Pallet Pools Pump Up Productivity
Keep spindles turning to win the production game
By Mike Jouglard, CMTSE
and Paul Spink, CMTSE
Mori Seiki USA Inc.
Horizontal machining centers equipped with pallet changers have traditionally been applied in high-volume production environments. Although high-volume machining is still prevalent in some industries, the general trend is toward smaller lot sizes and higher flexibility.
The key to using a HMC to manufacture small lot sizes is keeping the machine spindle cutting while tooling up the second pallet for the next job. One way to achieve this is through the use of pallet pools.
A pallet pool eliminates the setups between jobs and creates a queue of jobs to feed the machine and keep the spindle cutting.
Another advantage of a pallet pool is a reduced number of parts made for inventory. In a conventional machining setup, you do that to try to amortize the setup time over more parts, and to reduce the number of times you set up the same job in a year's time.
But, those parts are essentially cash sitting on the shelf. There are costs to carrying inventory, and an engineering change could scrap the inventory. With a pallet pool, parts are machined when you need them.
Pallet pools consist of several components. These include:
- A device to move pallets inside the system. This can be a chain-type carrier that moves all the pallets around the system at the same time like a small train. Or, it can be a device that rotates and extends to reach any pallet on a random or sequential basis. Another example is a rail-guided vehicle moving up and down the row of pallets, picking a new pallet or delivering a finished pallet.
- Some form of storage to hold the pallet in a known location. Without the ability to identify the location of a pallet, the transporter isn't able to pick up pallets or place them reliably.
- At least one station where an operator can load unfinished parts and remove finished parts.
- A cell controller that manages the movement of a pallet to and from machines, initiates the machining sequence for the parts on the pallet, and keeps records of pallet pool activity.
When starting the pallet pool, each part has to be set up, indicated in, and inspected. Next, the pallet is stored in the pallet pool, leaving the fixture untouched. The next time you need to run that job, the pallet is exactly as you left it. There's no further fixture setup, no indicating to reference the fixture and set offsets, and no need for first article inspection, because the first part you run now is exactly like the last part you ran a week or a month ago. Nothing has changed. That lost time in inspection is now production time. The time needed to start a job is reduced to the time it takes to move the pallet to the setup station.
Because the pool can hold a number of pallets, users need to decide how the pallets will be used. By evaluating production levels, cycle times, frequency of reorders, and the number of machines, engineers can determine approximately how many jobs would be needed to keep the machines loaded during the working day. Some parts may need one pallet, others several pallets, and still others only one face of a tombstone. All the fixtures make up a queue of jobs waiting to be machined by the one, two, or more machines in the pallet pool. It's common to run several jobs at one time.
The cell controller schedules jobs based on the priority given that job by production control. If all the high-priority fixtures are loaded and the setup station is idle, the system delivers lower-priority parts to the operator for loading. Loaded fixtures are held on a pallet stand to wait until they're sent to a machine.
As long as the operator loads high-priority fixtures, the cell controller will continue to send them to the HMCs. If the operator goes to lunch, takes some personal time or--in long-cycle operations--goes home, the cell controller finishes the high-priority parts. When there are none left, the controller schedules the lower-priority parts for machining. The cell controller will continue to run all the fixtures that have parts on them before the machine spindles stop, even if there are no operators at the pallet pool.
Completed parts are held in the pool until the operator returns and reloads the first high-priority fixture. Then the cell controller sends that fixture to a HMC and brings the next high-priority fixture for reloading to rebuild the buffer or queue for the machines. The result is much higher spindle utilization than a stand-alone machine.
Investing in a pallet system will require advance planning and preparation to facilitate installation and implementation. The time between ordering and delivery can be used to put all the required pieces in place. Pallet pool configuration and placement within the manufacturing facility is critical. Engineers need to consider efficiency and future expandability.
The pallet pool and machines may require a special foundation or anchoring to ensure performance and accuracy. In this situation, the machine builder can supply detailed drawings of the foundation specifications.
Components of a pallet pool include a device to move pallets inside the system (light blue); pallet storage stands (purple); an operator load/unload station (orange); a cell controller (darker blue); at least one machine tool (green); and a cell enclosure (yellow).
Required services, such as electrical, air, and fluids, must be determined and supplied. In operation, the system will generate large amounts of chips. A central chip conveyor system may be the solution. System layout and positioning must consider access to chip containers and allow adequate space for removing the chip conveyor from the machine.
Pallet pools can be prolific producers, and careful consideration must be given to material flow. How will raw material and finished product be handled? Will the material be on pallets, or in baskets, bins, or some other container? Will it be delivered by forklift, hand truck, belt conveyor, or some other method?
If the workpiece material is too heavy to be manually loaded, an assisted lifting device may be required. The placement of this device should provide maximum efficiency and minimum interference. If workpieces are machined in multiple operations on different workholding devices, plans must be made to store the work-in-process until the next operation is available.
How will data for part programs and fixture offsets be managed? Potential solutions range from the tried and true method of using the CNC memory to a networked solution for multi-machine systems. Because pallets can be shared by machines in most pallet pool systems, program management and revision control is critical. Centralized data storage and retrieval provides greater security.
Whether part-program data are stored in the CNC memory or on some network device, file maintenance should be conducted on a regular basis. Old data should be deleted or archived. Only the most current part-program revisions should be stored to minimize confusion.
Tooling availability for each machine in the system is a prime consideration. Loading tools can be extremely time-consuming, and will negate the setup time advantages of a pallet pool. Consideration must be given to each machine's magazine capacity, including redundant tooling capability.
Part inspection may require on-machine probing--something many engineers are reluctant to implement. However, probing cycles are relatively short, and their use should be based on overall pallet cycle time and judged on a value-added basis. A spindle probe can provide a wide variety of capabilities, from part location and fixture-offset to finding surfaces on workpieces with relatively large dimensional variations. Further, the spindle probe allows surface and feature measurement. A table-mounted probe is useful for automatically adjusting tool-length offsets and checking tools for breakage.
The cell controller manages the movement of pallets to and from machines, initiates machining for parts on the pallet, and keeps records of pallet pool activity.
Personnel need to be trained before the system is delivered. If you wait until the system is installed, the process of learning the operation of each component within the system could be overwhelming, and adversely affect productivity. All personnel involved in the daily operation should be well-trained in the operation of the machine tools. These same people may need varying levels of training in the total system operation. Maintenance staff should be trained to repair both the machines and the pallet pool.
Although it's a cliché, there's no substitute for proper training. In the case of a pallet pool system, training can make or break you. When the pallet pool system is delivered to your facility, it's a good idea to have your personnel involved with the installation process as much as possible. The knowledge they'll gain on how the system is assembled can prove invaluable in the event of trouble.
Once the system is installed, all cutting tools should be loaded into the tool magazines and tool length set into the CNC memory. Workholding fixtures need to be mounted to pallets and aligned. The zero point on the fixture should be measured at the machine to determine the fixture offset values. These values should be recorded on paper or in digital form.
You'll need to monitor and maintain cutting tools at the machines. Make sure the cutting tools are in proper working order, and that tool life is sufficient. In most pallet pool systems, unlike stand-alone machines, the operator is typically some distance from the cutting action. You can't always rely on the operator's eyes and ears to tell when a tool is about to give out. Valuable productivity can be lost due to a catastrophic tool failure.
Fixtures and workholding devices should be inspected and maintained daily to ensure that the workpiece can be properly located and clamped. Inadequate fixturing will affect part quality and increase the potential for accidents.
Preventive maintenance (PM) and good housekeeping practices are the key to problem-free operation. Often, PM schedules are neglected because production is critical. But lack of common-sense care for your system can lead to unpredictable downtime.
There are other considerations when working with a pallet pool. For example, how you fixture a part can affect production flow and minimize potential operator errors.
Consider as an example a part that requires two machining operations, Op 10 and Op 20. Assume the part is relatively small, and we can fit two parts on a pallet using a narrow tombstone fixture. Many manufacturing engineers will place two Op 10 parts on opposite sides of the pallet, then index the machine table 180º during the process to use the same tool on both parts and minimize tool-change time. On the second fixture, Op 20 would also have two parts on the pallet. Between the two operations, all six sides of the part would be machined.
In operation, parts fixtured on the Op 10 pallet are machined before the transporter moves the pallet back to the setup station. The operator removes the finished Op 10 parts, places them on a bench to wait for the Op 20 pallet, and puts new material on the fixture. The pallet transporter then moves the Op 10 pallet to a pallet stand to wait for a machine or directly to a machine if one is available.
The process is repeated when the machine finishes the Op 20 pallet, except the operator removes completed parts to the finished material bin. The operator then takes the two Op 10 parts and puts them on the empty fixture to await Op 20 machining. The pallet transporter moves the pallet to a stand or machine.
Those Op 10 parts sitting on the bench waiting for the next fixture are in-process parts. If you are only running one or two pieces, this may not be a problem. But if you are running five or ten parts in the system, these in-process parts take up space, can be damaged, and may allow the operator to place the wrong part in a fixture. The process becomes more daunting if the parts require more than two operations and the operator must separate them by operation and type.
An alternative fixturing method can eliminate the in-process parts. Start by putting Op 10 on one side of the tombstone fixture and Op 20 on the opposite side. When the pallet moves to the setup station, the operator removes the finished part from the Op 20 position, moves the half-finished part from the Op 10 position to the Op 20 position, and loads new material onto the Op 10 position.
This setup eliminates in-process material. Raw material goes into the fixture, and finished parts come out. Depending on part size, there may be two or three different parts on different faces of the tombstone. If production quantities require more than one fixture, there may be two or more fixtures in the pallet pool for that part. Otherwise, one fixture will make the part complete.
It may take slightly longer to machine one part at a time rather than running two parts at a time, because tool changes are a second or two longer than the table index. But this approach saves money and leaves space for an additional part in the pallet pool. In-process material is eliminated, and each pallet cycle yields a finished part.
This article was first published in the February 2004 edition of Manufacturing Engineering magazine.