Verifying CNC Program Correctness: Step 3

In my last two columns, we covered steps one and two of verifying CNC programs. Now, let's move on to step three: verifying the correctness of the first workpiece.

The objective of this step is to carefully run the first workpiece in such a way that it will pass inspection. For our purposes, let's assume this is very important to the company that justified the first workpiece.

As long as the program is correct—it is a proven program, or all programming mistakes have been found in steps one and two—there is no excuse to scrap the first workpiece during step three. No method is completely failsafe, but if you follow the suggestions we make, then each tool will be forced to machine properly. Thus, when the first workpiece is finished, it will pass inspection.

Each tool approaches the workpiece at the machine's rapid rate. Many programmers use an approach distance of 0.1 inch or less (about 2.5 mm). It may be impossible to confirm that cutting tools approach properly during step two, and a small mistake with an offset setting can be disastrous.

Performing step three for a new program presents another concern. Cutting tools will often make rapid movements during machining. While you can confirm that none of these will collide with fixture and clamping components during step two, it may be impossible to tell whether all cutting tool rapid movements will avoid workpiece obstructions during a dry-run.

For reasons such as these, the operator must be careful when running each tool. Unfortunately, machine tool builders vary when it comes to how easy they make it to confirm approach motions and rapid motions internal to the tool.

The best method we've seen involves the single block switch. When the single block is turned on for this type of machine, the feed rate override switch controls the motion rate of rapid movements. When each tool is approaching (because the single block is on), the setup person can easily control the motion rate for the approach. If anything looks wrong, he/she can stop the motion by pressing the feed hold button.

During each tool's machining motions (for a new program), the setup person will leave the single block turned on and step through the cutting tool's motion. Whenever a rapid positioning movement is commanded, he/she will have control of the motion rate with the feed rate override switch.

If your single block switch does not behave in this fashion, you'll have to use another method to take control of the motion rate during the approach movement. Some machines allow adequate control of rapid motion with the rapid override switch. By providing a switch that is similar to the feed rate override switch, they allow precise control of rapid motion down to a crawl. This function is almost as good as the single block switch method just described.

If you don't have these functions, use the dry-run switch to control approach movements. While cumbersome (you'll be turning it on and off during each tool), it enables the user to take full control of the motion rate (usually with the feed rate override switch) during each tool's approach and during rapid motions within the tool.

As each cutting tool comes into position and the machine stops (single block or optional stop is on), one must consider what the tool will be doing. If you are concerned that your initial offset settings are not accurate enough to stay within tolerance limits, you must use trial machining techniques. This involves making an adjustment that forces the cutting tool to leave a little extra material on the critical surfaces. Once the cutting tool has machined the surfaces, the machine will be stopped and the surfaces will be measured. Another adjustment will be made based upon the measurement, and the tool will be re-run. This time the surfaces will be right at target values.