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Palletizing for greater efficiency.

Often, machine shops consider two options when trying to increase productivity: add machines, which take more floor space and pose the risk of under-utilized spindle capacity; or retrofit an existing machines with pallets.

Pallets are an excellent way to raise the productivity of a machine tool or machining cell. For example, in most situations, vertical machining center output can be doubled through the use of a pallet changer. This occurs primarily from the reduction of machine-spindle idle time. Part loading, unloading, chip cleanup, and setup operations take place on the offline pallet while the machine is cutting work on the other pallet. The spindle is constantly machining parts except for the one-minute time required to switch pallets. When changeover to a new workpiece occurs, a few more minutes are required to switch machining programs and change tools.

In cases where output is doubled, the user gets the equivalent of a new machine for the price of a pallet changer. Manual pallet changers are available for less than $10,000-a fraction of the cost of a new machine. Retrofitting pallets to an existing machine reportedly takes less than three hours, and complete installation and operator training are supplied by the pallet changer manufacturer.

The simplicity of the pallet changer itself and the ease with which it can be retrofitted to an existing machine are also major pluses. Rotary indexing-style pallet changers and sideways shuttle-style changers such as those from SMW Systems Inc, contain switching devices enclosed in a floor mounted housing that is not directly connected to the machine. Both styles use a low-height locating and clamping fixture, which is mounted on the machine table. The pallet travels on rollers mounted in rails attached to the fixture and rolled on and off manually.

Surprising to many people is the fact that pallet changers are very effective on short-run work. This is true because the machine is not down during setup, which is taking place on the offline pallet while machining of the existing run is being completed. Therefore, the more frequently changeover occurs, the greater the savings become.

Unlike some capital investment programs, implementing a pallet system is not an all-or-nothing proposition. A machine shop can make an impact by retrofitting a single machine with two pallets for added productivity. Or, the shop can palletize and interconnect two or more machines to form a manufacturing cell. As the scope of an operation grows, a shop can interconnect several cells or machines with a system of pallets and rail-guided vehicles (RGVs), which will also enhance total shop productivity.

Automated pallet changers

Pallet changers in a sequential machining environment can be made even more efficient through the addition of automated systems and rail-guided vehicles (RGVs). An operator sets up a workpiece on a pallet. The pallet is engaged at the first machine tool; then an automated system transfers the pallet with the workpiece still affixed to a second machine, third machine, and so on until the piece has been completed and unloaded at a park station. Assuming the operator positioned the part correctly on the pallet fixture at the beginning of the sequence, the probability of workholding error along the path is greatly reduced and subsequent setup time is eliminated.

Eliminating time-intensive parts transfers from machine to machine shrinks handling time. Operators spend less time on workpiece setup, so productivity is improved because operators are then capable of doing more in less time. For example, if palletizing produces a 10% improvement at each of three spindles, this could result in a 30% boost in that particular manufacturing-cell sequence.

In an automated pallet system, where two or more pallets are integrated with software, an operator can split time efficiently between monitoring the controls and offline setup of parts on pallets. As pallets are off-loaded from a machining center, the operator can remove parts from the pallet fixtures. The operator then reloads the pallets at the front end of the machine sequence.

Safety is also enhanced in a palletized environment, because the operator is removed from the spindle area and more involved in parts setup. If enough pallets are provided, depending on part run time, one operator may be capable of supervising multiple machines without being overburdened. This can maximize operator efficiency and particularly benefit shops who cross-train their work-force.

Automated cells multiply productivity across several machine spindles. Two or more automated pallet systems retrofitted to machining centers and connected by RGVs can create a machining cell that offers flexible expandability and allows a shop to grow its business and process more end product without investing in new machine tools.

Setups on the shelf

Shops that have had time to thoroughly evaluate the potential of pallet changers invariably store pallets or sub-fixtures on the shelf for repeat jobs. This practice can apply to both custom and multi-use fixtures and is due in part to the relatively low cost of pallets. The big payoff in this approach comes from total elimination of setup time and elimination of the need to zero-in fixtures each time they are used on the pallet changer and machine.

In today's world, few shops are able to obtain a continuous flow of orders in consistent lot sizes. As Jim Tankersly of Laser-Tronics, San Marcos, California puts it: "There's always some customer--usually a good one--who needs a few parts, or even a single part, yesterday. Accommodating these requests is part of life if you want to survive. Embracing this reality was a major influence in how we set up our company."

Mr Tankersly continues, "From day one, every piece of equipment we bought for our shop and every process we designed has been aimed at being able to make money on orders for a single part. Pallet changers were part of this strategy. For example, if we're in the middle of a run and we get an overnight order, we can set it up on the offline pallet, interrupt the run, switch pallets, switch programs, and run. When the short run is over, we switch the original pallet back, switch programs and pick up where we left off."

Another example of Laser-Tronics' short run capability is the larger machine table on its Fadal VMC 6030. A larger-than-normal table was purchased so the extra space could be dedicated to high-frequency, short-run orders. With this space and each of the two pallets set up for a different job, the machine can run any one of three jobs on a moment's notice. To further add to the flexibility of this arrangement, Laser-Tronics has mounted the pallet changer on rails in the floor. When total access to the machine table is needed, the changer can be merely pushed aside.

Full table utilization

The traditional use of machine vises can severely restrict the capabilities of a VMC. Normally, a machine vise is used to, clamp one or two parts. Most shops will mount two vises on a machine table, thus producing two or four parts per machine cycle. Ray Fitzpatrick of CAD/CAM Services, Santa Fe Springs, California states: "Putting more parts on the machine table is what led us to purchase a pallet changer, although we saw other advantages as well.

"Vises take up too much room on the table so we developed a grid pattern of locating and hold-down holes that is standard throughout our shop. This allows us to achieve high-density fixturing on pallets. Putting more parts on the table shortens the cycle time per part by reducing the tool changes per part. For example, each tool can machine 20 parts (before being changed) compared to only four parts when vises are used.

"This grid pattern system also has the advantage of reducing setup time. In some cases we fixture parts directly on the pallet surface. In others we mount self-contained sub-fixtures on the pallet surface. In both cases the need to zero-in each new setup is eliminated because our CNC programs are pre-zeroed to the grid pattern locating holes."

Another major advantage that accrues from high-density fixturing is the ability to have machine operators perform other tasks. More parts on the table translates into longer unattended machine cycles. Among the duties that can be assigned to machine operators are inspection, deburring, setting up the next job, and running other machines. In many cases, unattended machine cycles are long enough to run through coffee, lunch, and washup breaks. In a few situations an unattended night shift is possible.

Improved flexibility

At Coltec Industries' diesel engine assembly plant in Wisconsin, Eimeldingen Corp solved the flexibility and productivity problems of a five-axis Sundstrand Omnimil. Coltec manufactures diesel engines for ships, submarines, and nuclear and electric power generation.

The Omnimil processes work-pieces ranging in weight from ounces to more than one ton. Operator setup time previously ranged from 20 minutes to more than one hour, depending on the part size. Consequently, setup caused significant spindle downtime. To improve spindle productivity on the Omnimil, the machine was retrofitted with one pallet receiver and one pallet loader designed to rotate 270 deg with service to three park stands that are several feet from the machine. Material is transferred on three ISO-standard pallets, which are 46"-dia with positioning accuracy of 0.0004" and a load bearing capacity of 5000 lb.

Now, one operator can set up multiple parts and do changeovers away from the Omnimil while the spindle runs a part. Cycle times vary because of the wide array of parts presented at the spindle. But the pallets average a transfer time of 60 seconds and the automation allows the operator to focus on part setup. Coltec received a 10% improvement in throughput, which is significant on such a machine. As a result, the retrofit positioned the shop for greater operator efficiency as well as the expansion of the pallet system to other tools or cells.

For more information on manual pallet changers from SMW Systems Inc, circle 230. For more information on automated pallet changers from Eimeldingen Corp, circle 231.
COPYRIGHT 1992 Nelson Publishing
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1992 Gale, Cengage Learning. All rights reserved.

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Title Annotation:machine shops
Author:Stovicek, Donald R.
Publication:Tooling & Production
Date:Jul 1, 1992
Previous Article:A profile on optical comparators.
Next Article:Turning to CNC modeling.

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