Published 10/19/2011

Five CNC Myths and Misconceptions

Managers that wind up working in companies that use CNCs often start their careers in other areas. While this isn’t necessarily a bad thing, managers with limited shop experience can’t always understand what’s actually going on in the shop. Here are five frustration-causing miscues I’ve noticed between management and shop employees.

Mike Lynch

Founder and President, CNC Concepts Inc.

• Myth 1: Anybody can run a CNC machine.

While there are applications that have been engineered so that a person with minimal training can operate a CNC machine, these applications are few and far between. The reality is that a CNC operator must perform many tasks that require knowledge and skill.

Common operator tasks include workpiece loading and unloading, cycle activation, cycle monitoring, workpiece measurement, acceptability evaluation, statistical process control reporting, sizing (offset) adjustments, dull-tool recognition and dull-tool replacement. The more we expect operators to do, the more they must know—and the more training we must provide.

• Myth 2: If a job has been run before, it will run without problems in the future.

When a setup person is having problems with a job, it’s not unusual to hear a manager say something like “I thought we’d run that job before.” The assumption is that just because the job has run in the past, it will run flawlessly in the future. However, any difference between the way the job was run in the past and the way it currently is running could potentially cause a problem. Variations must be eliminated in order for jobs to run consistently without problems.

• Myth 3: Once a new job is running, you should be able to predict when a production run will be completed.

With hot jobs, it’s tempting for managers to start predicting when the machine will finish a production run. After all, managers are in contact with demanding customers. Maybe the machine has a five-minute cycle time, and it takes 30 seconds to unload and load. With 200 parts to run, the job will be finished in about 18 hours, right? (That’s 200 parts times 5.5 minutes divided by 60 minutes in an hour.) Yes, the job should be completed in about 18 hours theoretically, but other factors determine how long it will actually take to complete a production run. How long will cutting tools last? How long will it take to replace dull tools? Can the operator keep up with the machine? With repeating jobs, managers should be able to make this prediction. However, accurately predicting when a new job will be completed may be impossible.

• Myth 4: G-code programming is a thing of the past.

Many managers believe that all CNC programming is done using the company’s CAM system. Indeed, almost all companies using CNCs have some kind of CAM system. Managers must remember that the output from any CAM system is G code—the same kind of G code that manual programmers write out long-hand. Of course, it is the G-code program that the CNC machine actually runs. If a setup person has a problem that requires a program to be changed, it is often quicker and easier to change the G-code at the machine than to have the programmer make the changes in the CAM system, recreate the G-code program and send it back to the machine. Changing programs at the machine requires an understanding of G-code programming.

There are other reasons why CNC people should understand G-code programming. If a programmer is to judge the quality and efficiency of G-code programs generated by a CAM system or modify the way G code is created, he or she must understand G code. If some jobs are very simple, it is often quicker to manually correct the program than to use a CAM system. Also, if your company wants to take advantage of any custom macro B functions, the programmer must understand G code.

• Myth 5: CAM system programmers don’t need a machine shop background.

New programmers seem to have one of two backgrounds: shop experience or computer experience. Admittedly, CAM systems do simplify the task of developing programs—a person with limited shop experience can easily get through the creation of a CNC program. However, programmers with limited shop experience tend to take what they’re given while those with extensive shop experience will make the CAM system give them what they want.

To create the most efficient CNC programs, a programmer must know about processing, machining operations, cutting tools, cutting conditions and how setups are made. They must also be able to direct the setup people and operators who use the programs they create. Again, a CNC programmer must possess a good understanding of basic machining practices to the win respect and confidence of experienced shop personnel.