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Increased flexibility through palletizing.

Increased flexibility through palletizing

Pallet-shuttle systems introduced in recent years are offering a cost-effective alternative to the expensive, all-or-nothing proposition of an elaborate FMS. This is especially the case with shrinking lot sizes and Just-In-Time methodologies. Many manufacturers who operate in low-volume, high-changeover situations often face this dilemma: purchase an additional machining center and risk underusing it, or palletize existing machines to boost their flexibility and capacity. Unlike a dedicated FMS, which is based on a total refabrication of an entire manufacturing operation, a pallet system can begin with a single machine, and be expanded later into more elaborate systems. The throughput economies realized on one machine can multiply when more palletized machines are added to form interconnected, palletized machining centers and manufacturing cells.

An automated pallet-shuttle system evens out the on-line running cycle by minimizing downtime caused by workpiece setup, parts changeovers, tool changes, and parts loading and unloading.

In a pallet system, the operator controls the machining process (manually or through CNC) while the automated system performs the loading, positioning, and unloading functions at the machining center. The operator's tasks are off-line, involving setup and removal of finished parts from pallets during the machining process.

How pallet systems operate

Pallets are round, rectangular, or square, and can range in size from 12" to 120". Either the pallet itself or a fixture holds an actual workpiece. The underside of each pallet will have either standard T-slots or a special mounting device.

Once set in motion, pallets engage with pallet receivers, the independent precision subplates mounted on machine tables. Pallet receivers can be coupled to vertical- and horizontal-spindle machining centers, including inspection machines and wash stations. Many pallet receivers can be specified as original equipment on new machine tools or retrofit on existing machines. This is a key to their value to a machine shop: infusing new capacity in machine tools.

Designs vary, but most receivers are mounted on machine tables by clamping, bolting, or dowel pins. The union between machine and pallet receiver is critical to palletizing precision and is considered permanent. At the appropriate time in the shuttle cycle, the receiver grips and locks the pallet into precise positioning for machining.

The pallet loader transfers a pallet from a loading station to a pallet receiver. The pallet is then transferred back to an unloading station, or to another machining, washing, or measurement area. The loader is a floor-mounted, motorized device that performs pallet interchanges, pulling part-loaded pallets out of the park mode and delivering them to the machine's pallet receiver. Frequently, the loader head will be designed with 180 deg or more rotational capability and a synchronized chain-and-sprocket assembly. The loader head moves along calibrated guide rails that span the gap between the loader and the pallet receiver. The loader also triggers subsequent cycles of pallet engagement, mounting, clamping, and unclamping.

When a workpiece has been completed, the loader retrieves the pallet from the machine and shuttles it to an unloading station. The loader functions either manually or via CNC control where sensor pins on the loader relay position signals for sequencing unclamping, movement, positioning, and unclamping movements.

For added flexibility and capacity, a basic pallet system can be upgraded with a Rail Guided Vehicle (RGV) or Automated Guided Vehicle (AGV) and additional park stands. These transport pallets from floor-mounted park stands and deliver them to a pallet loader. An RGV or AGV can also off-load machined pallets and transport them to measuring stations, wash stations, or inventory areas.

A basic pallet sequence

Assume a vertical machining center is scheduled to process three connecting rods. Conventionally handled, each rod would require setup time of 40 min and machining time of 20 min. Throughput would be one rod/hr and uptime is 20 min.

Add three off-line pallets and throughput improves three-fold. The operator sets up the first two workpieces on individual pallets and engages the control. The pallet loader transport and positions the first pallet on the receiver mounted at the machine spindle. While the machine is boring the first workpiece, the second pallet is on a park stand waiting its turn. Meanwhile, the operator is setting up the third workpiece on a pallet on another off-line park stand.

When the first workpiece is completed, the pallet loader removes the first pallet, and shuttles it to a waiting unload station and shuttles the second pallet to the machine. The process is repeated on the second and third pallets. The interval of the pallet exchange can be as little as one min.

The machine tool thus operates continually by exchanging the first, second, and third pallets. At that rate, uptime is maximized and throughput is increased from one rod to three in an hour interval. Operator time is optimized by focusing manual efforts on off-line setup. For parts changeovers, the operator does not have to shut down the machine, because he is working ahead of the machining process by setting up new pallets. By adding pallets for the run of three parts, the machining cost per unit is cut by 66%, or $16 for an operation cost of $50/hour.

In JIT manufacturing situations, nonautomated machine shops experience added downtime because of frequent part and tool changeovers. An expensive option is to install more machines to plug the production gap created by this downtime. Palletizing attacks the JIT/productivity problem. Pallet receivers, pallets, and loaders can retrofit to existing machines to increase machine output by 40% to 200%.

Standardization

Attention should be given to the need for some type of pallet standardization. In the absence of an international pallet standard, a committee of the NMTBA and various international manufacturers is trying to create an ISO specification. So far, the US contingent has proposed a single T-slot as standard for pallets up to 800 mm. For pallets larger than 800 mm, the group is considering a double T-slot configuration.

Standardization is important for two main reasons: to ensure that all pallets in a shop connect with loaders and receivers of similar or different make, and to reduce clamping distortion. Opinions vary on slotting configurations and their impact on distortion.

Quality benefits

Since pallets, loaders, and receivers are precision instruments, automated pallet systems should improve positioning accuracy and throughput performance. It's not uncommon for pallet transfer repeatability variation to range from no more than 0.01 mm on small pallets to no more than 0.02 mm on larger pallets.

Better use of a machine's spindle time maximizes the time available for parts changeover, a key to functioning in JIT conditions. As a machine runs continually from pallet to pallet, the off-line operator is setting up additional parts from the same lot, repositioning or off-loading them, or changing over pallet fixtures to accomodate different workpieces. The machine can complete its lot run uninterrupted.

The addition of park stands and RGVs can further increase spindletime productivity, the flexibility of a machining center, a cell, or a series of interconnected cells.

Case Studies: Pallets

boost large-scale productivity

Cummins Engine Co mobilized itself for JIT manufacturing in 1988. The primary product is connecting rods for heavy-duty internal combustion engines. The firm took a large-scale approach, containing its operation into a smaller area, and expanding its capacity from less than 200 rods/day to more than 600.

The firm's Scotland operation installed an FMS of nine cells that combined robotic machines and automated, synchronized pallet conveyors to produce four models of connecting rods, 6" to 18" long. The FMS blends nine cells, 27 machines, and an RGV in an area of 7500 sq ft, about 25% less than the former conventional line.

For Cummins, the pallet-oriented FMS reduced the plant's production area and tripled throughput. Prior to FMS, the conventional dedicated transfer line required eight hours for parts changeover. Application of FMS reduced the changeover sequence to 15 min, which has proved to be efficient with small parts lots and high-mix machining situations.

Pallet changer adds flexibility

Gibson Anesthesia of California improved its productivity through better use of a retrofitted, vertical machining center. The firm manufacturers two basic brass or stainless-steel parts for medical applications.

In the palletized sequence, 12 steel or brass parts are attached to a fixture, six on each side. The parts are sent through a vertical machining sequence that includes milling, beveling, drilling, and deburring. Six pallet-integrated machining stations are used. The modernized pallet arrangement allows the operator to load an alternate pallet while workpieces on another pallet are at the spindle.

Before the application of a pallet changer and two pallets, setup normally required 30 to 45 min of downtime. Actual machining involved about seven min. Under the new arrangement, downtime is reduced to 15 min maximum.

As a result of palletizing, productivity has increased nearly 100%. Pallet transfer takes only 20 sec. Capacity has increased to 200 to 400 parts/day; previous capacity was 130 to 150 parts/day. Also, the cost of a pallet changer was one tenth the cost of a new machine.

PHOTO : Advanced pallet-shuttle system integrated with one vertical machining center. System includes round pallets, an RGV, and multiple pallet loaders.

PHOTO : Eimeldingen retrofit application involving one pallet loader with rotating head and three pallets. Pallet system is interfaced with Sundstrand Omnimill machining center.

PHOTO : In foreground from left to right, two pallet loaders with rotational capability and three stationary loaders. In background, eight park stands, where workpieces are loaded and off-loaded.

by Hans Geppert General Manager Eimeldingen Corp Indianapolis, IN
COPYRIGHT 1990 Nelson Publishing
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1990 Gale, Cengage Learning. All rights reserved.

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Title Annotation:pallet shuttle systems
Author:Geppert, Hans
Publication:Tooling & Production
Date:Nov 1, 1990
Words:1576
Previous Article:A leading company returns.
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