Breaking router bits
My experience is with odd shaped hardwood components on vac pods. What I've learned is I snap bits when I set up in such a way as to allow the wood to vibrate a bit under cutting pressure. This will almost always result in a bit snap.
One possibility is that your collet and/or your toolholder is worn or dirty. In order to achieve the best performance from any bit, it is critical that the bit be turned perfectly on-center. We'll never achieve perfection, so we have to do the best we can.
Router spindles, toolholders and collets are made with matching tapers that, when clean and new, will keep the tool centered in the spindle. Any tiny amount of debris (like sawdust) that interferes with the mating of these tapers will cause the tool to "run out", or turn off-center. In turn, this will lead to vibration, poor finish, poor tool life, possibly tool breakage and spindle damage.
The amount of wear that causes a toolholder or collet to be worn is invisible to the naked eye. That's why I recommend replacing a collet that has been in daily every 6 months, or a toolholder after a year. If a tool ever slips in a collet, replace it immediately. The drawings below show a tool with run-out equal to the radius of the bit. This is obviously enhanced to demonstrate a point. It only takes .002" run out at the tool tip to ruin a tool's performance.
When you change a tool, completely disassemble the collet and nut, and use a solvent to clean all the parts, including the tool shank before re-assembly.
It seems to me on the .625 tool you are doing a groove/dado cut. A down shear is what I would recommend if you require a smooth top. If not, use the up/shear to assist in drawing the chips away. The above post is correct in the toolholder/collet problem. Your part layout and size will determine if you use an up or down shear or a combination shear.
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