Artigo Comparativo - Troca de Pallets e Células

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Feature Article

An Approach To Boosting Shop Production Capacity

The increased availability of modular manufacturing cells has added another means of increasing a shop’s capacity. This article looks at a practical

approach to evaluating when and how much automation is appropriate for a shop.

By John Wiese

Cincinnati Machine, A UNOVA Company

What’s the most effective way of adding assets to increase shop production capacity? The

traditional way is through the purchase or lease of additional machine tools, but that may not

always be the best way. Throughput and piece cost reduction should also be parts of the

equation.

One approach that considers throughput and cost reduction is the use of palletized workholding

systems, and, as needs dictate, progression to flexible machining cells. This approach offers

advantages that go beyond the simple productivity gains offered by putting more machine tools

online.

Reducing Non-Value-Added Time

Pallets, or more specifically, pallet changing systems, significantly reduce non-value-added time

caused by machine setup. For example, a manually operated two-pallet changing system can

reduce machine setup downtime by allowing load and setup operations to be performed on the

off-line pallet while work is being machined on the pallet clamped to the machine table.

When the machining cycle ends, pallets can be easily

switched, the machined workpiece removed, and a newsetup started. Combined with common workholding devices,

such as vises and clamps or modular fixtures rather than

custom fixtures, a palletized system can significantly reduce

part changeover time and extend the flexibility of the

machining operation. The manual system can also be

enhanced with additional pallets so that repeat setups can be

stored and delivered to the machine when needed.

The Goal: Maximizing Spindle Utilization

Pallet changing systems, both manual and automated, are often associated with long production runs; however, because they allow off-line setup, thare very effective for short runs, since they maximize spindle utilization.

Improving spindle utilization is a key objective in any type of automatic work handling or work changing system. Surveys indicate that machine toolutilization is generally in the 25 to 35 percent range, which is 15 to 21 minutes per hour. An automated pallet changer allows operator load/unload ti

to proceed in parallel with machining time, improving spindle utilization and increasing throughput.

Spindle utilization and throughput increase even more by combining a fixtured tombstone with a twin-pallet changer. Assume a 30-minute cycle time

one completed part. In this example, 15 tools are required per part during the 30-minute cycle, and there is a 10-second, metal-to-metal tool change

time.

That’s a total of 150 seconds or 2.5 minutes per part. When combined with tool dominant programming techniqtombstone fixturing of eight parts on a twin pallet system can amortize tool changes, saving 2.5 minutes per paFor each 30 minute cycle, 2.5 minutes represents 8.33 percent more spindle utilization, and that’s 8.33 percenteach part on the tombstone.

Combining pallet changing systems with tombstone fixtures can also dramatically compress “floor-to-door” time

when a ship set approach to scheduling and programming is used. For example, batch processing eight parts th

make up a ship set 20 at a time takes a half day, or four days to complete one ship set. Setting up tombstones

that all eight parts are programmed at one time allows a complete ship set to be produced in a half day.

When is a pallet changing system the right choice? The answer is whenever operators spend more time setting

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Cellular Manufacturing On MMS OnlineFollow the links below to four articles related to theuse of cellular manufacturing in metalworking shops

High Automation for Low Volume

Keep Your Spindles Cutting

Implementing A Machining Cell Dispelling Small Machine Shop Myth For more on the topic, perform a keyword search o"cellular manufacturing" from either MMS' ArticleArchive or Home Page.

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a job than they do running it. Parts best suited for a palletized workholding/work handling system are small to

medium sized prismatic parts.

Pallet changing systems can be f itted to virtually any type of production machine, including measuring machines

and vertical and horizontal machining centers. Vertical machining centers are generally limited to a two-pallet

changing system. Horizontal machining centers are not limited in the number of pallets that can be installed and

can easily be expanded to flexible, multi-machine cells with the addition of pallet transporters, load/unload stati

and a central controller.

Taking A Closer Look At Flexible Manufacturing

Flexible, cellular manufacturing offers even more production and throughputefficiencies than pallet changing systems. A cell can be a single machiningcenter with an automated work handling system, or it can be a close groupingof machines connected by an automatic work handling system.

Whether a single machine or several machines, a cell is characterized by its

ability to completely produce a workpiece so that the entire processing

sequence is compressed in terms of space and time. This “Start-a-Part/Finish-

a-Part” processing concept reduces the overall flow time required for the

complete machining operation.

A properly designed cell can reduce part travel time around the shop and reduce manufacturing staging time, or in-line queue time. In effect, a cell becomes a manufacturing solution configured with the type and quantity of

equipment needed to meet specific production requirements, whether they be high volume with low variety, low

volume with high variety or varying volume with high variety.

What are the benefits of flexible manufacturing using the cell approach? Certainly improved part throughput. The

primary advantage of cellular manufacturing is that automated pallet delivery keeps the spindle in the cut. In most

cell applications, spindle utilization averages 90 to 95 percent.

Improved resource utilization is another advantage. Cellular manufacturing reduces part travel, and because of that, improves operator utilization by

to 50 percent. Because fixtures can be shared across multiple machines, fixturing costs also go down.

Reduced labor costs are also possible with cellular manufacturing. Because operators can control multiple machines, direct labor costs go down, and

operators can be used in other value-added tasks. Cellular machining operations continue the cutting process through breaks and lunch, and they can

operate unattended or lightly attended during off shifts. For example, with the addition of a cell, shops running two 10-hour shifts per day can run thremaining four hours unattended for full, 24-hour-per-day operations.

Moving Away From Sequential Processing

Generally, a cell approach is considered as a replacement for station-style, operation-oriented manufacturing.Sequential, manual movement and part flow characterize this style of manufacturing. Operation-orientedmanufacturing is usually operator intense at each station and offers a limited variety of processing capabilities and

setup flexibility. It is basic manual transfer manufacturing.

A cell approach offers the flexibility of process-oriented manufacturing. It features an automated part/pallet delive

system and a pool of online fixtures that are already set up.

It offers high variety and varying volume capability and features computerized workload and process management

pre-staging and automatic job execution. Redundancy, or randomness, is also an important feature of cellularsystems.

Since processing is not sequential, parts can be routed to any available machine in the cell. The cell can also be se

up so that all machining operations can be performed on one machine. As an added benefit, cellular, process-

oriented, manufacturing is less direct-labor intensive than operation-oriented manufacturing.

Accounting For Unaccounted Costs

In a typical shop, part movement and in-queue times are the largest uncounted cost items. Parts can spend as much as 95 percent of processing tim

being moved or in-queue.

This time is usually taken up by waiting for raw goods inventory, moving parts from machine to machine, waiting for parts at each station, waiting for

finished goods inventory, and inspecting the same features multiple times. Sequential production does not maximize throughput.

Because of work handling downtime, sequential manufacturing also does not provide the most efficient spindle utilization. For example, a typical four-

operation sequential part processing application might have individual machine uptimes of 95, 94, 95, and 96 percent. But, when the four processes

combined (0.95 × 0.94 × 0.95 × 0.96), the result is 81 percent uptime.

An increase in spindle utilizationand throughput can be seen withthe use of a fixtured tombstone inconjunction with a twin-palletchanger. It’s a basic but importantstep to increasing productioncapacity.

Horizontal machining centers arreadily adaptable to various leveof automation. They are generalnot limited to the number of pallets that can be installed andcan be easily expanded.

A cell can be a single machinewith an automated workhandling system, or a closegrouping of machinesconnected by a work handling

system.


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Capital utilization is another concern in sequential operations. In the same operation, assume an average machine utilization of 64 percent. To deter

the capital utilization, multiply average machine utilization by uptime by typical efficiency (0.64 × 0.81 × 0.80 = 41 percent). Examples vary widely b

shop and by application, but the message is nonetheless clear. Because of time wasted in part movement and in queues for operations, sequential

operations create inefficiencies that are difficult to overcome simply by adding more equipment and working harder.

When Is Cellular Machining Right?

There are certain symptoms that indicate the need for a cell. For example, does your shop operate with aprogressive, dedicated station-style of manufacturing, typically producing too much work-in-progress identified bypartially machined parts being transported all over the shop? Do the parts spend more time in non-value-addedtime traveling between operations than being machined? Do production requirements change faster than your shopcan react? Are your parts plagued with engineering changes? Are your tooling costs too high? Do you purchasemultiple machines to produce the same part or part families because setup time severely limits machining time andthroughput?

If the answer is yes to any of these questions, then a cell arrangement may improve operations. Just as with

palletized workholding systems, small to medium sized, prismatic parts—the types normally considered for

horizontal machining center applications—are the ideal candidates for cellular manufacturing.

The first step in determining if a cell is the right approach for you is to perform an asset analysis. The asset value

of any machining operation is a total of inventory and capital equipment.

The proper weighting between the two determines shop profitability. For example, each dollar in inventory

increases expenses, costing as much as, or more than, 37 cents per year in taxes, lost revenue on inventorydollars, floor space cost and obsolescence.

On the other hand, dollars invested in capital equipment decrease expenses. Each dollar invested in capital equipment can produce an ROI of more th

33 cents per year due to investment tax credits, reduced direct labor costs, depreciation and better quality parts.

Consequently, each asset dollar shifted from inventory to capital equipment has the potential to produce 70 cents more profit per year. Machining cell

replace stand-alone machines at ratios of up to four-to-one, making them extremely cost effective capital investments. A cell arrangement can be an

efficient means of capital equipment expansion that has the added advantage of providing a highly flexible manufacturing capability that will give sho

owners the ability to take on a wider variety of work.

Not All Of The Benefits Are Quantitative

There are advantages to cellular manufacturing that go beyond dollars and cents. For example, cellular manufacturing often fosters a team spirit whe

employees work together rather than as individuals responsible for the operation of a single machine.

Cells also optimize information retrieval through relational databases. Remote access through various interface options allows supervisors to call up

status displays, generate reports, import/export NC programs, review tool data, workloads and routes, and exchange messages with operators from

PC.

It’s clear that any shop seriously considering increasing its capacity should investigate pallet changing systems and machining cells. A careful analysis

setup and queue times, capital utilization, direct labor rates and work-in-progress inventory could be all of the justification you need to open up a wid

new world of possibilities. MMS

About the author: John Wiese is Industrial Equipment Marketing Director at Cincinnati Machine (Cincinnati, Ohio).

Incremental steps from a single

machine to a full blown integratflexible manufacturing cell canproceed at a pace dictated by thshop’s business. Modularity is kto a graduated capacity growth

plan.

MMS Online is a trademark of Gardner Publications, Inc, copyright 1997-2007.MMS Online and all contents are properties of Gardner Publications, Inc.

All Rights Reserved.


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Artigo Comparativo - Troca de Pallets e Células

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