Pocket Milling

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The combination of CNC machines and CAD/CAM software allows machinists to cut materials faster and more accurately than what is possible when cutting by hand. One of the most common types of cutting is pocket milling (or pocketing), which sees extensive use in the aerospace and shipyard industries along with the uses a hobbyist has for the technique. Pocketing milling can be used with 2D or 2.5D projects and roughly 80% of all mechanical parts can be created with this kind of toolpath. Using this type of toolpath can make a big difference in the quality of a finished project.

Pocket milling

Pocket milling

Pocket milling allows you to use an end mill and machine away large parts of your raw material in a roughing process prior to finishing your project. This process allows you to save time and money while maximizing the amount of material removed. There are several styles of pocket milling such as traditional lace or zig-zag, concentric or offset In/Out and high speed cutting paths. The type of toolpath you pick can be categorized as linear or non-linear and there are parameters that can be set in many CAD/CAM software programs to control the pocket milling process.
Linear tool paths are unidirectional and the two major types are zig-zag tool paths and zig tool paths. Zig-zag milling causes the tool on your machine to move back and forth while removing material. Cutting is done with and against the rotation of the spindle. This method makes machining faster but also increases tool wear and machine chatter. Zig milling causes your tool to move in only one direction which results in better surface quality. However, you will have to lift and retract the tool after each cut so it takes more time to finish machining.
Non-linear tool paths are multi-directional with contour-parallel tool paths being an good example of this type of tool path. Contour-parallel tool paths use the pocket boundary to derive the tool path. Using this method your cutter is always in contact with your material, allowing you to eliminate time that would be needed to position and retract your tool using other methods. This method is used frequently for large-scale material removal and there are two different approaches that can be classified as contour-parallel tool paths. Pair-wise intersection approaches bring the boundary of the pocket inwards in steps. Offset segments intersect at concave corners and intersections are trimmed off to obtain the required contour. Voronoi diagram approaches segment the pocket boundary and the voronoi diagram is constructed for the entire pocket boundary. Voronoi diagrams are used to generate tools paths and this method is considered to be efficient since it avoids the topological problems machinists can run into using traditional offsetting algorithms.
Toolpath parameters for pocket milling in your CAD/CAM software will include data about roughing and finishing tools, pattern selections, cutting direction options (climb vs. conventional), toolpath cutting angle and step over % input fields. There should also be data that allows you to control side and bottom allowances for finishing as well as depth controls for single or multiple roughing steps. Many CAD/CAM software programs allow you to define single, even cutting depths based on the overall depth and define or force depths. Finally, there should be lead-in and lead-out options to control how the tool enters and exits the cut. Some CAD/CAM software programs will offer all or at least some of these options and some will not; it will depend on what software you are using. There are CAD/CAM programs that offer explanations and tutorials on pocket milling and these programs are usually the ones that are well equipped for pocket milling operations. Examples include BobCAD-CAM, CIMCO CNC-Calc and SimCam (CNCSimulator Pro).

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Article Name
Pocket Milling
Description
Pocket milling allows you to use an end mill and machine away large parts of your raw material in a roughing process prior to finishing your project. This process allows you to save time and money while maximizing the amount of material removed.
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