Most machines have a cycle completion light that alerts operators when a machine is ready for the next workpiece to be loaded. As long as this light cannot be turned off, a manager can quickly scan the shop to see how many machines are sitting idle.
The first switch a manager should know is the rapid override switch. Rapid override is often a multi-position (10, 25, 50 and 100 percent) switch that is used during setup to slow rapid motion rate during each tool’s initial approach to the workpiece.
Once a program is verified and the machine begins a production run, rapid override should be set to 100 percent. This ensures quick movement to and from the machine’s approach positions. A manager should know the location of the switch to confirm that it is appropriately positioned.
The following scenario describes the efficiency impact of an improperly set switch on a 10-tool program running on a machining center with a 1,000 ipm rapid rate. Each of the 10 tools travel a total of 14 inches while they retract from and approach the workpiece. So, approach and retract distance for tool changes totals 140 inches.
If the program runs with the rapid override switch set to 100 percent, rapid motions required for tool changing will take 0.14 minute, or about 8.5 seconds. (To calculate time in minutes, divide travel distance by motion rate in inches per minute.) However, if the operator has the rapid override switch set to 50 percent, tool-change time will double to 0.28 minute (about 17 seconds). In a 1,000-part production run, this will add more than 2 hours and 20 minutes to the time. Worse, if the switch is set to 25 percent, production run time will increase by seven hours for the 1,000-part production run.
The second two switches that a manager must know are the feed-rate override and spindle override (though not all machines have a spindle override switch). The feed-rate override is a multi-position switch that commonly ranges from 0 to 200 percent. It enables the setup person to slow (or actually stop) cutting motions on one end of the spectrum and double the programmed feed rate on the other.
These two switches are also helpful for program verification during the first few workpiece runs. They help the setup person and programmer confirm that the programmed speed and feed rate for each tool is appropriate.
Similar to the rapid override switch, these switches can also have a dramatic impact on program execution time if they are not properly set. The program will execute much slower if they are set at 50 percent. A 10-inch motion programmed at 10 ipm will actually take two minutes to complete, even though it should take only one. On the other hand, if the feed-rate override switch is set to 200 percent, this motion will take only 30 seconds, but tool life will suffer.
In my experience, operators have the tendency to turn down the feed-rate override switch when they’re trying to make tools last longer between dull tool replacements. They turn up the feed-rate override switch if they’re trying to achieve some kind of rate (as when doing piece work).
I’ve even seen some operators hide what they’re doing. For rotary switches (commonly used for feed-rate override), they’ll turn up the feed-rate override switch to 200 percent, remove the switch top and replace it at the 100-percent setting. This makes it look like the machine is running at 100 percent when it is actually running at 200 percent. Note that the opposite can also be done. An operator can make the machine appear to be running at 100 percent when it is actually running at a lower rate. Again, a manager must be familiar with these functions to confirm that switches are appropriately set.
One last function I’ll mention has to do with machining centers that have two-pallet automatic pallet changers. These machines have a safety-button, commonly called the standby button. When operators are finished loading the pallet in the loading area, they push this button. A light close to the button will come on as they do so. This tells the machine that it’s okay to actually make the pallet change. If the standby button has not been pressed, the machine will stop and wait.
If the operator forgets to press this button, the machine will not immediately change pallets. You can’t predict how long it will take for someone to notice. As you walk the shop floor, take notice of this function. Question the operator when you notice a machine with the standby button light off when the operator isn’t loading workpieces on the pallet.blog comments powered by Disqus