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CNC grinding for the US.

CNC grinding for the US

Market penetration of CNC grinding trails that of CNC turning and machining centers by 12 to 15 years. Although machine-tool builders have long recognized that users want to obtain the same benefits in grinding that are available with CNC turning and milling, high development and product costs have kept them from satisfying customer demands.

This seemingly slow development in the US can be attributed to factors such as: * Initial capital investment. A CNC-type grinder may cost three to five times as much as a comparably sized manual grinder or CNC turning center. * The "black art" syndrome. Grinding involves a much higher number of performance variables than encountered in turning, milling, drilling, etc. Because of this, misapplication is more likely to occur and, in fact, has resulted in misconceptions of the true nature of the grinding process. * Traditional justification procedures. These alone don't always provide good economic numbers. They don't address the intagible and indirect savings that CNC grinding provides.

To explain the last point, most CNC turning and machining-center justifications tend to focus on non-cutting events such as tool changing and workpiece handling. But these variables have less impact on CNC grinding.

From product to training

When did NC grinding begin? It's hard to pinpoint a date, but Okuma began applying numerical controls to grinding machines in 1964. Our continued development of such machines for Japan's domestic needs led to over 1000 installations in automotive, appliance, and other large-scale industries. In the US, our successful entry into the CNC turning and machining-center market paved the way for the introduction of CNC grinders in 1985.

Like most machine-tool builders, we've added features to meet specific customer demands as well as overall trends, and we've stepped up our support assistance. For example, we developed quick-setup features for just-in-time (JIT) environments, and we offer pre-and post-installation training activities.

For many users, CNC grinding sounds good, but they don't know where to begin. Through workshops and demonstrations, we help users choose the right combination of product and training for their needs.

There are still applications that CNC grinding cannot perform, and it's very important to inform customers of any shortcomings. This in turn assists them in future project evaluations and continues the education process.

Grinder design

The industry has taken many different approaches to CNC grinding. On the manufacturing side, our engineering goal is simple: Create a grinder with as few separations as possible. Reducing the number of separations between the foundation and grind point is critical for accuracy and strength.

Elimination of metal-to-metal contact between all moving points is another goal. Slideways and wheel spindles are two areas where high-pressure oil bearings (hydrostatic, hydrodynamic) can reduce transmission of vibrations and thus improve surface finishes and accuracies. In continuing this solution, engineers have developed a fluid-drive system that drives the ball screw without metal-to-metal contact.

Of course, state-of-the-art grinding relies heavily on computer technology. Our own OSP control system, example, can handle everything from simplified software packages for first-time users to advanced variable programming and subprogram execution.

The key is to assign values to as many inherent grinding conditions as possible. Wheel porosity, thermal change, and coolant viscosity all can affect the grinding process. Thus, an effective control system should monitor and change the program as process variables change, optimizing each grinding cycle.

Double cost savings

The role CNC grinding plays in manufacturing is twofold. First, it affects the processing costs of the part, and secondly, it provides indirect savings as a result of this process. Grinding operations usually occur in the final stages of a part's manufacturing process. Such processes as sawing, turning, drilling, and milling naturally add costs to the part. Table I shows generic processes and their accumulated costs.

As the table shows, costs peak in the grind process. If CNC implementation produces a 4-percent scrap reduction, then we can calculate savings at certain stages. For instance, in a 200-piece run, the savings would be $176 for turning, $256 for milling and drilling, and $1120 for grinding. Note that scrap reduction will play an important role in the overall justification.

Table : Table I Generic processes and accumulated costs
 Individual Accumulated
Process Costs ($) Costs ($)
Stock 5.00 5.00
Cut to length 5.00 10.00
Turn 12.00 22.00
Mill/drill 10.00 32.00
Hob 10.00 42.00
Heat treat 2.00 44.00
Grit blast 2.00 46.00
Grind 10.00 56.00

Direct benefits from the use of CNC include the combination of various surfaces and contours for each operation. This reduces overall setup time and inspection costs. Furthermore, super-high resolution in digital encoders helps reduce scrap by increasing accuracies.

Adaptive control of the grinding process is viable today. Typical systems monitor performance variables and automatically make control changes accordingly. The controls alter the grinding process to optimize parameters and reduce cycle time. Finally, advanced setup features save time and provide greater flexibility when making changeovers.

Indirect benefits of CNC include canned grinding processes that require less dependence on the machine operator - allowing more efficient use of manpower. Combined operations promote reduced lead time, contributing to JIT manufacturing - especially important where grind operations immediately precede assembly of components.

CNC also offers greater flexibility in workcenter scheduling, so that the centers revolve around scheduling requirements and not vice versa. Finally, increased capabilities of CNC grinders tend to draw applications toward them. In many cases, end users find better applications than were originally planned. The tough metalworking job at Packer Plastics, Lawrence, KS, is keeping up with mold making and repair. The firm produces containers for food suppliers and convenience-store outlets - involving the production of hundreds of different types of plastic containers.

The toolroom must provide a high level of support for building and reworking most of the cavities and cores used in this production, and new products plus emergency rework provide an unpredictable schedule of workloads. The operation is small-lot oriented, with the number of mold inserts rarely exceeding 16.

The toolroom got involved with CNC four years ago, using both turning and vertical machining centers. But the grinding area was considered too specialized to convert to CNC. Tool and Die Supervisor George Ginter explains, "We felt CNC grinding required large-lot runs to be justified. In our toolroom environment, the ability to react quickly is important, and, in those days, we thought CNC would hold us back.

"More recently, the increasing volume of work created a bottle-neck in the grind area, so we had to consider additional equipment. Our first thought was manual universal, but the people at Okuma convinced us to try a CNC grinder.

"It was a good decision. What impressed me was the repeatability and process control the machine gave us. It forced us to examine how we grind parts, and allowed us to create better overall processes. Even one-piece repair work became easier."

The payback numbers include grinding-time improvement ratios of 2.71:1 for eight-piece runs, and 3.68:1 for 16-piece runs. We developed these figures from a study of eight core and cavity-mold combinations ground on manual equipment and then on our Okuma GI-20N CNC grinder.

For eight different workpieces, total processing time (setup and grind) came to 131.66 hr using manual grinders. This figure dropped to 47.66 hr when the same batch ran on the CNC machine. For 16-piece lots, total time dropped from 277.0 hr to 75.16 hr.

Looking at a typical setup, grinding the stack-shoulder area of a cavity required only 40 min setup time on the manual machine, but grinding time was 720 min, for a total time of 12.6 hr. The same job required 90 min set-up time on the CNC grinder, but only 80 min grind time, for a total of only 2.8 hr.

Packer calculates a one-year payback of $199,800 for one-shift operation, and $399,600 for two shifts. This is based on 2000 available hours per shift, a shop rate of $45/hr, and a grinding-improvement ratio of 3.5 to 1.

Ginter concludes, "Manual universals are best at one-of-a-kind work. But I was surprised to find that CNC pays off with just two-or three-of-a-kind work. It's definitely worth considering if you want improved grinding capabilities."

Packer found that CNC grinders provide intangible benefits, too. Sales increased because engineers could offer a 60-percent reduction in lead time to produce molds. And, consistency of process execution lead to better scheduling, thus boosting efficiency of all equipment, including existing manual grinders.

PHOTO : Okuma GI-20N CNC grinder at Packard Plastics. Workpieces are tooling components for injection molding machines.
COPYRIGHT 1989 Nelson Publishing
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1989 Gale, Cengage Learning. All rights reserved.

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Title Annotation:computer numerical control
Author:TenClay, Dave
Publication:Tooling & Production
Date:Nov 1, 1989
Previous Article:Trends in laser measurement.
Next Article:Grinding thin-wall cylinders.

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