Monday, December 9, 2013
Most industrial metal fabrication is now done by computer numerical control--CNC machines. CNC programming allows the machines to quickly and precisely manufacture parts to tolerances of thousandths of an inch, at a much lower cost than older manual methods. Where earlier machines required a master machinist to painstakingly measure parts to match numbers read from a mechanical drawing, a CNC program uses the drawing itself to precisely position the working end of the tool.

The earliest numerical control machines date to the early 1800s; French gun manufacturers turned gun stocks on lathes positioned by cam cylinders. A cam cylinder is a round bar with studs or notches at intervals which move a pin or key as the cylinder is turned. Older music boxes and player pianos used cam cylinders. Computer numerical control was developed along with the earliest computers, beginning in the late 1940s at the Massachusetts Institute of Technology (MIT). As engineers at MIT were developing computer-aided design (CAD) programs, they immediately saw the advantages of computer-aided manufacturing (CAM). Their work led to today's CNC machines, which became widely available and commonly used by the 1970s.

Almost any type of tool--lathe, router, cutter, drill, sander--can be controlled by a CNC program. On the most advanced CNC machines, the mechanical part drawing from a CAD program such as AutoCAD™ is uploaded to the CNC machine, the raw material for the part is loaded on the machine platform, and the machine does the rest. A machine operator does have to monitor the process, but much of the earlier tedious, painstaking measurement is no longer necessary. Most CNC machines, however, require programming in a specialized language called G-code. A G-code program tells the machine how far and how fast to move the tool on up to 5 axes of motion. Once a CNC machine is programmed, the operator needs less skill than a machinist operating manual machines; however programming a CNC machine requires a high degree of training. Some companies that use CNC machines have two categories of workers: CNC programmers or technicians, who create and load the programs from mechanical part drawings; and CNC operators, who simply load the raw material into the machine, turn it on and monitor its progress after it is programmed.

Companies or individuals who are considering purchasing CNC machinery need to do some research before making a decision, just as for any capital expenditure. Things to consider:
  • The size of each part run and the degree of precision required. CNC machines are most cost-effective for large part runs that require high precision.
  • The skills of the workers and any additional training they may need to learn how to program and operate CNC machines.
  • The organization of the work flow--who will program the machines and who will operate and monitor them.
Once a purchase decision has been made and the new machine is in place, there are some pitfalls to be avoided. The first pitfall is a false sense of security. CNC machines are more accurate than manual ones, but they're not foolproof. Quality-control procedures still need to be strictly followed. An error in the CNC programming will cause, at minimum, faulty parts that need to be scrapped. At worst-case, a programming error can cause the machine to "crash" and damage itself. For example, if the program tells the tool to move too far toward a guard, the tool could hit the guard and damage itself, the guard, or both.

A related misconception is that CNC operators don't need as much training as manual machinists. While the machine does most of the processes that a manual machinist would do, the CNC operator needs to have an understanding of those processes in order to monitor the machine's function and the quality of the parts it produces.

A CNC machine that is chosen after adequate research can greatly enhance the profit, productivity and competitiveness of any business. Managers need to have a thorough understanding of their existing processes and their company's goals in order to get the best results from a CNC machine.

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