Restarting after breaking a tap

One of the most frustrating problems a CNC operator can have is breaking a tap in the middle of taping many holes. The broken tap, of course, must be removed from the workpiece and the hole repaired (re-tapped), but it is not often feasible or desirable to repair the hole during the CNC cycle. Most companies will leave the broken tap in the hole and will remove it and re-tap the hole by hand after the workpiece comes out of the machine.


But a major problem still exists. How do you tap the rest of the holes that were to be tapped after the tap broke? Consider, for example, having 200 holes to tap. On the 150th hole, the tap breaks. This means, of course, that there are still 50 holes to tap (after replacing the tap).


The operator must have a way to run the tap in these holes. One way is to rerun the entire tapping cycle, but this would mean re-tapping 150 holes. And if the broken tap remains in the hole, what then? Some operators will place a block delete code (slash code) in at the beginning of the command that taps the hole with the broken tap. If only a few holes must be re-tapped, this may not be too bad an idea. But if there are many holes – and especially when the possibility exists that a hole will be cross threaded if the tap reenters each hole (machines without rigid tapping are notorious for cross threading), a better way must be found.


Unfortunately, my suggestion requires a good understanding of manual programming and rerunning tools. In essence, we’re going to be modifying the program in such a way that the tapping cycle will be instated for the first hole, but the first hole will not be tapped. We’ll also ensure that the tap begins at a high enough Z location that the tap will remain over the remnant of the broken tap if it happens to go near it. Finally, we’ll need to confirm that the restart hole (where you want the tap to continue) has both an X and Y coordinate (many of the commands that specify hole locations have only one coordinate), and that the Z surfaces (Z and R) are appropriate.


Here is an example of how the program must be modified:

Original program:

  • (3/8-16 TAP)

  • N405 T08 M06

  • N410 G54 G90 S400 M03 T01

  • N415 G00 X2.5 Y1.0

  • N420 G43 H08 Z0.1

  • N425 M08

  • N430 G84 R0 Z-1.0 F25.0

  • N435 Y1.5

  • N440 Y2.0

  • N445 Y2.5

  • N450 Y3.0

  • N455 Y3.5 (Tap breaks here)

  • N460 Y4.0

  • N465 Y4.5

  • N470 Y5.0

  • .

  • .

  • .

  • N555 G80 M09

  • N560 G91 G28 Z0 M19

Admittedly, it’s a simple program – having just a few holes – but it should work nicely to stress the technique. Notice that it’s pretty efficient – the R plane is 0.1 and only the Y coordinates are specified.


Say the tap breaks when tapping the hole commanded by line N455. Here’s the modified program. Again, these modifications would be done “on the fly” after replacing the broken tap. And of course, the tap remains in the hole commanded by N455:

  • (3/8-16 TAP)

  • N405 T08 M06

  • N410 G54 G90 S400 M03 T01

  • N415 G00 X2.5 Y1.0

  • N420 G43 H08 Z2.0

  • N425 M08

  • N430 G84 R0 Z-1.0 F25.0 L0 G98

  • M99 P460

  • N435 Y1.5

  • N440 Y2.0

  • N445 Y2.5

  • N450 Y3.0

  • N455 Y3.5

  • N460 X2.5 Y4.0 R0.1 Z-1.0 L1 G99

  • N465 Y4.5

  • N470 Y5.0

  • .

  • .

  • .

  • N555 G80 M09

  • N560 G91 G28 Z0 M19

Once the tap is replaced and the program is modified (a good operator can modify the program in about a minute), the operator can restart the cycle. Notice that we’ve reset the initial plane in line N420 to Z2.0. This will ensure that if the tap moves over the hole with the broken tap, it will not collide with it. The tapping command in line N430 will instate the tapping cycle but the L0 will keep a hole from actually being tapped. Also, G98 ensures that the tap will retract to the initial plane (Z2.0) at this point.


The “M99 P460” tells the control to jump to line N460. Again, the operator must know which command to restart on.


In line N460, we’ve added the X (though it wasn’t necessary in this example), the R, Z, L1 (to machine a hole for each command from now on), and the G99 to reinstate the R plane as the retract position.


Once the workpiece is finished, of course, the program must be changed back to its initial state.

One last point. We’re assuming that all tapped holes are on one side of the workpiece. If you are tapping holes on several sides of a workpiece – using a rotary device of some kind – you must also confirm that the correct side of the workpiece is facing the spindle as part of the program’s modification.


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