Verify CNC Program Correctness

All CNC programs must be verified. While new programs present more challenges than proven programs, operators must be careful and alert during every step of a program's verification.

Step 1: Verify the correctness of the CNC program. This step is required for new programs or for programs that have been modified since the last time they were run (possibly because of engineering changes). It is also necessary to do this step if there is any doubt as to whether you are working with the current version of the program (after making changes at the machine the last time the job was run, perhaps the setup person forgot to save the program).

The objective of Step 1 is solely to confirm the correctness of motions commanded in the program. Other potential problems will require further verification at the machine; however, when Step 1 is successfully completed, the setup person will have confidence in the motions made by the program.

Some operators perform this step on the CNC machine during setup, which requires time. Many current model CNC machine tools have built-in toolpath displays, and as long as you verify the new program while the machine is running, you won't interfere with production. Not all CNC machines allow you to view one program's toolpath while another program is running. In this case, Step 1 will add to the setup time. If mistakes are found, the time it takes to correct them will also add to setup time.

Not all CNC machines provide toolpath display, and it is difficult to see a program's true motions by watching a CNC machine run a program. You may not be able to achieve the objective of Step 1 in this casebecause there might be serious mistakes to be found and corrected in Steps 2 and 3.

With the affordable off-line G-code level toolpath verification systems available, Step 1 can be performed for upcoming jobs, while the machine is running production shortly after a CNC program is created or modified. With these desktop computer-based systems, users can gain a better view of the program's movements than they could by watching the machine move.

If using an off-line system, the programmer is usually responsible for this step. They will perform this step shortly after the program is created. While most CAM systems have toolpath verification that is done as the CNC program is created, if changes are made to the G-code level program, many CAM systems cannot display the changes.

Even if changes are not made to the G- code level program, I recommend using a G-code level off-line program verification system to check the program's motions. If nothing else, this gives the programmer another way to see the motions a program is going to make before it is run on the CNC machine.

It takes a watchful eye to catch mistakes with an off-line system. Because the job is not currently on the machine, there is no real urgency, so mistakes can slip by. It might help to have someone else perform this step (another programmer or a setup person). Because the original programmer is so familiar with the job, he or she might not catch obvious mistakes. A setup person can be the best bet, since he or she will be responsible for actually running the program at the machine.

Many off-line systems don't show the location of clamps and other obstructions, so the person verifying the program must be able to visualize the placement of workholding components around the workpiece. The more problems they catch, the fewer problems there will be for the setup person to find and correct.

There may still be problems with the program's motions after Step 1 is completed, but these problems should not be severe. Even with a toolpath display, it can be difficult to catch small motion mistakes. Some solid model-type program verification systems allow performing measurements on the virtual workpiece machined in the system; however, you must suspect that a problem exists before taking a measurement. For instance, with a mistake of less than 0.01 inch, it is likely that you may not suspect that anything is wrong.

After completing Step 1, the setup person can proceed to Step 2, which will be explained in next month's column. They must still be extremely careful, but there will be no big problems in the program's motions.