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Chances are, CNC can help your grinding operation.

Chances are, CNC can help your grinding operation

Many manufacturers of parts requiring precise final size and finish have wondered if CNC could enhance their grinding operations. A few implemented grinding systems that overcame previous limitations of speed and economy associated with this age-old machining process. In most cases, CNC system purchases were justified on their ability to produce parts faster, more economically, and with greater accuracy than ever before.

What can CNC grinding do?

Grinding machine-tool builders have applied lessons learned in developing CNC turning and milling equipment to produce improved grinding systems with more rigid machines. Today's CNC is interactive and converses with the operator to quickly guide him through correct entry of cycle data. CNC controls wheel feed and speed, automatically initiates wheel-dressing at programmed intervals, and compensates for wheel wear. It also can perform machine-diagnostics and maintenance monitoring, alerting an operator to problems and their source. Stored part programs and canned cycles further enhance the speed at which the operator can set up new jobs.

Before CNC, grinders usually were equipped with a single-point diamonddressing tool that could be traversed across the wheel face by hand or guided by a template. The procedure was to true the wheel, restore the surface, and create the desired part profile.

Now, CNC creates and stores commands for profiling in its memory. Old part-dressing programs can be loaded in seconds. New profiles can be created in minutes for simple parts, in an hour or two for more complicated workpieces. While this is useful for any lot size, maximum cost savings result on runs under 100 pieces.

Tracing CNC's evolution

Figure 1 depicts the effect NC and CNC systems have had on job-task times for turning and grinding operations since 1955. The driving force for increased implementation of first NC and then CNC has been a steady stream of improvements alternating among machine, cutting tools, and control. NC programs could use more tools, so tool turrets became more common.

Improved single-point tooling, higher-horsepower drive motors, faster feeds and speeds, and greater use of better and less-costly servo-positioning systems provided quicker stock removal.

In turning, actual cutting time was reduced to a small percentage of total cycle time. A point was reached where, to increase productivity further, users had to shorten setup time. Thus, NC turning moved from a shop-floor novelty to a sophisticated CNC process with economic benefits that could be easily justified.

When using multiple operations to process parts, actual machine time becomes a smaller fraction of floor-to-floor time. Setup is now the weakest link and the target for further process improvement. It includes: Parts handling from raw-material inventory to finished parts, tooling and fixturing, process writing, staging for each operation, and multiple inspections. Machine setup and actual in-cut time can represent as little as 25 percent of total part-processing time, and, of that, in-cut time is about 10 percent.

Why grinding lagged CNC

If machining could be eliminated, there would be less capital investment and maintenance costs, fewer interfaces (sources of potential problems), and perhaps better accuracy resulting from fewer setups. As a result of eliminating processing operations, Figure 2, some manufacturers modified their view of grinding as merely a finishing operation. Full-depth form grinding was implemented as the sole machining step to produce finished parts, beginning with a forging or hardened blank. This was economical only on expensive parts or for large lot sizes. Could its benefits be scaled down?

Grinding technology has quietly kept pace with developments in turning and milling, but fell far short in one category --setup time! The rapid revolution in single-point tooling for turning was in contrast with a slow evolution in grinding wheels. In a parallel development, cutting time in grinding has improved to where machine setup time must be reduced to boost grinding's productivity. Today's CNC grinding systems accomplish this reduction. They also solve the problem of wheel-wear compensation.

Full-depth form grinding

Full-depth form grinding begins with the desired finished part shape, size, and finish. The next step is to dress the face of the grinding wheel to a profile that will yield the final part shape when the wheel is plunged into the workpiece to the full depth of the contour, Figure 3. Better accuracy and concentricity can be obtained because all diameters are completely machined in one operation.

Conventional grinding remained a "sleeper' technology because of several problems that were perceived as insurmountable: 1) it was radically different from conventional machining and traditionally slow, 2) the cost of grinding wheels per cubic inch of metal removed was high, and 3) really fast stock removal required a soft, friable wheel needing frequent dressing to achieve close accuracy. Also, it required frequent wheel replacement.

For many, these perceptions changed in the '50s with the introduction of crush dressing and the jet wheel cleaner. These innovations addressed problems associated with slow stock removal and soft wheels.

Insufficient stock removal. Removal rates are determined by the grinding wheel's openness. This is because metal swarf can only exit the grinding zone via pores in the wheel. The dressing method dictates how open-structured the wheel will be, and the most free-cutting grinding wheel is one that has been crush dressed. This consists of plunging the grinding wheel into a crush dressing roll to impart the desired form on the wheel. The crush dressing procedure fractures the bonds and grains of the wheel surface, providing an open-structured, freecutting wheel.

Soft wheels. Soft, friable wheels require frequent dressing to maintain an accurate profile. Also, they must be replaced frequently because of high wear rates. Hard grinding wheels can be used if they can be made to grind cool and not become loaded. Solving this problem requires effective application of coolant throughout the grinding zone and keeping the wheel's cutting edges sharp and clean.

The hard-wheel system includes a jet wheel cleaner that uses a needle-like stream of coolant at pressures as high as 10,000 psi to keep the wheel clean and sharp. In addition to blasting swarf out of the wheel, this high-pressure coolant dressing because they retain their shape longer than soft wheels. Savings from reduced operations and ability to use hard, free-cutting grinding wheels have enabled shops to economically use the onestep, full-depth form-grinding process instead of first turning and then finish grinding their parts separately.

CNC dressing alternatives

Full-depth form grinding uses three dressing methods to create the final part becomes entrained in the wheel pores and is transferred to the grinding zone, where it aids in dissipating heat. The result is a substantial gain obtained by pushing the wheel to its limit.

Hard wheels reduce downtime for wheel changes and require less frequent profile in the wheel face. A guide to dressing economics for various lot sizes is shown in Figure 4.

Crush dressing produces the most free-cutting wheel and is usually the most economical dressing method for lot runs between 100 and 100,000 parts. For lots over 100,000 pieces, the benefits of faster metal removal are outweighed by the economics of longer-lasting diamond-roll dressing.

Now, CNC-driven single-point diamond dressers are replacing manual and template-controlled versions, providing the economic breakthrough required for one-step, full-depth form grinding on many jobs with runs less than 100 pieces. The resulting dressed wheel surface is not quite as open as that obtained with crush-roll dressing, but the advantages of quick setup more than offset the lower stock-removal rate.

An edge for smaller shops

With today's CNC grinders, tooling cost for small lots is mainly in the software. The method of dressing the wheel with CNC is to follow menu-driven screen prompts and enter numbers for commands and dimensions. The key benefit of CNC is system versatility, particularly when machining requirements are uncertain or for mixed or smaller-and medium-sized lots. Versatile CNC systems provide capability to accurately use all three dressing methods on the same machine.

Many small shops are asked to bid on a variety of short-run jobs. Ideally, they want to duck the money-losing parts and select only the most profitable new work for the shop. A CNC plunge-grinding system often can provide this capability. Yet, if the job turns into a long runner, the CNC plunge grinder also can wear its high-production hat.

With today's CNC grinding, shops can gain a marketing edge. Process engineers can get a direct tie-in to estimates used for bidding new work, short or long run. CNC menu-driven screens can include all factors to consider in bidding profitable jobs. The process engineer is equipped with a part program that is divisible into easy-to-optimize elements-- dressing intervals, feed rates, speeds, and allowance for spark-out. And, the same planned part-process worksheet serves as a guide for the operator.

Part programming, on or off a grinder, is comparable to interactive CNC on any other machine. The control will do complex mathematical computations instantaneously and repeatably, guiding the wheel reliably without supervision. For example, in thread grinding, the sinefunction computation that links the rotary axis and the table feed is made 125 times/sec.

When comparing accuracies obtained from grinding versus turning, the differences are significant. Required accuracies for grinding operations can be as much as 10 times the accuracy required of turning. Today's CNC can deliver accuracies associated with grinding. Positive positioning feedback from the workpiece, rather than a lead screw, makes such production accuracy possible. As a result, 3-sigma standard deviation specs are appearing in more and more requests for quotations. For users, 6-sigma performance ensures the capability of proposed grinding systems.

Moreover, CNC gives grinding users ability to capitalize on the benefits of servomotors that reduce cycle time. Servomotors have a wide speed range and can move quickly from one speed to the next. The CNC monitors and controls speed and position of the servo for critically close control of wheel feeds, table movement, and other variables. Because of the accuracies involved, grinding needs the benefits of servomotor technology, and CNC is essential to reap the rewards.

Benefit summary

CNC grinding machines don't require the skill of a toolmaker's touch to get accurate parts repeatably. CNC mated to a structurally rigid grinding machine delivers accurate parts with minimum setup, maximum adaptability, and low cost.

Use of extremely hard grinding wheels, cleaned with a jet wheel cleaner, reduces tool cost and downtime for tool changes. CNC permits a given tool to incorporate all dressing methods, thus allowing use of the most economical one for handling a variety of jobs.

CNC grinding is flexible on a variety of parts. It is economical because it eliminates the need for multiple operations. Part forms can be produced quickly because the part is completely finish ground in one operation.

Setup economy, accuracy, and versatility are the major justifications managers are using to implement new CNC grinding systems.

For more information about CNC grinding, circle E9.

Photo: 1. Advances in machines, cutting tools, and controls have significantly reduced incut time. Managers are increasingly justifying today's CNC equipment because it addresses the weakest link--setup time. Graphs are based on lots between 100 and 200 pieces.

Photo: 2. For many users, full, depth form grinding with hard, open-structured grinding wheels has proved more economical and faster than conventional part processing. Today's CNC grinding systems shorten setup time and expand the applications considered lucrative with this one-step process.

Photo: 3. Photo shows crush-dressing roll (upper right) and finished part have the same shape. The grinding wheel is plunged into the roll to impart an accurate form to the wheelface. This also fractures bonds and grains providing the most free-cutting wheel of any dressing method.

Photo: 4. Bar graph shows optimum dressing method changes with lot sizes.
COPYRIGHT 1985 Nelson Publishing
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1985 Gale, Cengage Learning. All rights reserved.

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Title Annotation:computerized numerical control
Author:Lord, Robert E.
Publication:Tooling & Production
Date:Jul 1, 1985
Words:1939
Previous Article:Rx for downtime: machine diagnostics.
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