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Teaching CNC with the Key Concepts approach - part eleven

Part eleven - Key concept number Ten: You must know how to safely verify programs

Here are some links that allow you to review other parts of this article:


This is the last part of Teaching CNC with the Key Concepts Approach. Along with other information we provide on our website, we hope you've found this material to be helpful.


The final Key Concept draws together much of what has been presented in this class. Students must know how to verify new programs as well as programs that have been run before. They must also, of course, be able to machine acceptable workpieces.


Though companies vary in this regard, we’re assuming that it is quite important to make the first workpiece being machined a good one. They must, of course, be able to find and correct mistakes as they are found. And mistakes can be related to the program or to the setup that has been made. This means they must be able to recognize the cause of problems being encountered – and again – this requires a good understanding of what has been presented so far.


To this end, provide a series of procedures for verifying CNC programs (dry run, air cutting normal run, and cautiously running the first workpiece). These procedures are not overly specific – and are somewhat complex. And again, they require students to understand many of the points you’ve made so far.


Don't have them try to memorize each procedure - it doesn't work. Students quickly become confused and frustrated. Admittedly, there are some procedures that are so often used that students will soon have them memorized. But don't assume they can remember how to perform even the simplest procedures for very long. So prepare a written set of procedures for:

  • Single block dry run

  • Free flowing dry run

  • Actually running the first workpiece (using trial machining for critical tools)

  • Rerunning tools

Though you have introduced - and probably discussed in detail - the topic of trial machining, you might want to review it here. Point out that they must go through the program tool by tool and:

  • Ensure that Optional Stop is turned on (so the machine will stop at the end of each tool)

  • Consider what the tool will be doing

  • If it will be machining a tight tolerance, they must:

  • Adjust the tool's offset in such a way that excess material will be left on the surfaced machined by the tool

  • Let the tool cut under the influence of the trial machining offset

  • Measure what the tool has done and re-adjust the offset accordingly

  • Rerun the tool

Be sure students understand the importance of going through the program step by step. If they use this method, the first workpiece they machine will be a good one.


I give an example of not following this recommendation: Say there are two tools in a turning center program - a rough turning tool and a finish turning tool. The rough turning tool machines the workpiece 0.01 undersize. But since the setup person is not closely monitoring the operation and allowing the whole program to run, the undersize condition will not be noticed. The finish turning tool comes in an misses the workpiece all together. When the cycle ends, the workpiece will be 0.01 undersize. I'll ask students at this point "What do you think the setup person will do?" Of course, they'll increase the offset for the finishing tool and proceed to scrap their second workpiece.


Point out that offset setting can be very difficult after the fact. When a completed and scrap workpiece is in hand, it can be very difficult to determine which offset/s must be changed in order to make the next workpiece a good one. Did the facing tool take off too much stock or did the cutoff tool machine the part too short? Did a boring bar go too deep or did the facing tool not machine enough stock?

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