Milling Metric Cat - Jan Havelka

Technical InformationSurface Finish (cont’d.)FACE MILLSINSERTSSOLID CARBIDEMeasuring Finish Produced in a Milling OperationDo not rely on your eye or fingernail to determine surface finish.Fingernails are about 25 times as thick as the stylus tip of a surfacemeasuring instrument. They will skid across surface peaks, missingthe valleys. Use a surface measuring instrument since theappearance of the surface finish may be deceiving. For example,reflected light on a uniformly milled surface pattern will looksmoother than a random pattern. Also, a shiny surface will appearsmoother than a dull surface.Placement of the measuring device in a specific area on the milledsurface will affect the reading. Also, a surface finish measuredperpendicular to the feed direction is better than measuring parallelto the feed direction. This is usually the case regardless of workpiececondition and material (see figure 8).Ra finish2.52.01.51.0.500.25 0.76 2.5 mmcut-off widthChanging the cut-off width of the surface measuring instrument willaffect the Ra value of the measurement.SLOTTING90˚ MILLSSurface Profile RecordFigure 9 shows the variation in roughness created by increasing thecut-off width on the instrument. The greater the cut-off, (see figure 9),the higher the Ra finish. For example, figure 10 illustrates that a 0,25mm cut-off width will produce a 0.6 Ra finish; whereas, a 0,76 mmcut-off width will produce a 2.0 Ra finish.Also, figure 10 demonstrates that most measuring devices areprovided with 0,25 mm, 0,76 mm, and 2,54 mm cut-offs. In mostcases, the 0,76 mm width cut-off is preferred.DIE AND MOLDFigure 11 shows the standard surface symbols specifying maximumand minimum roughness, waviness, and lay, which is the direction inwhich the measurement is taken.CERAMIC MILLSPreferred Methodwaviness heightmax. roughness1.6 Ra0,05-2 mm0,25 mmwaviness widthroughness width cut-offCLASSIC MILLSmin. roughness 0.8Ra0,12 mmlayroughness widthfeed directionFigure 11: Standard surface symbolsTHREAD MILLSFigure 8: Measuring surface finishcut-off2.54 mmINDEXTECHNICAL DATA0,25 mm width cut-offFigure 9: Increasing the cut-off width creates a variation in roughness.530To place an order, contact Kennametal or your authorized Kennametal distributor, or visit www.kennametal.com.