CNC lathe:The Manufacturing Process

Until recently, most machining centers were built to customer specifications by the machine tool builder. Now, standardized tooling design has allowed machines to be built for stock or later sale, since the new designs can perform all the needed operations of most users. The cost of a new CNC machine runs from about $50,000 for a vertical center to $5 million for a Flexible Machining System for engine blocks. The actual manufacturing process proceeds as follows.


Welding the base

* The base of the machine is either cast or welded together. It is then heat treated to remove casting or welding stresses and to "normalize" the metal for machining. The base is fixtured into a large machining center, and the mounting areas for the ways are machined to specification.

* The ways are ground flat, bolted, and pinned to the base.


Bolting the bollscrews

* The mechanisms that move the bed or spindle are called ballscrews. These change rotary motion of the drive motors into linear motion and consist of a screw shaft and support bearings. As the shaft turns, a bearing mount follows the spiral grooves in the shaft and produces a very accurate linear movement that moves either the worktable under the spindle, or the spindle carrier itself. These ballscrews are bolted to the base with the bearing mount bolted to the worktable or spindle carrier.



Mounting the spindle

* The spindle is machined and ground, mounted to its drive motor, and then bolted to the movable spindle carrier. Each axis of motion has a separate ballscrew and set of ways in most machining centers.



The controller

* The computer, or controller, is an electronic assembly separate from the rest of the machine. It has a climate-controlled enclosure mounted on the side of the frame or in an operator's console. It contains all of the operating memory, computer boards, power supplies, and other electronic circuitry to operate the machine. Assorted wiring connects the controller to the machine motors and positional slides. The slides continuously send the axis location information to the controller, so the exact position of the worktable in relationship to the spindle is always known. The front of the controller has a video screen that displays the program information, position, speeds and feeds, and other data required for the operator to monitor the machine's performance. Also on the front panel are the data entry keys, data connection ports, and start-stop switches.

* The assembled machine is test run for accuracy. Each machine has slight physical differences that are mathematically corrected in the computer operating system. These correction values are stored in a separate memory, and the machine checks these continuously. As the machining center wears from use, these parameters can be recalibrated to assure accuracy. After testing, the finished machine is painted and prepared for shipment.



Quality Control

Quality in a machining center must be built in from the design through delivery and set-up. Careful instruction to the operators is also important to prevent a crash, the unintentional collision of the work with the tool. Crashes can result in tool damage or machine failure. Many controllers have subprograms to sense an impending crash and place the machine into emergency stop. All CNCs are shipped with special handling to avoid shocks, and are set up carefully by factory-trained technicians. The original correction factors are recorded for later reference. Complete programming, operation, and maintenance manuals are provided.



The Future

The future of CNC machines is exploding. One idea under development is a spider-like machine whose spindle is suspended by six telescoping ballscrew struts. The struts are like the ways in a conventional machine, but they are round with the ballscrew assembly in the center. The motions of the spindle are controlled by a sophisticated computer performing millions of calculations to assure proper part contour. Costing several million dollars to develop and using high level, proprietary mathematics, this machine promises to perform previously unheard of operations in metal machining. Advancement in computers and artificial intelligence will make CNC machines of the future faster and easier to operate. This will not come cheaply, and the price of sophisticated CNC machines will be beyond the reach of many companies. It will, how-ever, reduce the prices of the basic CNC machines performing the original three-axis movements.