In the methodology, the metal to be tested is cast into a sand mold in the shape of an 8-in. (20.3-cm) diameter circular disc that is 1.5-in. (5.1-cm) thick. The disc has a 2-in. (5.1-cm) diameter and 1.25-in. (3.2-cm) long hub for gripping in the chuck of a CNC center (Fig. 1). The interface of the cutting tool in contact with the workpiece disc is flooded with a 5 volume percent, water soluble oil cutting fluid to help maintain a constant tempera- ture at the tip of the cutting tool between interruptions of the fac- ing passes. An uncoated tungsten carbide insert is located in a cutting tool holder with a zero degree lead to minimize wear and catastrophic failure of the nose. The insert has a rhombic shape with an interior 0.5-in. (12.7 mm) diameter with a thickness of 0.1875 in. (4.76-mm), zero degree relief and a 0.03125-in. (0.8-mm) diameter nose radius. T e cutting tool catalog numbers
used in developing the machinability testing methodology are given in the us- er’s manual, although it may be necessary to select other sizes and grades for other ferrous and nonferrous castings. T e sizes and grades may be selected based on the following considerations: • Select simple geometries • Select simple chip breakers that minimize built-up-edge
• Select nose radii that are less than the depth of cut
• Select lead angles that minimize cutting with the nose of the cutting tool insert and wear the fl ank uni- formly to the depth of notch. A minimum of three cutting speeds with a feed rate of 0.008 ipr (0.203mm/rev) and depth of cut of 0.075 in. (1.91 mm) are used in the testing. Cutting speeds listed in ma- chining handbooks are used to select the following: • Low cutting speed that provides a tool life of 60 minutes
• High cutting speed that provides a tool life of 10 or more minutes
• Medium cutting speed between the low and high selected cut- ting speeds.
Feed rates for other cast met- als may be selected from machining Mar/Apr 2013 | METAL CASTING DESIGN & PURCHASING | 35
handbooks. A depth of cut up to 0.1 in. (2.54 cm) may be used for other cast metals if the disc does not slip in
2 Procedure
After clamping the hub in a three or four-jaw chuck of a turning center, the operator removes
0.02 in. (0.51 mm) off the face of the disc in a truing or skin pass to remove scale and solidifi cation eff ects on the subsurface microstructure. One of the selected cutting speeds is programmed for the fi rst test campaign or test insert run and the cutting tool/workpiece interface is fl ooded with the water soluble cutting fl uid. An up-sharp nose or corner of the cutting tool insert is used for each campaign. T e cutting tool path is programmed to take a cut or pass length leaving 0.5-in. diameter of uncut metal at the disc’s center (Fig. 2). T e tip of the cutting tool is pro- grammed to disengage quickly without
impacting the uncut metal at the disc’s center. T e test campaign is stopped frequently, usually after a completed pass, to remove and measure the fl ank wear on the cutting tool insert at a 30x magnifi cation. T e width of the wear land is measured in a zone of the cutting edge plane perpendicular to the major cutting edge, according to the measuring procedure in the International Standards Organization Specifi cation 3685-1977 (E). T e time to complete one facing pass is measured and recorded. T e fl ank wear is recorded, along with the number of completed passes on the test sheet. After measuring the insert, it is replaced, properly seated and rec- lamped in the tool holder, which has been cleared of chips and cutting fl uid
the chuck. Deeper cuts increase the amount of fl ank wear, which provides more area for measurements.
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