Deliverable: Machine is set up so you can run the Part Program and the cutter will only cut air.

Cutting Air? What is this "Cutting Air" of which you speak?

Set your Part Zero so the Cutters only Cut Air. Rather than setting Part Zero to be the real Part Zero, you need to set it to a point that is higher in Z, so the tip of the lowest cutter is well above your workpiece and workholding. This is done so you can watch the program run with no chance it'll crash into anything.

You could just estimate this from the length of your longest tool--just eyeball it. Or, you if you use Simulation Software, it will often list the lowest Z in the program statistics. If the program is set up so Part Zero is the top of the material, the lowest Z will be a negative number. Just set your Part Zero Z to be a positive value equal to that magnitude of that negative.

Deliverable: The machine will move slowly enough you can see what it will do in time to react if it is about to crash.

Most CNC Controls have a feature called "Feedrate Override" that allows you to set Feedrate to some percentage of the actual commanded feedrate--either faster or slower. Turn it way down, almost to zero. This will give you plenty of time to react if the program looks like it is going to crash.

Most cuts follow a pattern:

- Retract to a safe height

- Move at rapids speed to a position above where the cut begins

- Drop down to cut height

- Make the cut

- Rinse and repeat for the next cut

You'll get used to this cadence. On a simulator, you'll look for cases where the pattern was not followed properly. When cutting air, you keep the FRO (Feedrate Override) turned down until the cutter reaches safe height and starts to move in its cut pattern, then you turn it back up until that particular cut finishes and turn it down for the retract until you see it move safely and slowly to the next cutting position.

You'll see most operators keep one hand on the FRO as they watch the program Cut Air for the first time.

Depending on the machine, there may also be a separate rapids override. Make sure you're not running full rapids speeds during the Proofing Run.

Tip: Use the Machine's Dry Run Options, if available.

Many machines have a dry run lock. This will run the program through tool changes and other activity, but the axes won't move. This run allows the control to check for syntax errors and other alarms. The operator can also follow along visually to make sure the right tools are being indexed relative to the current position in the g-code program as expected.

Deliverable: Be prepared to push the E-Stop before the program can get into trouble.

Before any run, I always look carefully for where the E-Stop button is located. I reach out and touch it, mimicking the motion I will make if there is a problem. If my hand will be on the FRO, I start there and act out the motion of moving my hand from FRO to E-Stop. I close my eyes and visualize that motion too, several times.

Sound crazy?

I am just trying to make sure that if I have to press E-Stop, there'll be no fumbling for it. I want my muscle memory to know exactly what to do without having to stop and think about it.

Note: On some machines Feed Hold is a better choice than E-Stop. This is because it can be easier to get the program restarted after a Feed Hold than an E-Stop. Make sure you know which one is best and that it will always stop what's happening as soon as possible.

Tip: Use Optional Stops in the G-Code to make Proofing easier.

Experienced CNC'ers often put Optional Stops in the G-Code aimed at making the Proofing Run go smoothly. They stop the machine automatically so the operator can check what's happening at that point in the program and then restart.

Deliverable: Having watched the Program Cut Air, you'll know whether it's ready to make chips.

Before you even start to run the program, look at the backplot display on your controller. Are there any unexpected moves? Does it look like the correct program you expected?

If it all looks good, you're ready to cut air. At any point during a run, the operator can press the Feed Hold in order to stop progress and check on something. For example, if you know every tool is exactly 2" above the work when cutting air, you could use Feed Hold and a 1-2-3 block to check tool height after each tool change.

Deliverable: A proofed program read to cut chips.

You'll want to update your G-Code Program or other areas (for example, you notice the wrong tool is loaded and need to change it) based on what you see during Proofing to fix any problems.

And remember, proofing is also a test of your documentation. Update your Setup Sheet to make it more accurate and complete enough that setup is as easy as possible.

Deliverable: Proofing programs can be slow and expensive. You'll learn how to proof them faster and more cheaply.

The best way to save on proofing is with Simulation...

It should be ovbious that Proofing programs can be slow and expensive. The more expensive the CNC Machine, and the more work that is backed up waiting for that machine, the more expensive it becomes. If you're going to make a lot of parts, you can amortize the cost of proofing over all those parts. But if you're only going to make the part once, proofing can more than double the machine time needed to make the part.

How can we do better? How do we proof programs faster and more cheaply?

The alternative to proofing programs by hand in the manner described above is to have a system that ensures that by the time a program lands on the machine, it can be run without the need for proofing.

Most CNC operations can't eliminate proofing entirely, but those that do, report they nearly doubled their profitability. What are the major areas we need to address if we are to save on proofing:

- We need to make sure the G-Code Programs don't have errors. Even the best CAM software sometimes generates g-code with errors. This can happen for a variety of reasons, such as problems with the postprocessor. But, those errors are usually very obvious in a Simulator.

- We need to make sure the right tools are loaded and the right tool data is there to go with the tools. This is also something that could be carefully checked and even automated via tool touch setters (one example) to avoid the need for proofing.

- We need to make sure the right fixtures are loaded, the workpieces are properly in the fixtures, and the right Part Zero or Work Offsets are set up. Again, this can either be carefully checked by the operator or automated to an extent with, for example, in-process probing.

Resources:

- See our Series on Tool Data Management for information on how to automate Tool Data needs.