Machinability of composite materials

With the exception of the sliding layer, SKF composite bushings can be machined using conventional methods. 

If bushings are required to have a smaller width than the standard size, this can easily be achieved by turning. It is also possible to drill lubrication holes. However, any burrs must be removed, particularly from the sliding surface. 

Strips can be bent, pressed, cut, or drilled to fit the individual application. When cutting or drilling, SKF recommends to work from the sliding surface side to avoid creating burrs in the sliding layer. 

Any metal surfaces that are exposed as a result of machining should be protected against corrosion. 

Machining the bore of PTFE composite bushings

PTFE composite bushings are ready for installation. Any resizing of the bore, i.e. the sliding surface, reduces the service life of the bushings and should only be done in exceptional cases. If machining of the bore is necessary, the bore should be resized and calibrated. Because various criteria, like material and wall thickness of the housing as well as the deviation of cylindrical form, have to be taken into consideration, SKF recommends using a calibrated mandrel to resize the bore (fig. 1). After resizing, bushing bore expansion tests and application field tests should be undertaken prior to the release of a final product. When machining the calibrating mandrel, the calibrating mandrel diameter dk should be manufactured in a first step about 60 to 80 μm larger than the maximum value listed in the column “Bore diameter of mounted bushing” in table 1 for metric or in table 2 for inch PTFE composite bushings.

Guideline values for the requisite interference of the calibrating mandrel as a function of the desired widening of the bushing bore for various bushing bore diameters can be obtained from diagram 1. This diagram is based on tests with bushings of different sizes and valid for bushings mounted in a housing with a housing outside diameter DG = 2 x bushing outside diameter D (fig. 1). If mandrel expansion tests show that the bore is too wide, the calibrating mandrel should be reground and the tests repeated on new bushings.

Machining the bore of POM composite bushings

Bushings made of POM composite material are ready for installation. However, if necessary, the bushing bore can be machined to size while mounted in its housing. After machining the bore, the sliding surface should have an Ra value of max. 2,5 μm. The following machining recommendations provide good results when a poly-crystalline diamond cutting tool is used:

  • cutting speed > 150 m/min 
  • slow feed rate (0,05 to 0,1 mm/revolution)
  • cutting depth 0,1 mm

To prevent excessive heat build-up of the acetal resin, and to remove the swarf produced during machining, a constant supply of compressed air at a low pressure should be applied to the bushing during any machining operation.

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