noted from the projection screen, however, are easily keyed into a CNC program, allowing the number-crunching and positioning power of the CNC to take over the tedious handwheel traversing that can make grinding a drain on skilled toolmakers' time.

Optical/CNC grinders, with the addition of cylindrical grinding attachments, are being used in a variety of toolmaking applications, including the machining of forming rolls, crush rolls, and similar kinds of precision, cylindrical tools.

CNC is well developed in turning and milling, but, with the exception of some production-type cylindrical-grinding applications, most CNC grinding is done with patched-up milling programs. Two- and three-axis grinding applications are now exploiting simplified programming methods and producing dramatic productivity improvements.

With the addition of optical profile projection, "teaching mode" programming has come into its own in grinding. Teaching mode records a single, manually controlled dry run of the part profile and stores it as a CNC program that can be perfected and re-run as often as needed, even long after the initial run, for making multiple tools or for re-grinding a worn tool to its original profile. It's especially practical when the part and grinding wheel can be seen magnified on a projection screen.

All of today's programming tools are available, in fact, allowing downloading of tool programs from CAD/CAM systems or manual data-entry at the CNC. A trick from the earlier days of conventional optical profile grinders makes both the teaching mode and the manual-entry mode simpler exercises: a scaled drawing of the part, made on Mylar or drafting vellum, can be laid directly over the optical screen for use as a template. Watching the projected shadow of the grinding wheel on the screen, the operator can dry-run a program in minutes and see exactly where adjustments or wheel-wear compensation have to be made.

In the United States, several of these machines are being used for making crush rolls for forming production grinding wheels. This is the purpose for which AlliedSignal Propulsion Engines, Phoenix, AZ, has been running its Wasino GLS-130AS since acquiring it in the mid '80s. The machine was among the first optical/CNC profile grinders in the United States that was equipped for cylindrical grinding, and the company has accumulated considerable experience with it.

Operators overlay the screen with a scaled vellum drawing of the part. AlliedSignal uses 1/4"-width grinding wheels with "ball-nosed" wheel profiles (the actual shape is a uniform radius, but it looks like a ball-nosed end mill in profile on the screen), so the operator draws a half-round shape on the vellum to locate the starting position for the wheel. The numbers for tool movement are punched into the program, and a dry run is made to see if corrections are necessary.

The steel cylinder, up to 6" in diameter, usually an M43 high-speed steel in AlliedSignal's case and sometimes rough-turned to approximate shape, is fixed on an arbor and mounted in the cylindrical grinding attachment. Using a digital readout (DRO) to measure dimensions from a positioning block, the operator sets the offset for the roll diameter. The part is then ready for the rough-grinding pass.

Operators at AlliedSignal prefer to cut the critical slot sides in the crush rolls with a plunge rather than a retracting cut, for better wheel performance. CNC simplifies this by keeping track of its position; backing the wheel out and re-starting a plunge is another tedious step, and another chance for error, in manual optical grinding.

After finishing, the optical projector and DRO come into play again, for inspecting the part. A stylus is affixed to the part and its profile is checked against the template. Deviations from nominal dimensions are noted on the DRO. If wheel wear should happen to have been greater than expected, keeping the part in its fixture allows another finishing pass with ease. The variances taken from the DPO readings are treated as offsets, keyed into the CNC, and another pass can be made without interruption or refixturing.

Wasino notes that this is one way to inspect a part, but that it's more commonly done simply by comparing the roll profile with the template profile. Magnification of 20X and 50X is standard (44X and 110X are available as options), which is more than adequate for directly measuring variances from nominal part dimensions. Still, the alternate system that AlliedSignal uses illustrates the flexibility of having a projected image of the part and the tool, a scaled template on the screen, and a DRO. Needed adjustments are immediately obvious. The operator can correct them in several ways and view them at high magnification while they're in progress. The offsets are made at the machine in standard G-code programming, treating them as tool expansions or retractions. It's also common, says Wasino, for operators who use the teaching mode of programming to simply "teach" the machine through necessary offsets, as well as through the basic program.

AlliedSignal typically grinds ten rolls per week on its single machine, operating through two active shifts and often roughing parts unattended at night. In the experience of both operators and managers, the training is exceptionally quick and intuitive.

This contrasts sharply with the fine-tuned experience required for both manual- and conventional-CNC profile grinding. Even with the aid of CNC, operators not aided with a magnified image of the work in progress have to develop a sense of sound to determine the depth of cut being made by the wheel, and learning such things as proper wheel RPM and feedrates come by making big mistakes. With the optical projector, the actual cutting depth is immediately visible, and excessive wheel wear is apparent before the damage occurs.

Although AlliedSignal uses 1/4" radii on its wheels, it's possible, with Borazon or diamond wheels, to dress them to a radius as small as 0.002".

Wasino has only six or so of their machines equipped for cylindrical grinding in the United States. In contrast, it has over 100 of them in use in Japan. This fits the general pattern of differences in toolmaking between the two countries. Japan, unlike the US, has no strong history or tradition of toolmaking, and, until recently, has had relatively fewer skilled tool and die makers. The shortage in the US has been worsening for some years, as well, and the introduction of easier, more productive toolmaking technologies, such as optical/CNC tool grinding, serve an ever more important competitive need.

Multi-head grinding

A promising new process for grinding the lobes of automotive camshafts is being used in a large-scale production application. The process, which is embodied in Litton Industrial Automation's Landis multi-head cam lobe grinder, replaces the traditional single grinding wheel with multiple contouring heads using pulley-driven coated abrasive belts. In suitable applications, the multi-head design offers capabilities for increased grinding productivity.

Currently, 14 Landis multi-head cam lobe grinders are in operation at General Motors' Bay City, MI, powertrain plant, which manufactures engine and transmission components for GM cars and trucks. In the production setup, ten of the grinders are used to process shafts for a V-6 3. 1-L engine; the other four process shafts for a four-cylinder, 2.2-L engine.

During preproduction testing of the machines, Landis did run-offs on the four-cylinder shaft which combined rough and finish grinding in a single operation. In these operations, 0.010" of stock per side (0.020" total) was removed from all eight lobes in a single 45-second cycle.

In an automobile or truck engine, the camshaft is the component which mechanically controls the intake and exhaust valves. Because these functions are crucial to engine performance, shaft journals and lobes must be ground to precise geometric accuracy. Camshaft lobe grinding has always been an especially difficult process because of the lobes' non-cylindrical configuration. A major advance in cam-lobe grinding occurred in the early 1980s, however, when the first CNC masterless cam-lobe grinders were introduced. These machines made improvements possible in lobe-grinding productivity, accuracy, and production flexibility. They also increased the consistency of finished lobe contours through the wear life of the grinding wheel. Today, the CNC masterless cam-lobe grinders remain the technology best suited to meet the requirements of many production operations. For other applications, however, the multi-head grinders may offer a more productive alternative.

In contrast to the conventional CNC cam-lobe grinders, which use a single grinding wheel to machine each cam lobe independently, the multi-head grinders machine up to eight lobes simultaneously in a single operation--or "plunge." Thus, in the case of a four-cylinder camshaft, which has eight lobes, all of the lobes are ground in one plunge. In cases where six- or eight-cylinder shafts are involved, presenting 12 or 16 lobes, respectively, two plunges are performed successively, each simultaneously machining half of the lobes. This multiple machining capacity can result in up to a tripling of the production rate typically obtained with traditional single-wheel cam lobe grinders.

In general, the new multi-head cam lobe grinding system is best suited to applications characterized by two conditions. The first is high-volume production. Although a CNC single-wheel cam-lobe grinder can remove much more stock per unit time, the multi-head grinder offers the advantage of machining up to eight lobes at a time. In suitable applications, the balance of these factors can give the belt grinder as much as a 3:1 advantage in overall productivity. In many high-volume operations, fewer multi-head grinders than single-wheel machines would be needed to maintain the same level of production. In such cases, the multi-head system can be the preferred investment choice.

In addition to a high-volume production, the multi-head system is best suited to processing a single-camshaft style or a family of shafts with a common length and a common lateral positioning of lobes. The multi-head grinder is every bit as agile as the CNC masterless single-wheel grinder, meaning that it can efficiently machine any lobe contour. It is less flexible, however, in its capacity for changeover to different shaft styles.

In comparing the two systems, it can be found that the CNC masterless single-wheel grinder can be quickly reprogrammed to accommodate shafts of different lengths or lateral lobe positionings, as well as shafts with lobes of different contours or radial positions. By contrast, the multi-head grinder can only be reprogrammed to accommodate shafts with different lobe contours or profiles (the belt shoes can be adjusted up to an inch). The system cannot be easily changed over for shafts of different lengths or lateral lobe positionings, since such an accommodation would require a change of tooling.

Currently, the multi-head system is best suited to applications where softer workpiece materials are involved and smaller amounts of metal need to be removed. With present belt technology, the system performs most effectively at a maximum stock removal of about 0.02011 on diameter per lobe.

However, development work on super-abrasive belts is underway at Landis which is aimed at broadening the system's application potential to include hardened steel shafts.

When these belts are optimized, we can look forward to even greater productivity from the multi-head system, especially in terms of metal-removal capabilities," comments Rick Mowen, Landis Grinding Operations project engineer. "We'll also get better tool life--more parts between dressings and belt replacements."

In addition to its productivity advantages in suitable applications, the new multi-head grinding system opens the door to development of new camshaft designs that can help improve engine performance.

Previously, automotive engineers were limited in exploring more fuel-efficient designs, for example, by the inability of traditional single-wheel grinders to cost-effectively machine cam-lobe re-entry curves with very small radii. The major problem, of course, has been the size of the conventional single grinding wheel--typically, 24" in diameter.

It is true that single-wheel cam-lobe grinders can be equipped with small-diameter CBN grinding wheels; however, since grinding efficiency drops with reductions in wheel diameter, this adaptation is not entirely satisfactory. Smaller wheels have less abrasive material and must be dressed and replaced more often.

Now, with the new multi-head cam-lobe grinding process, severe re-entry curves can be machined with improved contouring accuracy and at greatly reduced costs.

Creep feed on a budget

It's no secret that creep-feed grinding provides some distinct advantages over conventional milling and secondary grinding operations. The first and most obvious is the elimination of one machining process: CNC milling. This alone slashes machining time, as well as providing additional benefits such as reduced fixturing and increased accuracy. Creep-feed grinding is also ideal for machining small parts and shapes within a workpiece that would try the patience of even the most ardent milling-machine operator.

Add to this the fact that creep-feed grinding is a CNC process in which the operator just sets up the work and lets it run, and it may be hard to understand why creep-feed grinding isn't taking the industrial world by storm. That is until you investigate the cost and find that a typical full-blown creep-feed grinder can run upwards of $500,000. In fact, on some full blown machines, add-ons such as a continuous wheel dresser with wheel wear compensation can carry a price tag in the $150,00 range.

So what can you do if you have applications that can benefit from creep-feed grinding but don't want to bear the expense of a full-blown machine? If your workpieces are small to medium in size and the required cut is 2" or less in width, and you can deal with some situations that require a more conservative approach to stock removal, there is an alternative offered by Okamoto Corp, manufacturers of a diverse line of grinding machines (including the expensive full-blown creep-feed grinders).

The company has taken its ACC-DXNC line of CNC saddle-type surface grinders and modified them to efficiently perform creep-feed grinding--with limitations. "We are not trying to compete with the full-blown creep-feed grinders on the market today," said Jim Asakawa, sales manager, Okamoto. "We found that many creep-feed applications didn't require full-blown machines. There are many customers that could benefit from the creep-feed process if they could only purchase a more reasonably priced machine."

Okamoto's engineers determined that the company's ACC-DXNC series of saddle grinders had the necessary rigidity for many creep-feed grinding applications. They took the company's ACC-12X24DXNC 2-axis simultaneous (crossfeed and downfeed) saddle-type CNC surface grinder and equipped it with a NC/ballscrew table drive that uses a high-resolution AC servomotor. This provided the simultaneously controlled third axis necessary for the process. Control is provided by a Fanuc OM CNC.

The control/motor combination produces table feedrates that are adjustable from 0 to 33 ft/min, a more than adequate range since most creep-feed applications fall in the 6"/min to 10"/min feedrate range.

The ACC-DXNC's standard 5 hp spindle motor was replaced with a 10 hp motor, and a high pressure coolant system and nozzles were added to provide proper coolant and efficient delivery of the coolant to the workpiece. A fully enclosed splash guard system was also installed, along with a wheel dresser.

The modified grinder can be outfitted with a variety of wheel dressing devices ranging from table mounted units available in single point, two-way, or three-way types to rotary and CNC swing type dressers. The shape of the cut will generally dictate the type of dresser required.

How much does this all add up to? Surprisingly, the cost comes in at a fraction of the cost of a full-blown creep-feed grinder. A typical setup as outlined, with a chuck capacity of 12" x 18", carries a price of between $150,000 and $200,000, depending on the type of dresser ordered. Available chuck capacities range from 8" x 14" to 20" x 34".

Concessions do have to be made. The amount of single-pass stock removal is dictated by two factors: The hardness of the workpiece material and workpiece length. The machining of tough materials may require multiple passes, which increases machining time.

Long workpieces generally require a multiple pass stock removal approach--a drawback resulting from not having continuous wheel dress. This is because the depth of the cut must be reduced to enable a full, uninterrupted cut. Once committed, the operator cannot stop the cycle to accommodate a wheel dress. What's more, the machine's 12" diam wheel has a shorter life span than the large wheels found on many full-blown systems.

In the final analysis, the user has to make the determination as to whether the cost savings are worthy of the concessions that must be made. However, the Okamoto ACC-DXNC series, adapted for creep-feed grinding, certainly fills a niche in providing a low cost alternative to full-blown creep-feed grinding systems.

For information from the companies mentioned in this article, circle the appropriate number:
Litton 284
Okamoto 285
Wasino 286

Source File

For additional grinding information, circle the appropriate number on the reader service card.
Apex Machinery 287
Blohm Grinding System 288
CCI Div, Enprotech 289
Chevalier Machinery 290
Colonial saw 291
Cutting Edge Technologies 292
Drake Mfg 209
Ferrostaal Machine Tool 293
Grinding Technology 294
Grindingmaster Linden 295

Gustav Gockel
Maschinenfabrik 296
Hermes Machine Tool 297
Hertlein Special Tool 298
Jones & Shipman 299
Junker Maschinenfabrik 300
KO Lee 301
Loroch GmbH 302
Maegerle 303
Mattison Machine Works 304
Mitsui High-Tec 305
Nicco Machine America 306
Novatech 307
NSK America 374
Pittler Grinding 308
Prema International 309
SBS Balancing Systems 310
Schmitt Industries 267
Stoffel Grinding Systems 268
Unison Corp 269
COPYRIGHT 1993 Nelson Publishing
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1993 Gale, Cengage Learning. All rights reserved.

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Title Annotation:advances in grinding technology
Author:Stovicek, Donald R.
Publication:Tooling & Production
Date:Aug 1, 1993
Previous Article:Automated turning meets auto challenge.
Next Article:It takes more than a fast spindle.

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