Pocket toolpaths: clearing, stepover and finishing
A pocket clears everything inside a closed shape down to a depth. Offset patterns follow the shape inward and give better walls; raster patterns sweep back and forth and clear faster. A visible pattern in the floor is almost always a tramming problem, not a stepover one.
Clearing patterns
| Offset | Raster | Adaptive / trochoidal | |
|---|---|---|---|
| Pattern | Follows the shape, spiralling inward | Back and forth lines at an angle | Curved passes keeping engagement constant |
| Wall quality | Very good | Needs a separate finishing pass | Good, usually plus a finishing pass |
| Speed | Moderate | Often fastest on simple shapes | Fast on large areas, with deeper cuts |
| Tool load | Rises sharply in corners | Variable at the ends of each pass | Constant by design |
| Best for | Shapes with curved or irregular walls | Large simple rectangles | Big clearing jobs, hard material, small tools |
For most hobby work: offset for anything where the wall is visible, raster for a big rectangle you will not see the inside of, adaptive when you have a lot to remove and your CAM supports it.
The corners a round tool cannot reach
Same constraint as profile cutting: an inside corner cannot be sharper than half the cutting diameter. In a pocket this leaves a small amount of uncut material in every corner. Your options are to design the radius in, to run a rest machining pass with a smaller tool, or to add a dogbone relief if a square part has to fit.
A floor that looks flat
A flat end mill should leave a flat floor. When the floor shows a regular pattern of ridges, the causes in order of likelihood:
- The spindle is not trammedMost common
A spindle tilted even a fraction of a degree cuts a shallow dish rather than a flat circle, and the dishes leave ridges where they overlap. See tramming. - Stepover too largeCommon
It does not create the ridges, it makes existing ones taller and further apart. Reducing it hides the symptom. - Tool deflection under loadOccasional
The bit tips slightly in the cut. Lighter depth or a finishing pass helps. - The bit is not flatOccasional
Some end mills have a slight dish or a corner radius by design. Check what you are actually using. - The workpiece is not flatOccasional
A cupped board machined flat on top will still read as uneven if it flexes under clamping and relaxes after.
Islands
A shape inside a pocket that should not be cut, such as a raised letter in a recessed field. CAM handles this by selecting both the outer boundary and the inner shape. If your island gets cut away, the usual causes are that the vectors were not both selected, that the inner vector is open rather than closed, or that they overlap. See vector troubleshooting.
Deep pockets
- Chip evacuation becomes the limiting factor. Use an upcut bit and good extraction.
- A downcut bit in a deep pocket packs chips downward into a hole it is blocking. Rough with an upcut, finish shallow with a downcut.
- Tool deflection compounds with depth. A finishing pass at full depth is worth more here than anywhere else.
- Consider clearing the bulk with a larger tool and detailing with a smaller one via rest machining.