Hi all,
Best to think in terms of how the domino cutter works. It is somewhat akin to a birdsmouth bit in that it cuts while moving side to side but cannot simply plunge without moving side to side as there is no way to clear the chips. It is not a drill bit. The Domino machine itself provides the side to side action and the user is providing the plunge action.
If you make a full depth cut and then try to move the machine sideways, only the tip of the bit at the bottom of the slot can cut and side to side movement of the machine will only serve to lever the shank of the bit against the uncut side wall in the work piece. If you try to plunge faster than the bit tip can cut and faster than the DC can remove the chips you will be levering the shank of the bit and overheating it.
If you cut one mortise and then want to make it wider you are best off to cut from the same center line at a wider cut setting and plunge slowly since the bit is spinning in free air (while in the original mortise) until it slams into the uncut side wall at which point all the leverage is on the cutting tip. It cuts to the width of the mortise that direction, then reverses through free air until it slams into the uncut side wall at the other side of the mortise. There is a lot of shock to the bit each time it hits the uncut wall after spinning in free air so minimize the trauma to the bit by plunging more slowly while making an existing mortise wider than you did while cutting the original mortise in the first place. it is possible to widen a mortise all from one side as Dave did, but you do need to go slowly and let the bit do its job.
Like other posters here, I can say that I have cut a lot of Domino mortises in all sizes in all kinds of very hard woods and have never broken a bit. I thought I would wear them out, especially the 5mm one, cutting exotic woods with lots of sand and mineral take up, but am still going on the very first bit set that I received along with one of the very early Domino units to come into the US market. The bits in that set may be getting dull, but I can't feel it and they seem to perform every bit as well today as they did day one.
One other consideration. If you want to change an mortise from, say 5mm to like 6 or 8mm proceed slowly. Let the machine do all the cutting. In this case the cutting tip is only partially supported by the uncut wood. Each cutting edge is hitting uncut wood, but during the time the bit is over the original mortise the center of the bit is unsupported. With all the cutting action taking place just at the bit tip, that is a lot of leverage on the bit shank and it is being subjected to a constant diet of in-thrust and out-thrust forces as the bit tip cuts first on one side where it is climb cutting and then on the other where it is down grain cutting. Again, plunge more slowly making a cut like this than you do making an original mortise in uncut wood.
Nearly all bit breakage will be a function of pilot error, but not always for the same reason so one person's experience will not necessarily translate directly to another persons problem. Hope this helps.
Jerry
