Related Glossary Terms
Cone-shaped pins that support a workpiece by one or two ends during machining. The centers fit into holes drilled in the workpiece ends. Centers that turn with the workpiece are called “live” centers; those that do not are called “dead” centers.
- computer numerical control ( CNC)
computer numerical control ( CNC)
Microprocessor-based controller dedicated to a machine tool that permits the creation or modification of parts. Programmed numerical control activates the machine’s servos and spindle drives and controls the various machining operations. See DNC, direct numerical control; NC, numerical control.
- computer-aided design ( CAD)
computer-aided design ( CAD)
Product-design functions performed with the help of computers and special software.
- computer-aided manufacturing ( CAM)
computer-aided manufacturing ( CAM)
Use of computers to control machining and manufacturing processes.
- family of parts
family of parts
Parts grouped by shape and size for efficient manufacturing.
Drilling process using a self-guiding tool to produce deep, precise holes. High-pressure coolant is fed to the cutting area, usually through the gundrill’s shank.
- inner diameter ( ID)
inner diameter ( ID)
Dimension that defines the inside diameter of a cavity or hole. See OD, outer diameter.
Turning machine capable of sawing, milling, grinding, gear-cutting, drilling, reaming, boring, threading, facing, chamfering, grooving, knurling, spinning, parting, necking, taper-cutting, and cam- and eccentric-cutting, as well as step- and straight-turning. Comes in a variety of forms, ranging from manual to semiautomatic to fully automatic, with major types being engine lathes, turning and contouring lathes, turret lathes and numerical-control lathes. The engine lathe consists of a headstock and spindle, tailstock, bed, carriage (complete with apron) and cross slides. Features include gear- (speed) and feed-selector levers, toolpost, compound rest, lead screw and reversing lead screw, threading dial and rapid-traverse lever. Special lathe types include through-the-spindle, camshaft and crankshaft, brake drum and rotor, spinning and gun-barrel machines. Toolroom and bench lathes are used for precision work; the former for tool-and-die work and similar tasks, the latter for small workpieces (instruments, watches), normally without a power feed. Models are typically designated according to their “swing,” or the largest-diameter workpiece that can be rotated; bed length, or the distance between centers; and horsepower generated. See turning machine.
- modular tooling
1. Tooling system comprised of standardized tools and toolholders. 2. Devices that allow rapid mounting and replacement of tools. Commonly used with carousel toolchangers and other computerized machining operations. See toolchanger; toolholder.
- numerical control ( NC)
numerical control ( NC)
Any controlled equipment that allows an operator to program its movement by entering a series of coded numbers and symbols. See CNC, computer numerical control; DNC, direct numerical control.
- numerical control ( NC)2
numerical control ( NC)
Any controlled equipment that allows an operator to program its movement by entering a series of coded numbers and symbols. See CNC, computer numerical control; DNC, direct numerical control.
- outer diameter ( OD)
outer diameter ( OD)
Dimension that defines the exterior diameter of a cylindrical or round part. See ID, inner diameter.
- payload ( workload)
payload ( workload)
Maximum load that the robot can handle safely.
- statistical process control ( SPC)
statistical process control ( SPC)
Statistical techniques to measure and analyze the extent to which a process deviates from a set standard.
- steady rest
Supports long, thin or flexible work being turned on a lathe. Mounts on the bed’s ways and, unlike a follower rest, remains at the point where mounted. See follower rest.
Minimum and maximum amount a workpiece dimension is allowed to vary from a set standard and still be acceptable.
Secures a cutting tool during a machining operation. Basic types include block, cartridge, chuck, collet, fixed, modular, quick-change and rotating.
Workpiece is held in a chuck, mounted on a face plate or secured between centers and rotated while a cutting tool, normally a single-point tool, is fed into it along its periphery or across its end or face. Takes the form of straight turning (cutting along the periphery of the workpiece); taper turning (creating a taper); step turning (turning different-size diameters on the same work); chamfering (beveling an edge or shoulder); facing (cutting on an end); turning threads (usually external but can be internal); roughing (high-volume metal removal); and finishing (final light cuts). Performed on lathes, turning centers, chucking machines, automatic screw machines and similar machines.
- work envelope
Cube, sphere, cylinder or other physical space within which the cutting tool is capable of reaching.
Dowty Aerospace is organized for efficiency and clear communication.
In Southern California, a thriving aerospace firm is as rare as a white Christmas. Southern California, with its high concentration of aerospace firms, was the hardest hit region when military contracts dried up and civilian aircraft demand dwindled. Many firms that served the aerospace industry have withered, and many more are struggling to stay alive.
But Dowty Aerospace, in the Los Angeles suburb of Duarte, seems to be beating the odds. Dowty has improved its fitness for survival in an increasingly hostile market by developing an organizational plan that defines tasks and areas of responsibility and establishes lines of communication and control. Dowty’s managers have thought long and hard about the manufacturing process. They’ve broken it down into a series of steps and then arranged those steps into groups that increase efficiency, reduce duplicated effort, and capitalize on the workers’ expertise. In such a well-regulated environment, little gets lost, forgotten, or overlooked.
Dowty specializes in aircraft hydraulics. Its chief product is actuators for aircraft thrust reversers. These actuators move the thrust reverser panels into position behind a plane’s jet engines as it lands. The panels direct the engines’ thrust forward and brake the plane. Each engine requires several panels along with the actuators to move them into position.
Actuators are made principally of stainless steel components. Dave Knapik, manager of NC operations for Dowty, says that, in addition to stainless steel, the company works often with aluminum and titanium. A finished actuator represents up to 20 hours of machining time and 20 to 30 operations. Dowty uses 5-axis machining centers, slant-bed lathes, CNC EDMs, drill presses, CNC ID and OD grinders, precision hones, and gundrilling machines to produce its parts.
Dowty’s customers include Boeing Aircraft, Seattle. The company produces actuators for Boeing’s 737, 757, and 777 airplanes. Other major aerospace firms, such as McDonnel Douglas, also use parts made by Dowty.
Division of Labor
Aerospace work leaves little room for error. Passengers’ lives depend on the reliable performance of parts made by companies like Dowty. To maintain this level of reliability, aircraft builders impose tight controls on every manufacturing process. The builders Dowty makes parts for expect their suppliers to monitor and document every step from the design of the part to its assembly. Dowty has satisfied these demands by structuring its operations to minimize the uncertainties and inefficiencies that lead to waste, mistakes, and negligent workmanship. To keep the lines of communication and responsibility clear, the company ensures that employees have clearly defined duties. These include part design, programming, machining, inspection, and tool maintenance.
Dowty designs most of the parts it makes, modifying existing designs or designing new parts from scratch to meet its customers’ specifications. The CAD files its designers produce are passed on to part programmers, who plan the sequence of operations, program the machines that will perform these operations, and prove out the part.
Most of the programs for the company’s EDMs and OD grinders are written on the machines themselves. All other part programs are conceived and written on workstations running CAM software in offices away from the shop floor. Once these programs are finished, the programmers send them to the machines they have been written for to test and fine-tune the programs. At this point, the operators may set up the test parts, but the programmers actually operate the machines. Knapik says this separation of functions frees the operators to work on other jobs, producing parts with programs that have already been perfected. This ensures that the operators, who are being paid a straight hourly wage, are producing good parts during the hours for which they are being paid.
The programmers work with the machines until they are making parts that will pass inspection. At this point, they are ready to work with the operators for a day or two to familiarize them with the part and the process. Then the programmers turn over the part to the operators for production.
If it is a part Dowty has not previously manufactured, it will have to be qualified before production can begin. This involves a series of tests to determine if the part will function properly. Dowty has a number of test rigs capable of taking a part through its cycle thousands of times or stressing it until it fails.
The Information Artery
Dowty’s separate departments act like individual organs working in concert for the benefit of the entire manufacturing body. Coursing through this body is a steady supply of documents and electronic data, the body’s lifeblood and nerve impulses. Through this communications network, workers are assigned tasks and told how to perform them.
The network is the link that allows departments to monitor each other. Copies of the communication that passes back and forth and printouts of the data that is compiled also provide the documentation Dowty’s aerospace customers demand.
All documents relate to what Knapik calls “frozen processes.” A process becomes frozen once the programs and processes developed by the manufacturing engineers are able to produce good parts. Finished part programs are stored on the company’s central computer system, which is linked to most of the machines on the shop floor. The part and its associated programs are assigned an identification number. By keying in this number, the operator can call up the program and download it to the machine tool’s CNC.
Once a process is frozen and the part is being made on a production basis, the operator is responsible only for entering offsets to compensate for normal tool wear. He also must change tools when they have reached the total cutting time that the company’s records say they are good for. The operators typically are allowed a maximum of 0.025" offset. If a larger adjustment is needed, it indicates that there is a problem with the application.
It is at this point that the programmers create the process’ numerical control instructions (NCI), a collection of documents that includes the part program’s revision history, cutting tool and fixturing requirements, the written sequence of operations (generally, these are documented with pictures of the setup), operators’ notes, offset data, and the program readout. This information is supplemented on an ongoing basis by the operators’ notes and the offset adjustments they have had to make. All documentation is bound in a notebook that is kept in Dowty’s NC programming department. Each process used to manufacture a complete part has its own notebook, and a notebook is kept for every operation Dowty has undertaken.
Because Dowty’s aerospace customers demand such strict control over the production process, changes to frozen processes can be made only through a strictly defined set of procedures. Documentation must be added to the record kept in the NCI book to show that parts made with the revised program and procedures were able to pass inspection. The manufacturing engineers send forms describing the changes to other departments to alert them to update the documents and procedures they keep separately from those recorded in the NCI book.
The NCI book presents a snapshot of the operation when it was making good parts, according to Knapik. “If the operator runs into a problem at a later time, he knows that the cause isn’t the program and it isn’t the use of the wrong tool, because he has a written record showing that, at one time, the program and tools were able to manufacture the part properly,” says Knapik. With this assurance, the operators and manufacturing engineers can narrow their search for the problem’s cause to the variables that couldn’t be foreseen when the NCI book was compiled, such as inconsistent machine performance, or poor tool or workpiece quality.
Dowty’s library of NCI books for past parts serves as a reference to programmers and operators who are asked to make more of the same parts. The books also provide the documentation that is frequently demanded by the customer or the Federal Aviation Administration.
Other information flows through the communications network as well. When a part is assigned to an operator, the operator receives a router. This document contains part specifications and all processing information. It also tells the operator what NCI number to use to access and download the part program from the central computer. At the beginning of the job, the operator is given the router, the NCI book, and all the tools and fixtures he will need to run the job.
On the Cellular Level
For aerospace companies, efficiency and productivity are matters of life and death. In such a competitive market, they can’t afford any wasted effort. Even though they are producing complex, high-precision parts, their operations must make it possible to shave man-hours to the very minimum. When Dowty’s engineers were looking for ways to organize operations more efficiently, among the factors they considered was the physical layout of the plant. By grouping machines into cells or mini-cells, they made it easier for operators to run more than one machine at a time.
The simplest arrangements feature two machines placed where the operator can reach the controls of both machines with a minimum of motion. Sometimes the machine tools are accompanied by a bank of drill presses. These mini-cells were established to increase the operators’ productive time. Having a second machine close by, the operator can oversee one operation while setting up another.
When Dowty’s engineers were putting together these mini-cells, they chose controls that would operate in a similar fashion on all the machines in the cell. For the most part, the company has used Fanuc units, because the operators are most familiar with these controls.
Knapik says the company’s manufacturing engineers had to learn through experience what operations they could successfully marry in a mini-cell. “When we put two complex 5-axis jobs within one mini-cell, it was too hard for the operator,” says Knapik. When both parts in the mini-cell had a number of geometric features, the operator couldn’t gage them all and still keep up with the workload, he explains. Dowty’s engineers have found that in an ideal mini-cell a relatively easy application is paired with a more difficult application. Sometimes these will be two operations on the same part, but in other situations, two different parts will be machined at the same time.
Dowty’s manufacturing engineers also have set up full-blown cells dedicated to producing a single family of parts. The company currently has mills, lathes, and honing and gundrilling machines grouped into cells for the manufacture of thrust reverser actuators for the Boeing aircraft. These cells have all the machine tools needed to produce every component of the actuator except the housing, according to Knapik.
The cells not only make it easier for the operators to perform multiple operations, they also simplify setups. The parts being machined in these cells are similar enough that they all can be machined with the same tools. The operators don’t have to change tools between part runs. When a new run begins, they simply call up the part program, and they’re ready to make chips. Occasionally, the setup will take additional work. “If the lathe uses a steady rest or a swing-out tailstock, the operator might have to position it,” says Knapik. “But that’s an extremely easy part of the setup.”
To maintain the efficiency of the cell, Dowty’s manufacturing engineers must limit jobs run within the cell to compatible parts. If a part is too disruptive, requiring too many tools that are not already on the machine, it will have to be done on other machines, or the part processing may be modified. Knapik says the benefits Dowty gains from its cell systems justify changing the way the part is made to fit it in. “Sometimes we’ll change an entire process to get the part into a particular cell environment to reduce costs,” says Knapik. “In most cases, these alterations reduce the number of operations as well.”
A palleted workholding system complements Dowty’s cellular machine arrangements. Parts set up on pallets can be shuttled easily from machine to machine. Knapik says Dowty’s engineers have developed probing routines that allow the operator to quickly determine the location of the part once it and its pallet are on a machine. The routines employ probes to touch off datum points on the pallet. These readings are used to calculate the proper offsets to compensate for deviations in the location of the pallet and part as they are moved from machine to machine.
Palleting lets Dowty’s operators machine one part while setting up the next part. They can also work on parts that are too large for a machine’s work envelope by machining one end and then turning the pallet around to machine the other end of the part.
Palleting also allows Dowty’s operators to store their setups for future use. These setups will include all the fixturing necessary to run a particular part, sometimes including a 5th axis if it is used. Dowty’s workflow makes such an arrangement especially helpful. When Dowty contracts with a customer to make a part, it may end up making thousands of units. But these units generally won’t all be made in one continuous run. When Dowty’s operators are preparing to run a part that has run before, the ability to pull a ready-made setup from a rack allows them to make chips a lot sooner. Knapik says the only task the operator may have to do if he is rerunning a job is to program in the offsets to compensate for deviations between the machine the part is running on and the machine it was run on originally.
Dowty has designated certain operators to work in its cells. These operators are not assigned any other projects, and operators from other areas are rarely brought in to work in the cells. Absences are handled by other workers in the cell or their supervisors. According to Knapik, operators who work only in one cell develop a great deal of expertise in manufacturing the parts that are produced in that cell. Their familiarity with the operations reduces their mistakes, and it shortens the learning curve when they are given a new part in the same family to machine. The operators also become familiar with the workflow, and they know what jobs to expect from month to month. Knowing what’s coming up, the operators can arrange their tasks to avoid scheduling conflicts.
Dowty has another group of operators for its mini-cells. These operators, designated multimachine operators, or MMOs, are trained to run more than one machine at a time. When an MMO is working in a mini-cell, he is paid a 20% premium. “They get a bonus out of it, and we get a significant benefit, as well as a real win-win situation,” says Knapik.
Tool management plays a key role in Dowty’s efforts to efficiently produce high-quality parts. Its tool-management operations have been established to keep the machines supplied with a steady flow of tools while insulating its operators from concerns about tool availability or performance.
Some of Dowty’s machines use standardized tools based on modular quick-change toolholders. With these modular toolholders, operators can use tools preset by toolcrib attendants to replace worn tools. They don’t have to use their time, or the machine’s time, to set the tool.
In many cases, the repeatability of the modular system makes it unnecessary for the operators to reset tool offsets when they change tools. If it is a high-precision operation, the operators may still have to adjust the offset, but with modular tooling this becomes a simple math exercise. Dowty requires its operators to check offsets whenever tolerances are tighter than 0.005". With modular toolholders, the operator can do this by reading the label the toolcrib attendant attached to the tool when it was preset. The label lists the tool’s z- and x-axes. The operator compares these measurements to the measurements recorded on the label of the worn tool that is being replaced. The difference between the tools’ measurements will determine the amount of offset the new tool will require. This method of calculating offsets requires only a bit of simple math and a certain degree of faith in the repeatability of the toolholders and the skills of the toolcrib attendant. The operator won’t have to perform tool touch-offs to establish its location in the spindle.
All of the presetting, as well as the storage and maintenance of the tools, is handled through the centralized toolcrib. Toolcrib attendants set the tools and label them with information about offsets and tool life.
On tools fitted with electronic read/write chips, the toolcrib attendants record this information electronically. The chips are glued into shallow holes Dowty employees drill into the toolholders. The hole and the chip are too small to have a significant effect on the balance or performance of the tool. The toolroom attendant uses a writer on the preset machine to record up to 500 bytes on the chip. (One byte is basically the equivalent of one alphanumeric character.) This is enough memory to hold the tool’s identification number, gage-line data, and life-cycle information. A printout of this information also is produced.
Reading heads on some machines can access this information and use it to automatically adjust offsets. The machine tools also can calculate when the tool will need to be replaced from the tool life information on the chip. When the machine is finished with the tool, it records how much additional time the tool has spent cutting.
Knapik says the read/write system works well, but it is a costly way to keep track of the tools. Each chip costs $40, which is nearly half the cost of the toolholder itself.
The location and status of every tool in Dowty’s inventory is recorded in a central database. Terminals in the toolcrib can access this information. Only a few keystrokes generally are needed to find out if a tool is available. Tools that are in stock can be fetched by a computer-run storage and retrieval system. Tools are identified by code numbers. This is how they are referenced in the NCI documents as well as in the tool-management system. Entering a tool’s code number into the system prompts the machine to bring the bin containing the tool to the front of the machine where a toolroom attendant can pick it up. Behind this area is an enclosed room-sized storage facility that houses the tools and the mechanism to move the bins from place to place.
The tool-management system is organized to run smoothly with little intervention from the operator. At the beginning of an assignment, the operator is issued all the tools he will need to run the application. These are pulled from stock by a toolroom attendant who reads the tool numbers from a list prepared by Dowty’s production-control department. The kit of tools contains everything, including spare inserts for the indexable tools. Replacements for the tools will be delivered by the toolroom attendant. Unless the tool has a read/write chip to keep track of time in cut, the operator uses his own judgment of tool performance to determine when it needs to be replaced.
The toolroom attendant also is responsible for troubleshooting at the machine tool. An operator can call for help whenever tool breakage or premature tool wear occurs. This frees up the operator to continue machining with the redundant tools he has loaded into the turret or magazine. “A lot of our operators are running multiple machines,” says Knapik, “So it’s real important to keep them at the machine as much as possible.”
Passengers on planes equipped with Dowty parts depend on the quality of these components to get the aircraft on the ground and stopped safely. Because the performance of Dowty’s actuators is vitally important, the company’s customers have little tolerance for part failures. Typically, customers and government regulators demand rigorous testing to ensure that the part will continue to work for the life of the plane. These parties also demand assurances that the parts have been properly made. Dowty’s customers are very specific in their quality-control demands. In most contracts, these customers require the company to implement SPC and send them its SPC data regularly. A typical contract will also mandate a certain Cpk for the processes Dowty uses to make the part. Some customers are also demanding ISO 9000 certification, so Dowty has begun this registration process as well.
To satisfy these demands, Dowty has implemented a system that closely monitors and documents part and production quality. Quality-control data is recorded and tracked by the company’s central computer system. Terminals at the workstations are networked through a wireless system with the central computer. When the operator gages a part, this information is entered into the computer at the terminal. Electronic gages that are linked directly to the terminal can send data automatically when the part is measured. If the operator is using a manual gage, he will have to key in the information. All the gages the operator will need are delivered to him at the beginning of the operation.
Dowty also maintains two CMMs in a separate department to gage intricate part features such as compound-angle holes. “There’s still a certain amount of checking that has to be done by hand,” says Knapik, “But the CMMs really reduce the inspection time substantially. We have parts, like the rudder for the F-15, that used to take two days to inspect. Now we can do it in a couple of hours.”
Data from the CMMs and the hand gages are compiled by the central computer and graphically represented in a series of screen charts showing the progress of the operation. These charts can be seen at terminals available to the operator, his supervisors, and Dowty management. Anyone reading these charts can tell if the process is in control or if tolerances are nearing their limits.
An operation’s gaging procedures are as carefully documented as the manufacturing steps. When the part is first programmed, a set of coordinate measurement instructions (CMI) are developed by the quality-control department. Like the NCI, the CMI is a collection of documents that detail the procedures that must be followed. A CMI will tell the machine tool and CMM operators which features are to be checked and whether these features are to be gaged on the floor or on the CMM. The gages and CMM probes that are to be used, and the gaging procedures that are to be followed, are listed in the CMI as well. The CMI also records any changes that are made to these procedures.
The inspection process begins with the first part off the machine each shift. The operator is responsible for producing a part that passes this initial inspection. Knapik calls this “buying off” the part. After hand-gaging the part, the operator sends it to the CMM department for gaging. If the part passes inspection, the job can proceed. If not, the discrepancies must be corrected and a second part is evaluated. The operator is given three chances to produce a good part. “But if it goes beyond that, a supervisor will step in and try to figure what the problem is and help him at that point,” says Knapik.
Dowty’s organizational structure would not be appropriate for every shop. It works for Dowty, because it is a relatively large company producing long runs of parts. Its work volume and size justify the use of cells and mini-cells dedicated to single families of parts. Dowty’s organization is the result of a lot of effort and planning. The company’s survival in an increasingly tough market is proof that the investment has paid off.