We’ve had several instances of bits slipping out of the spindle lately, and messing up people’s stock, digging into the spoilboards, etc. The cause of most of these is almost certainly incorrectly inserting the bit into the collet before tightening.
Most bits do not have a seating depth line on them, and you will need to visually identify the depth BEFORE placing the collet/nut/bit into the spindle.
The first diagram in the above picture is what you are aiming for - the top of the bit should be about .25 inch below the top of the collet. If you insert too deep, or too shallow, the collet cannot effectively grip the bit, and a slip is likely to occur. Note that the last bit above uses a special plug, which we do not have. ignore 4th example. #1 is the correct seating depth for all our bits.
DO:
Verify your depth visually before loading collet into the spindle.
Mark the correct depth, either with your fingers, or a black marker
Double check your seating depth before running spindle
Even if there is a line, double check it visually
Thoroughly clean the collet and nut with compressed air BEFORE and AFTER using them.
DON’T
Use the top of the cutting flutes as a reference
Jam it in there and hope for the best
Poorly seated bits can also cause broken tools, or increased runout (wallowing of cut).
It is extremely important to note that the flutes on the bit should not engage the collet. Plugs are used (as shown in #4 above) when the grip surface of the bit is shorter than the length of the collet.
From Onsrud:
The loss of gripping pressure is caused not only by repeated use, but also by heat transfer from the cutting tool and spindle. Cyclical heating and cooling cycles remove the original tempering of the steel resulting in a corresponding loss of elasticity and uniform grip. The smaller the collet, the faster this change happens – especially in air router collets. Over time, a worn collet will require increased tightening pressures to maintain a tool in proper position As the frequency of over-tightening increases, the collets become distorted and create eccentricities in the tool holder.
Edit: This could potentially be a product of people not properly seating the collet inside the nut before inserting the assembly in to the spindle. Again, Onsrud:
Proper positioning of the tool in the collet is critical. The tool should only be gripped on the shank portion of the tool. At no time should any portion of the flute fadeout be inside the collet. The collet should be snapped into the nut before tightening or a concentric grip will not be achieved. By not attaching the collet to the nut before tightening, the tapered mating surfaces between the collet and collet holder can be damaged when tightening occurs and the collet is jammed into the seating surface.
Another cause of this problem is dirty collets. Sometimes saw dust and dirt get in the groves of the collet, this can compromise the hold that the collets has on the bits. I try and blow out all collets before I install the router bit.
