Visible lines in a 3D carve

Three different problems

1. Parallel lines running with the finishing pass

These are scallops, the ridges a round cutter leaves between passes. Normal, and controlled by geometry.

1. Stepover too large for the ball nose diameterMost common
   Calculate the actual scallop height rather than using a percentage.
2. Ball nose larger than the detail requiresCommon
   A bigger ball allows a bigger stepover for the same scallop, but it also cannot reach fine detail.
3. Steep areasCommon
   The same horizontal stepover is a much larger distance along a steep surface, so steep regions always look worse.
4. Dull ball noseOccasional
   A dull tip burnishes rather than cuts, leaving a dull streaked surface.

2. Terraces following the shape

These are roughing steps the finishing pass did not remove.

1. Machining allowance too small or zeroMost common
   The finishing tool had nothing to cut in places, so the roughing steps survived. See allowance.
2. Finishing pass did not cover the whole modelCommon
   Check the boundary of the finishing toolpath.
3. Roughing depth per pass very largeCommon
   Big steps leave more for finishing to remove, and a light finishing pass cannot take that much.
4. Finishing tool could not reachOccasional
   Steep or narrow areas where the tool geometry does not fit. Consider a tapered ball nose.

3. Facets on a smooth curve

The model itself. An STL is made of triangles, and a coarse export shows its triangles no matter how fine your toolpath is. No machining setting can add detail the model does not contain. Re-export at a higher resolution, or use a better model. See 3D carving overview.

Diagnostic steps

1. Sight along the lines. Straight, evenly spaced and parallel to one direction: scallops. Following the contours in steps: roughing. Flat facets with sharp edges on a curve: the model.
2. Measure the spacing. Scallops sit at exactly your finishing stepover. Terraces sit at the roughing depth per pass.
3. Preview the model in CAM at full zoom before blaming anything mechanical. A faceted preview is a faceted carve.
4. Compare flat and steep areas. Scallops that are fine on flat areas and coarse on slopes are geometry doing what geometry does, and want a steep-area strategy rather than a smaller stepover everywhere.

Fixes

1. Calculate the scallop height rather than guessing a stepover percentage.
2. Set a proper allowance on the roughing pass, Starting point usually 0.01 to 0.03 inches in wood.
3. Add a second finishing pass at 90 degrees to the first, which evens out directional lines.
4. Use a tapered ball nose for detail, because a straight small ball nose flexes and the flex shows.
5. Check the model resolution before you blame the machine.
6. Sand. Rule of thumb A scallop of about 0.001 inches is meant to be sanded out, not machined away.

Why it happens

A ball nose is round, so two adjacent passes always leave a ridge between them; the only question is how tall. A roughing pass is a flat tool cutting in steps, so it always leaves terraces; the finishing pass is supposed to remove them, and can only do so if there is a consistent allowance for it to cut. And an STL is triangles, which the toolpath follows faithfully. None of these is a fault. They are three different things the process does by nature, and each has its own control.

Prevention

• Set finishing stepover from a target scallop height, every time, using the calculator below.
• Give the roughing pass a real allowance and let the finishing pass do the finishing.
• Check the model's resolution in CAM before you cut, and re-export from the source at a finer tolerance if it looks faceted.
• Plan a separate strategy or a second pass at 90 degrees for anything with steep areas.