Ridges or lines in a pocket floor
A flat end mill should leave a flat floor. Regular ridges mean the spindle is tilted, so the bit is cutting a shallow dish instead of a flat circle. Tram the spindle. A smaller stepover makes the ridges finer, not absent.
A regular pattern of parallel ridges across the floor of a pocket, or across a surfaced area. Evenly spaced, matching the stepover, and always running in the direction of the passes.
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Most likely causes
- Spindle not trammedMost common
A spindle tilted even a fraction of a degree cuts a shallow arc rather than a flat circle, and the overlapping arcs leave ridges. See tramming. - Stepover too largeCommon
It does not create the ridges, it makes them taller and further apart. Reducing it treats the symptom. - Tool deflection under loadCommon
The tool tips slightly in a heavy cut, producing the same effect as a tilt. - The bit is not actually flatOccasional
Some end mills have a small corner radius or a slight dish. Check what you are really using. - Workpiece not flat or flexingOccasional
A cupped board clamped flat springs back after machining. - Spoilboard not surfacedOccasional
The reference plane is not parallel to the machine's motion. See spoilboards.
Diagnostic steps
- Run a fingernail across the ridges. If every ridge catches the nail in the same direction, the spindle leans that way. If they alternate, the tool is deflecting under load rather than the spindle leaning.
- Measure a single pass with a straightedge and feeler gauges, or a dial indicator, across its width. A tilted spindle leaves one side measurably deeper.
- Cut a second pocket at half the depth per pass. If the ridges shrink, deflection is contributing. If they do not change, it is tram.
- Check the bit itself. Roll the tip on a flat surface under a light. A corner radius or a dished end shows immediately.
Fixes, in order
- Tram the spindle. The real fix. Everything else is compensation.
- Surface the spoilboard once the spindle is trammed, so your reference plane is right too.
- Reduce the stepover, to around 40 percent of the diameter or less, which makes any residual ridges much finer.
- Add a light finishing pass over the floor, Starting point taking about 0.01 inches at a slower feed.
- Reduce depth per pass so the tool deflects less.
- Cross-hatch the final pass at 90 degrees to the first, which averages out the pattern.
Why it happens
A flat end mill cuts with its bottom edge. If the spindle axis is perfectly vertical, that edge sweeps a flat plane. Tilt it slightly and the edge sweeps a very shallow cone instead, so each pass is a dish, deeper on one side than the other. Lay overlapping dishes side by side and there is a ridge wherever two meet.
The error is magnified by the tool diameter, which is why a 1/4 inch end mill in a pocket might look fine while a 1.5 inch surfacing bit produces obvious corduroy from the same tilt. If you have just bought a surfacing bit and suddenly discovered this problem, the tilt was there all along.
Prevention
- Tram after assembly, after moving the machine, and after any crash or router remount.
- Surface the spoilboard after tramming, not before, so the reference plane inherits the corrected axis.
- Keep a surfacing bit as your tram gauge: one wide pass in scrap shows a lean faster than any indicator setup.