The operation panels of a typical CNC machine have lots of buttons and switches. Setup people and operators must, of course, know the function of each button and switch and be able to set it properly. While a manager need not be as intimately familiar with buttons and switches as a setup person or operator, there are some machine functions a manager should know enough about to be able to judge whether they are appropriately set.
The first switch I mention is the Rapid Override switch. Rapid Override is often a multi-position switch having settings of 10%, 25%, 50%, and 100%. This switch is used during setup to allow the setup person to slow rapid motion rate during each tool’s initial approach to the workpiece. This takes some of the scare-factor out of running the first part or two.
Once a program is verified and the machine begins a production run, however, Rapid Override should be set to – and remain at – 100%. This ensures that the machine moves as quickly as possible to and from its approach positions.
If you’re wondering about the efficiency-impact of having this switch set improperly, consider this simple scenario – a ten tool program running on a machining center having a 1,000 inches per minute rapid rate. We’ll say that each tool travels 14 inches (total) during each tool change while it retracts from and approaches to the workpiece.
This totals 140 inches of rapid motion distance during the program’s execution just for tool changing purposes. Note that we’re not including any rapid motion that each tool includes during cutting motions (as would be required when drilling several holes) – we’re just showing the impact during tool changes.
If the program is run with the Rapid Override switch set to 100% – as it should be – rapid motions required for tool changing will take 0.14 minutes (about 8.5 seconds). But if – for whatever reason – the operator has the Rapid Override switch set to 50%, tool changing time will increase to 0.28 minutes (about 17 seconds). This, of course, doubles the time required for rapid motions needed for tool changing. And in a 1,000 part production run, will add over 2 hours and 20 minutes (actually 141.6 minutes) to the time required to complete the production run. Worse, it this switch is set to 25%, production run time will be increased for the 1,000 part production run by seven hours!
The second two switches that a manager must understand and be able to question are the Feedrate Override and Spindle Override switches (though not all machines have a Spindle Override switch). The Feedrate Override switch is a multi-position switch that commonly ranges from 0% to 200%. So it allows the setup person to slow (actually stop) cutting motions on one end of the spectrum, and double the programmed feedrate on the other.
These two switches are also helpful during the program’s verification – and during the running of the first few workpieces. They help the setup person or programmer confirm that the programmed speed and feedrate for each tool is appropriate. And I recommend that the person verifying the program sticks to it until they determine speeds and feeds that allow machine to run at 100% speed and feedrate at all times.
Like the Rapid Override switch, these switches can have an impact on program execution time if they are not properly set. If set to 50%, of course, the program will execute much slower. A 10 inch motion programmed at 10 inches per minute (that should take one minute to complete) will actually take two minutes. On the other hand, if the Feedrate Override switch is set to 200%, this motion will take only thirty seconds, but tool life will probably suffer dramatically.
In my experience, operators will have the tendency to turn down the Feedrate Override switch when they’re trying to make tools last longer between dull tool replacements. They’ll turn up the Feedrate Override switch if they’re trying to achieve some kind of rate (as when doing piece-work).
I’ve even seen such operators turn up the Feedrate Override switch to 200%, remove the switch top, and replace it at the 100% setting. This makes it look like the machine is running at 100% when it is actually running at 200%! Note that the opposite can also be done as well – an operator can make the machine appear to be running at 100% when it is actually running at a lower rate.