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Originally Posted by [527]phil
You don't always have to put the 0 on the m5 or m3 command, it will confuse some machines (trust me, confused machines aren't fun to be around). and i don't know why you'd want to reverse the spindle on a mill because the bit won't cut backwards. You only might use that once in a blue moon on cnc lathes, but never on mill unless your making reverse threads. And the spindle speed can be manually controlled on most machines using a knob in the back, you have to be carefull of the spindle speed, to fast and you could snap the bit, too slow and you'll either bind up the bit or make an extremely sloppy cut, and noone likes sloppy cuts  . |
I'm not too sure either, the only thing I can think of off hand is if your using a wire brush and you want a CCW brush pattern, but it doesnt hurt to know.
As for threading, the machines Ive worked with have a seperate G codes for rigid tapping (tap is held solid in the tool holder), and for the kind of tapping that allows the tap holder to move up and down independantly of the spindle, to a point. Its kinda neat watching the machines run the taps down into the hole so fast, and then reverse and back right out...
Jim, yes knowing how to run the CAM software is definitely important. When I worked over the summer, I saw the programmers add a toolpath to debur a part in a matter of minutes, and doing so manually would have taken hours. Once you get beyond basic parts, CAM software is pretty much absolutely needed.