Richard, when the collet nut is tightened it forces the collet into the shaft which causes it to tighten on the bit. This also causes the bit to move backward into the shaft. If the bit is bottomed out and can't move back the collet nut gets tight but the bit is not gripped properly by the collet. A bit bottomed out can also transfer the heat build up directly to the motor shaft. The 1617 manual requires that the bit be inserted into the collet 3/4"; deeper is better since it helps reduce vibration and deflection. You can insert a 1/8" diameter cord O ring into the shaft, it should be snug enough that it wont fall out. This will allow you to fully insert a bit to the O ring. The O ring will compress when the collet nut is tightened allowing the collet to properly secure the bit.
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Last edited by Mike; 10-10-2011 at 01:01 AM.