I use the Surebonder swivel head hand rivet tool supplied by Cleaveland Aircraft Tool for pulling blind rivets, and I've had problems start at about the same point in the service life of the tool. A few hundred rivets into the life of the tool, and it will begin to break the mandrel at the pulling jaws instead of near the mandrel head. Eventually one will break long... then after a few more successful pulls, another will break long... and the failure frequency gradually increases from there. Cleaveland Aircraft Tool really stands behind their sales, and has insisted on sending me a new one when the old one starts failing. I'm on my third tool now, and I've discussed it at length with Annette and Mark. My pulling technique is proper; both tools seemed to be adjusted correctly and disassembly and examination showed no anomalies. I've sent both my old ones back for their examination. What surprises me is that according to Cleaveland, the issue hasn't occurred with the Surebonder tool for any other builders as far as they know; the tool should always work, and shouldn't wear out. We discussed the possibility of a bad batch of mandrels, but that seems unlikely due to the randomness of occurrence, using both CS4-4 and LP4-3 blind rivets. I can also tell, just by the feel of the tool in action, when it is more likely to occur. I'd like to hear from other builders who have had similar experiences, and what may have been learned.