CNC Program Correctness - STEP 1
Author: Mike Lynch
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.
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).
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.
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.
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 case
because there might be serious mistakes to be found and corrected
in Steps 2 and 3.
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.
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.
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.
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.
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.
may still be problems with the program's motions alter 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.
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.
See Mike Lynch on MODERN MACHINE SHOP Online at: www.mmsonline.com/experts/lynch.html