by The Saw Doctor
A few weeks ago I had two customers experience the same problem with the Vee Groove Bit they were using on their CNC Routering machine. Both routers broke the tungsten tip in exactly the same position on the router bit as you can see in the pictures below!
Apart from other factors such as wrong feed and speeds rates and incorrect cutters etc which cause their own problems, both of these routers had one thing in common as to why the tips broke in exactly the same position - right on the tip of the cutter.
The problem was caused from excessive vibration from using a worn chuck collet. The collet becomes worn over time from constantly tightening and loosening the collet and over this time a flat worn spot appears on the collet.
The wear on the collet happens over a period of time from unloosening and tightening the collet where it is clipped into the chuck locking nut. As this nut is loosened and tightened several times a week or month the collet gets worn.
Even if you cannot see any damage to the collet, it's being constantly heated and cooled when used. The collet is made from spring steel which allows it to have a certain amount of elasticity to grip the tool.
With the constant heating and cooling of the steel this reduces the elasticity and then over time the collet needs increased tightening to keep the router in the correct position. Both the increased tightening and the flat worn spot on the collet can lead to the collet tightening unevenly onto the shank of the cutter which then puts the cutter out of alignment.
The problem the customers experienced with the cutter tip breaking off occurs because the cutter is not properly tightened and aligned in the collet and this is then magnified as the cutter is being used at full operating speeds. It causes the cutter to vibrate which puts extra stress on the cutting point which leads the tungsten tip to break off. This is a sign that not all is well with the spindle chuck and therefore with the collet being a wearing part it should be replaced. It is recommended that the collet be replaced every 6 -12 months depending on use.
It is a good idea to check all the wearing parts regularly and replace if there is any sign of wear. It may seem expensive or time consuming but the cost of a new collet is a lot cheaper than unnecessarily breaking tools or damaging the material for the job. Each time the cutter is replaced the collet should be removed from the chuck and all dust particles should be removed as this also compounds the problem of the cutter tips breaking off whilst cutting.
The dust particles are abrasive which contribute to the collet wearing out sooner than it normally would. As the CNC Router machines are a precision machine which operate at high speed with little tolerances it is very important to keep all working parts extremely clean.
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