3D finishing passes

Raster versus offset

	Raster (parallel)	Offset (3D offset)	Spiral
Pattern	Back and forth straight lines, at an angle you choose	Follows the model's own contours inward	Continuous spiral over the surface
Surface character	Consistent directional texture	Follows the form, looks organic	Even, no direction change marks
Steep walls	Poor. Passes spread out on steep areas.	Usually better, because passes follow the contour instead of crossing it	Depends on the model
Direction changes	Many, at the end of each pass	Fewer	Almost none
Common use	The default for most reliefs	Shapes with strong contours, round subjects	Round models and medallions

For a typical relief, raster is the normal choice. Where a raster pass leaves visible lines on steep areas, adding a second finishing pass at 90 degrees to the first, or switching those regions to offset, usually solves it.

Raster angle, and the grain

The angle of the raster lines relative to the workpiece matters for two separate reasons:

• The wood grain. Passes that run along the grain cut cleanly. Passes across the grain fuzz more. On a relief you will get both, but the dominant direction is yours to choose. Running the raster at an angle across the grain, rather than exactly along or exactly across, often gives the most even result.
• The model's shape. Raster passes are widely spaced in real terms on a steep slope, because the horizontal stepover translates into a much larger distance along the surface. Aligning the raster so it climbs steep areas rather than traversing them reduces this.

45 degrees is a common choice for a reason: it is never exactly aligned with the grain and never exactly across it, and it handles most shapes reasonably. See grain direction.

Stepover for finishing

Do not use a percentage. The number that matters is the scallop height, the ridge left between passes, and the relationship between percentage and scallop depends on the tool radius.

Spending the time where it shows

Finishing is usually the majority of a 3D carve's run time, and most of that time goes into areas that did not need it. The strategies that actually reduce it:

1. Finish the whole model at a moderate stepover with a 1/8 inch ball nose.
2. Add a second finishing pass with a small tapered ball nose, restricted to a selected region: the face, the lettering, the detailed foreground.
3. Leave flat backgrounds out of the 3D finishing pass entirely. A flat area does not need a ball nose sweeping it at a fine stepover. A flat end mill levels it in a fraction of the time.
4. Use the shallow and steep split if your CAM offers it, applying different strategies to each.

Done well, this cuts many hours off a detailed carve with no visible loss. See 3D carve time.

Settings for the finishing pass

Starting point A finishing pass removes very little material, so tool load is not the constraint. What limits you is machine accuracy and how cleanly the tip cuts.

• Higher RPM than roughing, because the effective cutting diameter at the ball's contact point is small.
• Feed rate as fast as the machine can hold without the surface degrading. On detailed models the machine will not reach the programmed feed anyway.
• Full depth in one pass. Finishing is not a stepped operation.
• A genuinely sharp ball nose. This matters more here than the settings do.

Judging the result

• Run a thumb across it. Rule of thumb If you feel ridges, the scallop is above about 0.0015 inches.
• Look at it in raking light. Side lighting reveals machining lines that are invisible head on, and it is how the piece will be seen on a wall.
• Check steep areas separately. They will always be the worst part of a raster finish.
• Expect to sand. Even a fine finish benefits from a light hand sanding, and in a relief that means a soft sanding pad and patience rather than a power sander.