Figure 2-22 shows a clamp cylinder, CYLA, and three self-contained automatic air powered drills controlled with air logic.

Some circuits clamp a part then start the drills with a "one shot" element. As long as all the drills start there is no problem. However, if any drill fails to cycle, parts may come off the fixture with one or more holes missing. When double drilling is necessary, part costs’ and scrap increase. The circuit in Figure 2-22 eliminates this problem with air logic elements and piping.

When the anti-tie down circuit shifts "flip flop" FF, a signal from its top port goes to extend clamp cylinder CYLA. FF top port output also supplies the normally closed port of limit valve LVA1. CYLA extends, clamps the part, and shifts limit valve LVA1. Use a limit valve here since the drills could sling a loosely clamped part out of the fixture. A pressure operated "not" circuit could allow premature cycling of the drills, resulting in damage and safety concerns.




















After clamping the part, FF output goes through limit valve LVA1 to the drills’ start ports. When the drills start they give a run signal when they move from home position. This run signal remains on until the drills fully retract. On most of these types of air-operated drills, the output is the same air that turns the air motor in the drill. One brand of air drill has an output when at rest and exhausts that signal when it starts.

The drill run signals go to the input ports of two "ands" and two "ors". When the two cascaded "ands" have three signals indicating all the drills are moving, their output shifts FF back to its starting position and exhausts the drill start signal. The three inputs to the two cascaded "ors" pass through to close "not3" so the clamp will not open until all drills have retracted. Output from the lower port of FF goes to the inlet of "not3" to set up clamp CYLA open sequence.

The drills will continue forward until they meet their internal limit valves and retract. The run signals drop out as each drill finishes and retracts to home position. When the last drill is home, the run signal from the last "or" element exhausts and "not3" opens. When "not3" opens, its output shifts the clamp valve to retract CYLA.

If starting of one of the drills is sluggish, the run start signal stays on until it moves. If a drill fails to start, the run signal stays on and the running drill stay extended. In either case, the operator knows when a problem exists. If one of the drills hangs in the part, the clamp will not open until the drill is free to retract. For every added drill, use another "and" and "or" element. With air indicators installed in each drill run signal line, picking out a nonrunning drill is easy.