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When not to use a dwell command!

As you know, G04 is used to specify a dwell – or pause – in your CNC program’s execution. With Fanuc controls, the format for the dwell command can vary based upon what character you choose to specify the period for the dwell. The commands

  • G04 X0.5

  • G04 U0.5

  • G04 P500

all specify a half second dwell (assuming dwell is specified in time and not the number of spindle revolutions).

While the dwell command is necessary when you wish to relieve tool pressure – as would be the case just after plunging an end mill into a surface and before starting to mill a pocket in XY – there are certain times when you should not use a dwell command.

The first time has to do with programming around machine problems. I’ve seen, for example, programmers that include a dwell command in their programs to allow time for the coolant system to kick in. Maybe there’s a bad check-valve in the coolant system – and the programmer wants to allow time for the coolant to be flowing at its maximum before allowing the machining operation to start. While this may be a reasonable temporary fix, the better long term solution is to fix the machine. In my opinion, this is an inappropriate use of the dwell command.

While it may be inappropriate, at least it isn’t dangerous. And there are times when I’ve seen some pretty dangerous applications for the dwell command. In one instance, for example, the programmer was trying to provide time for the operator to polish the workpiece during the CNC cycle. To this end, they included a twenty-second dwell in the program right after the finish turning operation. During this dwell, the operator was supposed to open the door and perform the manual polishing operation.

The obvious (and dangerous) problem with this technique, of course, is that if the operator didn’t finish the polishing during the twenty seconds, the machine would continue anyway, indexing the turret and bringing the next tool into position for the next machining operation. And of course this would be very dangerous for the operator.

Never use a dwell command to allow time for the operator to do something during the CNC cycle. Always consider the worst that could happen if they don’t finish what it is you intend them to do during the dwell. If it is at all dangerous, don’t use the dwell technique.

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