Why Am I Breaking Bits?

There a likely hundreds of potential reasons for this problem.  Some are not diagnosable, but several are. When working with our customers, here are some of the more popular areas we have uncovered.

1.  Improperly mounted equipment
   Effective use of bits require little or no movement in the overall structure of the CNC platform.  If the equipment moves, it is hard to maintain straight edge cutting. Please make sure that your equipment is properly mounted in a manner that provides a stable platform for the equipment to complete its work.

2.  Incorrect feeds and speeds
   There is a tendency to run routers at feed rates that are too low and spindle speeds (RPM) that are too high.  PreciseBits bits were designed to run at high feed rates with spindle speeds of 20k RPMs or greater.  Low feed rates do not allow the efficient removal of swarfe (cutting debris). At best this will make the bit run hotter, at worst, the debris packs the flutes and causes the bit to break.  For every material, there is an optimum combination of cutting depth, spindle speed and feedrate that yields the best cut and the longest bit life. The process of finding that "magic" combination can be quite straightforward if approached in a logical, methodical manner.  For more information, see the tutorial on "Finding the Sweet Spot in Microtool Woodworking"
   
   Please consult our feeds and speeds charts for proper calibration (go to PreciseBit Feeds and Speeds Chart).  If your results differ from the charts, please let us know.  The more data we collect, the more effective our charts become.



3. The Wrong Type of Bit for the Material Being Cut
   For optimum performance, we have designed and tested bits for specific uses.  Bits designed for soft media (hardwoods, softwoods, polymers, ivory, horn and resin impregnated wood) should be used for cutting soft media.  Likewise for bits designed for composites (paper phenolic, fiberglass, carbon fiber, turquoise, malachite and Micarta) and bits for non-ferrous metals (aluminum, brass, copper, gold, silver, sterling, platinum).  
   
   Wood bits should not be used for cutting metal and metal bits should not be used for cutting wood.  



4. Plunge Rate
   Plunge rates that exceed the tip cutting capacity of a tool can cause the bit to deflect and break. With most PreciseBit microtools a plunge rate of 60 - 80 IPM is acceptable.
   


5. Excessive Runout
   "Runout" is defined as the deviation of the axis of a cutting tool from the rotational axis of the spindle.  It most often shows up as a side-to-side displacement of the entire bit or as an angular "wobble" of the cutting tip. Sources of runout in a high speed spindle can include asymmetric flute grinding, a worn / damaged collet, dirty / corroded collet, dirty / corroded spindle bore, poor collet seating during tool loading, bad spindle bearings, and/or loose spindle mounting. 
   
   Generally speaking, with carbide microtools, runout (designated TIR for Total Indicated Runout) that measures greater than 5 - 10% of the bit's cutting diameter can cause premature bit breakage in most hardwoods.
   


6. Variable Wood Thickness
   Different bits work better in different material thickness.  Please see Plunge Rate description 
   


7. Variable density



8. Backlash (hysteresis) in the X/Y drive