Rigidity is a key factor in the routing of plastic material. The problems associated with rigidity involve the part, as well as the machine. Parts must be held solidly with established fixturing techniques, and the machine must be appropriately maintained to insure the cutting tool is following the proper tool path in a rigid and concentric fashion. One of the elements that aids in this whole process lies in the area of proper colleting of the router bit and the on-going maintenance procedure associated with router collets.

Types of Collets
The half grip and full grip collets are the two basic types found in CNC routers. Half grip collets are identified by slits running from the bottom or mouth of the collet toward the top for about 80% of the collet length. These collets are often counter bored at the top not allowing the shank of the tool to contact the entire length of the collet. The force holding the tool is primarily generated at the mouth of the collet, and proves ideal in situations where the shanks of the router tools are not long enough to fill the entire collet.

(Figure 1)	(Figure 2)
Half Grip Collets	Full Grip Collets

The full grip collet is identified by slits running from both ends of the collet, which creates specific collet sections. Full grip type collets allow for squeezing pressure to be exerted over the entire length of the collet.

Proper Colleting
The proper method of colleting a router bit in the full grip collet is to fill at least 80% of the depth of the collet. This allows the tool to be equally distributed on all sections of the collet and provides an environment where the tool runs in a true circle or concentrically. Without concentricity, the finish of the plastic part is adversely effected, and tool failure can occur. There are situations where the 80% rule cannot be maintained because of inadequate shank lengths or extreme reach problems. Consequently, it becomes necessary to fill the void in the top of the collet with a filler or collet life plug. This is a practical solution to avoid collapsing of the collet, which may result from not following the 80% rule.

In all router bits, there is an area known as the flute fadeout section of the tools. This is formed when the grinding wheel utilized in the manufacturing of the tool exits the work piece. In order to properly collet a router bit, the mouth or bottom of the collet must contact the router bit slightly above the flute fadeout. Overcolleting or allowing the flute fadeout portion to extend inside the collet can damage the collet and is a common cause of tool breakage.

Collet Maintenance
Router bits and collets are expected to operate accurately in a work environment inundated with heat and grime. Plastic chips formed by the cutting action of the router bit carry with them resins that migrate through the slits of the collets and adhere to the inside of this closely toleranced mechanism. The resin build up usually concentrates nearest the mouth of the collet. At this point, the tool is no longer being equally gripped causing a loss in concentricity and tool run out. Once again, the lack of a router tool running in a true circle effects the finish of the part and may cause the ultimate demise of the tool.

(Figure 5)

Collet Examples

Fortunately, this problem is easily resolved by cleaning the collets after every tool change. The procedure involves the use of non-abrasive brass tube brush applied inside the collet in combination with a cleaner such as Rust Free. All surfaces inside and outside the collet, inside the spindle taper, and matching and mating surfaces of quick- change toolholders, should be thoroughly cleaned and dried before being reassembled. Also, the collet nut should be cleaned of resin and chip buildup, and regularly replaced to insure the integrity of the whole collet system.
 

(Figure 6)

Spindle Types

Collet Replacement
Collets are manufactured from spring steel and regular usage causes a loss of elasticity. Therefore, it becomes necessary to replace collets on a regular interval as a part of an on-going maintenance procedure. With diligent attention to proper collet maintenance, the average collet should be replaced about every 400-600 hours of run time. Avoiding regular replacement can lead to brittle collets, which may crack or break, and cause permanent damage to the spindle. Replacement of collets is a much more economical alternative than replacing router bits or expensive spindles.

Rigidity and concentricity are the key elements in any routing application. The simple process of properly colleting router tools, maintaining collets, and replacing them at regular intervals will safeguard the productivity of the operation and insure that the finish of plastic parts is not jeopardized.


To view previous articles by Van Niser, visit http://www.PlasticRouting.com

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