Copyright Goodheart-Willcox Co., Inc. 196 CNC Manufacturing Technology The first method entails creating a pilot hole. Pilot holes are often created by drilling a hole in the center of the area to be pocket milled first, then entering an end mill in that hole to get to Z depth and start machining. This effective method allows for less tool pressure upon entry. It has downsides, however, in that it requires pro- gramming of an extra tool (the drill), and care must be taken not to hit the bottom of the pocket and damage the floor. The second method is a helical entry, Figure 9-19. As in circle programming, an end mill can use a helical interpolation around the center of the area to be pocket milled for a smooth transition into the pocket. Often, feed rates are reduced upon entry and then turned up once depth is achieved. End mill size and rigidity of the holder may be factors in deciding how fast the tool can enter the pocket area. The last strategy is a ramp angle entry, Figure 9-20. The tool is positioned above the part and then moved in an X,Z line or Y,Z line. Normally, the angle of entry is 3° to allow for the trailing edge of the end mill to clear the cut as it progresses. This is a very effective entry move, but it requires more lines of code. The feed rate can be increased significantly on the ramp and then reduced for the pocket roughing depending on the ramp depth and angle. 9.7.2 Roughing Method Once entry has been considered, the tool must now be programmed to remove the material from the pocket. This simply involves making straight- line moves in either the X or Y axis, then moving the tool over an amount in the opposite axis, and then traveling back. A good starting number for the step-over amount is the radius of the end mill. It is best not to mill all the way out to the edge of the pocket initially, but instead leave a small amount of material for a finish pass. Once the pocket is roughed out to depth, use a contour path to finish to print specifications. Consider the pocket radii when selecting a tool. The tool radius must be smaller than the part radii or it will not be able to cut the correct corners. Analyze this sample program, which utilizes a ramp-angle entry: O1426 (POCKET) (MATERIAL - 6061 ALUMINUM) (T1 | 1/2 FLAT END MILL | H1) G0 G17 G20 G40 G80 T1 M6 (Rough pocket) G0 G90 G54 X.05 Y−.4 S3820 M3 G43 H1 Z.25 M8 Z.2 G1 Z.1 F100. X.1 Z.0967 X−.9 Z.0443 Goodheart-Willcox Publisher Figure 9-19. A side view of a helical-entry strategy. Goodheart-Willcox Publisher Figure 9-20. A ramped-entry strategy.