Pocket milling is one of the most common CNC operations -- removing material from an enclosed area to create a recess or cavity. Whether you are machining a simple rectangular slot or a complex contoured pocket, the right strategy determines surface finish, tool life, and cycle time. This guide covers everything you need to know about pocket milling.
The most basic pocket shape, defined by length, width, and depth. Rectangular pockets are common in fixture plates, housings, and structural parts. Corner radii are limited by the tool diameter -- a 10mm end mill leaves a minimum 5mm corner radius.
Round pockets used for bearing seats, O-ring grooves, and counterbores. These can be machined with a helical toolpath that spirals outward from the center, providing smooth and consistent tool engagement.
Free-form pockets defined by DXF contours or CAD geometry. These require CAM software to generate the toolpath, as manual G-Code programming would be extremely time-consuming.
The direction the tool engages the material significantly affects cut quality:
| Aspect | Climb Milling | Conventional Milling |
|---|---|---|
| Tool rotation vs feed | Same direction | Opposite direction |
| Chip formation | Thick to thin | Thin to thick |
| Surface finish | Better | Rougher |
| Tool life | Longer | Shorter |
| Cutting forces | Pull tool into work | Push tool away |
| Heat generation | Less (chips carry heat away) | More (rubbing at entry) |
| Machine requirement | Low backlash needed | Works on older machines |
| Recommended for | CNC machines (default) | Manual mills, poor fixturing |
Best practice: Use climb milling on CNC machines for better finish and longer tool life. Only use conventional milling on manual machines or when the workpiece fixturing is weak.
How the tool enters the material for the first cut is critical. Plunging straight down puts enormous axial load on the tool. Better entry methods exist:
The tool feeds straight down into the material. Only suitable for center-cutting end mills and shallow depths. High axial forces can break tools and leave poor surface finish on the pocket floor.
The tool moves down at an angle while cutting forward. This distributes the load between axial and radial directions, reducing stress. Typical ramp angles are 2-5 degrees. This is the most common entry method for pocket milling.
The tool spirals downward in a circular path. This is the gentlest entry method and best for hard materials and deep pockets. The tool engages gradually with minimal shock loading. Requires the pocket to be large enough for the helix diameter (typically 50-75% of pocket width).
The tool moves back and forth across the pocket, stepping over after each pass. Fast but alternates between climb and conventional cutting, which can affect surface finish.
The tool always cuts in the same direction (climb or conventional), lifting and repositioning between passes. Better surface finish but longer cycle time due to repositioning moves.
The toolpath follows the pocket boundary, spiraling inward with each pass. Provides consistent tool engagement and the best surface finish on pocket walls. This is the preferred strategy for most CNC work.
The tool follows a series of circular arcs with constant radial engagement. Excellent for hard materials and deep pockets because it maintains consistent chip load. Requires CAM software to generate.
After roughing removes the bulk material, a finishing pass improves surface quality:
| Parameter | Roughing | Finishing |
|---|---|---|
| Step Over | 40-60% of tool diameter | 5-15% of tool diameter |
| Step Down | 0.5-1.0x tool diameter | Full pocket depth |
| Feed Rate | Standard | 70-80% of roughing feed |
| Spindle Speed | Standard | 10-20% higher than roughing |
| Stock to Leave | 0.1-0.3mm on walls | 0 (final size) |
CAM232 supports rectangular pockets, circular pockets, and DXF-based contour pockets with automatic roughing and finishing passes. Preview in 3D before cutting.
Try CAM232 FreeEffective pocket milling requires choosing the right entry method, cutting direction, and toolpath strategy for your material and machine. Use helical or ramp entry instead of plunging, prefer climb milling on CNC machines, and always include a finishing pass for critical surfaces. CAM232 handles all of these decisions automatically, generating optimized pocket toolpaths from your DXF geometry.