Collets are an extremely important component to the cutting head of the router, but their proper care and maintenance tend to fall by the wayside.
As a refresher on collets, there are two types. The first are half-grip collets that have slits running from the bottom of the collet towards the top, stopping at about 80% of the collet length. Typically the shank of the router bit cannot contact the entire length of the collet, thus the force holding the tool in place is generated at the mouth of the collet. These collets are ideal for router bits with shorter shank lengths.
The second kind of collet is a full-grip collet, with slits that run on both sides of the collet. These types of collets allow for squeezing pressure to be exerted over the entire shank of the router bit.
A router bit should fill 80% of a collet when inserted. The reason for this is to ensure the tool is equally distributed throughout the collet, and thus an equal exertion of pressure. This ensures concentricity of the tool, resulting in equal wear and proper cutting finish. If the collet were not filled to at least 80%, the collet could partially collapse, thus causing a loss of concentricity, and possible damage to the spindle or tool bit.
Proper collet maintenance and regular replacement are necessary to ensure accurate cut quality, and to decrease the likelihood of damaged parts. It is recommended to clean your collets every time there is a tool change. Plastic resin and other debris can become lodged in the slits of the collets, eventually causing the router tool to not be gripped uniformly throughout the collet. Just use a non-abrasive brass tube brush and some sort of rust cleaner.
It is recommended to replace your collet after 400-600 hours of use, as regular use will eventually cause a reduction in the elasticity of the collet. 17,000 RPM for over 600 hours can certainly take a toll on any moving part! Old collets will become brittle and could break, thus causing damage to more expensive parts such as tool bits or the spindle itself. At less than $35 a collet, timely replacement is certainly a worthwhile investment. So remember proper collet maintenance can save your company a lot of money on replacement parts and technical support.