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Thorough investigation of each step in its manufacturing processes is a way of life at Hartzell Propeller Inc. (Piqua, Ohio). Since the days Hartzell built propellers for the Wright Brothers, it has succeeded in maintaining a dominant position in its market by consistently revising its production methods to incorporate the latest proven technology.
Like many companies whose production lot sizes have been shrinking, the need to find ways to reduce setup/changeover downtime took on paramount importance at Hartzell. Manufacturing director Joe Brown explains the situation: "We had structured our processes primarily around dedicated machines that delivered the most cost-effective performance on historical lot sizes. However, changeover times on these machines were longer compared to CNC equipment.
"More recently, our customers began asking us to produce a greater amount of new model prototypes and have placed more small quantity orders. This has seriously increased machine changeover downtime. The problem was compounded by the fact that our largely dedicated-machine manufacturing process required changeover of several different machines involved in producing each given product. Based on our prior investigations of CNC methods, we knew the time to install these machines was now."
This decision led the company to divide its products into family groups and configure manufacturing cells to produce a complete component in one small area of the plant. Lot sizes now vary from one to 200 pieces with the average in the 15 to 20 piece range. Further adding to this changeover frequency is the need to be responsive to AOG (Aircraft On Ground) emergency orders, which often must be filled in 24 hours. The quick-changeover capabilities of CNC machines help meet these requirements, but the complexity of many of the setups forced Hartzell to seek quick-changeover fixturing and tooling methods as well.
Quick-Change Fixturing System
One example of how this goal was accomplished involves the installation of an SMW Setup-Switcher system on one of the company's new Cincinnati Machine Sabre CNC vertical machining centers. The SMW system is a manually operated pallet changer that utilizes a power-operated pallet clamping fixture mounted on the machine table, and this allows pallets to be switched by the machine operator in approximately one minute. Pallet positioning repeatability is within 0.0002 inch.
The fundamental advantage of this system is simple: Part setup, part loading and unloading and chip cleaning, which are operations that are normally performed on the machine table, no longer take place on the machine table. Instead, they are being performed on the off-line pallet while parts are being machined on the on-line pallet that is clamped on the machine table. When the machining cycle ends, the only time lost is the one minute required to switch pallets.
In the switching sequence, pallets are rolled on and off the clamping fixture on roller rails. Two sideways indexing pallet shuttles are contained in an independent, floor-mounted housing. Each shuttle has a pair of roller rails on which the pallet rests. When the machining cycle ends, the operator indexes the empty shuttle to the center position aligning it with the rails on the clamping fixture. The completed pallet is then unclamped and rolled onto the empty shuttle, and both shuttles are then indexed sideways to position the new pallet in front of the clamp fixture. The new pallet is rolled on and clamped, and therefore a new machining cycle is ready to begin.
Depending on workpiece machining and lot size, several different fixturing schemes can be employed: Both pallets can be dedicated to the same workpiece, sometimes using each pallet to present different faces of the workpiece to the spindle; each pallet can be dedicated to a different workpiece, or multiple workpieces can be fixtured in different positions on the same pallet. Further adding to this flexibility is the option to use additional pallets that can be switched into the system by using a cart.
These pallets can be dedicated to a specific workpiece that is ordered on a repetitive basis several times each year. The relatively inexpensive pallets allow storing these pre-aligned setups on shelves. When an order is received, the appropriate pallet is removed from the shelf onto an adjustable-height cart. It is then transported to the machine where it is exchanged with one of the two pallets in the switching system on the machine. Shelves store the pallets that are required by the family of parts that are produced by the cell.
This technique ensures that the next setup is always ready to be switched into the system when the machining cycle ends--the machine never sits idle while the next setup is prepared. As a result, machine output typically is doubled. Manufacturing engineer Dave Loerke sums it up this way: "Changeovers that used to take two hours, now take about 15 minutes."
In search of similar economies in its turning operations, Hartzell installed an SMW Ultimate CNC Chuck on its Cincinnati Machine Avenger CNC lathes. This chuck provides two primary advantages: It allows jaws to be changed, reversed or adjusted in approximately one minute, and it allows soft jaws to be remounted without requiring reboring.
Historically, jaw change or adjustment took 15 to 20 minutes using conventional chucks. Thus, averaging four to five chuck changeovers per day, the time savings from this feature is significant. However, approximately 40 percent of Hartzell's work requires soft jaws that are remounted again and again for short runs throughout the year. Because reboring usually takes an additional 20 to 30 minutes, the savings in this area also is very significant. To further expedite changeover, soft jaws for repeat orders are stored in drawer cabinets that are adjacent to the lathe.
Aside from the quick-changeover feature of the chuck, it offers several operating advantages compared to conventional chucks. For example, the design of the jaw actuating mechanism is highly resistant to deflection from centrifugal force. This results in a fairly flat grip force/speed curve, which means that there is minimal loss of grip force at today's high spindle speeds. In conventional chucking practice, in order to compensate for grip force loss at high speeds, the workpiece usually has to be "overclamped," which frequently contributes to workpiece deflection and a decline in accuracy of the machined surfaces.
Another feature that contributes to higher workpiece accuracy is the chuck's ability to be programmed for two different grip forces within the same machining cycle, which is accomplished without having to release or remove and reclamp the workpiece. This allows the application of a heavy grip force for the roughing portion of the machine cycle and a lighter force for the finishing operations that minimizes workpiece distortion and resulting accuracy loss. This a particularly significant benefit when turning fragile work.
Quick-Changeover Bar Feed
Another example of Hartzell's willingness to investigate and apply new technology is the use of an automatic-load bar feed system employed on another of its Avenger lathes. The system, also produced by SMW, is called SpaceSaver. It processes bars up to but no greater than five feet long. The unit requires eight feet of space behind the machine compared to 20 feet or so required by full-length (12-foot bar) systems.
Because bar feeding is considered by most people to be a high production or large batch-size process, one might ask why a company like Hartzell, which has average runs in the 15 to 20 piece range, has invested in a bar feed system. The answer is that changeover of this system takes approximately two to five minutes, which accommodates relatively frequent changeovers, yet allows the company to take advantage of the many benefits of bar feeding.
Traditional chucking practice is relatively complex: Blanks must be sawed and transported to the work area. Normally, the chuck must be fitted with hard jaws for the first-end machining operations, followed by changing to soft jaws, which have to be rebored to accommodate the workpiece for second-end operations. The workpiece must also be handled and run through the machine twice. In the case of bar feeding, the entire workpiece is machined in one setup. And, because the bar feeder is automatically loaded, the entire batch can be machined without the operator in attendance. On longer runs, the system's magazine can hold enough bars to allow the machine to run up to 24 hours unattended.
The short bars processed in the bar feeder are supported for almost their entire length in a spindle liner tube, which has a slightly larger ID than the bar diameter. Because the bar, the liner and the spindle all rotate together, machine speed is not restricted, as it is with some full-length bar feeds. As a result, the bar can be turned at maximum machine speed. Furthermore, the short bars are easier to handle, less bar straightness is required, and with SpaceSaver, no bar end preparation is required.
True to companies that retain leadership positions in their fields, Hartzell's progressive attitude permeates every part of its operations, the company's employees included. As Mr. Brown puts it, "Embracing these radical manufacturing process changes has required our employees to accept some major role changes, but they've once again responded very favorably. We try not to lose sight of the fact that ultimately, it's people that make the system work."